WO2022264956A1

WO2022264956A1 - Magnetic recording medium, magnetic tape cartridge, and magnetic recording reproduction device

Info

Publication number: WO2022264956A1
Application number: PCT/JP2022/023580
Authority: WO
Inventors: 成人笠田; 稔生多田
Original assignee: 富士フイルム株式会社
Priority date: 2021-06-17
Filing date: 2022-06-13
Publication date: 2022-12-22
Also published as: JPWO2022264956A1

Abstract

The present invention provides a magnetic recording medium that has a non-magnetic support and a first magnetic layer that includes a ferromagnetic powder and a binding agent, the magnetic recording medium also having a second magnetic layer between the non-magnetic support and the first magnetic layer, the second magnetic layer including a ferromagnetic powder that has a coercivity Hc of no more than 50 Oe and an average particle size of no more than 50 nm and a binding agent. The present invention also provides a magnetic tape cartridge and a magnetic recording reproduction device that include the magnetic recording medium.

Description

Magnetic Recording Media, Magnetic Tape Cartridges and Magnetic Recording/Reproducing Devices

The present invention relates to a magnetic recording medium, a magnetic tape cartridge, and a magnetic recording/reproducing device.

Magnetic recording media are roughly divided into two types: the metal thin film type and the coating type. A metal thin film magnetic recording medium is a magnetic recording medium having a magnetic layer of a metal thin film formed by sputtering or the like (see, for example, Patent Document 1). On the other hand, a coating type magnetic recording medium is a magnetic recording medium having a magnetic layer containing ferromagnetic powder together with a binder.

Patent No. 6531764

Magnetic recording methods include the perpendicular recording method and the longitudinal recording method. A hard disk drive (HDD), which is a representative example of a magnetic recording/reproducing device equipped with a metal thin film magnetic recording medium, generally employs a perpendicular recording method. Further, Japanese Patent No. 6531764 (Patent Document 1) discloses that recording is performed by a perpendicular recording system on a metal thin film magnetic recording medium having a magnetic layer formed by using a sputtering apparatus. (see paragraph 0102 of Patent Document 1).

On the other hand, for coating type magnetic recording media, the longitudinal recording method has conventionally been the mainstream recording method. However, the perpendicular recording method is a desirable recording method for further high-density recording because it can be expected to dramatically improve the recording density. Further, magnetic recording media are desired to exhibit excellent electromagnetic conversion characteristics.

In view of the above, it is an object of one aspect of the present invention to provide a coating type magnetic recording medium capable of exhibiting excellent electromagnetic conversion characteristics by magnetic recording in a perpendicular recording system.

One aspect of the present invention is as follows.
[1] having a non-magnetic support and a first magnetic layer containing a ferromagnetic powder and a binder;
A second magnetic layer containing a ferromagnetic powder having a coercive force Hc of 50 Oe or less and an average particle size of 50 nm or less and a binder between the non-magnetic support and the first magnetic layer. A magnetic recording medium further comprising:
[2] The number of equivalent circle diameters of a plurality of bright regions in a binarized secondary electron image obtained by imaging the surface of the first magnetic layer with a scanning electron microscope at an accelerating voltage of 5 kV. Distribution A is the following (1) to (3):
(1) 10,000 or more and 30,000 or less bright regions having an equivalent circle diameter of 1 nm or more and 50 nm or less,
(2) 7000 or more and 25000 or less bright regions with an equivalent circle diameter of 51 nm or more and 100 nm or less;
(3) 1000 or more and 3000 or less bright regions with an equivalent circle diameter of 101 nm or more;
and the number of circle-equivalent diameters of a plurality of dark areas in a binarized secondary electron image obtained by imaging the surface of the first magnetic layer with a scanning electron microscope at an accelerating voltage of 2 kV. Distribution B is the following (4) to (6):
(4) 200 or more and 50,000 or less dark regions with an equivalent circle diameter of 1 nm or more and 50 nm or less;
(5) 200 or more and 25,000 or less dark regions with an equivalent circle diameter of 51 nm or more and 100 nm or less;
(6) 0 or more and 2000 or less dark regions having an equivalent circle diameter of 101 nm or more;
The magnetic recording medium according to [1], which satisfies:
[3] The magnetic recording medium according to [1] or [2], wherein the perpendicular squareness ratio of the magnetic recording medium is 0.60 or more.
[4] The magnetic recording medium according to any one of [1] to [3], wherein the ferromagnetic powder contained in the second magnetic layer has a coercive force Hc of 10 Oe or more and 50 Oe or less.
[5] The magnetic recording medium according to any one of [1] to [4], wherein the ferromagnetic powder contained in the second magnetic layer has an average particle size of 5 nm or more and 50 nm or less.
[6] The magnetic recording medium according to any one of [1] to [5], wherein the ferromagnetic powder contained in the second magnetic layer is spinel ferrite powder.
[7] The magnetic recording medium according to any one of [1] to [6], wherein the ferromagnetic powder contained in the first magnetic layer is hexagonal barium ferrite powder.
[8] The magnetic recording medium according to any one of [1] to [6], wherein the ferromagnetic powder contained in the first magnetic layer is hexagonal strontium ferrite powder.
[9] The magnetic recording medium according to any one of [1] to [6], wherein the ferromagnetic powder contained in the first magnetic layer is ε-iron oxide powder.
[10] The magnetic recording according to any one of [1] to [9], further comprising a nonmagnetic layer containing nonmagnetic powder and a binder between the nonmagnetic support and the second magnetic layer. medium.
[11] The non-magnetic support further has a back coat layer containing a non-magnetic powder and a binder on the surface opposite to the surface having the first magnetic layer and the second magnetic layer, [ 1] The magnetic recording medium according to any one of [10].
[12] The magnetic recording medium according to any one of [1] to [11], which is a magnetic tape.
[13] A magnetic tape cartridge containing the magnetic recording medium according to [12].
[14] A magnetic recording/reproducing device including the magnetic recording medium according to any one of [1] to [12].

According to one aspect of the present invention, it is possible to provide a coating-type magnetic recording medium capable of exhibiting excellent electromagnetic conversion characteristics by recording in a perpendicular recording system. Further, according to one aspect of the present invention, it is possible to provide a magnetic tape cartridge and a magnetic recording/reproducing apparatus including such a magnetic recording medium.

An example arrangement of data bands and servo bands is shown. An example of servo pattern arrangement for an LTO (Linear Tape-Open) Ultrium format tape is shown.

[Magnetic recording medium]
A magnetic recording medium according to one aspect of the present invention has a non-magnetic support and a first magnetic layer containing ferromagnetic powder and a binder. The magnetic recording medium includes a ferromagnetic powder having a coercive force Hc of 50 Oe or less and an average particle size of 50 nm or less, and a binder between the non-magnetic support and the first magnetic layer. It further has a second magnetic layer comprising:

The magnetic recording medium has the first magnetic layer and the second magnetic layer.
The first magnetic layer is a layer in which data is recorded by magnetic recording, that is, a layer that can function as a recording layer. Data is recorded on the recording layer by applying a magnetic field from the magnetic head to magnetize the ferromagnetic powder particles in the recording layer. The longitudinal recording method is a recording method in which the direction of the magnetic field applied to the recording layer for magnetization is controlled so as to be parallel to the surface of the recording layer. The in-plane recording method is also generally called a horizontal recording method, a longitudinal recording method, or the like. On the other hand, the perpendicular recording method is a recording method in which the direction of the magnetic field applied to the recording layer for magnetization is controlled so as to be perpendicular to the surface of the recording layer. In the present invention and this specification, the term "perpendicular" does not necessarily mean only verticality in a strict sense, but includes the range of error normally permitted in the technical field to which the present invention belongs. A margin of error can mean, for example, a range of less than ±10 degrees of strict vertical. This point is the same for "horizontal".
In data recording by the perpendicular recording method, a soft magnetic underlayer is generally used to collect the magnetic field sent from the magnetic head in the direction perpendicular to the surface of the recording layer through the recording layer and back to the recording head. (Soft-magnetic Under Layer; SUL) is required. In the magnetic recording medium, the second magnetic layer can be a so-called soft magnetic layer, and can function as the soft magnetic underlayer. The ferromagnetic powder contained in the second magnetic layer that can function as a soft magnetic underlayer in the magnetic recording medium has a coercive force Hc of 50 Oe or less and an average particle size of 50 nm or less. However, it is believed that the magnetic recording medium, which is a coating type magnetic recording medium, can contribute to exhibiting excellent electromagnetic conversion characteristics (hereinafter also simply referred to as "electromagnetic conversion characteristics") by recording in the perpendicular recording system. The inventor speculates. Specifically, the inventor believes that the fact that the coercive force Hc of the ferromagnetic powder contained in the second magnetic layer is 50 Oe or less can contribute to suppressing the noise derived from the second magnetic layer. there is Further, the present inventors speculate that the fact that the average particle size of the ferromagnetic powder contained in the second magnetic layer is 50 nm or less can contribute to suppressing the decrease in output. The present inventor believes that this is because the small average particle size of the ferromagnetic powder contained in the second magnetic layer of 50 nm or less contributes to the suppression of roughening of the second magnetic layer. there is However, the invention is not limited to the speculations described herein. Further, as described above, the magnetic recording medium is suitable as a magnetic recording medium for perpendicular recording. However, it is not excluded that magnetic recording is performed on the magnetic recording medium by the longitudinal recording method.

The magnetic recording medium will be described in more detail below.

<Second magnetic layer>
(Various physical properties of ferromagnetic powder)
Coercive force Hc
In the present invention and herein, the coercive force Hc of ferromagnetic powder can be measured by a vibrating sample magnetometer. In the present invention and this specification, coercive force Hc is a value measured at a measurement temperature of 25°C ± 1°C. The measurement temperature is the temperature of the ferromagnetic powder at the time of measurement. By setting the temperature of the atmosphere around the ferromagnetic powder to be measured to the measurement temperature, temperature equilibrium is established, whereby the temperature of the ferromagnetic powder to be measured can be set to the measurement temperature. The unit Oe (Oersted) of the coercive force Hc is 1 [Oe]=10 ³ /4π [A/m].

The ferromagnetic powder contained in the second magnetic layer can be ferromagnetic powder called soft magnetic powder, and its coercive force Hc is 50 Oe or less. The coercive force Hc of the ferromagnetic powder contained in the second magnetic layer is preferably 45 Oe or less from the viewpoint of further improving the electromagnetic conversion characteristics, and is preferably 40 Oe or less, 35 Oe or less, 30 Oe or less, 25 Oe or less, or 20 Oe or less. is more preferable in that order. The coercive force Hc of the ferromagnetic powder contained in the second magnetic layer can be, for example, 1 Oe or more, 3 Oe or more, or 5 Oe or more. A low coercive force Hc of the ferromagnetic powder contained in the second magnetic layer is preferable from the viewpoint of further improving the electromagnetic conversion characteristics. The ferromagnetic powder having the coercive force Hc can be spinel ferrite powder, for example. In the present invention and in this specification, the term "spinel ferrite powder" refers to ferromagnetic powder in which the crystal structure of spinel ferrite is detected as the main phase by X-ray diffraction analysis. In the present invention and the specification, the term "main phase" refers to a structure to which the highest intensity diffraction peak is assigned in an X-ray diffraction spectrum obtained by X-ray diffraction analysis. For example, when the highest intensity diffraction peak in an X-ray diffraction spectrum obtained by X-ray diffraction analysis is attributed to the crystal structure of spinel ferrite, it is determined that the crystal structure of spinel ferrite has been detected as the main phase. When only a single phase is detected by X-ray diffraction analysis, this detected structure is taken as the main phase.

Average Particle Size In the present invention and this specification, unless otherwise specified, the average particle size of various powders such as ferromagnetic powder is a value measured by the following method using a transmission electron microscope.
The powder is photographed using a transmission electron microscope at a magnification of 100,000 times, and the photograph of the particles constituting the powder is obtained by printing on photographic paper or displaying it on a display so that the total magnification is 500,000 times. . The particle of interest is selected from the photograph of the obtained particle, the outline of the particle is traced with a digitizer, and the size of the particle (primary particle) is measured. Primary particles refer to individual particles without agglomeration.
The above measurements are performed on 500 randomly selected particles. The arithmetic mean of the particle sizes of the 500 particles thus obtained is taken as the average particle size of the powder. As the transmission electron microscope, for example, Hitachi's H-9000 transmission electron microscope can be used. Further, the particle size can be measured using known image analysis software such as Carl Zeiss image analysis software KS-400. Unless otherwise specified, the average particle size shown in the examples below was measured using a transmission electron microscope H-9000 manufactured by Hitachi, and image analysis software KS-400 manufactured by Carl Zeiss as image analysis software. value. In the present invention and herein, powder means a collection of particles. For example, ferromagnetic powder means a collection of ferromagnetic particles. In addition, the aggregation of a plurality of particles is not limited to the form in which the particles constituting the aggregation are in direct contact, but also includes the form in which binders, additives, etc., which will be described later, are interposed between the particles. be. The term particles is sometimes used to describe powders.

In the present invention and this specification, unless otherwise specified, the size of the particles constituting the powder (particle size) is the shape of the particles observed in the above particle photographs.
(1) In the case of needle-like, spindle-like, columnar (however, the height is greater than the maximum length of the bottom surface), etc., the length of the long axis constituting the particle, that is, the length of the long axis,
(2) In the case of a plate-like or columnar shape (where the thickness or height is smaller than the maximum major diameter of the plate surface or bottom surface), it is expressed by the maximum major diameter of the plate surface or bottom surface,
(3) If the particle is spherical, polyhedral, irregular, or the like, and the long axis of the particle cannot be specified from the shape, it is represented by an equivalent circle diameter. Equivalent circle diameter means the diameter obtained by circular projection method.

In addition, the average acicular ratio of the powder is obtained by measuring the length of the minor axis of the particles in the above measurement, that is, the minor axis length, and obtaining the value of (long axis length / minor axis length) of each particle. It refers to the arithmetic mean of the values obtained for the particles. Here, unless otherwise specified, the minor axis length is the length of the minor axis constituting the particle in the case of (1) in the definition of the particle size, and the thickness or height in the case of (2). In the case of (3), since there is no distinction between the major axis and the minor axis, (long axis length/short axis length) is regarded as 1 for convenience.
Unless otherwise specified, when the particle shape is specific, for example, in the case of the definition (1) of the particle size, the average particle size is the average major axis length, and in the case of the definition (2), the average particle size is Average plate diameter. In the case of the same definition (3), the average particle size is the average diameter (also referred to as average particle size or average particle size).

The average particle size of the ferromagnetic powder contained in the second magnetic layer is 50 nm or less, preferably 45 nm or less from the viewpoint of further improving the electromagnetic conversion characteristics, The order of 25 nm or less is more preferable. The average particle size of the ferromagnetic powder contained in the second magnetic layer can be, for example, 3 nm or more, 5 nm or more, or 10 nm or more. A small average particle size of the ferromagnetic powder contained in the second magnetic layer is preferable from the viewpoint of further improving the electromagnetic conversion characteristics.

Regarding the measurement of coercive force Hc and average particle size, if powder samples are available, the above measurements can be performed on such powder samples. Alternatively, sample powder can be collected from the magnetic recording medium for various measurements. As a method for collecting a powder sample from a magnetic recording medium, for example, the method described in paragraph 0015 of JP-A-2011-048878 can be adopted.

The coercive force Hc and average particle size of the ferromagnetic powder can be controlled, for example, by the preparation conditions of the ferromagnetic powder. This point will be further described later.

(Method for preparing ferromagnetic powder)
A glass crystallization method will be described below as an example of a method for preparing spinel ferrite powder that can be used as the ferromagnetic powder for the second magnetic layer. However, the ferromagnetic powder contained in the second magnetic layer is not limited to the spinel ferrite powder obtained by this preparation method.

A glass crystallization method generally includes the following steps.
(1) a step of melting a raw material mixture containing at least a spinel ferrite-forming component and a glass-forming component to obtain a melt (melting step);
(2) a step of quenching the melt to obtain an amorphous body (amorphization step);
(3) A step of heat-treating an amorphous body to obtain a crystallized product containing spinel ferrite particles precipitated by the heat-treating and a crystallized glass component (crystallization step);
(4) A step of collecting spinel ferrite particles from the crystallized product (particle collecting step).

The above steps will be explained in more detail below.

Melting Process The raw material mixture used in the glass crystallization method for obtaining spinel ferrite powder contains a spinel ferrite-forming component and a glass-forming component. Examples of the glass-forming component include oxides containing atoms that form constituent atoms of the later-described glass component. When using a glass crystallization method to obtain a spinel ferrite powder that can be used as the ferromagnetic powder of the second magnetic layer, the glass-forming components can be, for example, B ₂ O ₃ and XO. Examples of X atoms include alkaline earth metal atoms such as calcium atoms (Ca), strontium atoms (Sr), and barium atoms (Ba). By using the B ₂ O ₃ component and the XO component together, a glass component represented by the composition formula: XB ₂ O ₄ can be formed as a glass component described later. Each component contained in the raw material mixture in the glass crystallization method exists as an oxide or as various salts that can be converted to an oxide during a process such as melting. In the present invention and the specification, the term "B ₂ O ₃ component" is intended to include B ₂ O ₃ itself and various salts such as H ₃ BO ₃ that can be converted to B ₂ O ₃ during the process. do. This point also applies to other components.

Examples of the spinel ferrite-forming component contained in the raw material mixture include oxides containing atoms that form constituent atoms of the spinel ferrite crystal structure. Examples of spinel ferrite include spinel ferrite represented by the composition formula: AFe ₂ O ₄ . Examples of A atoms include divalent metal atoms such as nickel atoms (Ni), manganese atoms (Mn), zinc atoms (Zn), and copper atoms (Cu). A divalent metal atom refers to a metal atom that can become a divalent cation as an ion. Specific examples of spinel ferrite-forming components include Fe ₂ O ₃ component and AO component. Examples of the AO component include NiO component, MnO component, ZnO component, and CuO component.

The content of various components in the raw material mixture is not particularly limited. Regarding the coercive force Hc of the spinel ferrite powder, as a result of examination by the present inventors, for example, for a spinel ferrite powder containing nickel atoms and zinc atoms as A atoms, the higher the content of zinc atoms, the lower the coercive force Hc tends to be. seen. Therefore, increasing the content of the ZnO component in the raw material mixture containing the NiO component and the ZnO component can lead to a decrease in the coercive force Hc of the spinel ferrite powder. The raw material mixture can be prepared by weighing and mixing various components. Next, the raw material mixture is melted to obtain a melt. The melting temperature may be set according to the composition of the raw material mixture, and is usually 1000 to 1500°C. The melting time may be appropriately set so that the raw material mixture is sufficiently melted.

Amorphization Step Next, the obtained melt is rapidly cooled to obtain an amorphous body. The above quenching can be carried out in the same manner as the quenching process usually performed to obtain an amorphous body by the glass crystallization method. etc. can be performed by a known method.

Crystallization Step After the rapid cooling, the obtained amorphous material is heat-treated. By this heat treatment, spinel ferrite particles and crystallized glass components can be precipitated. The particle size of the spinel ferrite particles to be precipitated can be controlled by heating conditions. When the heating temperature for crystallization (crystallization temperature) is increased, the grain size of the precipitated spinel ferrite grains tends to increase, and as a result, the average grain size of the prepared spinel ferrite powder tends to increase. . Further, for spinel ferrite powders, when the composition of the raw material mixture is the same, the larger the average particle size, the higher the coercive force Hc tends to be. In consideration of the above points, it is preferable to control the heating conditions so as to obtain a spinel ferrite powder having an average particle size within the above range and a coercive force Hc within the above range. In one form, the crystallization temperature is preferably in the range of 600°C to 690°C. The heating time for crystallization (holding time at the crystallization temperature) is, in one form, for example 0.1 to 24 hours, preferably 0.15 to 8 hours.

Particle Collection Step The crystallized material obtained by heat-treating the amorphous body contains spinel ferrite particles and crystallized glass components. Therefore, when the crystallized product is subjected to an acid treatment, the crystallized glass component surrounding the spinel ferrite particles is dissolved and removed, so that the spinel ferrite particles can be collected. Prior to the acid treatment, coarse pulverization is preferably carried out in order to increase the efficiency of the acid treatment. Coarse pulverization may be carried out by either a dry method or a wet method. Conditions for coarse pulverization can be set according to a known method. The acid treatment for particle collection can be carried out by a method commonly used in glass crystallization, such as acid treatment under heating. Thereafter, spinel ferrite powder can be obtained by performing post-treatment such as classification (for example, centrifugal separation, decantation, magnetic separation method, etc.), washing with water, and drying, if necessary.

An example of a method for preparing spinel ferrite powder has been described above. However, the ferromagnetic powder contained in the second magnetic layer of the magnetic recording medium is not limited to that obtained by the above preparation method.

(Binder)
The above magnetic recording medium is a coating type magnetic recording medium and contains a binder in the second magnetic layer. A binder is one or more resins. As the binder, various resins commonly used as binders for coating-type magnetic recording media can be used. Examples of binders include polyurethane resins, polyester resins, polyamide resins, vinyl chloride resins, acrylic resins obtained by copolymerizing styrene, acrylonitrile, methyl methacrylate, etc., cellulose resins such as nitrocellulose, epoxy resins, phenoxy resins, polyvinyl acetal, A resin selected from polyvinyl alkylal resins such as polyvinyl butyral can be used alone, or a plurality of resins can be mixed and used. Preferred among these are polyurethane resins, acrylic resins, cellulose resins, and vinyl chloride resins. These resins may be homopolymers or copolymers. These resins can also be used as binders in the first magnetic layer, non-magnetic layer and/or back coat layer, which will be described later.
Regarding the above binders, paragraphs 0028 to 0031 of JP-A-2010-24113 can be referred to. The weight-average molecular weight of the resin used as the binder can be, for example, 10,000 or more and 200,000 or less.

The second magnetic layer contains the ferromagnetic powder and binder, and may contain one or more additives. For other details of the second magnetic layer, such as additives that may be included in the second magnetic layer, see known techniques for magnetic layers commonly referred to as soft magnetic underlayers, known techniques for magnetic layers commonly referred to as recording layers, and/or A known technique for the non-magnetic layer can be applied. Further, with regard to the second magnetic layer, it is also possible to refer to the below-described description of the first magnetic layer and/or the following description of the non-magnetic layer.

The second magnetic layer described above can be provided directly on the surface of the non-magnetic support or indirectly via the non-magnetic layer.

<First magnetic layer>
In the magnetic recording medium, the first magnetic layer is a layer that can function as a recording layer. From the viewpoint of further improving the electromagnetic conversion characteristics, on the surface of the first magnetic layer, the number distribution A satisfies (1) to (3) described above and/or the number distribution B described above. It is preferable to satisfy (4) to (6). In the magnetic recording medium, the number distribution A preferably satisfies (1) to (3) described above, and the number distribution B satisfies (4) to (6) described above.

The scanning electron microscope used in the present invention and herein for determining the number distribution A and the number distribution B, respectively, is a Field Emission-Scanning Electron Microscope (FE-SEM). As the FE-SEM, for example, FE-SEM S4800 manufactured by Hitachi, Ltd. can be used, and this FE-SEM was used in the examples described later. As the magnetic recording medium to be measured, an unused magnetic recording medium which is not attached to the magnetic recording/reproducing apparatus is used. For example, magnetic tapes are usually distributed in magnetic tape cartridges. For example, as the magnetic tape to be measured, a magnetic tape taken out from an unused magnetic tape cartridge that is not attached to the magnetic recording/reproducing apparatus is used.
Further, when obtaining the number distribution A and the number distribution B, the surface of the first magnetic layer is not coated before taking the SEM image.
Each image is taken by selecting an unimaged area on the surface of the first magnetic layer.
The captured SEM image is a secondary electron image.
The circle-equivalent diameter is obtained in increments of 1 nm by rounding off to the first decimal place and omitting the second decimal place and beyond.
When calculating the number distribution A, in the measurement of the number of bright areas, only a part of the bright area is included in the binarized image and the remaining part is outside the binarized image. shall be excluded.
When calculating the number distribution B, in the measurement of the number of dark areas, dark areas that are partly included in the binarized image and the remaining part is outside the binarized image are excluded from the measurement target. shall be
In addition, in the present invention and this specification, the term "(the) surface of the first magnetic layer" is synonymous with the first magnetic layer side surface of the magnetic recording medium.

(Method for measuring number distribution A)
In the present invention and the specification, "number distribution A" is a number distribution measured by the following method.
Using a scanning electron microscope (FE-SEM), a secondary electron image of the surface of the first magnetic layer of the magnetic recording medium to be measured is captured. As imaging conditions, the acceleration voltage is 5 kV, the working distance is 5 mm, and the imaging magnification is 10,000. At the time of imaging, a non-imaging area on the surface of the first magnetic layer is selected, focus is adjusted under the imaging conditions described above, and a secondary electron image is captured. A secondary electron image with a pixel count of 960 pixels×1280 pixels is obtained by deleting the portion (micron bar, cross mark, etc.) indicating the size and the like from the imaged image.
The above operation is performed 100 times at different locations on the surface of the first magnetic layer of the magnetic recording medium to be measured.
The secondary electron image obtained in this manner is taken into image processing software, and binarization processing is performed according to the following procedure. As image analysis software, for example, ImageJ, which is free software, can be used. The binarization process divides the image into a bright area (white area) and a dark area (black area).
The threshold value for binarizing the secondary electron image obtained above has a lower limit of 210 gradations and an upper limit of 255 gradations, and binarization is performed using these two thresholds. After the binarization processing, noise component removal processing is performed using image analysis software. Noise component removal processing can be performed, for example, by the following method. In the image analysis software ImageJ, noise cut processing Despeckle is selected to remove noise components.
For the binarized image thus obtained, the number of bright regions (that is, white portions) and the area of each bright region are determined by image analysis software. The equivalent circle diameter of each bright region is determined from the area of the bright region determined here. Specifically, from the determined area A, the equivalent circle diameter L is calculated by (A/π)̂(1/2)×2=L. Here, the operator "^" represents exponentiation.
The above steps are performed on the binarized images (100 images) obtained above.
Thus, the number distribution A is obtained.

(Method for measuring number distribution B)
In the present invention and the specification, "number distribution B" is a number distribution measured by the following method.
Using a scanning electron microscope (FE-SEM), a secondary electron image of the surface of the first magnetic layer of the magnetic recording medium to be measured is taken. As imaging conditions, the acceleration voltage is 2 kV, the working distance is 5 mm, and the imaging magnification is 10,000 times. At the time of imaging, a non-imaging area on the surface of the first magnetic layer is selected, focus is adjusted under the imaging conditions described above, and a secondary electron image is captured. A secondary electron image with a pixel count of 960 pixels×1280 pixels is obtained by deleting the portion (micron bar, cross mark, etc.) indicating the size and the like from the imaged image.
The above operation is performed 100 times at different locations on the surface of the first magnetic layer of the magnetic recording medium to be measured.
The secondary electron image obtained in this manner is taken into image processing software, and binarization processing is performed according to the following procedure. As image analysis software, for example, ImageJ, which is free software, can be used.
The threshold value for binarizing the secondary electron image obtained above has a lower limit value of 0 gradation and an upper limit value of 75 gradation, and the binarization process is performed using these two threshold values. After the binarization processing, noise component removal processing is performed using image analysis software. Noise component removal processing can be performed, for example, by the following method. In the image analysis software ImageJ, noise cut processing Despeckle is selected to remove noise components.
In the binarized image thus obtained, the number of dark regions (that is, black portions) and the area of each dark region are determined by image analysis software. The equivalent circle diameter of each dark region is obtained from the area of the dark region obtained here. Specifically, from the determined area A, the equivalent circle diameter L is calculated by (A/π)̂(1/2)×2=L.
The above steps are performed on the binarized images (100 images) obtained above. Thus, the number distribution B is obtained.

(Number distribution A and number distribution B)
In the above magnetic recording medium, the number distribution A determined by the method described above is the following (1) to (3):
(1) 10,000 or more and 30,000 or less bright regions with an equivalent circle diameter of 1 nm or more and 50 nm or less,
(2) 7000 or more and 25000 or less bright regions with an equivalent circle diameter of 51 nm or more and 100 nm or less;
(3) 1000 or more and 3000 or less bright regions with an equivalent circle diameter of 101 nm or more,
is preferably satisfied.

Furthermore, in the magnetic recording medium, the number distribution B obtained by the method described above is the following (4) to (6):
(4) 200 or more and 50,000 or less dark regions with an equivalent circle diameter of 1 nm or more and 50 nm or less;
(5) 200 or more and 25,000 or less dark regions with an equivalent circle diameter of 51 nm or more and 100 nm or less;
(6) 0 or more and 2000 or less dark regions having an equivalent circle diameter of 101 nm or more;
is preferably satisfied.

The first magnetic layer can be formed, for example, using a magnetic layer-forming composition containing one or more non-magnetic powders in addition to ferromagnetic powder. With respect to the number distribution A and the number distribution B obtained by the method described above, the present inventors found that the number distribution A It is believed that the magnetic powder (hereinafter also referred to as "abrasive") can serve as an indicator of the state of existence on the surface of the first magnetic layer. The number distribution B is the number distribution of the non-magnetic powder (hereinafter also referred to as "filler") contained in the first magnetic layer to form moderate protrusions on the surface of the first magnetic layer for controlling the frictional characteristics. The present inventor believes that it can serve as an indicator of the state of existence on the surface of one magnetic layer. Controlling the number distribution A and the number distribution B as described above can contribute to further improvement of the electromagnetic conversion characteristics. From the viewpoint of achieving both a reduction in spacing loss and an improvement in the stability of the contact state between the surface of the first magnetic layer and the magnetic head, it is preferable that the number distribution A and the number distribution B are within the above ranges. presumed to be for this reason.

Regarding (1) above, there are 10,000 or more and 30,000 or less bright regions with equivalent circle diameters of 1 nm or more and 50 nm or less. The number of bright regions is preferably 15000 or more, more preferably 20000 or more. In addition, the number of bright regions is preferably 28000 or less, more preferably 25000 or less.

Regarding (2) above, the number of bright regions having equivalent circle diameters of 51 nm or more and 100 nm or less is 7000 or more and 25000 or less. The number of bright regions is preferably 8000 or more, more preferably 9000 or more. Also, the number of bright regions is preferably 24000 or less, more preferably 23000 or less.

Regarding (3) above, the number of bright areas with an equivalent circle diameter of 101 nm or more is 1000 or more and 3000 or less. The number of bright regions is preferably 1500 or more, more preferably 2000 or more. In addition, the number of bright regions is preferably 2800 or less, more preferably 2500 or less. In one aspect, the number of bright regions may be less than 3000. Also, in one form, the number of bright regions may be more than 1,000.

Regarding (4) above, the number of dark regions with an equivalent circle diameter of 1 nm or more and 50 nm or less is 200 or more and 50000 or less. The number of such dark regions is preferably 1000 or more, more preferably 2000 or more, and even more preferably 3000 or more. Also, the number of such dark regions is preferably 40,000 or less, more preferably 30,000 or less. In one form, the number of dark regions can be greater than 1000.

Regarding (5) above, the number of dark regions with an equivalent circle diameter of 51 nm or more and 100 nm or less is 200 or more and 25000 or less. The number of such dark regions is preferably 250 or more, more preferably 300 or more. Also, the number of such dark regions is preferably 20,000 or less, more preferably 15,000 or less.

Regarding (6) above, the number of dark regions with an equivalent circle diameter of 101 nm or more is 0 or more and 2000 or less. The number of such dark regions is preferably 10 or more, more preferably 20 or more. Also, the number of such dark regions is preferably 1500 or less, more preferably 1000 or less. In one aspect, the number of dark regions may be less than 200.

Number distribution A and number distribution B depend on the type of components added to the composition for forming the first magnetic layer used to form the first magnetic layer, the method of preparing such composition (e.g., dispersion method, classification method, etc.). method, etc.). For specific examples of the control method, the examples described later can also be referred to.

(ferromagnetic powder)
As the ferromagnetic powder contained in the first magnetic layer, one or a combination of two or more ferromagnetic powders known as ferromagnetic powders generally used in magnetic layers generally called recording layers in various magnetic recording media are used. be able to. From the viewpoint of improving the recording density, it is preferable to use ferromagnetic powder having a small average particle size. From this point of view, the average particle size of the ferromagnetic powder contained in the first magnetic layer is preferably 50 nm or less, more preferably 45 nm or less, even more preferably 40 nm or less, and 35 nm or less. more preferably 30 nm or less, even more preferably 25 nm or less, and even more preferably 20 nm or less. On the other hand, from the viewpoint of magnetization stability, the average particle size of the ferromagnetic powder contained in the first magnetic layer is preferably 5 nm or more, more preferably 8 nm or more, and preferably 10 nm or more. More preferably, it is 15 nm or more, and even more preferably 20 nm or more.

Hexagonal Ferrite Powder A preferred specific example of the ferromagnetic powder contained in the first magnetic layer is hexagonal ferrite powder. For details of the hexagonal ferrite powder, for example, paragraphs 0012 to 0030 of JP-A-2011-225417, paragraphs 0134-0136 of JP-A-2011-216149, paragraphs 0013-0030 of JP-A-2012-204726 and Paragraphs 0029 to 0084 of JP-A-2015-127985 can be referred to.

In the present invention and this specification, "hexagonal ferrite powder" refers to ferromagnetic powder in which the crystal structure of hexagonal ferrite is detected as the main phase by X-ray diffraction analysis. For example, when the highest intensity diffraction peak in an X-ray diffraction spectrum obtained by X-ray diffraction analysis is attributed to the crystal structure of hexagonal ferrite, it is determined that the crystal structure of hexagonal ferrite has been detected as the main phase. do. The crystal structure of hexagonal ferrite contains at least iron atoms, divalent metal atoms and oxygen atoms as constituent atoms. A divalent metal atom is a metal atom that can become a divalent cation as an ion, and examples thereof include alkaline earth metal atoms such as strontium, barium, and calcium atoms, and lead atoms. In the present invention and the specification, hexagonal strontium ferrite powder means that the main divalent metal atoms contained in this powder are strontium atoms, and hexagonal barium ferrite powder means that the main divalent metal atoms contained in this powder are a barium atom as a divalent metal atom. The main divalent metal atom means the divalent metal atom that accounts for the largest amount on an atomic % basis among the divalent metal atoms contained in the powder. However, rare earth atoms are not included in the divalent metal atoms for the above hexagonal ferrite. "Rare earth atoms" in the present invention and herein are selected from the group consisting of scandium atoms (Sc), yttrium atoms (Y), and lanthanide atoms. Lanthanide atoms include lanthanum atom (La), cerium atom (Ce), praseodymium atom (Pr), neodymium atom (Nd), promethium atom (Pm), samarium atom (Sm), europium atom (Eu), gadolinium atom (Gd ), terbium atom (Tb), dysprosium atom (Dy), holmium atom (Ho), erbium atom (Er), thulium atom (Tm), ytterbium atom (Yb), and lutetium atom (Lu) be.

The hexagonal strontium ferrite powder, which is one form of the hexagonal ferrite powder, will be described in more detail below.

The activated volume of the hexagonal strontium ferrite powder is preferably in the range of 800-1600 nm ³ . A finely divided hexagonal strontium ferrite powder exhibiting an activation volume within the above range is suitable for making a magnetic tape exhibiting excellent electromagnetic conversion characteristics. The activated volume of the hexagonal strontium ferrite powder is preferably greater than or equal to 800 nm ³ , for example it may be greater than or equal to 850 nm ³ . Further, from the viewpoint of further improving electromagnetic conversion characteristics, the activated volume of the hexagonal strontium ferrite powder is more preferably 1500 nm ³ or less, further preferably 1400 nm ³ or less, and 1300 nm ³ or less. is more preferable, 1200 nm ³ or less is even more preferable, and 1100 nm ³ or less is even more preferable. The same is true for the activation volume of hexagonal barium ferrite powder.

The "activation volume" is a unit of magnetization reversal, and is an index indicating the magnetic size of a particle. The activation volume and the anisotropy constant Ku described in the present invention and this specification were measured using a vibrating sample magnetometer at magnetic field sweep speeds of 3 minutes and 30 minutes at the coercive force Hc measurement unit (measurement Temperature: 23° C.±1° C.), which is a value obtained from the following relational expression between Hc and activation volume V. Note that the unit of the anisotropy constant Ku is 1 erg/cc=1.0×10 ⁻¹ J/m ³ .
Hc=2Ku/Ms{1−[(kT/KuV)ln(At/0.693)] ^1/2 }
[In the above formula, Ku: anisotropy constant (unit: J/m ³ ), Ms: saturation magnetization (unit: kA/m), k: Boltzmann constant, T: absolute temperature (unit: K), V: activity volume (unit: cm ³ ), A: spin precession frequency (unit: s ⁻¹ ), t: magnetic field reversal time (unit: s)]

An anisotropic constant Ku can be cited as an index for reducing thermal fluctuation, in other words, improving thermal stability. The hexagonal strontium ferrite powder can preferably have a Ku of 1.8×10 ⁵ J/m ³ or more, more preferably 2.0×10 ⁵ J/m ³ or more. Also, Ku of the hexagonal strontium ferrite powder can be, for example, 2.5×10 ⁵ J/m ³ or less. However, the higher the Ku value, the higher the thermal stability, which is preferable.

The hexagonal strontium ferrite powder may or may not contain rare earth atoms. When the hexagonal strontium ferrite powder contains rare earth atoms, it preferably contains 0.5 to 5.0 atomic % of rare earth atoms (bulk content) with respect to 100 atomic % of iron atoms. In one form, the hexagonal strontium ferrite powder containing rare earth atoms can have uneven distribution of rare earth atoms on the surface layer. In the present invention and in this specification, the term "rare earth atom surface uneven distribution" refers to the rare earth atom content ratio (hereinafter referred to as "Rare earth atom surface layer content" or simply "surface layer content" with respect to rare earth atoms.) is obtained by completely dissolving hexagonal strontium ferrite powder with acid. (hereinafter referred to as "rare earth atom bulk content" or simply "bulk content" with respect to rare earth atoms), and
Rare earth atom surface layer content/rare earth atom bulk content>1.0
means that the ratio of The rare earth atom content rate of the hexagonal strontium ferrite powder described later is synonymous with the rare earth atom bulk content rate. On the other hand, since partial dissolution using an acid dissolves the surface layer of the particles constituting the hexagonal strontium ferrite powder, the content of rare earth atoms in the solution obtained by partial dissolution is It is the rare earth atom content rate in the surface layer portion of the particles. The rare earth atom surface layer portion content ratio satisfies the ratio of "rare earth atom surface layer portion content/rare earth atom bulk content rate >1.0" means that the rare earth atoms are present in the surface layer portion of the particles constituting the hexagonal strontium ferrite powder. It means that it is unevenly distributed (that is, it exists more than inside). In the present invention and in this specification, the term "surface layer portion" means a partial region extending from the surface toward the inside of a particle that constitutes the hexagonal strontium ferrite powder.

When the hexagonal strontium ferrite powder contains rare earth atoms, the rare earth atom content (bulk content) is preferably in the range of 0.5 to 5.0 atomic % with respect to 100 atomic % of iron atoms. The fact that the rare earth atoms are contained in the bulk content in the above range and that the rare earth atoms are unevenly distributed in the surface layer of the particles constituting the hexagonal strontium ferrite powder contributes to suppressing the decrease in reproduction output during repeated reproduction. Conceivable. This is because the hexagonal strontium ferrite powder contains rare earth atoms with a bulk content within the above range, and the rare earth atoms are unevenly distributed in the surface layers of the particles constituting the hexagonal strontium ferrite powder. This is presumed to be due to the fact that The higher the anisotropy constant Ku, the more the occurrence of a phenomenon called thermal fluctuation can be suppressed (in other words, the thermal stability can be improved). By suppressing the occurrence of thermal fluctuation, it is possible to suppress a decrease in reproduction output in repeated reproduction. The uneven distribution of rare earth atoms in the particle surface layer of the hexagonal strontium ferrite powder contributes to stabilizing the spin of the iron (Fe) site in the crystal lattice of the surface layer, thereby increasing the anisotropy constant Ku. It is speculated that it will increase.
Further, the use of the hexagonal strontium ferrite powder having rare earth atoms unevenly distributed on the surface layer as the ferromagnetic powder of the first magnetic layer suppresses abrasion of the surface of the first magnetic layer due to sliding with the magnetic head. It is speculated that it also contributes to That is, it is presumed that the hexagonal strontium ferrite powder having rare earth atoms unevenly distributed on the surface layer can contribute to the improvement of the running durability of the magnetic recording medium. This is because the uneven distribution of rare earth atoms on the surfaces of the particles that make up the hexagonal strontium ferrite powder causes interaction between the particle surfaces and the organic substances (e.g., binders and/or additives) contained in the first magnetic layer. It is presumed that this is because it contributes to the improvement of the action and, as a result, the strength of the first magnetic layer is improved.
From the viewpoint of suppressing a decrease in reproduction output in repeated reproduction and/or from the viewpoint of further improving running durability, the rare earth atom content (bulk content) is in the range of 0.5 to 4.5 atomic %. is more preferably in the range of 1.0 to 4.5 atomic %, and even more preferably in the range of 1.5 to 4.5 atomic %.

The above bulk content is the content obtained by completely dissolving the hexagonal strontium ferrite powder. In the present invention and this specification, unless otherwise specified, the atomic content refers to the bulk content obtained by completely dissolving the hexagonal strontium ferrite powder. The hexagonal strontium ferrite powder containing rare earth atoms may contain only one kind of rare earth atoms as rare earth atoms, or may contain two or more kinds of rare earth atoms. When two or more rare earth atoms are included, the bulk content is determined for the total of two or more rare earth atoms. This point also applies to the present invention and other components in this specification. That is, unless otherwise specified, only one component may be used, or two or more components may be used. When two or more are used, the content or content refers to the total of two or more.

When the hexagonal strontium ferrite powder contains rare earth atoms, the contained rare earth atoms may be any one or more rare earth atoms. Preferred rare earth atoms from the viewpoint of suppressing a decrease in reproduction output in repeated reproduction include neodymium atoms, samarium atoms, yttrium atoms and dysprosium atoms, more preferably neodymium atoms, samarium atoms and yttrium atoms, and neodymium atoms. More preferred.

In the hexagonal strontium ferrite powder having rare earth atoms unevenly distributed on the surface layer, the rare earth atoms need only be unevenly distributed on the surface layer of the particles constituting the hexagonal strontium ferrite powder, and the degree of uneven distribution is not limited. For example, for a hexagonal strontium ferrite powder having rare earth atoms unevenly distributed in the surface layer, the surface layer content of rare earth atoms obtained by partially dissolving under the melting conditions described later and the rare earth atoms obtained by completely dissolving under the melting conditions described later The ratio of atoms to the bulk content, "surface layer content/bulk content", is greater than 1.0 and can be 1.5 or greater. When the "surface layer content/bulk content" is greater than 1.0, it means that the rare earth atoms are unevenly distributed in the surface layer (ie, more present than in the interior) in the particles constituting the hexagonal strontium ferrite powder. do. In addition, the ratio between the surface layer content of rare earth atoms obtained by partial dissolution under the dissolution conditions described later and the bulk content of rare earth atoms obtained by complete dissolution under the dissolution conditions described later, "surface layer content/ The "bulk content" can be, for example, 10.0 or less, 9.0 or less, 8.0 or less, 7.0 or less, 6.0 or less, 5.0 or less, or 4.0 or less. However, in the hexagonal strontium ferrite powder having rare earth atoms unevenly distributed in the surface layer portion, the rare earth atoms may be unevenly distributed in the surface layer portion of the particles constituting the hexagonal strontium ferrite powder. "Ratio" is not limited to the exemplified upper or lower limits.

Partial dissolution and total dissolution of hexagonal strontium ferrite powder are described below. For hexagonal strontium ferrite powders present as powders, sample powders for partial dissolution and total dissolution are taken from the same lot of powder. On the other hand, with respect to the hexagonal strontium ferrite powder contained in the first magnetic layer of the magnetic recording medium, part of the hexagonal strontium ferrite powder taken out from the first magnetic layer is partially melted, and the other part is melted. A portion is subjected to total lysis. The hexagonal strontium ferrite powder can be extracted from the magnetic layer, for example, by the method described in paragraph 0032 of JP-A-2015-91747.
The above-mentioned partial dissolution means dissolution to such an extent that residual hexagonal strontium ferrite powder can be visually confirmed in the liquid at the end of dissolution. For example, by partial dissolution, a region of 10 to 20% by mass of the particles constituting the hexagonal strontium ferrite powder can be dissolved out of 100% by mass of the entire particles. On the other hand, the above-mentioned complete dissolution means that the hexagonal strontium ferrite powder is dissolved to the point where no residue of the hexagonal strontium ferrite powder remains in the liquid at the end of dissolution.
The partial dissolution and the measurement of the surface layer content are performed, for example, by the following methods. However, the dissolution conditions such as the amount of sample powder described below are examples, and dissolution conditions that allow partial dissolution and complete dissolution can be arbitrarily adopted.
A container (for example, a beaker) containing 12 mg of sample powder and 10 mL of 1 mol/L hydrochloric acid is held on a hot plate with a set temperature of 70° C. for 1 hour. The resulting solution is filtered through a 0.1 μm membrane filter. Atomic analysis of the filtrate thus obtained is performed by an inductively coupled plasma (ICP) analyzer. In this way, the surface layer portion content of rare earth atoms relative to 100 atomic % of iron atoms can be obtained. When multiple types of rare earth atoms are detected by atomic analysis, the total content of all rare earth atoms is taken as the surface layer portion content. This point also applies to the measurement of the bulk content.
On the other hand, the measurement of the total dissolution and bulk content is carried out, for example, by the following method.
A container (for example, a beaker) containing 12 mg of sample powder and 10 mL of 4 mol/L hydrochloric acid is held on a hot plate with a set temperature of 80° C. for 3 hours. After that, the partial dissolution and the measurement of the surface layer portion content are carried out in the same manner as described above, and the bulk content with respect to 100 atom % of iron atoms can be obtained.

From the viewpoint of increasing the reproduction output when reproducing data recorded on a magnetic recording medium, it is desirable that the ferromagnetic powder contained in the magnetic layer that can function as a recording layer in the magnetic recording medium have a high mass magnetization σs. In this regard, hexagonal strontium ferrite powders containing rare earth atoms but not unevenly distributed in the surface layer of rare earth atoms tended to have a significantly lower σs than hexagonal strontium ferrite powders containing no rare earth atoms. On the other hand, hexagonal strontium ferrite powder having rare earth atoms unevenly distributed in the surface layer is considered preferable in terms of suppressing such a large decrease in σs. In one form, the σs of the hexagonal strontium ferrite powder can be 45 A·m ² /kg or greater, and can also be 47 A·m ² /kg or greater. On the other hand, from the viewpoint of noise reduction, σs is preferably 80 A·m ² /kg or less, more preferably 60 A·m ² /kg or less. σs can be measured using a known measuring device capable of measuring magnetic properties, such as a vibrating sample magnetometer. In the present invention and this specification, unless otherwise specified, the mass magnetization σs is a value measured at a magnetic field strength of 15 kOe.

With respect to the content of constituent atoms (bulk content) of the hexagonal strontium ferrite powder, the strontium atom content can be, for example, in the range of 2.0 to 15.0 atomic percent with respect to 100 atomic percent of iron atoms. . In one form, the hexagonal strontium ferrite powder can have strontium atoms as the only divalent metal atoms contained in the powder. In yet another form, the hexagonal strontium ferrite powder can also contain one or more other divalent metal atoms in addition to the strontium atoms. For example, it can contain barium atoms and/or calcium atoms. When other divalent metal atoms other than strontium atoms are contained, the barium atom content and calcium atom content in the hexagonal strontium ferrite powder are, for example, 0.05 to 5 atoms per 100 atomic percent of iron atoms. can be in the range of .0 atomic %.

As crystal structures of hexagonal ferrite, magnetoplumbite type (also called “M type”), W type, Y type and Z type are known. The hexagonal strontium ferrite powder may have any crystal structure. The crystal structure can be confirmed by X-ray diffraction analysis. The hexagonal strontium ferrite powder can have a single crystal structure or two or more crystal structures detected by X-ray diffraction analysis. For example, in one form, a hexagonal strontium ferrite powder can be one in which only the M-type crystal structure is detected by X-ray diffraction analysis. For example, M-type hexagonal ferrite is represented by a composition formula of AFe ₁₂ O ₁₉ . Here, A represents a divalent metal atom, and if the hexagonal strontium ferrite powder is M-type, A is only a strontium atom (Sr), or if A contains a plurality of divalent metal atoms, , as described above, strontium atoms (Sr) account for the largest amount on an atomic % basis. The divalent metal atom content of the hexagonal strontium ferrite powder is usually determined by the type of crystal structure of the hexagonal ferrite, and is not particularly limited. The same applies to the iron atom content and the oxygen atom content. The hexagonal strontium ferrite powder contains at least iron atoms, strontium atoms and oxygen atoms, and may also contain rare earth atoms. Furthermore, the hexagonal strontium ferrite powder may or may not contain atoms other than these atoms. As an example, the hexagonal strontium ferrite powder may contain aluminum atoms (Al). The content of aluminum atoms can be, for example, 0.5 to 10.0 atomic % with respect to 100 atomic % of iron atoms. From the viewpoint of suppressing a decrease in reproduction output in repeated reproduction, the hexagonal strontium ferrite powder contains iron atoms, strontium atoms, oxygen atoms and rare earth atoms, and the content of atoms other than these atoms is 100 atomic % iron atoms. On the other hand, it is preferably 10.0 atomic % or less, more preferably in the range of 0 to 5.0 atomic %, and may be 0 atomic %. That is, in one form, the hexagonal strontium ferrite powder may contain no atoms other than iron atoms, strontium atoms, oxygen atoms and rare earth atoms. The content expressed in atomic % above is the content of each atom (unit: mass %) obtained by completely dissolving the hexagonal strontium ferrite powder, and converted to the value expressed in atomic % using the atomic weight of each atom. It is required by conversion. Further, in the present invention and this specification, the phrase "not containing" an atom means that the content of the atom as measured by an ICP analyzer after total dissolution is 0% by mass. The detection limit of an ICP analyzer is usually 0.01 ppm (parts per million) or less on a mass basis. The above "does not contain" shall be used in the sense of containing in an amount below the detection limit of the ICP analyzer. The hexagonal strontium ferrite powder, in one form, can be free of bismuth atoms (Bi).

Metal powder Ferromagnetic metal powder is also a preferred specific example of the ferromagnetic powder. For details of the ferromagnetic metal powder, for example, paragraphs 0137 to 0141 of JP-A-2011-216149 and paragraphs 0009-0023 of JP-A-2005-251351 can be referred to.

ε-Iron Oxide Powder A preferred specific example of the ferromagnetic powder contained in the first magnetic layer is ε-iron oxide powder. In the present invention and the specification, "ε-iron oxide powder" means a ferromagnetic powder in which the crystal structure of ε-iron oxide is detected as the main phase by X-ray diffraction analysis. For example, when the highest intensity diffraction peak in the X-ray diffraction spectrum obtained by X-ray diffraction analysis is attributed to the crystal structure of ε-iron oxide, it is determined that the crystal structure of ε-iron oxide has been detected as the main phase. shall be As a method for producing ε-iron oxide powder, a method of producing from goethite, a reverse micelle method, and the like are known. All of the above manufacturing methods are known. Also, a method for producing ε-iron oxide powder in which a part of Fe is substituted with substitution atoms such as Ga, Co, Ti, Al, and Rh is described in J. Am. Jpn. Soc. Powder Metallurgy Vol. 61 Supplement, No. S1, pp. S280-S284, J.P. Mater. Chem. C, 2013, 1, pp. 5200-5206 and the like. However, the method for producing the ε-iron oxide powder that can be used as the ferromagnetic powder in the first magnetic layer of the magnetic recording medium is not limited to the methods mentioned here.

The activated volume of the ε-iron oxide powder is preferably in the range of 300-1500 nm ³ . A finely divided ε-iron oxide powder exhibiting an activation volume in the above range is suitable for making a magnetic tape exhibiting excellent electromagnetic conversion properties. The activated volume of the ε-iron oxide powder is preferably greater than or equal to 300 nm ³ and may eg be greater than or equal to 500 nm ³ . In addition, from the viewpoint of further improving the electromagnetic conversion characteristics, the activated volume of the ε-iron oxide powder is more preferably 1400 nm ³ or less, further preferably 1300 nm ³ or less, and 1200 nm ³ or less. is more preferable, and 1100 nm ³ or less is even more preferable.

An anisotropic constant Ku can be cited as an index for reducing thermal fluctuation, in other words, improving thermal stability. The ε-iron oxide powder can preferably have a Ku of 3.0×10 ⁴ J/m ³ or more, more preferably 8.0×10 ⁴ J/m ³ or more. Also, Ku of the ε-iron oxide powder can be, for example, 3.0×10 ⁵ J/m ³ or less. However, a higher Ku means a higher thermal stability, which is preferable, and thus is not limited to the values exemplified above.

From the viewpoint of increasing the reproduction output when reproducing data recorded on a magnetic recording medium, it is desirable that the ferromagnetic powder contained in the magnetic layer that can function as a recording layer in the magnetic recording medium have a high mass magnetization σs. In this regard, in one aspect, the σs of the ε-iron oxide powder can be 8 A·m ² /kg or greater, and can also be 12 A·m ² /kg or greater. On the other hand, σs of the ε-iron oxide powder is preferably 40 A·m ² /kg or less, more preferably 35 A·m ² /kg or less, from the viewpoint of noise reduction.

The ferromagnetic powder content (filling rate) in the first magnetic layer is preferably in the range of 50 to 90% by mass, more preferably 60 to 90% by mass, relative to the total mass of the first magnetic layer. is in the range of A high filling rate of the ferromagnetic powder in the first magnetic layer is preferable from the viewpoint of improving the recording density.

(binder)
The first magnetic layer contains ferromagnetic powder and a binder. The binder contained in the first magnetic layer can be referred to the above description of the binder in the second magnetic layer.

(curing agent)
The composition for forming the first magnetic layer may contain a curing agent together with a resin that can be used as a binder. The curing agent can be, in one form, a thermosetting compound which is a compound in which a curing reaction (crosslinking reaction) proceeds by heating, and in another form, a photocuring compound in which a curing reaction (crosslinking reaction) proceeds by light irradiation. can be a chemical compound. As the curing reaction progresses in the process of forming the first magnetic layer, the curing agent reacts (crosslinks) with other components such as the binder to form the first magnetic layer. can be included in layers. In this respect, when the composition used for forming other layers contains a curing agent, the same applies to layers formed using this composition. Preferred curing agents are thermosetting compounds, preferably polyisocyanates. For details of the polyisocyanate, paragraphs 0124 to 0125 of JP-A-2011-216149 can be referred to. The curing agent is contained in the composition for forming the first magnetic layer in an amount of, for example, 0 to 80.0 parts by weight per 100.0 parts by weight of the binder. .0 to 80.0 parts by weight.

(Additive)
The first magnetic layer may optionally contain one or more additives. Examples of additives include the curing agents described above. Additives contained in the magnetic layer include nonmagnetic powders, lubricants, dispersants, dispersing aids, antifungal agents, antistatic agents, antioxidants, and the like. For example, regarding lubricants, paragraphs 0030 to 0033, 0035 and 0036 of JP-A-2016-126817 can be referred to. A non-magnetic layer, which will be described later, may contain a lubricant. Paragraphs 0030, 0031, 0034 to 0036 of JP-A-2016-126817 can be referred to for lubricants that can be contained in the non-magnetic layer. Regarding the dispersant, paragraphs 0061 and 0071 of JP-A-2012-133837 can be referred to.

Dispersants that can be added to the first magnetic layer-forming composition include known dispersants for enhancing the dispersibility of ferromagnetic powders such as carboxy group-containing compounds and nitrogen-containing compounds. For example, the nitrogen-containing compound may be any of a primary amine represented by _NH2R , a _secondary amine represented by NHR2, and a _tertiary amine represented by NR3. In the above, R represents an arbitrary structure constituting the nitrogen-containing compound, and multiple Rs may be the same or different. The nitrogen-containing compound may be a compound (polymer) having multiple repeating structures in its molecule. The reason why the nitrogen-containing compound can work as a dispersing agent is considered to be that the nitrogen-containing portion of the nitrogen-containing compound functions as an adsorption portion to the particle surface of the ferromagnetic powder. Examples of carboxy group-containing compounds include fatty acids such as oleic acid. As for the carboxy group-containing compound, the reason why the carboxy group-containing compound can work as a dispersing agent is considered to be that the carboxy group functions as an adsorption site on the particle surface of the ferromagnetic powder. It is also preferable to use a carboxy group-containing compound and a nitrogen-containing compound in combination. The amount of these dispersants to be used can be set appropriately.

A dispersant may be added to the composition for forming the non-magnetic layer. See paragraph 0061 of JP-A-2012-133837 for the dispersant that can be added to the composition for forming a non-magnetic layer.

Abrasives As described above, the number distribution A can be considered as an indicator of the presence of abrasives on the surface of the first magnetic layer. Therefore, the number distribution A can be controlled by the type of non-magnetic powder added as an abrasive. As the abrasive, a non-magnetic powder having a Mohs hardness of more than 8 is preferable, and a non-magnetic powder having a Mohs hardness of 9 or more is more preferable. The maximum value of Mohs hardness is 10. The abrasive can be a powder of inorganic material or can be a powder of organic material. The abrasive can be an inorganic or organic oxide powder or carbide powder. Examples of carbide include boron carbide (eg, B ₄ C), titanium carbide (eg, TiC), and the like. Diamond can also be used as the abrasive. In one form, the abrasive is preferably an inorganic oxide powder. Specifically, examples of inorganic oxides include alumina (e.g. Al ₂ O ₃ ), titanium oxide (e.g. TiO ₂ ), cerium oxide (e.g. CeO ₂ ), zirconium oxide (e.g. ZrO ₂ ), etc. Among them, alumina is preferred. Alumina has a Mohs hardness of about 9. Regarding alumina powder, paragraph 0021 of JP-A-2013-229090 can also be referred to. Further, as an index of the particle size of the abrasive, the specific surface area can be used. It can be considered that the larger the specific surface area, the smaller the particle size of the primary particles constituting the abrasive. As the abrasive, it is preferable to use an abrasive having a specific surface area measured by the BET (Brunauer-Emmett-Teller) method (hereinafter referred to as "BET specific surface area") of 14 m ² /g or more. From the viewpoint of dispersibility, it is preferable to use a polishing agent having a BET specific surface area of 40 m ² /g or less. The content of the abrasive in the first magnetic layer is preferably 1.0 to 20.0 parts by mass, more preferably 1.0 to 15.0 parts by mass, per 100.0 parts by mass of the ferromagnetic powder. is more preferable. As the abrasive, only one type of non-magnetic powder can be used, or two or more types of non-magnetic powders having different compositions and/or physical properties (for example, size) can be used. When two or more kinds of non-magnetic powders are used as abrasives, the content of abrasives means the total content of these two or more kinds of non-magnetic powders. The above points also apply to the content of various components in the present invention and the present specification. The abrasive is preferably dispersed separately from the ferromagnetic powder (separate dispersion), and more preferably dispersed separately from the filler described later (separate dispersion). When preparing the first composition for forming the magnetic layer, two or more dispersions with different components and/or dispersion conditions are prepared as abrasive dispersions (hereinafter also referred to as "abrasive liquids"). This is preferable for controlling the number distribution A.

A dispersant can also be used to adjust the dispersion state of the abrasive dispersion. A compound that can function as a dispersant for enhancing the dispersibility of the abrasive includes an aromatic hydrocarbon compound having a phenolic hydroxy group. A "phenolic hydroxy group" refers to a hydroxy group directly attached to an aromatic ring. The aromatic ring contained in the aromatic hydrocarbon compound may be monocyclic, polycyclic, or condensed. From the viewpoint of improving the dispersibility of the abrasive, aromatic hydrocarbon compounds containing a benzene ring or a naphthalene ring are preferred. Moreover, the aromatic hydrocarbon compound may have a substituent other than the phenolic hydroxy group. Examples of substituents other than phenolic hydroxy groups include halogen atoms, alkyl groups, alkoxy groups, amino groups, acyl groups, nitro groups, nitroso groups, hydroxyalkyl groups and the like. Alkoxy groups, amino groups and hydroxyalkyl groups are preferred. The number of phenolic hydroxy groups contained in one molecule of the aromatic hydrocarbon compound may be one, two, three or more.

A preferred form of the aromatic hydrocarbon compound having a phenolic hydroxy group is the compound represented by the following formula 100.

[In Formula 100, two of X ¹⁰¹ to X ¹⁰⁸ are hydroxy groups, and the other six each independently represent a hydrogen atom or a substituent. ]

In the compound represented by Formula 100, the substitution positions of the two hydroxy groups (phenolic hydroxy groups) are not particularly limited.

In the compound represented by Formula 100, two of X ¹⁰¹ to X ¹⁰⁸ are hydroxy groups (phenolic hydroxy groups), and the other six independently represent hydrogen atoms or substituents. In addition, among X ¹⁰¹ to X ¹⁰⁸ , all of the portions other than the two hydroxy groups may be hydrogen atoms, or some or all of them may be substituents. As the substituent, the substituents described above can be exemplified. One or more phenolic hydroxy groups may be included as substituents other than the two hydroxy groups. From the viewpoint of improving the dispersibility of the abrasive, it is preferable that the hydroxy groups other than the two hydroxy groups among X ¹⁰¹ to X ¹⁰⁸ are not phenolic hydroxy groups. That is, the compound represented by formula 100 is preferably dihydroxynaphthalene or a derivative thereof, more preferably 2,3-dihydroxynaphthalene or a derivative thereof. Preferable substituents as the substituents represented by X ¹⁰¹ to X ¹⁰⁸ include halogen atoms (eg, chlorine atom, bromine atom), amino groups, alkyl groups having 1 to 6 carbon atoms (preferably 1 to 4), and methoxy groups. and ethoxy, acyl, nitro and nitroso groups, and —CH ₂ OH groups.

In addition, paragraphs 0024 to 0028 of JP-A-2014-179149 can also be referred to for the dispersant for enhancing the dispersibility of the abrasive.

The dispersant for enhancing the dispersibility of the abrasive is, for example, when preparing the abrasive liquid (for each abrasive liquid when preparing a plurality of abrasive liquids), per 100.0 parts by mass of the abrasive: For example, it can be used in a proportion of 0.5 to 20.0 parts by mass, preferably in a proportion of 1.0 to 10.0 parts by mass.

Filler As described above, the number distribution B is a magnetic layer of non-magnetic powder (filler) contained in the first magnetic layer in order to form moderate protrusions on the surface of the first magnetic layer for controlling friction characteristics. It is thought that it can be an index of the state of existence on the surface. Therefore, the number distribution B can be controlled by the type of non-magnetic powder added as a filler. One form of filler is carbon black. The BET specific surface area of carbon black is preferably 10 m ² /g or more, more preferably 15 m ² /g or more. The BET specific surface area of carbon black is preferably 50 m ² /g or less, more preferably 40 m ² /g or less, from the viewpoint of ease of improving dispersibility. Another form of filler is colloidal particles. The colloidal particles are preferably inorganic colloidal particles, more preferably inorganic oxide colloidal particles, and still more preferably silica colloidal particles (colloidal silica) from the viewpoint of availability. In the present invention and the specification, the term "colloidal particles" means methyl ethyl ketone, cyclohexanone, toluene, ethyl acetate, or at least one mixed solvent containing two or more of the above solvents in an arbitrary mixing ratio. A particle is defined as a particle that, when dispersed, does not settle but is capable of dispersing to provide a colloidal dispersion. The average particle size of the colloidal particles can be, for example, 30-300 nm, preferably 40-200 nm. The content of the filler in the first magnetic layer is preferably 0.5 to 20.0 parts by mass, more preferably 0.5 to 15.0 parts by mass, with respect to 100.0 parts by mass of the ferromagnetic powder. is more preferable. The filler is preferably dispersed separately from the ferromagnetic powder, and more preferably dispersed separately from the abrasive. When preparing the composition for forming the magnetic layer, preparing two or more types of dispersions with different components and/or dispersion conditions as filler dispersions (hereinafter also referred to as "filler liquids") can result in a number distribution It is preferable to control B.

From the viewpoint of improving the dispersibility of carbon black, in one form, a compound having an ammonium salt structure of an alkyl ester anion represented by the following formula 1 can be used when preparing the filler liquid. The "alkylester anion" can also be called an "alkylcarboxylate anion".

In Formula 1, R represents an alkyl group having 7 or more carbon atoms or a fluorinated alkyl group having 7 or more carbon atoms, and Z ⁺ represents an ammonium cation.

In addition, from the viewpoint of improving the dispersibility of carbon black, in one embodiment, two or more components capable of forming a compound having a salt structure can be used during preparation of the filler liquid. This allows at least a portion of these components to form a compound having the above salt structure during preparation of the filler liquid.

Unless otherwise specified, the groups described below may have a substituent or may be unsubstituted. In addition, the “carbon number” of a group having a substituent means the number of carbon atoms not including the number of carbon atoms of the substituent unless otherwise specified. In the present invention and the specification, substituents include, for example, alkyl groups (eg alkyl groups having 1 to 6 carbon atoms), hydroxy groups, alkoxy groups (eg alkoxy groups having 1 to 6 carbon atoms), halogen atoms (eg fluorine atom, chlorine atom, bromine atom, etc.), a cyano group, an amino group, a nitro group, an acyl group, a carboxy group, a salt of a carboxy group, a sulfonic acid group, a salt of a sulfonic acid group, and the like.

Formula 1 will be described in more detail below.

In Formula 1, R represents an alkyl group having 7 or more carbon atoms or a fluorinated alkyl group having 7 or more carbon atoms. A fluorinated alkyl group has a structure in which some or all of the hydrogen atoms constituting the alkyl group are substituted with fluorine atoms. The alkyl group or fluorinated alkyl group represented by R may have a linear structure, may have a branched structure, may be a cyclic alkyl group or fluorinated alkyl group, and has a linear structure. is preferred. The alkyl group or fluorinated alkyl group represented by R may have a substituent or may be unsubstituted, and is preferably unsubstituted. An alkyl group represented by R can be represented by, for example, C _n H _2n+1 -. Here, n represents an integer of 7 or more. Further, the fluorinated alkyl group represented by R can have a structure in which, for example, some or all of the hydrogen atoms constituting the alkyl group represented by C _n H _2n+1 - are substituted with fluorine atoms. The number of carbon atoms in the alkyl group or fluorinated alkyl group represented by R is 7 or more, preferably 8 or more, more preferably 9 or more, further preferably 10 or more, and 11 or more. more preferably, 12 or more, and even more preferably 13 or more. The number of carbon atoms in the alkyl group or fluorinated alkyl group represented by R is preferably 20 or less, more preferably 19 or less, and even more preferably 18 or less.

In Formula 1, Z ⁺ represents an ammonium cation. The ammonium cation specifically has the following structure. In the present invention and this specification, "*" in formulas representing part of a compound represents the bonding position between the structure of that part and an adjacent atom.

The nitrogen cation N ⁺ of the ammonium cation and the oxygen anion O 2 ⁻ in formula 1 can form a salt bridging group to form the ammonium salt structure of the alkyl ester anion represented by formula 1. The presence of the compound having the ammonium salt structure of the alkyl ester anion represented by Formula 1 in the first magnetic layer was confirmed by X-ray photoelectron spectroscopy (ESCA) for the magnetic recording medium. It can be confirmed by analyzing by infrared spectroscopy (IR) or the like.

In one form, an ammonium cation represented by Z ⁺ can be provided, for example, by a nitrogen atom of a nitrogen-containing polymer becoming a cation. A nitrogen-containing polymer means a polymer containing nitrogen atoms. In the present invention and the specification, the terms "polymer" and "polymer" are used in the sense of including homopolymers and copolymers. A nitrogen atom can be contained as an atom constituting a main chain of a polymer in one form, and can be contained as an atom constituting a side chain of a polymer in one form.

One form of nitrogen-containing polymer is polyalkyleneimine. Polyalkyleneimine is a ring-opening polymer of alkyleneimine, and is a polymer having a plurality of repeating units represented by formula 2 below.

The nitrogen atom N constituting the main chain in Formula 2 can become a nitrogen cation N ⁺ to provide an ammonium cation represented by Z ⁺ in Formula 1. and can form an ammonium salt structure with an alkyl ester anion, for example as follows.

Formula 2 will be described in more detail below.

In Formula 2, R ¹ and R ² each independently represent a hydrogen atom or an alkyl group, and n1 represents an integer of 2 or more.

Examples of the alkyl group represented by R ¹ or R ² include an alkyl group having 1 to 6 carbon atoms, preferably an alkyl group having 1 to 3 carbon atoms, more preferably a methyl group or an ethyl group. group, more preferably a methyl group. The alkyl group represented by R ¹ or R ² is preferably an unsubstituted alkyl group. The combination of R ¹ and R ² in Formula 2 includes a mode in which one is a hydrogen atom and the other is an alkyl group, a mode in which both are hydrogen atoms, and a mode in which both are alkyl groups (same or different alkyl groups). There is a form, preferably a form in which both are hydrogen atoms. As the alkyleneimine leading to polyalkyleneimine, the structure with the smallest number of carbon atoms constituting the ring is ethyleneimine, and the main chain of the alkyleneimine obtained by ring opening of ethyleneimine (ethyleneimine) has 2 carbon atoms. . Therefore, n1 in Formula 2 is 2 or more. n1 in Formula 2 can be, for example, 10 or less, 8 or less, 6 or less, or 4 or less. The polyalkyleneimine may be a homopolymer containing only the same structure as the repeating structure represented by Formula 2, or may be a copolymer containing two or more different structures as the repeating structure represented by Formula 2. . The number average molecular weight of the polyalkyleneimine that can be used to form the compound having the ammonium salt structure of the alkyl ester anion represented by Formula 1 can be, for example, 200 or more, preferably 300 or more, It is more preferably 400 or more. The number average molecular weight of the polyalkyleneimine may be, for example, 10,000 or less, preferably 5,000 or less, and more preferably 2,000 or less.

In the present invention and the specification, the average molecular weight (weight average molecular weight and number average molecular weight) is measured by gel permeation chromatography (GPC; Gel Permeation Chromatography) and refers to a value determined by standard polystyrene conversion. Unless otherwise specified, the average molecular weight shown in the examples below is a value obtained by converting the value measured under the following measurement conditions using GPC into standard polystyrene (polystyrene conversion value).
GPC device: HLC-8220 (manufactured by Tosoh Corporation)
Guard column: TSKguardcolumn Super HZM-H
Column: TSKgel Super HZ 2000, TSKgel Super HZ 4000, TSKgel Super HZ-M (manufactured by Tosoh Corporation, 4.6 mm (inner diameter) × 15.0 cm, 3 types of columns connected in series)
Eluent: Tetrahydrofuran (THF) containing stabilizer (2,6-di-t-butyl-4-methylphenol) Eluent flow rate: 0.35 mL/min Column temperature: 40°C
Inlet temperature: 40°C
Refractive index (RI; Refractive Index) measurement temperature: 40°C
Sample concentration: 0.3% by mass
Sample injection volume: 10 μL

Another form of nitrogen-containing polymer is polyallylamine. Polyallylamine is a polymer of allylamine and is a polymer having a plurality of repeating units represented by Formula 3 below.

The nitrogen atom N constituting the amino group of the side chain in Formula 3 can become a nitrogen cation N ⁺ to provide an ammonium cation represented by Z ⁺ in Formula 1. and can form an ammonium salt structure with an alkyl ester anion, for example as follows.

The weight average molecular weight of the polyallylamine that can be used to form the compound having the ammonium salt structure of the alkyl ester anion represented by Formula 1 can be, for example, 200 or more, preferably 1,000 or more. , 1,500 or more. The weight average molecular weight of the polyallylamine may be, for example, 15,000 or less, preferably 10,000 or less, and more preferably 8,000 or less.

The presence of a compound having a structure derived from polyalkyleneimine or polyallylamine in the first magnetic layer as the compound having the ammonium salt structure of the alkyl ester anion represented by Formula 1 is effective for flying on the surface of the first magnetic layer. It can be confirmed by analysis by time-of-flight secondary ion mass spectrometry (TOF-SIMS) or the like.

The compound having an ammonium salt structure of an alkyl ester anion represented by Formula 1 is a nitrogen-containing polymer and at least one fatty acid selected from the group consisting of fatty acids having 7 or more carbon atoms and fluorinated fatty acids having 7 or more carbon atoms. can be a salt of The salt-forming nitrogen-containing polymer can be one or more nitrogen-containing polymers, such as nitrogen-containing polymers selected from the group consisting of polyalkyleneimines and polyallylamines. Fatty acids that form salts can be one or more fatty acids selected from the group consisting of fatty acids having 7 or more carbon atoms and fluorinated fatty acids having 7 or more carbon atoms. A fluorinated fatty acid has a structure in which some or all of the hydrogen atoms constituting the alkyl group bonded to the carboxyl group COOH in the fatty acid are substituted with fluorine atoms. For example, by mixing the nitrogen-containing polymer and the fatty acid at room temperature, the salt formation reaction can proceed easily. Room temperature is, for example, about 20 to 25.degree. In one embodiment, one or more of the nitrogen-containing polymer and one or more of the above fatty acids are used as components of the filler liquid, and the salt formation reaction can be advanced by mixing them in the preparation process of the filler liquid. . In one embodiment, one or more of the nitrogen-containing polymer and one or more of the fatty acids are mixed to form a salt before preparing the filler liquid, and then the salt is used as a component of the filler liquid. A filler liquid can be prepared. When the nitrogen-containing polymer and the fatty acid are mixed to form the ammonium salt of the alkyl ester anion represented by the formula 1, the nitrogen atom constituting the nitrogen-containing polymer and the carboxy group of the fatty acid are also The following structures may be formed upon reaction, and forms containing such structures are also included in the above compounds.

Examples of the above fatty acids include fatty acids having an alkyl group described above as R in Formula 1 and fluorinated fatty acids having a fluorinated alkyl group described as R in Formula 1 above.

The mixing ratio of the nitrogen-containing polymer and the fatty acid used to form the compound having the ammonium salt structure of the alkyl ester anion represented by Formula 1 is 10:10 as a mass ratio of the nitrogen-containing polymer:the fatty acid. It is preferably from 90 to 90:10, more preferably from 20:80 to 85:15, even more preferably from 30:70 to 80:20. Further, the compound having an ammonium salt structure of the alkyl ester anion represented by Formula 1 is added to 100.0 parts by mass of carbon black at the time of preparing the filler liquid (for each filler liquid when preparing multiple filler liquids). For example, 1.0 to 20.0 parts by mass can be used, and 1.0 to 10.0 parts by mass is preferably used. Further, for example, when preparing a filler liquid (for each filler liquid when preparing a plurality of filler liquids), 0.1 to 10.0 parts by mass of a nitrogen-containing polymer is used per 100.0 parts by mass of carbon black. It is preferable to use 0.5 to 8.0 parts by weight of the nitrogen-containing polymer. The fatty acid can be used, for example, in an amount of 0.05 to 10.0 parts by mass, preferably 0.1 to 5.0 parts by mass, per 100.0 parts by mass of carbon black.

The first magnetic layer described above can be directly provided, for example, on the second magnetic layer formed on the non-magnetic support.

<Nonmagnetic layer>
Next, the nonmagnetic layer will be explained. The magnetic recording medium may have the second magnetic layer directly on the surface of the non-magnetic support, or the second magnetic layer may be formed on the surface of the non-magnetic support via a non-magnetic layer containing non-magnetic powder. may have The non-magnetic powder used in the non-magnetic layer may be inorganic powder or organic powder. Carbon black or the like can also be used. Examples of powders of inorganic substances include powders of metals, metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, metal sulfides, and the like. These non-magnetic powders are commercially available and can be produced by known methods. For details, paragraphs 0146 to 0150 of Japanese Patent Application Laid-Open No. 2011-216149 can be referred to. For carbon black that can be used in the non-magnetic layer, see paragraphs 0040 and 0041 of JP-A-2010-24113. The nonmagnetic powder content (filling rate) in the nonmagnetic layer is preferably in the range of 50 to 90% by mass, more preferably in the range of 60 to 90% by mass, based on the total mass of the nonmagnetic layer. .

The non-magnetic layer contains a binder and can also contain one or more additives. Known techniques for nonmagnetic layers can be applied to other details such as binders and additives for the nonmagnetic layer. In addition, for example, the type and content of the binder, the type and content of the additive, and the like can be applied to known techniques related to magnetic layers.

In the present invention and in this specification, non-magnetic layers include non-magnetic powders as well as substantially non-magnetic layers containing a small amount of ferromagnetic powders, for example as impurities or intentionally. Here, the substantially non-magnetic layer means that the residual magnetic flux density of this layer is 10 mT or less, the coercive force is 7.96 kA/m (100 Oe) or less, or the residual magnetic flux density is 10 mT or less. and a coercive force of 7.96 kA/m (100 Oe) or less. The non-magnetic layer preferably has no residual magnetic flux density and no coercive force.

<Back coat layer>
In one form of the magnetic recording medium, a backcoat layer containing a nonmagnetic powder and a binder is provided on the surface of the nonmagnetic support opposite to the surface having the first magnetic layer and the second magnetic layer. can have In another form, the magnetic recording medium may be a magnetic recording medium without a back coat layer. When the magnetic recording medium has a back coat layer, the non-magnetic powder in the back coat layer is preferably carbon black or inorganic powder, or both. The backcoat layer includes a binder and may also include one or more additives. As for the binder and various additives that can be contained in the backcoat layer, known techniques relating to the backcoat layer can be applied, and known techniques relating to the formulation of the magnetic layer and/or the non-magnetic layer can also be applied. For example, paragraphs 0018 to 0020 of JP-A-2006-331625 and US Pat. .

<Nonmagnetic support>
Next, the non-magnetic support (hereinafter also simply referred to as "support") will be described.
Examples of the non-magnetic support include known materials such as biaxially stretched polyethylene terephthalate, polyethylene naphthalate, polyamide, polyamideimide, and aromatic polyamide. Among these, polyethylene terephthalate, polyethylene naphthalate and polyamide are preferred. These supports may be previously subjected to corona discharge, plasma treatment, adhesion-enhancing treatment, heat treatment, or the like.

<Various thicknesses>
The thickness of the nonmagnetic support is preferably 3.0 to 5.0 μm.
The thickness of the first magnetic layer can be optimized according to the saturation magnetization amount of the magnetic head to be used, the head gap length, the recording signal band, etc., and is, for example, 0.01 μm to 0.15 μm. From the point of view, it is preferably 0.02 μm to 0.12 μm, more preferably 0.03 μm to 0.1 μm. At least one first magnetic layer is sufficient, and the first magnetic layer may be separated into two or more layers having different magnetic properties, and a known multilayer magnetic layer configuration can be applied. The thickness of the first magnetic layer when separated into two or more layers is the total thickness of these layers. This point also applies to the second magnetic layer.
The thickness of the second magnetic layer is, for example, 0.1 to 1.5 μm, preferably 0.1 to 1.0 μm.
The thickness of the nonmagnetic layer is, for example, 0.1 to 1.5 μm, preferably 0.1 to 1.0 μm.
The thickness of the backcoat layer is preferably 0.9 μm or less, more preferably 0.1 to 0.7 μm.
The various thicknesses described above can be determined, for example, by the following methods.
After exposing a section of the magnetic recording medium in the thickness direction with an ion beam, the exposed section is observed with a scanning electron microscope or a transmission electron microscope. Various thicknesses can be determined as the arithmetic mean of the thicknesses determined at two arbitrary locations in cross-sectional observation. Alternatively, various thicknesses can be obtained as design thicknesses calculated from manufacturing conditions and the like.

<Manufacturing process>
(Preparation of each layer-forming composition)
The steps of preparing a composition for forming the first magnetic layer, second magnetic layer, non-magnetic layer, or backcoat layer usually include at least a kneading step, a dispersing step, and before and after these steps. Optionally, a mixing step may be included. Each step may be divided into two or more stages. The components used for preparing each layer-forming composition may be added at the beginning or in the middle of any step. As the solvent, one or more of various solvents commonly used in the production of coating-type magnetic recording media can be used. Regarding the solvent, for example, paragraph 0153 of JP-A-2011-216149 can be referred to. Alternatively, individual components may be added in two or more steps. For example, the binder may be dividedly added in the kneading step, the dispersing step, and the mixing step for viscosity adjustment after dispersion. In order to manufacture the magnetic tape, known manufacturing techniques can be used in various steps. In the kneading step, it is preferable to use a kneader having a strong kneading force such as an open kneader, a continuous kneader, a pressure kneader or an extruder. Details of the kneading process can be referred to JP-A-1-106338 and JP-A-1-79274. A known disperser can be used. Filtration may be performed by a known method at any stage of preparing each layer-forming composition. Filtration can be performed, for example, by filter filtration. As a filter used for filtration, for example, a filter having a pore size of 0.01 to 3 μm (eg, glass fiber filter, polypropylene filter, etc.) can be used.

The abrasive liquid used to prepare the first magnetic layer-forming composition is preferably dispersed separately from the ferromagnetic powder and filler. The dispersion state of the abrasive in the abrasive liquid depends on whether or not a dispersant is used to improve the dispersibility of the abrasive, the amount of the dispersant used, the treatment conditions for dispersion treatment such as bead dispersion, and the classification treatment such as centrifugation. can be adjusted depending on the processing conditions, etc. It is preferable to control the number distribution A to adjust the dispersed state of the abrasive. The abrasive liquid is preferably prepared as one or more abrasive liquids containing an abrasive, a solvent, and preferably a binder separately from the ferromagnetic powder and the filler, and the composition for forming the magnetic layer is prepared. can be used for the preparation of A commercially available device can be used for the dispersion treatment and the classification treatment. The conditions for performing these treatments are not particularly limited, and may be set according to the type of apparatus to be used so that the number distribution A satisfies (1) to (3) described above. .

In addition, it is preferable to disperse and prepare the filler liquid used to prepare the first magnetic layer-forming composition separately from the ferromagnetic powder and the abrasive. The dispersion state of the filler in the filler liquid depends on whether or not a component is used to improve the dispersibility of the filler, the amount of such component used, the processing conditions for dispersion treatment such as bead dispersion, and the processing conditions for classification treatment such as centrifugation. Adjustable. In one embodiment, one or more of the nitrogen-containing polymer and one or more of the fatty acids described above are used as components of the filler liquid, and by mixing them in the preparation process of the filler liquid, the salt formation reaction is induced. can proceed. In one embodiment, one or more of the nitrogen-containing polymer and one or more of the fatty acids are mixed to form a salt before preparing the filler liquid, and then the salt is used as a component of the filler liquid. A filler liquid can be prepared. It is preferable to control the number distribution B to adjust the dispersed state of the filler. The filler liquid is preferably prepared as one or more abrasive liquids containing a filler, a solvent, and preferably a binder separately from the ferromagnetic powder and the abrasive. Can be used for preparation. Commercially available equipment can be used for stirring, dispersion treatment and classification treatment. The conditions for performing these treatments are not particularly limited, and may be set according to the type of apparatus used, etc. so that the number distribution B satisfies (4) to (6) described above. .

Regarding the first dispersing treatment of the composition for forming the magnetic layer, in one embodiment, the ferromagnetic powder is dispersed in two stages, and the ferromagnetic powder is dispersed in the first stage to form coarse agglomerates. After pulverizing, a second stage dispersion treatment can be performed in which the impact energy applied to the particles of the ferromagnetic powder by collision with the dispersion beads is smaller than that in the first dispersion treatment. It is believed that such dispersion treatment can both improve the dispersibility of the ferromagnetic powder and suppress the occurrence of chipping (partial chipping of particles). This point is also preferable for controlling the vertical squareness ratio, which will be described later.

As an example of the two-step dispersion treatment, a first step of dispersing the ferromagnetic powder, the binder and the solvent in the presence of the first dispersing beads to obtain a dispersion liquid; and a second stage of dispersing the dispersion liquid obtained in (1) in the presence of second dispersing beads having a bead diameter and density smaller than those of the first dispersing beads. The above distributed processing will be further described below.

In order to improve the dispersibility of the ferromagnetic powder in the first magnetic layer-forming composition, the first step and the second step should be a dispersion treatment before mixing the ferromagnetic powder with other powder components. It is preferable to perform as For example, before mixing with an abrasive and a filler, as a dispersion treatment of a liquid (magnetic liquid) containing a ferromagnetic powder, a binder, a solvent and optionally added additives, the above first step and second step It is preferred to perform steps.

The bead diameter of the second dispersed beads is preferably 1/100 or less, more preferably 1/500 or less of the bead diameter of the first dispersed beads. Also, the bead diameter of the second dispersed beads can be, for example, 1/10000 or more of the bead diameter of the first dispersed beads. However, it is not limited to this range. For example, the bead diameter of the second dispersed beads is preferably in the range of 80-1000 nm. On the other hand, the bead diameter of the first dispersed beads can range, for example, from 0.2 to 1.0 mm.
The bead diameter in the present invention and this specification is a value measured by the same method as the method for measuring the average particle size of the powder described above.

The above-mentioned second step is preferably carried out under conditions where the second dispersed beads are present in an amount of 10 times or more that of the ferromagnetic hexagonal ferrite powder on a mass basis, and in an amount of 10 to 30 times More preferably under existing conditions.
On the other hand, the amount of the first dispersed beads in the first stage is also preferably within the above range.

The second dispersed beads are beads with a lower density than the first dispersed beads. "Density" is determined by dividing the mass (unit: g) of dispersed beads by the volume (unit: cm ³ ). Measurements are made by the Archimedes method. The density of the second dispersed beads is preferably 3.7 g/cm ³ or less, more preferably 3.5 g/cm ³ or less. The density of the second dispersed beads may be, for example, greater than or equal to 2.0 g/cm ³ or less than 2.0 g/cm ³ . Preferred second dispersed beads from the viewpoint of density include diamond beads, silicon carbide beads, silicon nitride beads, etc. Preferred second dispersed beads from the viewpoint of density and hardness include diamond beads. can be done.
On the other hand, as the first dispersion beads, dispersion beads with a density of more than 3.7 g/cm ³ are preferable, dispersion beads with a density of 3.8 g/cm ³ or more are more preferable, and dispersion beads with a density of 4.0 g/cm ³ or more are preferable. Beads are more preferred. The density of the first dispersed beads may be, for example, less than or equal to 7.0 g/cm ³ or greater than 7.0 g/cm ³ . As the first dispersed beads, zirconia beads, alumina beads, etc. are preferably used, and zirconia beads are more preferably used.

The dispersion time is not particularly limited, and may be set according to the type of dispersion machine used.

(Coating process)
The second magnetic layer is formed, for example, by directly coating the second magnetic layer-forming composition on the surface of the non-magnetic support, or by sequentially or simultaneously coating the second magnetic layer-forming composition with the non-magnetic layer-forming composition. can do.
The first magnetic layer can be formed, for example, by coating the first magnetic layer-forming composition and the second magnetic layer-forming composition sequentially or simultaneously.
The backcoat layer comprises a backcoat layer-forming composition applied to the surface of a non-magnetic support having a first magnetic layer and a second magnetic layer or a further non-magnetic layer (or those layers are subsequently provided). It can be formed by coating the opposite surface.
For details of coating for forming each layer, paragraph 0066 of JP-A-2010-231843 can be referred to.

(Other processes)
Known techniques can be applied to other various steps for manufacturing the magnetic recording medium. For various steps, for example, paragraphs 0067 to 0070 of JP-A-2010-231843 can be referred to.
For example, the coating layer of the composition for forming the first magnetic layer can be subjected to orientation treatment in the orientation zone while the coating layer is in a wet state. Various known techniques including those described in paragraph 0052 of JP-A-2010-24113 can be applied to the alignment treatment. For example, the vertical alignment treatment can be performed by a known method such as a method using opposed magnets with different polarities. In the orientation zone, the drying speed of the coating layer can be controlled by the temperature and air volume of the drying air and/or the conveying speed in the orientation zone. Also, the coated layer may be pre-dried before being conveyed to the orientation zone.

The magnetic recording medium according to one aspect of the present invention can be a tape-shaped magnetic recording medium (magnetic tape), or can be a disk-shaped magnetic recording medium (magnetic disk). A magnetic tape is housed in, for example, a magnetic tape cartridge, and the magnetic tape cartridge is loaded into a magnetic recording/reproducing apparatus. For example, a long magnetic tape material obtained through various processes can be cut (slit) into the width of the magnetic tape to be wound on the magnetic tape cartridge by a known cutting machine. The above widths are determined according to standards and are, for example, 1/2 inch. 1 inch = 12.65 mm.

(servo pattern)
Servo patterns can also be formed on the magnetic recording medium by a known method in order to enable head tracking in a magnetic recording/reproducing apparatus. "Formation of servo patterns" can also be called "recording of servo signals." The formation of a servo pattern will be described below using a magnetic tape as an example.

A servo pattern is usually formed along the longitudinal direction of the magnetic tape. Methods of control using servo signals (servo control) include timing-based servo (TBS), amplitude servo, frequency servo, and the like.

As shown in ECMA (European Computer Manufacturers Association)-319 (June 2001), a magnetic tape conforming to the LTO (Linear Tape-Open) standard (generally called "LTO tape") adopts a timing-based servo system. ing. In this timing-based servo system, a servo pattern is composed of a plurality of non-parallel pairs of magnetic stripes (also called "servo stripes") arranged continuously in the longitudinal direction of the magnetic tape. A servo system is a system that performs head tracking using a servo signal. In the present invention and in this specification, the term "timing-based servo pattern" refers to a servo pattern that enables head tracking in a timing-based servo system servo system. The reason why the servo pattern is composed of a pair of non-parallel magnetic stripes as described above is to inform the servo signal reading element passing over the servo pattern of its passing position. Specifically, the pair of magnetic stripes are formed so that the interval between them changes continuously along the width direction of the magnetic tape. and the relative position of the servo signal reading element. This relative position information enables tracking of the data tracks. For this reason, a plurality of servo tracks are usually set on the servo pattern along the width direction of the magnetic tape.

A servo band is composed of servo patterns that are continuous in the longitudinal direction of the magnetic tape. A plurality of servo bands are usually provided on the magnetic tape. For example, in LTO tape, the number is five. A data band is an area sandwiched between two adjacent servo bands. The data band is composed of a plurality of data tracks, each data track corresponding to each servo track.

In one form, as disclosed in JP-A-2004-318983, each servo band includes information indicating the number of the servo band ("servo band ID (identification)" or "UDIM (Unique Data Band Identification)"). Method (also called information) is embedded. This servo band ID is recorded by shifting a specific one of a plurality of pairs of servo stripes in the servo band so that the position thereof is relatively displaced in the longitudinal direction of the magnetic tape. Specifically, the method of shifting a specific one of a plurality of pairs of servo stripes is changed for each servo band. As a result, the recorded servo band ID is unique for each servo band, so that one servo band can be uniquely specified only by reading one servo band with a servo signal reading element.

It should be noted that as a method for uniquely specifying a servo band, there is also a method using a staggered method as shown in ECMA-319 (June 2001). In this staggered method, groups of non-parallel pairs of magnetic stripes (servo stripes) arranged continuously in the longitudinal direction of the magnetic tape are recorded so as to be shifted in the longitudinal direction of the magnetic tape for each servo band. do. Since this combination of shifts between adjacent servo bands is unique for the entire magnetic tape, the servo band can be uniquely identified when reading the servo pattern with two servo signal reading elements. It is possible.

In each servo band, information indicating the position in the longitudinal direction of the magnetic tape (also called "LPOS (Longitudinal Position) information") is also usually embedded as indicated in ECMA-319 (June 2001). ing. Like the UDIM information, this LPOS information is also recorded by shifting the positions of a pair of servo stripes in the longitudinal direction of the magnetic tape. However, unlike the UDIM information, the same signal is recorded in each servo band in this LPOS information.

Other information different from the above UDIM and LPOS information can also be embedded in the servo band. In this case, the embedded information may be different for each servo band, such as UDIM information, or common to all servo bands, such as LPOS information.
Also, as a method of embedding information in the servo band, it is possible to adopt a method other than the above. For example, a predetermined code may be recorded by thinning out a predetermined pair from a group of paired servo stripes.

The servo pattern forming head is called a servo write head. A servo write head normally has a pair of gaps corresponding to the pair of magnetic stripes as many as the number of servo bands. Normally, a core and a coil are connected to each pair of gaps, and by supplying current pulses to the coils, a magnetic field generated in the core can generate a leakage magnetic field in the pair of gaps. When forming the servo pattern, the magnetic pattern corresponding to the pair of gaps is transferred onto the magnetic tape by inputting a current pulse while the magnetic tape is running over the servo write head, thereby forming the servo pattern. can be done. The width of each gap can be appropriately set according to the density of the servo pattern to be formed. The width of each gap can be set to, for example, 1 μm or less, 1 to 10 μm, or 10 μm or more.

Before forming the servo pattern on the magnetic tape, the magnetic tape is usually demagnetized (erase). This erasing process can be performed by applying a uniform magnetic field to the magnetic tape using a DC magnet or an AC magnet. The erase process includes DC (Direct Current) erase and AC (Alternating Current) erase. AC erase is performed by gradually decreasing the strength of the magnetic field while reversing the direction of the magnetic field applied to the magnetic tape. DC erase, on the other hand, is performed by applying a unidirectional magnetic field to the magnetic tape. There are two methods of DC erase. The first method is a horizontal DC erase that applies a unidirectional magnetic field along the length of the magnetic tape. The second method is perpendicular DC erase, in which a unidirectional magnetic field is applied along the thickness of the magnetic tape. The erase process may be performed on the entire magnetic tape, or may be performed on each servo band of the magnetic tape.

The direction of the magnetic field of the formed servo pattern is determined according to the erase direction. For example, when horizontal DC erasing is performed on a magnetic tape, the servo pattern is formed so that the direction of the magnetic field is opposite to the direction of erasing. As a result, the output of the servo signal obtained by reading the servo pattern can be increased. Incidentally, as disclosed in Japanese Patent Application Laid-Open No. 2012-53940, when a magnetic pattern is transferred to a perpendicular DC-erased magnetic tape using the gap, the formed servo pattern is read and obtained. The servo signal has a unipolar pulse shape. On the other hand, when a magnetic pattern is transferred to a magnetic tape that has been horizontally DC-erased using the gap, a servo signal obtained by reading the formed servo pattern has a bipolar pulse shape.

<Vertical squareness ratio>
In one form, the vertical squareness ratio of the magnetic recording medium is preferably 0.60 or more, for example. The upper limit of the squareness ratio is, in principle, 1.00 or less. The perpendicular squareness ratio of the magnetic recording medium may be 1.00 or less, for example, 0.95 or less, 0.90 or less, 0.85 or less, or 0.80 or less. A magnetic recording medium having a large squareness ratio in the perpendicular direction is preferable from the viewpoint of further improving the electromagnetic conversion characteristics. The perpendicular squareness ratio of the magnetic recording medium can be controlled by a known method such as performing a perpendicular orientation treatment.

In the present invention and in this specification, the "perpendicular squareness ratio" is the squareness ratio measured in the perpendicular direction of the magnetic recording medium. The "perpendicular direction" described with respect to the squareness ratio is the direction perpendicular to the surface of the first magnetic layer, and can also be called the thickness direction. In the present invention and in this specification, the vertical squareness ratio is obtained by the following method.
A sample piece of a size that can be introduced into the vibrating sample magnetometer is cut out from the magnetic recording medium to be measured. Using a vibrating sample magnetometer, this sample piece was measured at a maximum applied magnetic field of 3979 kA/m, a measurement temperature of 296 K, and a magnetic field sweep rate of 8.3 kA/m/sec. A magnetic field is applied in the direction perpendicular to the direction of ), and the magnetization intensity of the sample piece is measured with respect to the applied magnetic field. The measured value of the magnetization intensity shall be obtained as a value after demagnetization correction and as a value obtained by subtracting the magnetization of the sample probe of the vibrating sample magnetometer as background noise. The squareness ratio SQ is a value calculated as SQ=Mr/Ms, where Ms is the magnetization intensity at the maximum applied magnetic field and Mr is the magnetization intensity at zero applied magnetic field. The measurement temperature refers to the temperature of the sample piece, and by setting the ambient temperature around the sample piece to the measurement temperature, the temperature equilibrium is established, whereby the temperature of the sample piece can be made the measurement temperature.

[Magnetic tape cartridge]
One aspect of the present invention relates to a magnetic tape cartridge including the tape-shaped magnetic recording medium (that is, magnetic tape).

The details of the magnetic tape included in the magnetic tape cartridge are as described above.

A magnetic tape cartridge generally contains a magnetic tape wound on a reel inside the cartridge body. The reel is rotatably provided inside the cartridge body. As the magnetic tape cartridge, a single reel type magnetic tape cartridge having one reel inside the cartridge body and a dual reel type magnetic tape cartridge having two reels inside the cartridge body are widely used. When a single-reel type magnetic tape cartridge is mounted on a magnetic tape device for recording and/or reproducing data on the magnetic tape, the magnetic tape is pulled out from the magnetic tape cartridge and wound on the reel of the magnetic tape device. be taken. A magnetic head is arranged in the magnetic tape transport path from the magnetic tape cartridge to the take-up reel. The magnetic tape is fed out and taken up between the reel (supply reel) of the magnetic tape cartridge and the reel (take-up reel) of the magnetic tape device. During this time, data is recorded and/or reproduced by contact and sliding between the magnetic head and the surface of the magnetic layer of the magnetic tape. On the other hand, a twin-reel type magnetic tape cartridge has both a supply reel and a take-up reel inside the magnetic tape cartridge. The magnetic tape cartridge may be either a single-reel type magnetic tape cartridge or a twin-reel type magnetic tape cartridge. The magnetic tape cartridge may include the magnetic recording medium (magnetic tape) according to one aspect of the present invention, and other known techniques can be applied.

[Magnetic recording and reproducing device]
One aspect of the present invention relates to a magnetic recording/reproducing device including the magnetic recording medium. In the magnetic recording/reproducing apparatus, data is recorded on the magnetic recording medium and/or reproduced from the magnetic recording medium by, for example, contacting and sliding the magnetic layer surface of the magnetic recording medium and the magnetic head. It can be done by For example, the magnetic recording/reproducing device can detachably include a magnetic tape cartridge according to one aspect of the present invention.

<Magnetic head>
In the present invention and in this specification, the term "magnetic recording/reproducing apparatus" means a device capable of at least one of recording data on a magnetic recording medium and reproducing data recorded on the magnetic recording medium. do. Such devices are commonly called drives. The magnetic head included in the magnetic recording/reproducing device can be a recording head capable of recording data on a magnetic recording medium, and a reproducing head capable of reproducing data recorded on the magnetic recording medium. can also be In one form, the magnetic recording/reproducing apparatus can include both a recording head and a reproducing head as separate magnetic heads. In another form, the magnetic head included in the magnetic recording/reproducing device may have a configuration in which both the recording element and the reproducing element are provided in one magnetic head.
The recording head can be a magnetic head for perpendicular recording or a magnetic head for longitudinal recording. As for the magnetic head for perpendicular recording and the magnetic head for longitudinal recording, known techniques for these heads can be applied. The magnetic recording medium can exhibit excellent electromagnetic conversion characteristics when reproducing data recorded by the perpendicular recording method. Therefore, the recording head included in the magnetic recording/reproducing apparatus is preferably a magnetic head for perpendicular recording.
As a reproducing head, a magnetic head (MR head) including a magnetoresistive (MR) element as a reproducing element capable of reading information recorded on a magnetic recording medium with high sensitivity is preferable. As the MR head, various known MR heads (eg, GMR (Giant Magnetoresistive) head, TMR (Tunnel Magnetoresistive) head, etc.) can be used. A magnetic head for recording and/or reproducing data may also include a servo pattern reading element. Alternatively, the magnetic recording/reproducing apparatus may include a magnetic head (servo head) having a servo pattern reading element as a separate head from the magnetic head that records and/or reproduces data. For example, a magnetic head for recording data and/or reproducing recorded data (hereinafter also referred to as a "recording/reproducing head") may include two servo signal reading elements. can simultaneously read two adjacent servo bands across the data band. One or more data elements can be positioned between the two servo signal read elements. An element for recording data (recording element) and an element for reproducing data (reading element) are collectively referred to as a "data element".

When recording data and/or reproducing recorded data, first, head tracking using a servo signal can be performed. That is, by causing the servo signal reading element to follow a predetermined servo track, the data element can be controlled to pass over the target data track. The movement of the data track is performed by changing the servo track read by the servo signal reading element in the tape width direction.
The record/playback head can also record and/or play back other data bands. In this case, the above-described UDIM information is used to move the servo signal reading element to a predetermined servo band, and tracking for that servo band can be started.

Fig. 1 shows an example of the arrangement of data bands and servo bands on a magnetic tape. In FIG. 1, a plurality of servo bands 1 are sandwiched between guide bands 3 on the first magnetic layer of the magnetic tape MT. A plurality of areas 2 sandwiched between two servo bands are data bands. A servo pattern is a magnetized region that can be formed by magnetizing a specific region of the first magnetic layer with a servo write head. The area magnetized by the servo write head (the position where the servo pattern is formed) is defined by standards. For example, in the industry standard LTO Ultrium format tape, a plurality of servo patterns inclined with respect to the tape width direction as shown in FIG. 2 are formed on the servo band when the magnetic tape is manufactured. Specifically, in FIG. 2, the servo frame SF on servo band 1 is composed of servo subframe 1 (SSF1) and servo subframe 2 (SSF2). A servo subframe 1 is composed of an A burst (symbol A in FIG. 2) and a B burst (symbol B in FIG. 2). The A burst is composed of servo patterns A1 to A5, and the B burst is composed of servo patterns B1 to B5. On the other hand, servo subframe 2 is composed of a C burst (symbol C in FIG. 2) and a D burst (symbol D in FIG. 2). The C burst is composed of servo patterns C1 to C4, and the D burst is composed of servo patterns D1 to D4. Such 18 servo patterns are arranged in sets of 5 and 4 in subframes arranged in an array of 5, 5, 4, 4, and are used to identify servo frames. FIG. 2 shows one servo frame for explanation. In practice, however, a plurality of servo frames are arranged in the running direction in each servo band on the magnetic layer of the magnetic tape on which the head tracking of the timing-based servo system is performed. In FIG. 2, arrows indicate the direction of travel. For example, an LTO Ultrium format tape typically has 5000 or more servo frames per meter of tape length in each servo band of the magnetic layer.

The present invention will be described below based on examples. However, the present invention is not limited to the embodiments shown in Examples. "Parts" and "%" described below indicate "mass parts" and "mass%" unless otherwise specified. In addition, the processes and evaluations described below were performed in an environment with an ambient temperature of 23° C.±1° C. unless otherwise specified. Also, "eq" described below indicates an equivalent, which is a unit that cannot be converted into the SI unit system.

[Preparation of Ferromagnetic Powder for Second Magnetic Layer]
<Preparation of ferromagnetic powder A>
The raw materials shown in Table 1 were weighed in proportions (mol% basis) shown in Table 1 and mixed in a mixer to obtain a raw material mixture.
The raw material mixture thus obtained was melted in a platinum crucible at a melting temperature of 1480° C., and the melt was stirred while heating the outlet provided at the bottom of the platinum crucible to dispense the melt in the form of a rod at a rate of about 3 g/sec. . The tapped liquid was rolled and quenched with water-cooled twin rolls to obtain an amorphous body.
280 g of the obtained amorphous material was placed in an electric furnace, the temperature inside the electric furnace was raised to the crystallization temperature shown in Table 1, and the same temperature was maintained for 5 hours to deposit particles of the ferromagnetic powder (crystallization ).
Next, the crystallized product containing the precipitated particles was coarsely pulverized in a mortar, and 1000 g of zirconia beads having a bead diameter of 1 mm and 800 ml of 1% concentration acetic acid were added to the glass bottle containing the coarsely pulverized particles, followed by dispersion treatment for 3 hours using a paint shaker. After that, the dispersion was separated from the beads and placed in a stainless steel beaker. After the dispersion liquid was allowed to stand at a liquid temperature of 80°C for 3 hours to dissolve the glass component (CaB ₂ O ₄ ), it was precipitated in a centrifugal separator and washed by repeating decantation, and the internal atmosphere temperature was 110°C. was dried in a dryer for 6 hours to obtain a ferromagnetic powder.

<Preparation of ferromagnetic powders B to E, X and Y>
Each ferromagnetic powder was obtained by the method described for the preparation of ferromagnetic powder A, except that the composition of the raw material mixture and the crystallization temperature were changed as shown in Table 1.

To confirm that the ferromagnetic powders A to E, X and Y obtained above are each spinel ferrite, scan CuKα rays under the conditions of a voltage of 45 kV and an intensity of 40 mA, and measure the X-ray diffraction pattern under the following conditions ( X-ray diffraction analysis). Each of the ferromagnetic powders obtained above exhibited the crystal structure of spinel ferrite, and the crystal phase detected by X-ray diffraction analysis was a single phase of spinel ferrite.
PANalytical X'Pert Pro diffractometer, PIXcel detector Soller slits for incident and diffracted beams: 0.017 radians Fixed divergence slit angle: ¼ degree Mask: 10 mm
Anti-scattering slit: 1/4 degree Measurement mode: continuous Measurement time per step: 3 seconds Measurement speed: 0.017 degree per second Measurement step: 0.05 degree

<Measurement of average particle size of ferromagnetic powder>
For the ferromagnetic powders A to E, X and Y obtained above, the average particle size was determined by the method described above. A transmission electron microscope H-9000 made by Hitachi was used as the transmission electron microscope, and image analysis software KS-400 made by Carl Zeiss was used as the image analysis software.

<Measurement of coercive force Hc of ferromagnetic powder>
For the ferromagnetic powders A to E, X and Y obtained above, the coercive force Hc was determined by the method described above. As the vibrating sample magnetometer, a VSM (Vibrating Sample Magnetometer) manufactured by Toei Kogyo Co., Ltd. was used.

Table 1 shows the above results.

[Preparation of abrasive liquid]
<Preparation of Abrasive Liquid A>
For 100.0 parts of the abrasive (alumina powder) shown in Table 2, 2,3-dihydroxynaphthalene (manufactured by Tokyo Kasei Co., Ltd.) in the amount shown in Table ₂ , a polyester polyurethane resin (Toyobo) having an SO Na group as a polar group 31.3 parts of a 32% solution (solvent is a mixed solvent of methyl ethyl ketone and toluene) of UR-4800 (amount of polar group: 80 meq/kg) manufactured by Co., Ltd.; 0.0 part was mixed and dispersed with a paint shaker in the presence of zirconia beads (bead diameter: 0.1 mm) for the time shown in Table 2 (bead dispersion time).
After dispersion, the dispersion obtained by separating the dispersion and the beads with a mesh was subjected to centrifugal separation. For the centrifugal separation, CS150GXL manufactured by Hitachi Koki Co., Ltd. (rotor used is S100AT6 manufactured by Hitachi Koki Co., Ltd.) is used as a centrifuge, and the rotation speed (rpm; rotation per minute) shown in Table 2 is used for the time shown in Table 2 (centrifugation time ),Carried out. By this centrifugation treatment, particles having a relatively large particle size are precipitated, and particles having a relatively small particle size are dispersed in the supernatant liquid.
After that, the supernatant was collected by decantation. This collected liquid is called "abrasive liquid A".

<Preparation of Abrasive Liquids B and C>
Abrasive liquid B and abrasive liquid C were each prepared by the method described for the preparation of abrasive liquid A, except that various items were changed as shown in Table 2.

[Preparation of filler liquid]
<Preparation of filler liquid D>
Per 100.0 parts of the filler (carbon black) shown in Table 3, polyethyleneimine in the amount shown in Table 3, stearic acid in the amount shown in Table 3, and a mixed solution of methyl ethyl ketone and cyclohexanone 1:1 (mass ratio) as a solvent 570 0.0 part was mixed and dispersed with a paint shaker in the presence of zirconia beads (bead diameter: 0.1 mm) for the time shown in Table 3 (bead dispersion time).
After dispersion, the dispersion obtained by separating the dispersion and the beads with a mesh was subjected to centrifugal separation. For the centrifugation treatment, CS150GXL manufactured by Hitachi Koki Co., Ltd. (rotor used is S100AT6 manufactured by Hitachi Koki Co., Ltd.) is used as a centrifuge, and the rotation speed (rpm; rotation per minute) shown in Table 3 is used for the time shown in Table 3 (centrifugation time ),Carried out. By this centrifugation treatment, particles having a relatively large particle size are precipitated, and particles having a relatively small particle size are dispersed in the supernatant liquid.
After that, the supernatant was collected by decantation. This collected liquid is called "filler liquid D".

The above polyethyleneimine is a commercial product (number average molecular weight 600) manufactured by Nippon Shokubai Co., Ltd.

<Preparation of filler liquids E to G>
Filler fluids EG were each prepared by the method described for the preparation of filler fluid D, except that various items were changed as shown in Table 3.

[Example 1]
<Preparation of composition for forming first magnetic layer>
(Magnetic liquid)
Ferromagnetic powder: 100.0 parts Hexagonal barium ferrite powder with an average particle size (average plate diameter) of 21 nm (“BaFe” in Table 4)
Oleic acid: 2.0 parts Vinyl chloride copolymer (MR-104 manufactured by Zeon Corporation): 10.0 parts SO ₃ Na group-containing polyurethane resin: 4.0 parts (weight average molecular weight: 70000, SO ₃ Na group: 0.0 parts) 07 meq/g)
Amine polymer (DISPERBYK-102 manufactured by BYK-Chemie): 6.0 parts Methyl ethyl ketone: 150.0 parts Cyclohexanone: 150.0 parts (abrasive liquid)
The abrasive liquid shown in Table 4 was used so that the amount of abrasive in the abrasive liquid was the amount shown in Table 4 (filler liquid)
Use the filler liquid shown in Table 4 so that the amount of filler in the filler liquid is the amount shown in Table 4 (other ingredients)
Stearic acid: 3.0 parts Stearic acid amide: 0.3 parts Butyl stearate: 6.0 parts Methyl ethyl ketone: 110.0 parts Cyclohexanone: 110.0 parts Polyisocyanate (Coronate (registered trademark) L manufactured by Tosoh Corporation): 3 .0 copies

(Preparation method)
Various components of the above magnetic liquid were dispersed for 24 hours using zirconia beads (first dispersion beads, density 6.0 g/cm ³ ) with a bead diameter of 0.5 mm in a batch-type vertical sand mill (first stage ), followed by filtration using a filter with a pore size of 0.5 μm to prepare Dispersion A. The zirconia beads were used in an amount 10 times the mass of the ferromagnetic powder.
After that, the dispersion liquid A was dispersed in a batch-type vertical sand mill using diamond beads with a bead diameter of 500 nm (second dispersion beads, density 3.5 g/cm ³ ) for 1 hour (second step), followed by centrifugation. A dispersion liquid (dispersion liquid B) was prepared by separating the diamond beads using a vessel. The amount of diamond beads used was 10 times the mass of the ferromagnetic powder.
Dispersion B obtained above, the abrasive liquid, the filler liquid, and the above-mentioned other components were introduced into a dissolver stirrer and stirred at a peripheral speed of 10 m/sec for 360 minutes. Then, after performing ultrasonic dispersion treatment for 60 minutes at a flow rate of 7.5 kg/min using a flow-type ultrasonic disperser, the mixture was filtered three times through a filter with a pore size of 0.3 μm to prepare a first magnetic layer-forming composition. did.

<Preparation of composition for forming second magnetic layer>
Various components of the composition for forming the second magnetic layer described below were dispersed for 24 hours using zirconia beads with a bead diameter of 0.1 mm in a batch-type vertical sand mill. A second magnetic layer-forming composition was prepared by filtering using

Ferromagnetic powder (see Table 4): 100.0 parts SO ₃ Na group-containing polyurethane resin: 18.0 parts (weight average molecular weight 70000, SO ₃ Na group content 0.2 meq/g)
Stearic acid: 1.0 parts Cyclohexanone: 300.0 parts Methyl ethyl ketone: 300.0 parts

<Preparation of composition for forming non-magnetic layer>
Various components of the following composition for forming a non-magnetic layer were dispersed for 24 hours in a batch-type vertical sand mill using zirconia beads with a bead diameter of 0.1 mm, and then filtered using a filter with a pore diameter of 0.5 μm. A composition for forming a non-magnetic layer was prepared by filtration.

Non-magnetic inorganic powder α-iron oxide: 100.0 parts (average particle size 10 nm, BET specific surface area 75 m ² /g)
Carbon black: 25.0 parts (average particle size 20 nm)
SO ₃ Na group-containing polyurethane resin: 18.0 parts (weight average molecular weight 70000, SO ₃ Na group content 0.2 meq/g)
Stearic acid: 1.0 parts Cyclohexanone: 300.0 parts Methyl ethyl ketone: 300.0 parts

<Preparation of Composition for Forming Backcoat Layer>
Of the various components of the composition for forming a backcoat layer described below, components other than lubricants (stearic acid and butyl stearate), polyisocyanate and 200.0 parts of cyclohexanone were kneaded and diluted in an open kneader, and then dispersed in a horizontal bead mill. Using zirconia beads with a bead diameter of 1 mm, 12 passes of dispersion treatment were performed with a bead filling rate of 80% by volume, a rotor tip peripheral speed of 10 m/sec, and a residence time of 2 minutes per pass. Thereafter, the remaining components were added and stirred with a dissolver stirrer, and the resulting dispersion was filtered through a filter having a pore size of 1 μm to prepare a composition for forming a backcoat layer.

Non-magnetic inorganic powder α-iron oxide: 80.0 parts (average particle size 0.15 μm, BET specific surface area 52 m ² /g)
Carbon black: 20.0 parts (average particle size 20 nm)
Vinyl chloride copolymer: 13.0 parts Sulfonic acid group-containing polyurethane resin: 6.0 parts Phenylphosphonic acid: 3.0 parts Cyclohexanone: 155.0 parts Methyl ethyl ketone: 155.0 parts Stearic acid: 3.0 parts Stearic acid Butyl: 3.0 parts Polyisocyanate: 5.0 parts Cyclohexanone: 200.0 parts

<Magnetic Tape and Magnetic Tape Cartridge Manufacturing Method>
On the surface of a 4.1 μm-thick polyethylene naphthalate support, the composition for forming a non-magnetic layer prepared above was applied so as to give a thickness of 0.7 μm after drying, and dried to form a non-magnetic layer. did.
Next, the composition for forming the second magnetic layer prepared above was coated on the non-magnetic layer so that the thickness after drying was 0.1 μm, and dried to form a second magnetic layer.
Next, the composition for forming the first magnetic layer prepared above was coated on the second magnetic layer so that the thickness after drying was 0.1 μm to form a coating layer.
Thereafter, while the coated layer of the first magnetic layer-forming composition is in a wet state, a magnetic field having a magnetic field strength of 0.3 T is applied in a direction perpendicular to the surface of the coated layer to perform a vertical alignment treatment. It was dried to form the first magnetic layer.
After that, the above-prepared back coat was applied to the surface of the support opposite to the surface on which the non-magnetic layer, the second magnetic layer and the first magnetic layer were formed so that the thickness after drying was 0.3 μm. The layer-forming composition was applied and dried to form a backcoat layer.
After that, using a calender roll composed only of metal rolls, the surface is smoothed (calendered) at a speed of 100 m / min, a linear pressure of 300 kg / cm, and a calender temperature of 90 ° C. (calender roll surface temperature). did
Then, after performing heat treatment for 36 hours at an ambient temperature of 70° C., the long magnetic tape original was slit into 1/2 inch width to obtain a magnetic tape. By recording servo signals on the first magnetic layer of the obtained magnetic tape using a commercially available servo writer, the magnetic tape has a data band, a servo band, and a guide band arranged according to the LTO (Linear Tape-Open) Ultrium format. , and a magnetic tape having a servo pattern (timing-based servo pattern) arranged and shaped according to the LTO Ultrium format on the servo band. The servo pattern thus formed is a servo pattern according to the descriptions of JIS (Japanese Industrial Standards) X6175:2006 and Standard ECMA-319 (June 2001). The obtained magnetic tape (tape length: 960 m) was accommodated in a single reel type magnetic tape cartridge.
By repeating the above steps, two magnetic tape cartridges were produced. One magnetic tape cartridge was used for measuring the following number distribution A and number distribution B, and the other magnetic tape cartridge was used for the following electromagnetic conversion characteristics. used for the evaluation of

It can be confirmed by the following method that the first magnetic layer of the magnetic tape contains the compound containing the ammonium salt structure of the alkyl ester anion represented by the formula 1 formed by polyethyleneimine and stearic acid.
A sample is cut out from the magnetic tape, and X-ray photoelectron spectroscopic analysis is performed on the surface of the magnetic layer (measurement area: 300 μm×700 μm) using an ESCA device. Specifically, wide scan measurement is performed with an ESCA device under the following measurement conditions. In the measurement results, peaks are confirmed at the positions of the binding energy of the ester anion and the binding energy of the ammonium cation.
Apparatus: Shimadzu AXIS-ULTRA
Excitation X-ray source: Monochromatic Al-Kα ray Scan range: 0 to 1200 eV
Pass energy: 160 eV
Energy resolution: 1 eV/step
Acquisition time: 100ms/step
Cumulative count: 5
In addition, a sample piece having a length of 3 cm was cut from the magnetic tape, and the surface of the first magnetic layer was subjected to ATR-FT-IR (attenuated total reflection-fourier transform-infrared spectrometer) measurement (reflection method). Absorption is confirmed at the wavenumber (1540 cm ⁻¹ or 1430 cm ⁻¹ ) corresponding to the absorption of ⁻ and the wavenumber (2400 cm ⁻¹ ) corresponding to the absorption of ammonium cation.

[Examples 2 to 28, Comparative Examples 1 to 25]
A magnetic tape cartridge was produced by the method described for Example 1, except that the items shown in Table 4 were changed as shown in Table 4.
In Table 4, for the comparative examples described as "no second magnetic layer", the second magnetic layer was not formed, and the first magnetic layer was formed on the non-magnetic layer.

In Table 4, "SrFe1" indicates a hexagonal strontium ferrite powder produced as follows.
1707 g of SrCO3 _, 687 g of _H3BO3 _, 1120 g of _Fe2O3 _, 45 g of Al(OH) ₃ , 24 g of BaCO3, 13 g of _CaCO3 _, and 235 g of _Nd2O3 _were weighed and mixed in a mixer. A raw material mixture was obtained by mixing.
The obtained raw material mixture was melted in a platinum crucible at a melting temperature of 1390° C., and while the melt was being stirred, a tap hole provided at the bottom of the platinum crucible was heated, and the melt was tapped in a rod shape at a rate of about 6 g/sec. . The tapped liquid was rolled and quenched with a water-cooled twin roller to prepare an amorphous body.
280 g of the produced amorphous material was placed in an electric furnace, heated to 635° C. (crystallization temperature) at a heating rate of 3.5° C./min, and held at the same temperature for 5 hours to produce hexagonal strontium ferrite particles. Precipitated (crystallized).
Next, the crystallized product obtained above containing hexagonal strontium ferrite particles was coarsely pulverized in a mortar, and 1000 g of zirconia beads having a particle size of 1 mm and 800 mL of 1% concentration of acetic acid aqueous solution were added to a glass bottle and dispersed for 3 hours using a paint shaker. did After that, the resulting dispersion was separated from the beads and placed in a stainless steel beaker. After the dispersion liquid was allowed to stand at a liquid temperature of 100°C for 3 hours to dissolve the glass component, it was precipitated in a centrifugal separator, washed by repeating decantation, and placed in a heating furnace at a temperature of 110°C for 6 hours. After drying for a few hours, hexagonal strontium ferrite powder was obtained.
The average particle size of the hexagonal strontium ferrite powder obtained above is 18 nm, the activation volume is 902 nm ³ , the anisotropy constant Ku is 2.2×10 ⁵ J/m ³ , and the mass magnetization σs is 49 A·m ² /. kg.
12 mg of sample powder was taken from the hexagonal strontium ferrite powder obtained above, and the sample powder was partially dissolved under the dissolution conditions exemplified above. The surface layer content was determined.
Separately, 12 mg of sample powder was taken from the hexagonal strontium ferrite powder obtained above, and the sample powder was completely dissolved under the dissolution conditions exemplified above. Atomic bulk content was determined.
The content of neodymium atoms (bulk content) with respect to 100 atomic % of iron atoms in the hexagonal strontium ferrite powder obtained above was 2.9 atomic %. The content of neodymium atoms in the surface layer was 8.0 atomic %. The ratio of the surface layer portion content rate to the bulk content rate, "surface layer portion content rate/bulk content rate", was 2.8, confirming that neodymium atoms were unevenly distributed in the surface layer of the particles.

The fact that the powder obtained above exhibits the crystal structure of hexagonal ferrite can be confirmed by scanning CuKα rays under the conditions of a voltage of 45 kV and an intensity of 40 mA and measuring the X-ray diffraction pattern under the following conditions (X-ray diffraction analysis). confirmed. The powder obtained above exhibited a crystal structure of magnetoplumbite type (M type) hexagonal ferrite. The crystal phase detected by X-ray diffraction analysis was a magnetoplumbite single phase.
PANalytical X'Pert Pro diffractometer, PIXcel detector Soller slits for incident and diffracted beams: 0.017 radians Fixed divergence slit angle: ¼ degree Mask: 10 mm
Anti-scattering slit: 1/4 degree Measurement mode: Continuous Measurement time per step: 3 seconds Measurement speed: 0.017 degree per second Measurement step: 0.05 degree

In Table 4, "SrFe2" indicates hexagonal strontium ferrite powder produced as follows.
1725 g of SrCO3, 666 g of _H3BO3 _, 1332 g of _Fe2O3 _, 52 g of Al(OH) ₃ _, 34 g of _CaCO3 and 141 g of BaCO3 _were weighed and mixed in a mixer to obtain a raw material mixture.
The obtained raw material mixture was melted in a platinum crucible at a melting temperature of 1380° C., and the melt was stirred while heating the outlet provided at the bottom of the platinum crucible to dispense the melt in a rod shape at a rate of about 6 g/sec. . The tapped liquid was rolled and quenched with water-cooled twin rolls to prepare an amorphous body.
280 g of the obtained amorphous material was placed in an electric furnace, heated to 645° C. (crystallization temperature), and held at the same temperature for 5 hours to precipitate (crystallize) hexagonal strontium ferrite particles.
Next, the crystallized product obtained above containing hexagonal strontium ferrite particles was coarsely pulverized in a mortar, and 1000 g of zirconia beads with a particle size of 1 mm and 800 mL of 1% concentration of acetic acid aqueous solution were added to a glass bottle, followed by dispersion treatment with a paint shaker for 3 hours. did After that, the resulting dispersion was separated from the beads and placed in a stainless steel beaker. After the dispersion liquid was allowed to stand at a liquid temperature of 100°C for 3 hours to dissolve the glass component, it was precipitated in a centrifugal separator, washed by repeating decantation, and placed in a heating furnace at a temperature of 110°C for 6 hours. After drying for a few hours, hexagonal strontium ferrite powder was obtained.
The obtained hexagonal strontium ferrite powder had an average particle size of 19 nm, an activated volume of 1102 nm ³ , an anisotropy constant Ku of 2.0×10 ⁵ J/m ³ , and a mass magnetization σs of 50 A·m ² /kg. there were.

In Table 4, "ε-iron oxide" indicates ε-iron oxide powder prepared as follows.
8.3 g of iron (III) nitrate nonahydrate, 1.3 g of gallium (III) nitrate octahydrate, 190 mg of cobalt (II) nitrate hexahydrate, 150 mg of titanium (IV) sulfate, and 4.0 g of an aqueous ammonia solution having a concentration of 25% was added to a solution of 1.5 g of polyvinylpyrrolidone (PVP) in an air atmosphere at an ambient temperature of 25° C. while stirring using a magnetic stirrer. , and the mixture was stirred for 2 hours while maintaining the ambient temperature of 25°C. An aqueous citric acid solution obtained by dissolving 1 g of citric acid in 9 g of pure water was added to the obtained solution, and the mixture was stirred for 1 hour. The precipitated powder after stirring was collected by centrifugation, washed with pure water, and dried in a heating furnace with an internal furnace temperature of 80°C.
800 g of pure water was added to the dried powder, and the powder was dispersed again in water to obtain a dispersion liquid. The obtained dispersion was heated to a liquid temperature of 50° C., and 40 g of an ammonia aqueous solution having a concentration of 25% was added dropwise while stirring. After stirring for 1 hour while maintaining the temperature at 50° C., 14 mL of tetraethoxysilane (TEOS) was added dropwise and the mixture was stirred for 24 hours. 50 g of ammonium sulfate was added to the obtained reaction solution, and the precipitated powder was collected by centrifugation, washed with pure water, and dried in a heating furnace at an internal temperature of 80°C for 24 hours to obtain a ferromagnetic powder precursor. Obtained.
The obtained ferromagnetic powder precursor was placed in a heating furnace having an internal temperature of 1000° C. in an air atmosphere, and subjected to a heat treatment for 4 hours.
The heat-treated ferromagnetic powder precursor is put into a 4 mol/L sodium hydroxide (NaOH) aqueous solution, and the liquid temperature is maintained at 70° C. and stirred for 24 hours to obtain a heat-treated ferromagnetic powder precursor. The silicic acid compound, which is an impurity, was removed from the
After that, the ferromagnetic powder from which the silicic acid compound was removed was collected by centrifugal separation and washed with pure water to obtain the ferromagnetic powder.
When the composition of the obtained ferromagnetic powder was confirmed by high-frequency inductively coupled plasma-optical emission spectrometry (ICP-OES), Ga, Co and Ti-substituted ε-iron oxide (ε-Ga _{0 .28} Co _0.05 Ti _0.05 Fe _1.62 O ₃ ). In addition, X-ray diffraction analysis was performed under the same conditions as those previously described for the hexagonal strontium ferrite powder SrFe1. From the peaks of the X-ray diffraction pattern, the obtained ferromagnetic powder had an α-phase and a γ-phase crystal structure. It was confirmed to have a single-phase ε-phase crystal structure (ε-iron oxide crystal structure) containing no
The resulting ε-iron oxide powder had an average particle size of 12 nm, an activated volume of 746 nm ³ , an anisotropy constant Ku of 1.2×10 ⁵ J/m ³ and a mass magnetization σs of 16 A·m ² /kg. there were.

The activation volume and anisotropy constant Ku of the hexagonal strontium ferrite powder and the ε-iron oxide powder were obtained by using a vibrating sample magnetometer (manufactured by Toei Kogyo Co., Ltd.) for each ferromagnetic powder, as previously described. It is a value obtained by the method of
The mass magnetization σs is a value measured with a magnetic field strength of 1194 kA/m (15 kOe) using a vibrating sample magnetometer (manufactured by Toei Industry Co., Ltd.).

[Measurement of number distribution A and number distribution B]
The magnetic tape housed in the magnetic tape cartridge was subjected to the following method using an FE-SEM S4800 manufactured by Hitachi Ltd. as a scanning electron microscope (FE-SEM) to determine the number distribution A and the number distribution A on the surface of the first magnetic layer. A number distribution B was obtained.

(Number distribution A)
Using a scanning electron microscope (FE-SEM), a secondary electron image of the surface of the first magnetic layer of the magnetic tape to be measured is taken. As imaging conditions, the acceleration voltage is 5 kV, the working distance is 5 mm, and the imaging magnification is 10,000. At the time of imaging, a non-imaging area on the surface of the first magnetic layer is selected, focus is adjusted under the imaging conditions described above, and a secondary electron image is captured. A secondary electron image with a pixel count of 960 pixels×1280 pixels is obtained by deleting the portion (micron bar, cross mark, etc.) indicating the size and the like from the imaged image.
The above operation is performed 100 times at different locations on the surface of the first magnetic layer of the magnetic tape to be measured.
The secondary electron image obtained in this way is loaded into image processing software (Free software ImageJ), and binarization processing is performed according to the following procedure.
The threshold value for binarizing the secondary electron image obtained above has a lower limit of 210 gradations and an upper limit of 255 gradations, and binarization is performed using these two thresholds. After the binarization process, the noise component is removed by selecting the noise cut process Despeckle in the image analysis software (Free software ImageJ).
For the binarized image thus obtained, the number of bright regions (that is, white portions) and the area of each bright region are determined by image analysis software (Free software ImageJ). From the area A of the bright region obtained here, the equivalent circle diameter L of each bright region is calculated by (A/π)̂(1/2)×2=L.
The above steps are performed on the binarized images (100 images) obtained above.
The number distribution A is obtained from the above.

(Number distribution B)
Using a scanning electron microscope (FE-SEM), a secondary electron image of the surface of the first magnetic layer of the magnetic tape to be measured is taken. As imaging conditions, the acceleration voltage is 2 kV, the working distance is 5 mm, and the imaging magnification is 10,000 times. At the time of imaging, a non-imaging area on the surface of the first magnetic layer is selected, focus is adjusted under the imaging conditions described above, and a secondary electron image is captured. A secondary electron image with a pixel count of 960 pixels×1280 pixels is obtained by deleting the portion (micron bar, cross mark, etc.) indicating the size and the like from the imaged image.
The above operation is performed 100 times at different locations on the surface of the first magnetic layer of the magnetic tape to be measured.
The secondary electron image obtained in this way is taken into image processing software (Free software ImageJ), and binarization processing is performed according to the following procedure.
The threshold value for binarizing the secondary electron image obtained above has a lower limit value of 0 gradation and an upper limit value of 75 gradation, and the binarization process is performed using these two threshold values. After the binarization process, the noise component is removed by selecting the noise cut process Despeckle in the image analysis software (Free software ImageJ).
In the binarized image thus obtained, the number of dark regions (that is, black portions) and the area of each dark region are determined by image analysis software (Free software ImageJ). From the area A of the dark region obtained here, the equivalent circle diameter L of each dark region is calculated by (A/π)̂(1/2)×2=L.
The above steps are performed on the binarized images (100 images) obtained above. The number distribution B is obtained from the above.

A sample piece for measuring the squareness ratio in the vertical direction was cut out from the magnetic tape contained in each of the magnetic tape cartridges described above. For this sample piece, the squareness ratio in the vertical direction was determined by the method described above using a TM-TRVSM5050-SMSL model manufactured by Tamagawa Seisakusho as a vibrating sample magnetometer. As a result, the vertical squareness ratio of the magnetic tapes taken out from the respective magnetic tape cartridges of Examples 1-28 and Comparative Examples 1-25 was 0.60 or more and 1.00 or less.

[Evaluation of electromagnetic conversion characteristics (perpendicular recording method)]
The magnetic tape in each magnetic tape cartridge is recorded by the perpendicular recording method by the method described in paragraph 0102 of Japanese Patent No. 6531764 (Patent Document 1), and the recorded data is reproduced to obtain an SNR (Signal- to-Noise Ratio) was obtained. The SNR was obtained as a relative value with the value of Comparative Example 1 as a reference (0 dB).

The above results are shown in Table 4 (Tables 4-1 and 4-2).

From the results shown in Table 4, it can be confirmed that the coated magnetic recording media of Examples 1 to 28 exhibited excellent electromagnetic conversion characteristics (high SNR) when reproducing data recorded by the perpendicular recording method.

One aspect of the present invention is useful in various magnetic recording applications where further improvement in recording density is desired.

Claims

a non-magnetic support and a first magnetic layer comprising a ferromagnetic powder and a binder;
A second magnetic layer containing a ferromagnetic powder having a coercive force Hc of 50 Oe or less and an average particle size of 50 nm or less and a binder between the non-magnetic support and the first magnetic layer A magnetic recording medium further comprising:
A number distribution A of equivalent circle diameters of a plurality of bright regions in a binarized secondary electron image obtained by imaging the surface of the first magnetic layer with a scanning electron microscope at an acceleration voltage of 5 kV is , the following (1) to (3):
(1) 10,000 or more and 30,000 or less bright regions having an equivalent circle diameter of 1 nm or more and 50 nm or less,
(2) 7000 or more and 25000 or less bright regions with an equivalent circle diameter of 51 nm or more and 100 nm or less;
(3) 1000 or more and 3000 or less bright regions with an equivalent circle diameter of 101 nm or more;
and the number of equivalent circle diameters of a plurality of dark regions in a binarized secondary electron image obtained by imaging the surface of the first magnetic layer with a scanning electron microscope at an acceleration voltage of 2 kV. Distribution B is the following (4) to (6):
(4) 200 or more and 50,000 or less dark regions with an equivalent circle diameter of 1 nm or more and 50 nm or less;
(5) 200 or more and 25,000 or less dark regions with an equivalent circle diameter of 51 nm or more and 100 nm or less;
(6) 0 or more and 2000 or less dark regions having an equivalent circle diameter of 101 nm or more;
2. The magnetic recording medium according to claim 1, which satisfies:
2. The magnetic recording medium according to claim 1, wherein said magnetic recording medium has a perpendicular squareness ratio of 0.60 or more.
2. The magnetic recording medium according to claim 1, wherein the ferromagnetic powder contained in said second magnetic layer has a coercive force Hc of 10 Oe or more and 50 Oe or less.
2. The magnetic recording medium according to claim 1, wherein the ferromagnetic powder contained in said second magnetic layer has an average particle size of 5 nm or more and 50 nm or less.
2. The magnetic recording medium according to claim 1, wherein the ferromagnetic powder contained in said second magnetic layer is spinel ferrite powder.
2. The magnetic recording medium according to claim 1, wherein the ferromagnetic powder contained in said first magnetic layer is hexagonal barium ferrite powder.
2. The magnetic recording medium according to claim 1, wherein the ferromagnetic powder contained in said first magnetic layer is hexagonal strontium ferrite powder.
2. The magnetic recording medium according to claim 1, wherein the ferromagnetic powder contained in said first magnetic layer is ε-iron oxide powder.
2. The magnetic recording medium according to claim 1, further comprising a non-magnetic layer containing non-magnetic powder and a binder between said non-magnetic support and said second magnetic layer.
2. The method according to claim 1, further comprising a back coat layer containing a non-magnetic powder and a binder on the surface side opposite to the surface side having the first magnetic layer and the second magnetic layer of the non-magnetic support. A magnetic recording medium as described.
A number distribution A of equivalent circle diameters of a plurality of bright regions in a binarized secondary electron image obtained by imaging the surface of the first magnetic layer with a scanning electron microscope at an acceleration voltage of 5 kV is , the following (1) to (3):
(1) 10,000 or more and 30,000 or less bright regions having an equivalent circle diameter of 1 nm or more and 50 nm or less,
(2) 7000 or more and 25000 or less bright regions with an equivalent circle diameter of 51 nm or more and 100 nm or less;
(3) 1000 or more and 3000 or less bright regions with an equivalent circle diameter of 101 nm or more;
and the number of equivalent circle diameters of a plurality of dark regions in a binarized secondary electron image obtained by imaging the surface of the first magnetic layer with a scanning electron microscope at an accelerating voltage of 2 kV. Distribution B is the following (4) to (6):
(4) 200 or more and 50,000 or less dark regions with an equivalent circle diameter of 1 nm or more and 50 nm or less;
(5) 200 or more and 25,000 or less dark regions with an equivalent circle diameter of 51 nm or more and 100 nm or less;
(6) 0 or more and 2000 or less dark regions having an equivalent circle diameter of 101 nm or more;
The filling,
The perpendicular squareness ratio of the magnetic recording medium is 0.60 or more,
The ferromagnetic powder contained in the second magnetic layer has a coercive force Hc of 10 Oe or more and 50 Oe or less,
The ferromagnetic powder contained in the second magnetic layer has an average particle size of 5 nm or more and 50 nm or less,
The ferromagnetic powder contained in the second magnetic layer is spinel ferrite powder,
The ferromagnetic powder contained in the first magnetic layer is ferromagnetic powder selected from the group consisting of hexagonal barium ferrite powder, hexagonal strontium ferrite powder and ε-iron oxide powder,
A nonmagnetic layer containing nonmagnetic powder and a binder is further provided between the nonmagnetic support and the second magnetic layer, and the first magnetic layer and the second magnetic layer of the nonmagnetic support are further provided. 2. The magnetic recording medium according to claim 1, further comprising a backcoat layer containing a non-magnetic powder and a binder on the surface opposite to the surface having the magnetic layer.
2. The magnetic recording medium of claim 1, which is a magnetic tape.
13. The magnetic recording medium of claim 12, which is a magnetic tape.
A magnetic tape cartridge comprising the magnetic recording medium according to claim 13 or 14.
A magnetic recording/reproducing apparatus comprising the magnetic recording medium according to any one of claims 1 to 14.