JP2019067477A - Magnetic recording medium and magnetic recording/reproducing device - Google Patents

Abstract

To provide a magnetic recording medium suitable for a recording medium for archive which can exhibit excellent electromagnetic conversion characteristics.SOLUTION: There is provided a magnetic recording medium in which a magnetic layer includes ferromagnetic hexagonal ferrite powder, binder and oxide abrasive, an intensity ratio (Int(110)/Int(114)) determined by an X-ray diffraction analysis of the magnetic layer using the In-Plane method is 0.5 or more and 4.0 or less, a perpendicular squareness ratio of the magnetic recording medium is 0.65 or more and 1.00 or less, a logarithmic decrement determined by a pendulum viscoelasticity test on the surface of the magnetic layer is 0.050 or less, and an average particle diameter of the oxide abrasive determined from a secondary ion image, which is obtained by irradiating the surface of the magnetic layer with a focused ion beam, is 0.04 μm or more and 0.08 μm or less. There is provided a magnetic recording/reproducing device including the magnetic recording medium.SELECTED DRAWING: None

Description

  The present invention relates to a magnetic recording medium and a magnetic recording and reproducing apparatus.

  Recording and / or reproduction of information on the magnetic recording medium is usually performed by bringing the surface of the magnetic recording medium (the surface of the magnetic layer) and the magnetic head (hereinafter, also simply referred to as "head") into contact and sliding. To be done.

  One of the performances required of the magnetic recording medium is that it can exhibit excellent electromagnetic conversion characteristics when reproducing information recorded on the magnetic recording medium.

  On the other hand, when the magnetic layer surface and the head slide, the reproducing element of the head is scraped (hereinafter, also referred to as "head element scraping"), the distance between the magnetic layer surface and the reproducing element increases, and the electromagnetic conversion characteristic Spacing loss may occur which causes a drop. Conventionally, it has been proposed to provide a protective layer on the head as a measure for suppressing the occurrence of the spacing loss (see, for example, Patent Document 1).

JP 2005-92967 A

  By the way, data recorded on various recording media such as magnetic recording media are referred to as hot data, warm data and cold data according to the access frequency (reproduction frequency). The access frequency decreases in the order of hot data, warm data and cold data, and cold data is usually stored as it is recorded on a recording medium for a long period of 10 years or more (for example, several decades). Recording and storing such cold data is called an archive. As the amount of cold data recorded and stored on magnetic recording media has increased with the recent rapid increase in the amount of information and the digitization of various information, attention has been paid to a magnetic recording and reproducing system suitable for archiving. It is rising.

  Under such circumstances, in recent years, a green tape test (GTT) has been conducted as a test of a magnetic recording and reproducing apparatus (generally referred to as a "drive"). In this GTT, assuming a usage pattern specific to archival applications for recording / reproducing cold data with low access frequency, multiple (for example, several hundred) new (for example, several hundred) new products (not yet available) while replacing the magnetic recording medium for one head. Sliding of the magnetic recording medium used). On the other hand, in the conventional head durability test, since the use mode in which the access frequency is higher than that in the archive application is assumed, normally, one magnetic recording medium is replaced with the same magnetic head without replacing the magnetic recording medium with a new one. Repeated sliding has been performed. In such a conventional durability test, since the surface of the magnetic layer wears while sliding is repeated, it is difficult to cause head element scraping to occur gradually. On the other hand, in GTT, the magnetic recording medium slid with the head is replaced with a new one, and the same head is repeatedly slid with a plurality of new magnetic recording media. It will be exposed to the severe conditions which are easily scraped off. In order to suppress head element scraping in such GTT, it is conceivable to take measures on the head side and measures on the magnetic recording medium side. For example, as a countermeasure on the head side, although it is conceivable to thicken the protective layer of the head, thickening the protective layer of the head increases the distance between the surface of the magnetic layer and the reproducing element of the head. It causes pacing loss. On the other hand, if measures can be taken on the magnetic recording medium side to suppress head element scraping in the GTT, the magnetic recording medium on which such measures have been taken will have head element scraping in the usage mode for archiving applications. It can be said that it is a magnetic recording medium suitable for an archival recording medium which is less likely to occur.

  Therefore, it is an object of the present invention to provide a magnetic recording medium suitable for an archival recording medium that can exhibit excellent electromagnetic conversion characteristics, and more specifically, can exhibit excellent electromagnetic conversion characteristics and is green An object of the present invention is to provide a magnetic recording medium capable of suppressing the occurrence of head element scraping in a tape test (GTT).

One aspect of the present invention is
A magnetic recording medium comprising a magnetic layer containing a ferromagnetic powder and a binder on a nonmagnetic support, comprising:
The ferromagnetic powder is ferromagnetic hexagonal ferrite powder, the magnetic layer contains an oxide abrasive,
Peak intensity of the diffraction peak of the (114) plane of the hexagonal ferrite crystal structure determined by X-ray diffraction analysis of the magnetic layer using the In-Plane method Peak intensity Int of the diffraction peak of the (110) plane relative to Int (114) The intensity ratio (Int (110) / Int (114); hereinafter, "XRD (X-ray diffraction) intensity ratio") of (110) is 0.5 or more and 4.0 or less,
The perpendicular direction squareness ratio of the magnetic recording medium is 0.65 or more and 1.00 or less,
The logarithmic attenuation factor determined by a pendulum visco-elastic test on the surface of the magnetic layer (hereinafter, also referred to as “logarithmic attenuation factor of the magnetic layer surface” or simply “logarithmic attenuation factor”) is 0.050 or less, and The average particle diameter of the above-mentioned oxide abrasive obtained from a secondary ion image obtained by irradiating a focused ion beam (FIB; focused ion beam) on the surface of the magnetic layer (hereinafter also referred to as “FIB abrasive diameter”) ) Is 0.04 μm or more and 0.08 μm or less,
About.

  In one aspect, the perpendicular squareness ratio can be greater than or equal to 0.65 and less than or equal to 0.90.

  In one aspect, the logarithmic attenuation factor can be 0.010 or more and 0.050 or less.

  In one aspect, the oxide abrasive can be alumina powder.

  In one aspect, the magnetic recording medium can have a nonmagnetic layer containing a nonmagnetic powder and a binder between the nonmagnetic support and the magnetic layer.

  In one aspect, the magnetic recording medium can have a backcoat layer containing nonmagnetic powder and a binder on the surface side of the nonmagnetic support opposite to the surface having the magnetic layer.

  In one aspect, the magnetic recording medium can be a magnetic tape.

  A further aspect of the present invention relates to a magnetic recording and reproducing apparatus comprising the above magnetic recording medium and a magnetic head.

  In one aspect, the magnetic head can be a magnetic head including a magnetoresistive (MR) element.

  According to one aspect of the present invention, a magnetic recording medium suitable for an archive application which can exhibit excellent electromagnetic conversion characteristics and can suppress the occurrence of head element scraping in a green tape test (GTT), and the magnetic recording medium A magnetic recording and reproducing apparatus including a recording medium can be provided.

It is explanatory drawing of the measuring method of logarithmic attenuation factor. It is explanatory drawing of the measuring method of logarithmic attenuation factor. It is explanatory drawing of the measuring method of logarithmic attenuation factor. An example (process schematic) of the specific aspect of a magnetic tape manufacturing process is shown.

[Magnetic recording medium]
One embodiment of the present invention is a magnetic recording medium having a magnetic layer containing a ferromagnetic powder and a binder on a nonmagnetic support, wherein the ferromagnetic powder is a ferromagnetic hexagonal ferrite powder, and the magnetic layer is a magnetic layer. The (110) plane relative to the peak intensity Int (114) of the diffraction peak of the (114) plane of the hexagonal ferrite crystal structure determined by X-ray diffraction analysis of the magnetic layer using an In-Plane method, including an oxide abrasive The intensity ratio (Int (110) / Int (114)) of the peak intensities Int (110) of the diffraction peaks of the magnetic recording medium is 0.5 or more and 4.0 or less. The logarithmic attenuation factor determined by the pendulum visco-elastic test on the surface of the magnetic layer is 0.050 or less, and the surface of the magnetic layer is irradiated with a focused ion beam. A magnetic recording medium secondary ion images the average particle diameter of the oxide abrasive obtained from (FIB abrasive diameter) is less 0.08μm or 0.04μm being.

  In the present invention and in the present specification, “the (magnetic) layer surface” is synonymous with the magnetic layer side surface of the magnetic recording medium. Further, in the present invention and this specification, “ferromagnetic hexagonal ferrite powder” means an assembly of a plurality of ferromagnetic hexagonal ferrite particles. Ferromagnetic hexagonal ferrite particles are ferromagnetic particles having a hexagonal ferrite crystal structure. Hereinafter, particles (ferromagnetic hexagonal ferrite particles) constituting the ferromagnetic hexagonal ferrite powder are also referred to as “hexagonal ferrite particles” or simply “particles”. "Assembly" is not limited to the embodiment in which the particles constituting the assembly are in direct contact, but also includes the embodiment in which a binder, an additive and the like are interposed between particles. The above points also apply to various powders such as the nonmagnetic powder in the present invention and the present specification.

  In the present invention and in the present specification, "oxide abrasive" means nonmagnetic oxide powder having a Mohs hardness of 8 or more.

  In the present invention and in the present specification, descriptions of directions and angles (e.g., vertical, orthogonal, parallel, etc.) include the range of allowable errors in the technical field to which the present invention belongs, unless otherwise specified. The range of the error means, for example, a range of less than the strict angle ± 10 °, preferably within the strict angle ± 5 °, and more preferably within the range of ± 3 °.

The present inventors' guess about the above magnetic recording medium is as follows.
The present inventors have found that the perpendicular direction squareness ratio and the XRD intensity ratio of the magnetic recording medium are in the above range, so that the magnetic recording medium can exhibit excellent electromagnetic conversion characteristics, and more specifically, recording on the magnetic recording medium It is thought that it mainly contributes to the ability to reproduce the received information at a high signal-to-noise (Ratio) ratio (SNR). This point is further described below.
The magnetic recording medium contains ferromagnetic hexagonal ferrite powder in the magnetic layer. The present inventors found that, among the ferromagnetic hexagonal ferrite powder contained in the magnetic layer, particles (hereinafter referred to as "particles of the former") that affect the magnetic properties of the ferromagnetic hexagonal ferrite powder (aggregation of particles). It is inferred that it includes particles (hereinafter, also referred to as “the particles of the latter”) which are considered to have little or no influence. The latter particles are considered to be fine particles generated, for example, by chipping of the particles (chipping) due to dispersion treatment performed at the time of preparation of the composition for forming a magnetic layer.
Among the particles contained in the ferromagnetic hexagonal ferrite powder contained in the magnetic layer, the present inventors are the particles of the former that give rise to diffraction peaks in X-ray diffraction analysis using the In-Plane method. Since the latter particles are fine, they do not bring about a diffraction peak or the influence on the diffraction peak is considered to be small. Therefore, based on the intensity of the diffraction peak provided by the X-ray diffraction analysis of the magnetic layer using the In-Plane method, the state of existence of particles in the magnetic layer that affects the magnetic properties of the ferromagnetic hexagonal ferrite powder is controlled. I guess I can do that. The XRD intensity ratio, which will be described in detail later, is considered by the present inventors as an indicator in this regard.
On the other hand, the perpendicular direction squareness ratio is the ratio of residual magnetization to saturation magnetization measured in the direction perpendicular to the magnetic layer surface, and the smaller the residual magnetization, the smaller the value. The latter particles described above are considered to be fine and difficult to maintain magnetization, so it is surmised that the larger the latter particles are included in the magnetic layer, the smaller the perpendicular squareness ratio tends to be. Therefore, the present inventors believe that the perpendicular direction squareness ratio can be an indicator of the amount of the above-mentioned latter particles (fine particles) in the magnetic layer. It is believed that the smaller the amount of such fine particles present in the magnetic layer, the better the magnetic properties of the ferromagnetic hexagonal ferrite powder.
Then, by reducing the abundance of the latter particles (fine particles) in the magnetic layer and controlling the existence state of the former particles in the magnetic layer, the perpendicular direction squareness ratio and the XRD intensity ratio of the magnetic recording medium can be obtained. The present inventors infer that the electromagnetic conversion characteristics can be improved by setting the above range.

  Furthermore, in the above magnetic recording medium, the fact that the logarithmic attenuation factor of the surface of the magnetic layer and the FIB abrasive diameter are respectively in the above ranges mainly contributes to suppressing the occurrence of head element scraping in GTT, the present inventors. Is thinking. This point is further described below.

In the present invention and this specification, the logarithmic attenuation factor of the magnetic layer surface is a value determined by the following method.
1 to 3 are explanatory diagrams of a method of measuring a logarithmic attenuation factor. Hereinafter, the method of measuring the logarithmic attenuation rate will be described with reference to these drawings. However, the illustrated aspect is an illustration and does not limit the present invention.
In the sample stage 101 in the pendulum visco-elasticity tester, the measurement surface (magnetic layer surface) on the substrate 103 in a state in which a part (sample for measurement) of the magnetic tape to be measured (sample for measurement) is clear. ) With the fixing tape 105 or the like in a state of being fixed upward.
The pendulum-equipped round rod cylinder edge 104 is placed on the measurement surface of the measurement sample 100 such that the major axis direction of the cylinder edge is parallel to the longitudinal direction of the measurement sample 100. An example of a state in which a pendulum-mounted round bar cylinder edge 104 is placed on the measurement surface of the measurement sample 100 (a view from above) is shown in FIG. In the embodiment shown in FIG. 1, a holder and temperature sensor 102 is provided, and the surface temperature of the substrate 103 can be monitored. However, this configuration is not essential. The longitudinal direction of the measurement sample 100 is the direction indicated by the arrow in the drawing in the embodiment shown in FIG. 1, and the longitudinal direction (in the case of a magnetic tape) or the radial direction (magnetic) in the magnetic recording medium from which the measurement sample was cut out. In the case of a disc) the same direction. In addition, as the pendulum 107 (see FIG. 2), a pendulum made of a material (for example, made of metal, made of an alloy, or the like) having a property of being adsorbed to a magnet is used.
The surface temperature of the substrate 103 on which the measurement sample 100 is placed is raised to 80 ° C. at a temperature rising rate of 5 ° C./min or less (an arbitrary temperature rising rate may be sufficient if 5 ° C./min or less) The pendulum motion is started (induces initial vibration) by releasing the attraction between the pendulum 107 and the magnet 106. An example of the state of the pendulum 107 in pendulum motion (the state viewed from the side) is shown in FIG. In the embodiment shown in FIG. 2, in the pendulum visco-elastic tester, the pendulum movement is started by stopping energization (turning off the switch) to the magnet (electromagnet) 106 disposed below the sample stage. The pendulum motion is stopped by causing the magnet 106 to adsorb the pendulum 107 to the magnet 106 by reenergizing the electromagnet (with the switch turned on). During pendulum movement, as shown in FIG. 2, the pendulum 107 repeats its amplitude. From the results obtained by monitoring the displacement of the pendulum by the displacement sensor 108 while the pendulum repeats the amplitude, the displacement is taken on the vertical axis, and the displacement time is taken on the horizontal axis. An example of a displacement-time curve is shown in FIG. FIG. 3 schematically shows the correspondence between the state of the pendulum 107 and the displacement-time curve. Repeat stationary (adsorption) and pendulum movement at a constant measurement interval, and use the displacement-time curve obtained at the measurement interval after 10 minutes or more (10 minutes or more may be any time). The logarithmic attenuation factor Δ (no unit) is determined from the following equation, and this value is taken as the logarithmic attenuation factor of the magnetic layer surface of the magnetic tape. The adsorption time of one adsorption is 1 second or more (it may be any time if it is 1 second or more), and the interval from the end of adsorption to the next adsorption start is 6 seconds or more (any time if 6 seconds or more) Time is good.) The measurement interval is an interval of time from the start of adsorption to the next start of adsorption. The humidity of the environment in which the pendulum movement is performed may be any relative humidity as long as the relative humidity is in the range of 40 to 70%. In addition, the ambient temperature of the environment in which the pendulum motion is performed may be any temperature within the range of 20 to 30 ° C.

In the displacement-time curve, the interval from the minimum to the minimum of the displacement is defined as one cycle of the wave. Let n be the number of waves included in the displacement-time curve in the measurement interval, and An be the difference between the minimal and maximal displacements of the nth wave. In Figure 3, the displacement of the n-th wave intervals up again becomes minimum of minimum, labeled Pn (e.g. P ₁ for the first wave, for second P _2, P ₃ for third) doing. The difference between the minimal displacement and the maximal displacement appearing next to the nth wave (A _{n + 1} in the above equation, A _{4 in the} displacement-time curve shown in FIG. 3) is also used to calculate the logarithmic attenuation factor. The part where the pendulum 107 is at rest (adsorption) after the maximum displacement is not used for counting the number of waves. Also, the portion where the pendulum 107 is at rest (adsorption) before the maximum displacement is not used for counting the number of waves. Thus, in the displacement-time curve shown in FIG. 3, the number of waves is three (n = 3).

The above logarithmic attenuation factor is considered to be a value indicative of the amount of adhesive component released from the magnetic layer surface and interposed between the magnetic layer surface and the head when the head and the magnetic layer surface contact and slide. Be It is considered that the adhesion between the surface of the magnetic layer and the head is enhanced as the adhesive component is more present, and smooth sliding between the surface of the magnetic layer and the head is hindered (slidability is lowered). On the other hand, in the above magnetic recording medium, when the logarithmic attenuation factor of the magnetic layer surface is 0.050 or less, that is, the adhesive component is reduced, the magnetic layer surface and the head can be made smooth. It is thought that it contributes to making it slide. As a result, the present inventors infer that it is possible to suppress the head element from being scraped by the sliding with the magnetic layer surface in the GTT.
The details of the above-mentioned adhesive component are not clear. The present inventors speculate that the adhesive component may be derived from a resin used as a binder. The details are as follows. As the binder, various resins can be used as described in detail later. The resin is a polymer (including homopolymers and copolymers) of two or more polymerizable compounds, and a component having a molecular weight lower than the average molecular weight (hereinafter referred to as "low molecular weight binder component"). Usually included. The present inventors believe that such a low molecular weight binder component may be released from the surface of the magnetic layer when it slides between the head and the surface of the magnetic layer and be interposed between the surface of the magnetic layer and the head. ing. The low-molecular-weight binder component is considered to be adhesive, and the low-molecular-weight binder liberates from the magnetic layer surface when the magnetic layer surface and the head slide, as determined by the pendulum visco-elasticity test. The present inventors speculate that it may be an indicator of the amount of ingredients. In one embodiment, the magnetic layer may be a composition for forming a magnetic layer containing a curing agent, in addition to the ferromagnetic hexagonal ferrite powder, the binder and the oxide abrasive, directly on the nonmagnetic support or in another layer. Applied and cured. By the curing treatment here, the binder and the curing agent can be cured (crosslinking reaction). However, although the reason for the low molecular weight binder component is not clear, the present inventors believe that the reactivity of the curing reaction may be poor. Therefore, the low molecular weight binder component does not easily stay in the magnetic layer and is easily released from the magnetic layer, and the low molecular weight binder component intervenes between the magnetic layer surface and the head when sliding between the magnetic layer surface and the head. The present inventors speculate that this is one of the reasons for

In the present invention and in the present specification, the FIB abrasive diameter is a value determined by the following method.
(1) Acquisition of Secondary Ion Image A focused ion beam apparatus acquires a secondary ion image of a region of 25 μm square (25 μm × 25 μm) on the surface of the magnetic layer of the target magnetic recording medium for which the FIB abrasive diameter is to be determined. As a focused ion beam apparatus, MI4050 manufactured by Hitachi High-Technologies Corporation can be used.
The beam irradiation conditions of the focused ion beam apparatus at the time of acquiring a secondary ion image are set to an acceleration voltage of 30 kV, a current value of 133 pA (picoampere), BeamSize of 30 nm and Brightness of 50%. The coating process prior to imaging on the magnetic layer surface is not performed. The secondary ion detector detects a secondary ion (SI) signal and picks up a secondary ion image. The imaging conditions of the secondary ion image are determined by the following method. ACB (Auto Contrast Brightess) is performed at three unimaged areas on the surface of the magnetic layer (that is, ACB is performed three times) to stabilize the tint of the image and determine the contrast reference value and the brightness reference value . A contrast value which is 1% lower than the contrast reference value determined by the present ACB and the above-described brightness reference value are used as imaging conditions. An unimaged area on the surface of the magnetic layer is selected, and a secondary ion image is imaged under the imaging conditions determined above. The portion (micron bar, cross mark, etc.) that displays the size and the like is erased from the captured image, and a secondary ion image with a pixel count of 2000 pixels × 2000 pixels is acquired. The specific examples of the imaging conditions can be referred to the examples described later.
(2) Calculation of FIB Abrasive Diameter The secondary ion image acquired in the above (1) is taken into image processing software, and binarization processing is performed according to the following procedure. As image analysis software, for example, free software ImageJ can be used.
The color tone of the secondary ion image acquired in (1) above is changed to 8 bits. The threshold for the binarization process is set to 250 tones for the lower limit value and 255 tones for the upper limit value, and the binarization process is executed with these two threshold values. After the binarization processing, noise component removal processing is performed by image analysis software. The noise component removal process can be performed, for example, by the following method. In the image analysis software ImageJ, noise cut processing Despeckle is selected, and Size 4.0-Infinity is set in AnalyzeParticle to remove noise components.
In the binarized image thus obtained, each white glowing portion is determined to be an oxide abrasive, the number of white glowing portions is determined by image analysis software, and the area of each white glowing portion is determined. The equivalent circle diameter of each portion is determined from the area of each portion that emits white light determined here. Specifically, the equivalent circle diameter L is calculated from the obtained area A by (A / π) ^ (1/2) × 2 = L.
The above steps were carried out four times at different locations (25 μm square) of the magnetic layer surface of the magnetic recording medium to be subjected to the determination of the FIB abrasive diameter, and from the results obtained, the FIB abrasive diameter was calculated as follows: FIB abrasive diameter = Calculated by Σ (Li) / Σi. Σi is the total number of white glowing portions observed in the binarized image obtained by four executions. Σ (Li) is the sum of the circle equivalent diameters L determined for each of the white glowing portions observed in the binarized image obtained by performing 4 times. It is possible that only a part of the whitening portion is included in the binarized image. In such a case, Σi and Σ (Li) are obtained without including the part.

  The above-mentioned FIB abrasive diameter is a value which can be used as an index of the existence state of the oxide abrasive in the magnetic layer, and from the secondary ion image obtained by irradiating the surface of the magnetic layer with the focused ion beam (FIB) Desired. The secondary ion image is generated by capturing secondary ions generated from the surface of the magnetic layer irradiated with FIB. On the other hand, a method using a scanning electron microscope (SEM) has conventionally been proposed as a method of observing the state of the abrasive in the magnetic layer. In the SEM, an electron beam is irradiated to the surface of the magnetic layer, secondary electrons emitted from the surface of the magnetic layer are captured, and an image (SEM image) is generated. Due to the difference in image formation principle as described above, even if the same magnetic layer is observed, the size of the oxide abrasive determined from the secondary ion image and the size of the oxide abrasive determined from the SEM image are different. It becomes. As a result of intensive studies, the present inventors use the FIB abrasive diameter determined by the method described above from the secondary ion image as a new indicator of the state of the oxide abrasive present in the magnetic layer. It came to control the existence state of the oxide polishing agent in a magnetic layer so that a diameter might be set to 0.04 micrometer or more and 0.08 micrometer or less. The present inventors believe that controlling the existence state of the oxide abrasive in the magnetic layer in this way also contributes to suppressing scraping of the head element by sliding with the magnetic layer surface in GTT.

  As described above, to the extent that the above magnetic recording medium can exhibit excellent electromagnetic conversion characteristics, it is mainly contributed that the XRD intensity ratio and the perpendicular direction squareness ratio are in the above range, and the occurrence of head element scraping in GTT is suppressed The present inventors speculate that the log attenuation factor of the surface of the magnetic layer and the diameter of the FIB abrasive mainly contribute to the above range. However, the present invention is not limited to the above estimation.

  Hereinafter, the magnetic recording medium will be described in more detail.

[XRD intensity ratio]
The magnetic recording medium contains ferromagnetic hexagonal ferrite powder in the magnetic layer. The XRD intensity ratio is determined by X-ray diffraction analysis of the magnetic layer containing ferromagnetic hexagonal ferrite powder using the In-Plane method. In the following, X-ray diffraction analysis performed using the In-Plane method is also described as "In-Plane XRD". In-Plane XRD is performed by irradiating the surface of the magnetic layer with X-rays under the following conditions using a thin film X-ray diffractometer. Magnetic recording media are roughly classified into tape-shaped magnetic recording media (magnetic tapes) and disk-shaped magnetic recording media (magnetic disks). The measurement direction is the longitudinal direction for the magnetic tape and the radial direction for the magnetic disk.
Cu source used (output 45 kV, 200 mA)
Scan condition: 0.05 degree / step, 0.1 degree / min in the range of 20-40 degrees
Optical system used: Parallel optical system Measurement method :: 2θχ scan (X-ray incident angle 0.25 °)
The above conditions are set values in the thin film X-ray diffractometer. A known device can be used as the thin film X-ray diffraction device. As an example of a thin film X-ray diffractometer, a Rigaku SmartLab can be mentioned. The sample to be subjected to the In-Plane XRD analysis is a medium sample cut out from the magnetic recording medium to be measured, and the size and the shape thereof are not limited as long as the diffraction peak described later can be confirmed.

  Methods of X-ray diffraction analysis include thin film X-ray diffraction and powder X-ray diffraction. Powder X-ray diffraction measures X-ray diffraction of a powder sample, whereas thin film X-ray diffraction can measure X-ray diffraction of a layer or the like formed on a substrate. Thin film X-ray diffraction is classified into In-Plane method and Out-Of-Plane method. The X-ray incident angle at the time of measurement is usually in the range of 0.20 to 0.50 in the In-Plane method, while it is in the range of 5.00 to 90.00 in the Out-Of-Plane method. . In the present invention and in the In-Plane XRD in the present specification, the X-ray incident angle is 0.25 ° as described above. The In-Plane method has a smaller X-ray incident angle than the Out-Of-Plane method, and therefore the penetration depth of X-rays is shallow. Therefore, according to X-ray diffraction analysis (In-Plane XRD) using the In-Plane method, X-ray diffraction analysis of the surface layer portion of the sample to be measured can be performed. With respect to the magnetic recording medium sample, X-ray diffraction analysis of the magnetic layer can be performed by In-Plane XRD. The above-mentioned XRD intensity ratio means the (110) plane relative to the peak intensity Int (114) of the diffraction peak of the (114) plane of the hexagonal ferrite crystal structure in the X-ray diffraction spectrum obtained by the In-Plane XRD. The intensity ratio (Int (110) / Int (114)) of the peak intensities Int (110) of the diffraction peaks of Int is used as an abbreviation of Intensity. In the X-ray diffraction spectrum (vertical axis: Intensity, horizontal axis: diffraction angle 2θχ (degree)) obtained by In-Plane XRD, the diffraction peak of the (114) plane is a peak detected in the range of 33 to 36 ° at 2θχ The diffraction peak of the (110) plane is a peak detected in the range of 29 to 32 degrees in 2θχ.

In the diffractive surface, the (114) plane of the hexagonal ferrite crystal structure is located near the easy magnetization axis direction (c-axis direction) of the particles of the ferromagnetic hexagonal ferrite powder (hexagonal ferrite particles). Further, the (110) plane of the hexagonal ferrite crystal structure is located in the direction orthogonal to the easy magnetization axis direction.
In the X-ray diffraction spectrum determined by In-Plane XRD, the present inventors show the peak intensity of the diffraction peak of the (114) plane of the hexagonal ferrite crystal structure to the peak intensity of the diffraction peak of the (110) plane with respect to Int (114). As the intensity ratio (Int (110) / Int (114); XRD intensity ratio) of Int (110) increases, the direction perpendicular to the magnetization easy axis direction is closer to being parallel to the magnetic layer surface. It is inferred that the smaller the XRD intensity ratio, the smaller the particles of the former present in such a state in the magnetic layer. The state in which the XRD intensity ratio is 0.5 or more and 4.0 or less is considered to mean that the particles of the former are properly aligned in the magnetic layer. The present inventors infer that this contributes to the improvement of the electromagnetic conversion characteristics.
The XRD intensity ratio is preferably 3.5 or less, more preferably 3.0 or less, from the viewpoint of further improving the electromagnetic conversion characteristics. From the same viewpoint, the XRD intensity ratio is preferably 0.7 or more, more preferably 1.0 or more. The XRD intensity ratio can be controlled, for example, by the processing conditions of the orientation processing performed in the manufacturing process of the magnetic recording medium. As the alignment treatment, it is preferable to perform vertical alignment treatment. The vertical alignment treatment can be preferably performed by applying a magnetic field perpendicular to the surface of the coated layer of the composition for forming a magnetic layer in the wet state (undried state). As the orientation conditions are strengthened, the value of the XRD intensity ratio tends to be larger. Examples of processing conditions for the alignment process include magnetic field strength in the alignment process and the like. The processing conditions of the orientation processing are not particularly limited. The processing conditions of the alignment processing may be set so that the XRD intensity ratio of 0.5 or more and 4.0 or less can be realized. As an example, the magnetic field strength in the vertical alignment process can be 0.10 to 0.80 T, or can be 0.10 to 0.60 T. As the dispersibility of the ferromagnetic hexagonal ferrite powder in the composition for forming a magnetic layer is enhanced, the value of the XRD intensity ratio tends to be increased by the vertical alignment treatment.

[Vertical squareness ratio]
The perpendicular direction squareness ratio is a squareness ratio measured in the perpendicular direction of the magnetic recording medium. The “perpendicular direction” described with respect to the squareness ratio means a direction perpendicular to the magnetic layer surface. For example, when the magnetic recording medium is a tape-shaped magnetic recording medium, ie, a magnetic tape, the perpendicular direction is also a direction perpendicular to the longitudinal direction of the magnetic tape. The perpendicular squareness ratio is measured using a vibrating sample type flux meter. More specifically, the perpendicular squareness ratio in the present invention and in the present specification is the maximum external magnetic field of 1194 kA / m (15 kOe) in the magnetic recording medium at a measurement temperature of 23 ° C. ± 1 ° C. in the vibrating sample type flux meter. And, it is a value obtained by sweeping under the condition of a scanning speed of 4.8 kA / m / sec (60 Oe / sec), which is a value after the demagnetizing field correction. The measurement value is obtained as a value obtained by subtracting the magnetization of the sample probe of the vibrating sample type flux meter as background noise.

  The perpendicular direction squareness ratio of the magnetic recording medium is 0.65 or more. The present inventors speculate that the perpendicular direction squareness ratio of the magnetic recording medium can be an indicator of the amount of the latter particles (fine particles) described above. A magnetic layer having a perpendicular direction squareness ratio of 0.65 or more of the magnetic recording medium is considered to have a small amount of such fine particles. The present inventors infer that this contributes to the improvement of the electromagnetic conversion characteristics. From the viewpoint of further improving the electromagnetic conversion characteristics, the above-mentioned perpendicular direction squareness ratio is preferably 0.70 or more, more preferably 0.73 or more, and still more preferably 0.75 or more. Also, the squareness ratio is at most 1.00 in principle. Therefore, the perpendicular squareness ratio of the magnetic tape is 1.00 or less. The vertical angular ratio may be, for example, 0.95 or less, 0.90 or less, 0.87 or less, or 0.85 or less. However, as the value of the above-mentioned perpendicular direction squareness ratio is larger, it is considered preferable from the viewpoint of improvement of the electromagnetic conversion characteristics because the above-mentioned fine particles of the latter are less in the magnetic layer. Therefore, the above-mentioned perpendicular direction squareness ratio may exceed the value illustrated above.

  In order to make the above-mentioned perpendicular direction squareness ratio 0.65 or more, in the preparation process of the composition for forming a magnetic layer, it is suppressed that fine particles are generated due to partial chipping (chipping) of particles. We believe that it is preferred. Specific means for suppressing the occurrence of chipping will be described later.

[Log decay rate]
The logarithmic attenuation factor determined by the pendulum viscoelasticity test on the surface of the magnetic layer of the magnetic recording medium is 0.050 or less. It is inferred that this contributes to suppressing the occurrence of head element scraping in the GTT. The logarithmic attenuation factor is preferably 0.048 or less, more preferably 0.045 or less, and further preferably 0.040 or less from the viewpoint of further suppressing the occurrence of head element abrasion in GTT. preferable. On the other hand, from the viewpoint of suppressing the occurrence of head element scraping in GTT, the lower the above-mentioned logarithmic attenuation factor, the lower the above-mentioned logarithmic attenuation factor is, and therefore the lower limit value is not particularly limited. As an example, the logarithmic attenuation factor can be, for example, 0.010 or more, or 0.015 or more. However, the logarithmic attenuation rate may be lower than the above-described values. Specific aspects of the means for adjusting the logarithmic attenuation factor will be described later.

[FIB abrasive diameter]
The diameter of the FIB abrasive determined from the secondary ion image obtained by irradiating the surface of the magnetic layer of the magnetic recording medium with FIB is 0.04 μm or more and 0.08 μm or less. The fact that the FIB abrasive diameter is 0.08 μm or less is considered to contribute to suppressing the head element being abraded by the oxide abrasive in GTT. In addition, it is surmised that the FIB abrasive diameter of 0.04 μm or more contributes to the removal of the component derived from the magnetic layer attached to the head by sliding with the magnetic layer surface in the GTT. This is considered to contribute to suppressing the scraping of the element of the head due to the head sliding on the surface of the magnetic layer in a state where the component derived from the magnetic layer adheres to the head in the GTT. From the viewpoint of further suppressing the occurrence of head element abrasion in GTT, the FIB abrasive diameter is preferably 0.05 μm or more, and more preferably 0.06 μm or more. Further, from the same viewpoint, the FIB abrasive diameter is preferably 0.07 μm or less. Specific embodiments of means for adjusting the FIB abrasive diameter will be described later.

  Hereinafter, the magnetic recording medium will be described in more detail.

[Magnetic layer]
Ferromagnetic hexagonal ferrite powder
The magnetic layer of the magnetic recording medium contains ferromagnetic hexagonal ferrite powder as ferromagnetic powder. With regard to ferromagnetic hexagonal ferrite powder, magnetoplumbite type (also called "M type"), W type, Y type and Z type are known as crystal structure of hexagonal ferrite. The ferromagnetic hexagonal ferrite powder contained in the magnetic layer may have any crystal structure. The crystal structure of hexagonal ferrite contains, as constituent atoms, an iron atom and a divalent metal atom. The divalent metal atom is a metal atom capable of becoming a divalent cation as an ion, and examples thereof include an alkaline earth metal atom such as a barium atom, a strontium atom, and a calcium atom, and a lead atom. For example, a hexagonal ferrite containing a barium atom as a divalent metal atom is a barium ferrite, and a hexagonal ferrite containing a strontium atom is a strontium ferrite. The hexagonal ferrite may be a mixed crystal of two or more hexagonal ferrites. As an example of mixed crystal, mixed crystal of barium ferrite and strontium ferrite can be mentioned.

An activated volume can be used as an indicator of the particle size of the ferromagnetic hexagonal ferrite powder. The "activation volume" is a unit of magnetization reversal. The activation volume described in the present invention and in the present specification is measured using a vibrating sample type flux meter under an environment of an atmosphere temperature of 23 ° C. ± 1 ° C. with a magnetic field sweep speed of 3 minutes and 30 minutes of the coercive force Hc measurement unit. And the value obtained from the following relationship between Hc and activation volume V:
Hc = 2 Ku / Ms {1-[(kT / KuV) ln (At / 0.693)] ^1/2 }
[In the above formula, Ku: anisotropy constant, Ms: saturation magnetization, k: Boltzmann constant, T: absolute temperature, V: activation volume, A: spin precession frequency, t: magnetic field reversal time]
High density recording is always desired for magnetic recording media. As a method for achieving high density recording, there is a method of reducing the particle size of the ferromagnetic powder contained in the magnetic layer and increasing the filling rate of the ferromagnetic powder of the magnetic layer. In this respect, the activation volume of the ferromagnetic hexagonal ferrite powder is preferably at 2500 nm ³ or less, more preferably 2300 nm ³ or less, and further preferably 2000 nm ³ or less. On the other hand, from the viewpoint of the stability of the magnetization, the activation volume is, for example, preferably 800 nm ³ or more, more preferably 1000 nm ³ or more, and still more preferably 1200 nm ³ or more.

  The shape of the particles constituting the ferromagnetic hexagonal ferrite powder is obtained by photographing the ferromagnetic hexagonal ferrite powder at a photographing magnification of 100,000 times using a transmission electron microscope and printing it on a printing paper so that the total magnification becomes 500,000 times. In the obtained particle photograph, the contour of the particle (primary particle) is traced and specified by the digitizer. Primary particles refer to independent particles without aggregation. Photographing using a transmission electron microscope is performed by a direct method using a transmission electron microscope at an acceleration voltage of 300 kV. Transmission electron microscopy and measurement can be performed using, for example, Hitachi Transmission Electron Microscope Model H-9000 and Carl Zeiss image analysis software KS-400. With regard to the shape of the particles constituting the ferromagnetic hexagonal ferrite powder, "plate-like" refers to a shape having two opposing plate surfaces. On the other hand, among particles having no plate surface, the shape having a distinction between the major axis and the minor axis is "elliptical". The major axis is determined as the axis (straight line) that can take the longest particle length. On the other hand, the minor axis is determined as the axis whose length is longest when the particle length is taken as a straight line orthogonal to the major axis. The shape in which the major axis and the minor axis are not distinguished, that is, the shape in which the major axis length = the minor axis length is “spherical”. A shape whose major and minor axes can not be identified from the shape is called irregular. The imaging using the transmission electron microscope for specifying the particle shape described above is performed without subjecting the powder to be imaged to an orientation process. The shape of the ferromagnetic hexagonal ferrite powder used for preparation of the composition for forming the magnetic layer and the ferromagnetic hexagonal ferrite powder contained in the magnetic layer may be any of plate, elliptical, spherical and amorphous.

  The average particle size for the various powders described in the present invention and in the specification is, unless stated otherwise, the values determined for 500 randomly extracted particles using particle photographs taken as described above. It is an arithmetic mean. The average particle size shown in Examples described later is a value obtained using a Hitachi transmission electron microscope H-9000 type as a transmission electron microscope and an image analysis software KS-400 manufactured by Carl Zeiss as an image analysis software.

  For details of the ferromagnetic hexagonal ferrite powder, for example, it is also possible to refer to paragraphs 0134 to 0136 of JP-A-2011-216149.

  The content (filling ratio) of the ferromagnetic hexagonal ferrite powder in the magnetic layer is preferably in the range of 50 to 90% by mass, and more preferably in the range of 60 to 90% by mass. The components other than the ferromagnetic hexagonal ferrite powder of the magnetic layer are at least a binder and an oxide abrasive, and may optionally contain one or more additives. A high filling factor of the ferromagnetic hexagonal ferrite powder in the magnetic layer is preferable from the viewpoint of improving the recording density.

<Binder, Hardener>
The magnetic recording medium contains a binder in the magnetic layer. The binder is one or more resins. The resin may be a homopolymer or a copolymer (copolymer). As the binder contained in the magnetic layer, polyurethane resin, polyester resin, polyamide resin, vinyl chloride resin, acrylic resin obtained by copolymerizing styrene, acrylonitrile, methyl methacrylate or the like, cellulose resin such as nitrocellulose, epoxy resin, phenoxy resin, Those selected from polyvinyl acetal, polyvinyl alkylal resins such as polyvinyl butyral and the like can be used alone, or a plurality of resins can be mixed and used. Among these, polyurethane resins, acrylic resins, cellulose resins and vinyl chloride resins are preferred. These resins can also be used as binders in the nonmagnetic layer and / or backcoat layer described later. Paragraphs 0029 to 0031 of JP-A-2010-24113 can be referred to for the above-mentioned binders. The weight average molecular weight of the resin used as a binder can be, for example, 10,000 or more and 200,000 or less. The weight average molecular weight in the present invention and the present specification is a value determined by polystyrene conversion of a value measured by gel permeation chromatography (GPC). The following conditions can be mentioned as measurement conditions. The weight average molecular weight shown in the below-mentioned Example is the value which calculated | required and converted the value measured on the following measurement conditions into polystyrene.
GPC apparatus: HLC-8120 (made by Tosoh Corporation)
Column: TSK gel Multipore HXL-M (Tosoh Corp., 7.8 mm ID (Inner Diameter) × 30.0 cm)
Eluent: Tetrahydrofuran (THF)

  Further, at the time of forming the magnetic layer, a curing agent can be used together with the resin usable as the above-mentioned binder. The curing agent may be a thermosetting compound which is a compound which causes a curing reaction (crosslinking reaction) to proceed by heating in one aspect, and a light curing in which a curing reaction (crosslinking reaction) proceeds in the other aspect. Can be a sex compound. The curing agent may be included in the magnetic layer in a state where it is reacted (crosslinked) with other components such as a binder, as the curing reaction proceeds in the manufacturing process of the magnetic recording medium. Preferred curing agents are thermosetting compounds, polyisocyanates being preferred. Paragraphs 0124 to 0125 of JP-A-2011-216149 can be referred to for details of the polyisocyanate. The curing agent is, for example, 0 to 80.0 parts by mass with respect to 100.0 parts by mass of the binder in the composition for forming a magnetic layer, and preferably 50.0 to 80. out of the viewpoint of improving the strength of the magnetic layer. It can be used by adding in an amount of 0 parts by mass.

<Oxide abrasive>
The magnetic recording medium contains an oxide abrasive in the magnetic layer. The oxide abrasive is a nonmagnetic oxide powder having a Mohs hardness of 8 or more, and is preferably a nonmagnetic oxide powder having a Mohs hardness of 9 or more. The maximum value of the Mohs hardness is 10. The oxide abrasive may be an inorganic oxide powder or an organic oxide powder, and is preferably an inorganic oxide powder. Specifically, examples of the abrasive include powders of alumina (Al ₂ O ₃ ), titanium oxide (TiO ₂ ), cerium oxide (CeO ₂ ), zirconium oxide (ZrO ₂ ), etc., among which alumina powder is preferred. Is preferred. The Mohs hardness of alumina is about 9. Paragraph 0021 of JP-A-2013-229090 can also be referred to for the alumina powder. Moreover, a specific surface area can be used as a parameter | index of the particle size of oxide abrasives. It can be considered that the larger the specific surface area, the smaller the particle size of the primary particles of the particles constituting the oxide abrasive. An oxide abrasive having a specific surface area (hereinafter referred to as "BET specific surface area") measured by a BET (Brunauer-Emmett-Teller) method of 14 m ² / g or more is used as the oxide abrasive. Is preferred. From the viewpoint of dispersibility, it is preferable to use an oxide abrasive having a BET specific surface area of 40 m ² / g or less. The content of the oxide abrasive in the magnetic layer is preferably 1.0 to 20.0 parts by mass, preferably 1.0 to 10.0 parts by mass, with respect to 100.0 parts by mass of the ferromagnetic hexagonal ferrite powder. It is more preferable that

<Additives>
The magnetic layer contains a ferromagnetic hexagonal ferrite powder, a binder and an oxide abrasive, and may further contain one or more additives as needed. As an additive, said hardening | curing agent is mentioned as an example. Moreover, as an additive which may be contained in the magnetic layer, nonmagnetic powders other than oxide abrasives, lubricants, dispersants, dispersion aids, mildew proofing agents, antistatic agents, antioxidants, etc. can be mentioned. . The additives can be used in any amount, suitably selected from commercially available products according to the desired properties, or manufactured by known methods. For example, for lubricants, paragraphs 0030 to 0033, 0035 and 0036 of JP-A-2016-126817 can be referred to. The nonmagnetic layer may contain a lubricant. Paragraphs 0030 to 0031, 0034, 0035 and 0036 of JP-A-2016-126817 can be referred to for the lubricant which may be contained in the nonmagnetic layer. With regard to the dispersant, paragraphs 0061 and 0071 of JP 2012-133837 A can be referred to. The dispersant may be contained in the nonmagnetic layer. With regard to the dispersant that may be contained in the nonmagnetic layer, reference can be made to paragraph 0061 of JP-A-2012-133837.

  Moreover, as a dispersing agent, the dispersing agent for improving the dispersibility of an oxide abrasive can be mentioned. As a compound which can function as such a dispersing agent, the aromatic hydrocarbon compound which has a phenolic hydroxy group can be mentioned. "Phenolic hydroxy group" refers to a hydroxy group directly bonded to an aromatic ring. The aromatic ring contained in the aromatic hydrocarbon compound may be a single ring, may be a polycyclic structure, or may be a condensed ring. From the viewpoint of improving the dispersibility of the polishing agent, aromatic hydrocarbon compounds containing a benzene ring or a naphthalene ring are preferred. The aromatic hydrocarbon compound may have a substituent other than the phenolic hydroxy group. Examples of substituents other than phenolic hydroxy group include halogen atom, alkyl group, alkoxy group, amino group, acyl group, nitro group, nitroso group, hydroxyalkyl group and the like, and halogen atom, alkyl group, An alkoxy group, an amino group and a hydroxyalkyl group are preferred. The number of phenolic hydroxy groups contained in one molecule of the aromatic hydrocarbon compound may be one, two, three or more.

  As a preferred embodiment of the aromatic hydrocarbon compound having a phenolic hydroxy group, a compound represented by the following general formula 100 can be mentioned.

[In general 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 General Formula 100, the substitution position of the two hydroxy groups (phenolic hydroxy groups) is not particularly limited.

Compound represented by the general formula 100, two of X ¹⁰¹ to X ¹⁰⁸ is a hydroxyl group (phenolic hydroxy group), the other six each independently represent a hydrogen atom or a substituent. Further, among the X ¹⁰¹ to X ^108, portions other than two hydroxy groups which all may be hydrogen atom, a part or all of it may be a substituent. As the substituent, the substituents described above can be exemplified. As substituents other than two hydroxy groups, one or more phenolic hydroxy groups may be included. From the viewpoint of improving the dispersibility of the polishing agent, it is preferable that the hydroxyl groups other than the two hydroxy groups of X ^{101 to} X ¹⁰⁸ are not phenolic hydroxy groups. That is, the compound represented by the general formula 100 is preferably dihydroxynaphthalene or a derivative thereof, and more preferably 2,3-dihydroxynaphthalene or a derivative thereof. Preferred examples of the substituent represented by X ^{101 to} X ¹⁰⁸ include a halogen atom (for example, chlorine atom, bromine atom), an amino group, an alkyl group having 1 to 6 carbon atoms (preferably 1 to 4), and a methoxy group And ethoxy groups, acyl groups, nitro groups and nitroso groups, and -CH ₂ OH groups.

  Further, with regard to a dispersant for enhancing the dispersibility of the oxide abrasive, it is also possible to refer to paragraphs 0024 to 0028 of JP-A-2014-179149.

  The dispersant for enhancing the dispersibility of the oxide abrasive is preferably used in 100.0 parts by mass of the abrasive during the preparation of the composition for forming the magnetic layer (preferably when preparing the abrasive liquid as described later). For example, it can be used in the ratio of 0.5-20.0 mass parts, and it is preferable to use in the ratio of 1.0-10.0 mass parts.

Moreover, as a dispersing agent, the well-known dispersing agent for improving the dispersibility of ferromagnetic hexagonal ferrite powder, such as a carboxy-group containing compound and a nitrogen-containing compound, can also be mentioned. For example, the nitrogen-containing compound may be any of a primary amine represented by NH ₂ R, a secondary amine represented by NHR ₂ and a tertiary amine represented by NR ₃ . In the above, R represents any structure constituting the nitrogen-containing compound, and a plurality of R may be the same or different. The nitrogen-containing compound may be a compound (polymer) having a plurality of repeating structures in the molecule. It is considered that the nitrogen-containing portion of the nitrogen-containing compound functions as an adsorption portion to the particle surface of the ferromagnetic hexagonal ferrite powder, which is the reason why the nitrogen-containing compound can function as a dispersant. Examples of carboxy group-containing compounds include fatty acids such as oleic acid. With respect to the carboxy group-containing compound, it is considered that the fact that the carboxy group functions as an adsorption part to the particle surface of the ferromagnetic hexagonal ferrite powder is the reason why the carboxy group-containing compound can function as a dispersant. It is also preferable to use a carboxy group-containing compound and a nitrogen-containing compound in combination. The amount of use of these dispersants can be set appropriately.

  As a nonmagnetic powder other than the oxide abrasive which may be contained in the magnetic layer, a nonmagnetic powder capable of forming a protrusion on the surface of the magnetic layer to contribute to control of the frictional characteristics (hereinafter also referred to as a "protrusion forming agent"). Can be mentioned. As the protrusion forming agent, various nonmagnetic powders generally used as a protrusion forming agent in the magnetic layer can be used. These may be powders of inorganic substances (inorganic powders) or powders of organic substances (organic powders). In one aspect, it is preferable that the particle size distribution of the protrusion forming agent is a monodispersion exhibiting a single peak, not a polydispersion having a plurality of peaks in the distribution, from the viewpoint of homogenization of friction characteristics. From the viewpoint of availability of monodispersed particles, the protrusion forming agent is preferably an inorganic powder. Examples of the inorganic powder include powders of metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, metal sulfides and the like. The particles constituting the protrusion forming agent (nonmagnetic powder other than the oxide abrasive) are preferably colloidal particles, and more preferably inorganic oxide colloidal particles. Further, from the viewpoint of the availability of monodispersed particles, the inorganic oxide constituting the inorganic oxide colloidal particles is preferably silicon dioxide (silica). The inorganic oxide colloidal particles are more preferably colloidal silica (silica colloidal particles). In the present invention and in the present specification, "colloidal particles" means 1 g per 100 mL of at least one organic solvent of methyl ethyl ketone, cyclohexanone, toluene or ethyl acetate, or a mixed solvent containing two or more of the above solvents in any mixing ratio. The term “particles” refers to particles that can be dispersed without settling to give a colloidal dispersion. In another aspect, it is also preferred that the protrusion forming agent is carbon black. The average particle size of the protrusion-forming agent can be, for example, 30 to 300 nm, and preferably 40 to 200 nm. Also, from the viewpoint that the protrusion forming agent can exhibit its function better, the content of the protrusion forming agent in the magnetic layer is 1.0 to 4 with respect to 100.0 parts by mass of the ferromagnetic hexagonal ferrite powder. It is preferable that it is 0 mass part, and it is more preferable that it is 1.5-3.5 mass parts.

  The magnetic layer described above can be provided directly on the nonmagnetic support surface or indirectly via the nonmagnetic layer.

[Nonmagnetic layer]
Next, the nonmagnetic layer will be described.
The magnetic recording medium may have a magnetic layer directly on the surface of the nonmagnetic support, and has a nonmagnetic layer containing nonmagnetic powder and a binder between the nonmagnetic support and the magnetic layer. May be The nonmagnetic powder contained in the nonmagnetic layer may be an inorganic powder or an organic powder. In addition, carbon black and the like can also be used. Examples of the inorganic powder include powders of metals, metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, metal sulfides and the like. These nonmagnetic powders are commercially available and can be produced by known methods. As for the details, paragraphs 0036 to 0039 in JP 2010-24113 A can be referred to. The content (filling ratio) of the nonmagnetic powder in the nonmagnetic layer is preferably in the range of 50 to 90% by mass, and more preferably in the range of 60 to 90% by mass.

  For other details such as the binder and additives for the nonmagnetic layer, known techniques relating to the nonmagnetic layer can be applied. Further, for example, regarding the type and content of the binder, the type and content of the additive, and the like, known techniques relating to the magnetic layer can also be applied.

  The nonmagnetic layer in the present invention and the present specification is intended to encompass a substantially nonmagnetic layer containing a small amount of ferromagnetic powder, for example, as an impurity or purposely together with the nonmagnetic powder. Here, a substantially nonmagnetic 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 lower, or the residual magnetic flux density is 10 mT or less And a layer having a coercive force of 7.96 kA / m (100 Oe) or less. The nonmagnetic layer preferably has no residual magnetic flux density and no coercivity.

[Non-magnetic support]
Next, the nonmagnetic support (hereinafter, also simply referred to as “support”) will be described.
Examples of the nonmagnetic support include publicly known polyethylene terephthalate subjected to biaxial stretching, polyethylene naphthalate, polyamide, polyamide imide, aromatic polyamide and the like. Among these, polyethylene terephthalate, polyethylene naphthalate and polyamide are preferable. These supports may be previously subjected to corona discharge, plasma treatment, easy adhesion treatment, heat treatment and the like.

[Back coat layer]
The magnetic recording medium may have a backcoat layer containing nonmagnetic powder and a binder on the surface side opposite to the surface side having the magnetic layer of the nonmagnetic support. The backcoat layer preferably contains one or both of carbon black and inorganic powder. With respect to the binder contained in the backcoat layer and various additives which may optionally be contained, known techniques relating to the backcoat layer can be applied, and application of known techniques relating to the formulation of the magnetic layer and / or the nonmagnetic layer You can also. For example, the description of paragraphs 0018 to 0020 of JP-A-2006-331625 and column 4, line 65 to column 5, line 38 of US Pat. No. 7,029,774 can be referred to for the backcoat layer. .

[Various thickness]
The thickness of the nonmagnetic support and each layer in the above magnetic recording medium will be described below.
The thickness of the nonmagnetic support is, for example, 3.0 to 80.0 μm, preferably 3.0 to 50.0 μm, and more preferably 3.0 to 10.0 μm.

  The thickness of the magnetic layer can be optimized according to the saturation magnetization of the magnetic head used, the head gap length, the band of the recording signal, and the like. The thickness of the magnetic layer is generally 10 nm to 100 nm, preferably 20 to 90 nm, and more preferably 30 to 70 nm from the viewpoint of high density recording. The magnetic layer may be at least one layer, and the magnetic layer may be separated into two or more layers having different magnetic properties, and the configuration regarding a known double magnetic layer can be applied. The thickness of the magnetic layer in the case of separation into two or more layers is the total thickness of these layers.

  The thickness of the nonmagnetic layer is, for example, 50 nm or more, preferably 70 nm or more, and more preferably 100 nm or more. On the other hand, the thickness of the nonmagnetic layer is preferably 800 nm or less, more preferably 500 nm or less.

  The thickness of the backcoat layer is preferably 0.9 μm or less, and more preferably 0.1 to 0.7 μm.

  The thickness of each layer of the magnetic recording medium and the nonmagnetic support can be determined by a known film thickness measurement method. For example, after a cross section in the thickness direction of the magnetic recording medium is exposed by a known method such as ion beam or microtome, the exposed cross section is exposed using an electron microscope such as a scanning electron microscope or a transmission electron microscope. Perform cross-sectional observation. Various thicknesses can be determined as an arithmetic mean of the thicknesses determined at one location in the thickness direction in cross-sectional observation or at two or more locations, for example, two locations randomly extracted. Alternatively, the thickness of each layer may be determined as a designed thickness calculated from manufacturing conditions.

[Manufacturing process]
<Preparation of composition for forming each layer>
The step of preparing the composition for forming the magnetic layer, the nonmagnetic layer or the backcoat layer usually includes at least a kneading step, a dispersion step, and a mixing step optionally provided before or after these steps. Each process may be divided into two or more stages. The components used for preparation of the composition for forming each layer may be added at the beginning or during any process. As the solvent, one or more of various solvents which are usually used in the production of a coating type magnetic recording medium can be used. About a solvent, Paragraph 0153 of Unexamined-Japanese-Patent No. 2011-216149 can be referred, for example. In addition, individual components may be divided and added in two or more steps. For example, the binder may be divided and added in the kneading step, the dispersing step, and the mixing step for viscosity adjustment after dispersion. In order to manufacture the above magnetic recording medium, conventional known manufacturing techniques can be used in various processes. In the kneading step, it is preferable to use one having a strong kneading power such as an open kneader, a continuous kneader, a pressure kneader or an extruder. JP-A-1-106338 and JP-A-1-79274 can be referred to for details of the kneading treatment. A known disperser can be used. At any stage of preparing the composition for forming each layer, filtration may be performed by a known method. The filtration can be performed, for example, by filter filtration. As a filter used for filtration, for example, a filter with a pore diameter of 0.01 to 3 μm (for example, a filter made of glass fiber, a filter made of polypropylene, etc.) can be used.

  The FIB abrasive diameter tends to decrease as the oxide abrasive is present in a finer state in the magnetic layer. One of the means for causing the oxide abrasive to exist in a finer state in the magnetic layer is to use the dispersant capable of enhancing the dispersibility of the oxide abrasive as described above. it can. In addition, in order to make the oxide abrasive to exist in a finer state in the magnetic layer, an abrasive having a small particle size is used to suppress aggregation of the abrasive and to suppress uneven distribution to uniformly form the magnetic layer. It is preferable to disperse. One of the means for that purpose is to strengthen the dispersion conditions of the oxide abrasive at the time of preparation of the composition for forming a magnetic layer. For example, separately dispersing the oxide abrasive from the ferromagnetic hexagonal ferrite powder is an aspect of strengthening the dispersion conditions. The separate dispersion more specifically includes an abrasive liquid containing an oxide abrasive and a solvent (but containing substantially no ferromagnetic hexagonal ferrite powder), a ferromagnetic hexagonal ferrite powder, a solvent and a binder. It is a method of preparing the composition for magnetic layer formation through the process mixed with a magnetic liquid. Thus, the dispersibility of the oxide abrasive in the composition for forming a magnetic layer can be enhanced by separately dispersing the oxide abrasive and the ferromagnetic hexagonal ferrite powder and then mixing them. The above-mentioned "substantially free of ferromagnetic hexagonal ferrite powder" means that the ferromagnetic hexagonal ferrite powder is not added as a component of the polishing solution, and is an impurity unintentionally mixed. The presence of a trace amount of ferromagnetic hexagonal ferrite powder is tolerable. In addition to separate dispersion, or along with separate dispersion, long-time dispersion processing, use of a dispersion medium of small size (for example, reduction in diameter of dispersion beads in bead dispersion), means such as high packing of dispersion media in a dispersion machine The dispersion conditions can be strengthened by combining. Dispersers and dispersion media can be used commercially. In addition, performing the centrifugal separation treatment of the abrasive liquid can remove the oxide particles in the magnetic layer by removing particles larger than the average particle size and / or aggregated particles among the particles constituting the oxide abrasive. It can contribute to the presence of the abrasive in a finer state. Centrifugation can be performed using a commercially available centrifuge. Further, it is preferable to filter the abrasive solution by filter filtration or the like in order to remove coarse aggregates in which particles constituting the oxide abrasive are aggregated. Removal of such coarse aggregates can also contribute to the finer presence of the oxide abrasive in the magnetic layer. For example, filtering with a smaller pore size filter may contribute to the finer presence of the oxide abrasive in the magnetic layer. In addition, various treatment conditions (for example, stirring conditions, dispersion treatment conditions, filtration conditions, etc.) after mixing the abrasive liquid with the components for preparing the composition for forming a magnetic layer such as ferromagnetic hexagonal ferrite powder are adjusted. Thereby, the dispersibility of the oxide abrasive in the composition for forming a magnetic layer can be enhanced. This can also contribute to the presence of the oxide abrasive in a finer state in the magnetic layer. However, if the oxide abrasive is present in a very fine state in the magnetic layer, the diameter of the FIB abrasive falls below 0.04 μm, so various conditions for preparation of the abrasive liquid should be 0.04 μm to 0.08 μm. It is preferable to adjust so as to realize the following FIB abrasive diameter.

  With regard to the dispersion treatment of the composition for forming a magnetic layer, as described above, it is preferable to suppress the occurrence of chipping. For that purpose, in the process of preparing the composition for forming a magnetic layer, the dispersion process of the ferromagnetic hexagonal ferrite powder is performed by the two-stage dispersion process, and the coarse process of the ferromagnetic hexagonal ferrite powder is performed by the dispersion process of the first stage. It is preferable to carry out the second stage dispersion treatment in which the collision energy applied to the particles of the ferromagnetic hexagonal ferrite powder by collision with the dispersion beads is smaller than the first dispersion treatment after crushing the aggregates. According to the dispersion process, it is possible to achieve both the improvement of the dispersibility of the ferromagnetic hexagonal ferrite powder and the suppression of the occurrence of chipping.

  In a preferred embodiment of the above two-step dispersion process, a first step of obtaining a dispersion by dispersing the ferromagnetic hexagonal ferrite powder, the binder and the solvent in the presence of the first dispersion beads, and And D. dispersing the dispersion obtained in the first step in the presence of a second dispersion bead having a bead diameter and a density smaller than that of the first dispersion bead. it can. Hereinafter, the dispersion process of the above-described preferred embodiment will be further described.

  In order to enhance the dispersibility of the ferromagnetic hexagonal ferrite powder, the above first and second steps may be performed as a dispersion treatment prior to mixing the ferromagnetic hexagonal ferrite powder with other powder components. preferable. For example, ferromagnetic hexagonal ferrite powder, binder, solvent and optionally added prior to mixing with the oxide abrasive (preferably before mixing with the oxide abrasive and the protrusion forming agent described above) It is preferable to carry out the first and second steps described above as dispersion treatment of a liquid (magnetic liquid) containing an additive.

The bead diameter of the second dispersion bead is preferably 1/100 or less, more preferably 1/500 or less of the bead diameter of the first dispersion bead. Also, the bead diameter of the second dispersion bead can be, for example, 1/10000 or more of the bead diameter of the first dispersion bead. However, it is not limited to this range. For example, the bead diameter of the second dispersion bead is preferably in the range of 80 to 1000 nm. On the other hand, the bead diameter of the first dispersion bead can be, for example, in the range of 0.2 to 1.0 mm.
In the present invention and in the present specification, the bead diameter 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 performed under the condition that the second dispersion beads are present in an amount of 10 times or more of the ferromagnetic hexagonal ferrite powder on a mass basis, and the amount is 10 times to 30 times It is more preferred to carry out under the conditions present.
On the other hand, the amount of the first dispersion beads in the first stage is also preferably in the above range.

The second dispersion beads are beads having a smaller density than the first dispersion beads. The “density” is obtained by dividing the mass (unit: g) of the dispersed beads by the volume (unit: cm ³ ). The measurement is performed by the Archimedes method. The density of the second dispersion beads is preferably 3.7 g / cm ³ or less, more preferably 3.5 g / cm ³ or less. The density of the second dispersion beads may be, for example, 2.0 g / cm ³ or more, and may be less than 2.0 g / cm ³ . Preferred second dispersion beads in terms of density include diamond beads, silicon carbide beads, silicon nitride beads and the like, and preferred second dispersion beads in terms of density and hardness are diamond beads. Can.
On the other hand, the first dispersion beads, preferably density is 3.7 g / cm ³ greater than the dispersion beads, density is 3.8 g / cm ³ or more dispersing beads more preferably, 4.0 g / cm ³ or more dispersion Beads are more preferred. The density of the first dispersion beads may be, for example, 7.0 g / cm ³ or less, or may be more than 7.0 g / cm ³ . As the first dispersion beads, zirconia beads, alumina beads or the like are preferably used, and zirconia beads are more preferably used.

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

<Coating step, cooling step, heating and drying step, burnishing step, curing step>
The magnetic layer can be formed by directly applying the composition for forming the magnetic layer on the nonmagnetic support, or by applying the composition for forming the nonmagnetic layer sequentially or simultaneously with the composition for forming the nonmagnetic layer. Paragraph 0066 of JP-A-2010-231843 can be referred to for details of the coating for forming each layer.

  In a preferred embodiment, a composition for forming a magnetic layer comprising a ferromagnetic hexagonal ferrite powder, a binder, an oxide abrasive, a curing agent and a solvent is coated on a nonmagnetic support directly or through a nonmagnetic layer. The magnetic layer can be formed through a coating step of forming a coating layer, a heating and drying step of drying the coating layer by heat treatment, and a curing step of curing the coating layer. The magnetic layer forming step preferably includes a cooling step of cooling the coated layer between the coating step and the heating and drying step, and further, the surface of the coated layer is burnished (between the heating and drying step and the curing step). It is preferred to include a burnishing step to treat.

It is considered that carrying out the cooling step and the burnishing step in the above magnetic layer forming step is a preferable means for setting the logarithmic attenuation factor to 0.050 or less. The details are as follows.
Performing the cooling step of cooling the coating layer between the coating step and the heating and drying step is to localize the adhesive component described above on the surface of the coating layer and / or the surface layer near the surface. It is guessed that it may contribute. This is not because, by cooling the coated layer of the composition for forming a magnetic layer before the heating and drying step, the tacky component is easily transferred to the surface and / or surface portion of the coating layer during solvent evaporation in the heating and drying step. It is thought that. However, the reason is not clear. Then, it is considered that the adhesive component can be removed by burnishing the surface of the coating layer in which the adhesive component is localized on the surface and / or the surface layer portion. It is surmised that performing the curing step after removing the adhesive component in this way leads to making the logarithmic attenuation factor 0.050 or less. However, the above is only an assumption and does not limit the present invention.

  As described above, the composition for forming a magnetic layer can be applied in multiple layers sequentially or simultaneously with the composition for forming a nonmagnetic layer. In a preferred embodiment, the magnetic recording medium can be manufactured by sequential multi-layer coating. The manufacturing process including sequential multi-layer coating can be preferably performed as follows. The nonmagnetic layer is formed through a coating step of forming a coating layer by coating the nonmagnetic layer-forming composition on a nonmagnetic support, and a heat drying step of drying the formed coating layer by heat treatment. A magnetic layer is formed by applying a composition for forming a magnetic layer on the formed nonmagnetic layer to form a coated layer, and a heat drying step of drying the formed coated layer by heat treatment. .

  Hereinafter, the specific aspect of the said manufacturing method is demonstrated based on FIG. However, the present invention is not limited to the following specific embodiments.

  FIG. 4 is a schematic view showing a specific embodiment of the process of producing a magnetic recording medium having a nonmagnetic layer and a magnetic layer in this order on one surface of a nonmagnetic support and a backcoat layer on the other surface. FIG. In the embodiment shown in FIG. 4, the nonmagnetic support (long film) is continuously wound up from the delivery section by the delivery and take-up section, and is applied at each section or each zone shown in FIG. A nonmagnetic layer and a magnetic layer are sequentially formed by multilayer coating on one surface of a traveling nonmagnetic support by performing various treatments such as drying and orientation, and a backcoat layer is formed on the other surface. it can. Such a manufacturing method is the same as the manufacturing method usually performed for manufacturing a coated magnetic recording medium except that it includes a cooling zone in the magnetic layer forming process and includes a burnishing process before the curing process. Can.

  The composition for forming the nonmagnetic layer is applied on the nonmagnetic support delivered from the delivery section in the first application section (application process for the composition for forming the nonmagnetic layer).

  After the application step, in the first heat treatment zone, the applied layer is dried by heating the applied layer of the composition for forming a nonmagnetic layer formed in the application step (heat-drying step). The heating and drying step can be carried out by passing the nonmagnetic support having the coating layer of the composition for forming a nonmagnetic layer in a heating atmosphere. The atmosphere temperature of the heating atmosphere here can be, for example, about 60 to 140 ° C. However, what is necessary is just to set it as the temperature which can volatilize a solvent and can dry an application layer, and is not limited to the said range. Optionally, a heated gas may be sprayed onto the surface of the coating layer. The above points are the same for the heating and drying step in the second heat treatment zone and the heating and drying step in the third heat treatment zone described later.

  Next, in the second application section, the composition for forming a magnetic layer is applied on the nonmagnetic layer formed by performing the heating and drying process in the first heat treatment zone (the composition for forming the magnetic layer) Coating process).

  After the application step, the coating layer of the composition for forming a magnetic layer formed in the application step is cooled in the cooling zone (cooling step). For example, the cooling step can be performed by passing the nonmagnetic support in which the coating layer is formed on the nonmagnetic layer in a cooling atmosphere. The atmosphere temperature of the cooling atmosphere can be preferably in the range of -10 ° C to 0 ° C, and more preferably in the range of -5 ° C to 0 ° C. The time for performing the cooling step (for example, the time from when an arbitrary part of the coating layer is carried into the cooling zone to when it is carried out (hereinafter, also referred to as "staying time")) is not particularly limited. Since the value of the logarithmic attenuation factor tends to decrease as the residence time is lengthened, it is preferable to perform adjustment by conducting preliminary experiments as necessary so as to realize the logarithmic attenuation factor of 0.050 or less. In the cooling step, the cooled gas may be sprayed on the surface of the coating layer.

  Thereafter, in the embodiment in which the orientation process is performed, the orientation process of the ferromagnetic hexagonal ferrite powder in the application layer is performed in the orientation zone while the application layer of the composition for forming a magnetic layer is in a wet state. Various known techniques including the description in paragraph 0067 of JP-A-2010-231843 can be applied to the alignment treatment. As described above, it is preferable to perform vertical alignment processing as alignment processing from the viewpoint of controlling the XRD intensity ratio. The above description can also be referred to for the orientation treatment.

  The coated layer after orientation treatment is subjected to a heating and drying process in a second heat treatment zone.

  Next, in the third coating portion, a composition for forming a backcoat layer is applied on the surface opposite to the surface on which the nonmagnetic layer and the magnetic layer of the nonmagnetic support are formed, whereby a coating layer is formed. (The application process of the composition for backcoat layer formation). Thereafter, in the third heat treatment zone, the coated layer is heat treated and dried.

  Thus, a magnetic recording medium having a coated layer of the composition for forming a magnetic layer heat-dried on the nonmagnetic layer on one surface of the nonmagnetic support and having a backcoat layer on the other surface is obtained. it can. The magnetic recording medium obtained here becomes a product magnetic recording medium after being subjected to various processes described later.

  The obtained magnetic recording medium is cut (slit) into the size of a product magnetic recording medium after being taken up by a take-up unit. Slitting can be performed using a known cutter.

  The slitted magnetic recording medium is heated and dried prior to curing (heating, light irradiation, etc.) depending on the type of curing agent contained in the composition for forming a magnetic layer. The surface of the applied layer is burnished (burning process between the heating and drying process and the curing process). The present inventors have found that the burnishing treatment can remove the adhesive component cooled in the cooling zone and transferred to the surface and / or the surface of the coating layer, which leads to the above-mentioned logarithmic attenuation factor of 0.050 or less. They guess. However, as described above, it is only speculation and does not limit the present invention.

  The burnishing process is a process in which the surface to be treated is rubbed with a member (for example, a polishing tape or a grinding tool such as a grinding blade or grinding wheel), and the burnishing process known in the art for producing coated magnetic recording media It can be done in the same way. However, it has not been conventionally performed to perform burnishing at the stage before the curing step after the cooling step and the heating and drying step. On the other hand, the above-mentioned logarithmic attenuation factor can be made into 0.050 or less by performing burnishing processing at the above-mentioned step.

  The burnishing is preferably performed by one or both of rubbing (grinding) the coated layer surface to be treated with an abrasive tape and rubbing (grinding) the coated layer surface to be treated by a grinding tool. It can be implemented. It is preferable to use an abrasive tape containing at least one abrasive having a Mohs hardness higher than that of the oxide abrasive contained in the composition for forming a magnetic layer. As an abrasive tape, a commercial item may be used and the abrasive tape produced by the well-known method may be used. In addition, as a grinding tool, a known grinding blade such as a fixed blade, a diamond wheel, or a rotary blade, a grinding wheel, or the like can be used. Moreover, you may perform the wiping process which wipes off the coating layer surface rubbed with abrasive tape and / or a grinding tool with a wiping material. For details of the preferred abrasive tape, grinding tool, burnishing treatment and wiping treatment, reference can be made to paragraphs 0034 to 0048 of JP-A-6-52544 and the examples of FIG. 1 and the same publication. As the burnishing process is intensified, the value of the above-mentioned logarithmic attenuation rate tends to be smaller. The burnishing treatment can be strengthened as the abrasive having a high hardness is used as the abrasive contained in the abrasive tape, and can be strengthened as the amount of the abrasive in the abrasive tape is increased. Moreover, it can strengthen, so that a highly rigid grinding tool is used as a grinding tool. With regard to the burnishing conditions, the higher the sliding speed between the coating layer surface to be treated and the member (for example, the abrasive tape or the grinding tool), the stronger the burnishing can be. The sliding speed can be increased by increasing one or both of the speed of moving the member and the speed of moving the magnetic tape to be processed.

  After the above-mentioned burnishing treatment (burning treatment step), the applied layer of the composition for forming a magnetic layer is subjected to curing treatment. In the embodiment shown in FIG. 4, the coated layer of the composition for forming a magnetic layer is subjected to surface smoothing treatment after burnishing treatment and before curing treatment. The surface smoothing process is preferably performed by calendering. For details of the calendar process, for example, paragraph 0026 of JP-A-2010-231843 can be referred to. As the calendering process is intensified, the surface of the magnetic recording medium can be smoothed. Calendering can be strengthened as the surface temperature (calender temperature) of the calender rolls is increased and / or as the calender pressure is increased.

  Thereafter, the coating layer of the composition for forming a magnetic layer is subjected to a curing treatment according to the type of the curing agent contained in the coating layer (curing step). The hardening process can be performed by the process according to the kind of hardening agent contained in the said application layers, such as heat processing and light irradiation. The curing treatment conditions are not particularly limited, and may be appropriately set according to the formulation of the composition for forming a magnetic layer used for forming the coating layer, the type of curing agent, the thickness of the coating layer, and the like. For example, when a coating layer is formed using the composition for magnetic layer formation containing polyisocyanate as a hardening | curing agent, it is preferable that hardening treatment is heat processing. In the case where a layer other than the magnetic layer contains a curing agent, the curing reaction of the layer can also be advanced by the curing treatment here. Alternatively, a curing step may be provided separately. Further, after the curing step, burnishing may be further performed.

  Thus, the magnetic recording medium according to one aspect of the present invention can be obtained. However, the above manufacturing method is an exemplification, and those values can be made to fall within the above ranges, respectively, by any means capable of adjusting the XRD intensity ratio, the perpendicular direction squareness ratio, the logarithmic attenuation factor of the magnetic layer surface and the FIB abrasive diameter. Such aspects can also be controlled and are encompassed by the present invention.

  The magnetic recording medium according to one aspect of the present invention can be a tape-shaped magnetic recording medium (magnetic tape), and can also be a disk-shaped magnetic recording medium (magnetic disk). For example, a magnetic tape is usually accommodated in a magnetic tape cartridge, and the magnetic tape cartridge is mounted on a magnetic recording and reproducing apparatus. Servo patterns can also be formed on the magnetic tape by known methods in order to enable head tracking servo to be performed in the magnetic recording and reproducing apparatus.

[Magnetic recording and reproducing device]
One aspect of the present invention relates to a magnetic recording and reproducing apparatus including the above magnetic recording medium and a magnetic head.

  In the present invention and this specification, “magnetic recording and reproducing device” means a device capable of performing at least one of recording of information on a magnetic recording medium and reproduction of information recorded on a magnetic recording medium. Do. Such devices are commonly referred to as drives. The magnetic head included in the magnetic recording and reproducing apparatus can be a recording head capable of recording information on a magnetic recording medium, and can be a reproducing head capable of reproducing information recorded on the magnetic recording medium. It is also possible. Also, in one aspect, the magnetic recording and reproducing apparatus can include both a recording head and a reproducing head as separate magnetic heads. In another aspect, the magnetic head included in the magnetic recording and reproducing apparatus may have a configuration in which both the recording element and the reproducing element are provided in one magnetic head. Also, the magnetic head for recording information and / or reproducing information may include a servo pattern reading element. Alternatively, a magnetic head (servo head) having a servo pattern reading element may be included in the magnetic recording and reproducing apparatus as a head separate from the magnetic head for recording and / or reproducing information.

  In the above magnetic recording and reproducing apparatus, the recording of the information on the magnetic recording medium and the reproduction of the information recorded on the magnetic recording medium are performed by bringing the magnetic layer surface of the magnetic recording medium and the magnetic head into contact and sliding. Can. The magnetic recording and reproducing device may be any one including the magnetic recording medium according to one aspect of the present invention, and other known techniques can be applied.

  The magnetic recording medium according to one aspect of the present invention can exhibit excellent electromagnetic conversion characteristics in the above magnetic recording and reproducing apparatus. That is, in the above magnetic recording and reproducing apparatus, the information recorded on the magnetic recording medium according to one aspect of the present invention can be reproduced at high SNR. Furthermore, in the above magnetic recording and reproducing apparatus, GTT can be performed while replacing the magnetic recording medium with a new one. In this GTT, according to the magnetic recording medium of one aspect of the present invention, the occurrence of head element scraping can be suppressed. The element whose occurrence of head element abrasion is suppressed may be one or more of elements selected from the group consisting of a reproducing element, a recording element, and a servo pattern reading element, and may be two or more of these elements. . The reproducing element is preferably a magnetoresistive (MR) element that can read information recorded on a magnetic recording medium with high sensitivity. In addition, an MR element is also preferable as a servo pattern reading element. Various known MR heads can be used as heads (MR heads) including an MR element as a read element and / or a servo pattern read element.

  Hereinafter, the present invention will be described based on examples. However, the present invention is not limited to the embodiments shown in the examples. Unless otherwise indicated, the display of "part" and "%" as described below shows "mass part" and "mass%." 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. Moreover, "eq" as described below shows equivalent which is a unit which can not be converted to SI unit system.

Example 1
The formulation of the composition for forming each layer is shown below.

<Preparation of Abrasive Liquid>
2,3-Dihydroxynaphthalene (manufactured by Tokyo Chemical Industry Co., Ltd.) in an amount shown in Condition C in Table 1 with respect to 100.0 parts of the oxide abrasive (alumina powder) shown in Condition C in Table 1, SO ₃ Na as a polar group 32% solution (solvent is a mixed solvent of methyl ethyl ketone and toluene) 31.3 parts of a polyester polyurethane resin having a hydroxyl group (Toyobo Co., Ltd. UR-4800 (polar group weight: 80 meq / kg)), methyl ethyl ketone and cyclohexanone 1: 1 as a solvent 570.0 parts of a mixed solution (mass ratio) was mixed, and dispersed in the presence of zirconia beads (bead diameter: 0.1 mm) with a paint shaker for the time shown in condition C in Table 1 (bead dispersion time) . After the dispersion, the dispersion obtained by separating the dispersion and the beads by mesh was centrifuged. Centrifugation processing uses CS150GXL manufactured by Hitachi Koki Co., Ltd. (the rotor used is S100AT6 manufactured by the company) as a centrifuge, and the rotation speed (rpm; rotation per minute) shown in condition C in table 1 is changed to condition C in table 1. The indicated time (centrifugation time) was performed. Then, it filtered with the filter of the hole diameter shown to the conditions C of Table 1, and obtained the alumina dispersion (abrasive liquid).

<Preparation of composition for forming magnetic layer>
(Magnetic liquid)
Plate-like ferromagnetic hexagonal ferrite powder (M-type barium ferrite): 100.0 parts (activation volume: 1500 nm ³ )
Oleic acid: 2.0 parts vinyl chloride copolymer (Nippon Zeon MR-104): 10.0 parts SO ₃ Na group-containing polyurethane resin: 4.0 parts (weight average molecular weight 70000, SO ₃ Na group: 0. 07 meq / g)
Amine polymer (DISPERBYK-102 manufactured by Bick Chemie): 6.0 parts methyl ethyl ketone: 150.0 parts cyclohexanone: 150.0 parts (abrasive liquid)
The above-mentioned abrasive liquid: 6.0 parts (protrusion-forming liquid (silica sol))
Colloidal silica: 2.0 parts (average particle size 80 nm)
Methyl ethyl ketone: 8.0 parts (other components)
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 (Toronso Coronate (registered trademark) L): 3 .0 copies

(Preparation method)
The various components of the above magnetic solution are dispersed for 24 hours using batch-type vertical sand mill using zirconia beads (first dispersion beads, density 6.0 g / cm ³ ) with a bead diameter of 0.5 mm (first stage) And then filtered using a filter with a pore size of 0.5 μm to prepare dispersion A. The zirconia beads were used in a 10-fold amount on a mass basis with respect to the mass of the ferromagnetic hexagonal ferrite powder.
Thereafter, Dispersion A was dispersed in a batch type vertical sand mill using diamond beads (second dispersion beads, density 3.5 g / cm ³ ) of bead diameters shown in Table 2 for the time shown in Table 2 (second A), a dispersion (dispersion B) was prepared by separating diamond beads using a centrifuge. The following magnetic liquid is the dispersion B thus obtained.
The magnetic liquid, the abrasive liquid, the protrusion forming liquid, and the other components described above were introduced into a dissolver stirrer, and stirred at a peripheral speed of 10 m / sec for the time shown in condition C of Table 1 (stirring time). Then, after performing ultrasonic dispersion treatment for a time (ultrasonic dispersion time) shown in condition C of Table 1 at a flow rate of 7.5 kg / min with a flow type ultrasonic disperser, a filter with the pore diameter shown in condition C of Table 1 The composition for forming a magnetic layer was prepared by filtration under the conditions shown in Table 1 under the conditions C.

  The activation volume of the ferromagnetic hexagonal ferrite powder described above is a value calculated and measured using the powder of the same powder lot as the ferromagnetic hexagonal ferrite powder used for the preparation of the composition for forming the magnetic layer. It is. The measurement was performed using a vibrating sample type flux meter (manufactured by Toei Kogyo Co., Ltd.) at magnetic field sweep speeds of 3 and 30 minutes in the coercive force Hc measurement unit, and the activation volume was calculated from the relational expression described above. The measurement was performed in an environment of 23 ° C. ± 1 ° C.

<Preparation of composition for forming nonmagnetic layer>
The following components of the composition for forming a nonmagnetic layer are dispersed for 24 hours using batch-type vertical sand mill using zirconia beads with a bead diameter of 0.1 mm, and then using a filter having a pore diameter of 0.5 μm The composition for forming the nonmagnetic layer was prepared by filtration.

Nonmagnetic 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 back coat layer formation>
After kneading and diluting the components except the lubricant (stearic acid and butyl stearate), polyisocyanate, and 200.0 parts of cyclohexanone among various components of the composition for forming a back coat layer described below with an open kneader, horizontal bead mill dispersion The sample was subjected to 12 passes of dispersion treatment using zirconia beads with a bead diameter of 1 mm by a machine, a bead filling rate of 80% by volume, and a residence time per pass of 2 minutes at a circumferential velocity of the rotor tip of 10 m / sec. Thereafter, the remaining components described above were added, the solution was stirred with a dissolver, and the resulting dispersion was filtered using a filter having a pore size of 1 μm to prepare a backcoat layer-forming composition.

Nonmagnetic 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 sulfonate 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

<Preparation of magnetic tape>
A magnetic tape was produced according to the specific embodiment shown in FIG. The details are as follows.
A polyethylene naphthalate support having a thickness of 5.0 μm is delivered from the delivery unit, and a composition for forming a nonmagnetic layer is applied to one surface so that the thickness after drying is 100 nm in the first application unit, The coated layer was formed by drying in one heat treatment zone (ambient temperature 100 ° C.).
Thereafter, the composition for forming a magnetic layer was applied onto the nonmagnetic layer so that the thickness after drying was 70 nm in the second application portion, to form an applied layer. After allowing the cooling process to pass through the cooling zone adjusted to the atmosphere temperature of 0 ° C for the staying time shown in Table 2 while the formed coating layer is in the wet state, the magnetic field of the strength shown in Table 2 is further The film was applied in a direction perpendicular to the surface of the coating layer to perform vertical alignment, and then dried in a second heat treatment zone (at an atmospheric temperature of 100 ° C.).
Then, in the third coating portion, a backcoat layer is formed on the surface opposite to the nonmagnetic layer and the surface on which the magnetic layer is formed of the polyethylene naphthalate support so that the thickness after drying becomes 0.4 μm. The composition for formation was applied to form a coated layer, and the formed coated layer was dried in a third heat treatment zone (at an atmospheric temperature of 100 ° C.).
After slitting the magnetic tape thus obtained to a width of 1/2 inch (0.0127 meter), the surface of the coated layer of the composition for forming a magnetic layer was subjected to a burnishing treatment and a wiping treatment. With regard to the burnishing treatment and the wiping treatment, in the processing apparatus described in FIG. 1 of JP-A-6-52544, a commercially available polishing tape as a polishing tape (trade name MA22000 manufactured by Fujifilm Corp., polishing agent: diamond / Cr ₂₎ A commercially available wiping blade (product made by Kuraray Co., Ltd.) is used as a wiping material, using a commercially available sapphire blade (manufactured by Kyocera, 5 mm wide, 35 mm long, tip angle 60 degrees) as a grinding blade using O ₃ / bengara. The name WRP 736) was used. As the processing conditions, the processing conditions in Example 12 of JP-A-6-52544 were adopted.
After the above burnishing treatment and wiping treatment, with a calender roll consisting only of metal rolls, at a velocity of 80 m / min, a linear pressure of 300 kg / cm (294 kN / m), and a calender temperature (surface temperature of the calender roll) of 90 ° C. Calendar processing (surface smoothing processing) was performed.
Thereafter, heat treatment (hardening treatment) is performed for 36 hours in an environment at an ambient temperature of 70 ° C., and then a servo pattern is formed on the magnetic layer by a commercially available servo writer.
Thus, the magnetic tape of Example 1 was obtained.

[Examples 2 to 9, Comparative Examples 1 to 13]
Magnetic tapes were produced in the same manner as in Example 1 except that various items shown in Tables 1 and 2 were changed as shown in the respective tables.
The oxide abrasives listed in Table 1 are all alumina powders.
The composition for forming a magnetic layer was prepared without performing the second step in the magnetic liquid dispersion treatment for the comparative examples described in Table 2 as “none” in the column of dispersed beads and the column of time.
In Table 2, for the example described as “none” in the column of the vertical alignment treatment magnetic field strength, the magnetic layer was formed without performing the alignment treatment.
In the comparative example described in Table 2 as “not performed” in the cooling zone staying time column and the pre-hardening varnishing column, the magnetic layer forming step does not include the cooling zone and the bar before hardening processing The magnetic tape was manufactured by the manufacturing process which does not perform a nick process and a wiping process.

[Physical evaluation of magnetic tape]
(1) XRD intensity ratio A tape sample was cut out from the prepared magnetic tape.
With respect to the tape sample cut out, X-rays were made incident on the surface of the magnetic layer using a thin film X-ray diffractometer (SmartLab manufactured by Rigaku Corporation), and In-Plane XRD was performed by the method described above.
From the X-ray diffraction spectrum obtained by In-Plane XRD, the peak intensities of diffraction peaks of (114) plane of hexagonal ferrite crystal structure, Int (114) and peak intensities of diffraction peaks of (110) plane, Int (110) It asked for and computed XRD intensity ratio (Int (110) / Int (114)).

(2) Vertical Direction Square Ratio The vertical direction square ratio was determined for each of the manufactured magnetic tapes using the vibrating sample type flux meter (manufactured by Toei Kogyo Co., Ltd.) by the method described above.

(3) Measurement of the logarithmic attenuation factor of the magnetic layer surface As a measuring device, rigid pendulum type physical property tester RPT-3000W manufactured by A & D Co., Ltd. (pendulum: rigid pendulum FRB-100 manufactured by A & D Co., Ltd.) Weight: None, Round bar type cylinder edge: RBP-040 manufactured by A & D Co., Ltd., substrate: glass substrate, substrate temperature rising rate 5 ° C./min), according to the method described above The log decay rate of the layer surface was determined.
As a glass substrate, what cut the commercially available slide glass in the size of 25 mm (short side) x 50 mm (long side) was used. With the magnetic tape mounted on the center of the glass substrate so that the longitudinal direction of the magnetic tape is parallel to the short side direction of the glass substrate, tapes for fixing the four corners of the magnetic tape on the glass substrate (Kapton tape made by Toray Dupont It fixed with the glass substrate by). Then, the magnetic tape of the part which overflowed from the glass substrate was cut. Thus, the measurement sample was fixed and mounted on the glass substrate at four places as shown in FIG. A displacement-time curve was created for the 86th measurement interval, with an adsorption time of 1 second and a measurement interval of 7 to 10 seconds, and this curve was used to determine the logarithmic attenuation factor. The measurement was performed under an environment of about 50% relative humidity.

(4) FIB Abrasive Diameter The FIB abrasive diameter of each of the prepared magnetic tapes was determined by the following method. As focused ion beam apparatus, MI4050 manufactured by Hitachi High-Technologies Corporation was used, and as image analysis software, free software ImageJ was used.
(I) Acquisition of Secondary Ion Image Commercially available double-sided carbon double-sided tape for SEM measurement (double-sided tape in which a carbon film is formed on a base material made of aluminum) of the backcoat layer surface of the measurement sample cut out from each magnetic tape prepared Stuck to the adhesive layer of The adhesive layer on the surface opposite to the backcoat layer surface and the pasted surface of this double-sided tape was affixed to the sample stage of the focused ion beam apparatus. Thus, the measurement sample was placed on the sample stage of the focused ion beam apparatus with the surface of the magnetic layer facing upward.
The imaging setting was not performed, the beam setting of the focused ion beam apparatus was set to an acceleration voltage of 30 kV, a current value of 133 pA, Beam Size of 30 nm and Brightness of 50%, and a secondary ion detector detected an SI signal. ACB was performed in three unimaged areas on the surface of the magnetic layer to stabilize the color tone of the image, and the contrast reference value and the brightness reference value were determined. A contrast value 1% lower than the contrast reference value determined by the present ACB and the above-mentioned brightness reference value were determined as imaging conditions. An unimaged area on the surface of the magnetic layer was selected, and imaging was performed at Pixel distance = 25.0 (nm / pixel) under the imaging conditions determined above. The image capture method was PhotoScan Dot × 4_Dwell Time 15 μsec (capture time: 1 minute), and the capture size was 25 μm square. Thus, a secondary ion image of a 25 μm square region on the surface of the magnetic layer was obtained. The obtained secondary ion image right-clicked the mouse on the loading screen after scanning, and saved the file format as JPEG using ExportImage. It was confirmed that the number of pixels of the image was 2000 pixels × 2100 pixels, and the cross marks and micron bars of the captured image were erased to obtain 2000 pixel × 2000 pixel images.
(Ii) Calculation of FIB Abrasive Diameter The image data of the secondary ion image obtained in the above (i) was dragged and dropped onto the image analysis software ImageJ.
The image data was changed to 8 bits using image analysis software. Specifically, I pressed Image in the operation menu of the image analysis software and selected 8 bits of Type.
In order to perform the binarization processing, the lower limit 250 tones and the upper limit 255 tones were selected, and the binarization processing with these two threshold values was executed. Specifically, on the operation menu of the image analysis software, pressing Image, selecting Adjust Threshold, selecting the lower limit 250 and the upper limit 255, and then selecting apply. About the obtained image, Process of the operation menu of image analysis software was pressed, Despeckle was selected from Noise, Size 4.0-Infinity was set by AnalyzeParticle, and the noise component was removed.
For the binarized image thus obtained, AnalyzeParticle was selected from the operation menu of the image analysis software, and the number and white area (unit: Pixel) of the white glowing portion on the image were determined. The area was determined by converting Area (unit: Pixel) into an area for each white lighted portion on the screen by image analysis software. Specifically, since 1 pixel corresponds to 0.0125 μm in an image obtained under the above imaging conditions, the area A [μm ² ] is calculated by the area A = Area pixel × 0.0125 2. Using the area thus calculated, the circle equivalent diameter L was determined for each part that glows white, according to the circle equivalent diameter L = (A / π) ^ (1/2) × 2 = L.
The above steps were carried out four times at different locations (25 μm square) on the surface of the magnetic layer of the measurement sample, and from the results obtained, the FIB abrasive diameter was calculated by FIB abrasive diameter = Σ (Li) / Σi did.

[Evaluation of electromagnetic conversion characteristics (SNR)]
The electromagnetic conversion characteristics of each of the manufactured magnetic tapes were measured by the following method using a 1/2 inch (0.0127 meter) reel tester with a fixed head. The following recording and reproduction were performed by sliding the surface of the magnetic layer of the magnetic tape and the head.
The traveling speed of the magnetic tape (magnetic head / magnetic tape relative speed) was 4 m / sec. A MIG (Metal-In-Gap) head (gap length 0.15 μm, track width 1.0 μm) was used as a recording head, and the recording current was set to the optimum recording current of each magnetic tape. As a reproducing head, a GMR (Giant-Magnetoresistive) head having an element thickness of 15 nm, a shield interval of 0.1 μm and a lead width of 0.5 μm was used. The signal was recorded at a linear recording density of 300 kfci, and the reproduction signal was measured by a spectrum analyzer manufactured by Shibasoku. The ratio of the output value of the carrier signal to the integral noise of the entire spectral band is taken as SNR. For the SNR measurement, the portion of the signal where the signal was sufficiently stable was used after traveling of the magnetic tape. The SNR is shown in Table 2 as a relative value when the SNR of Comparative Example 1 is 0.0 dB. The unit kfci is a unit of linear recording density (impossible to convert to SI unit system).

[Head element scraping amount in GTT]
The magnetic head (MR head) used in the IBM TS1140 tape drive is attached to a reel tester, and a one-roll 1000 m-long magnetic tape slides between the magnetic layer surface and the MR head while traveling speed (magnetic head Traveled 200 passes at 4 m / sec.
The same 200-pass travel as above was repeated after replacing the magnetic tape with a new one (using 1000 volumes of magnetic tape), and then the amount of scraping of the MR element of the MR head was measured by the following method.
A carbon film is deposited on the surface including the sliding surface with the magnetic layer surface of the MR head using a vacuum deposition apparatus (JEE-4X manufactured by JEOL), and an ion sputtering apparatus (Hitachi High) is deposited on the carbon film. The platinum film was sputter-deposited using Technologies E-1030). Thereafter, a cross section parallel to the sliding direction with the magnetic tape during traveling is exposed from the MR head using the MRBEI FIB-SEM multifunction machine Helios 400 S, and the MR element is included in this cross section. A sample for observation (section of 100 nm in thickness) was cut out. A sliding surface with the magnetic layer surface using a TEM image obtained by observing this cross-sectional observation sample with a transmission electron microscope (TEM; Transmission Electron Microscope) (FTI company Titan 80-300) at an accelerating voltage of 300 kV. And the vertical distance between the top of the MR element and the top of the MR element. The difference between the determined distance and the distance determined by the same method for the unused MR head is shown in Table 2 as the head element removal amount in GTT.

  From the results shown in Table 2, Examples 1 to 9 in which the XRD intensity ratio of the magnetic tape, the perpendicular direction squareness ratio, the logarithmic attenuation factor of the magnetic layer surface and the FIB abrasive agent diameter are in the ranges described above respectively have high SNR It is possible to confirm that the regeneration of the head element is possible (that is, the excellent electromagnetic conversion characteristic is exhibited), and the occurrence of head element scraping in the GTT is suppressed. In Comparative Examples 6 and 7, the reason for the lower SNR compared to Examples 1 to 9 is that the oxide abrasive was present in the magnetic layer in the state where the diameter of the FIB abrasive greatly exceeded 0.08 μm. It is considered that the distance between the surface of the magnetic layer and the reproducing element is increased and spacing loss is caused due to the surface of the magnetic layer being roughened.

  One aspect of the present invention is useful in the technical field of magnetic recording media used as archival recording media.

Claims (9)

A magnetic recording medium comprising a magnetic layer containing a ferromagnetic powder and a binder on a nonmagnetic support, comprising:
The ferromagnetic powder is a ferromagnetic hexagonal ferrite powder, and the magnetic layer contains an oxide abrasive.
Peak intensity of the diffraction peak of the (114) plane of the hexagonal ferrite crystal structure determined by X-ray diffraction analysis of the magnetic layer using the In-Plane method Peak intensity of the diffraction peak of the (110) plane relative to Int (114) The intensity ratio of (110), Int (110) / Int (114), is 0.5 or more and 4.0 or less,
The perpendicular direction squareness ratio of the magnetic recording medium is 0.65 or more and 1.00 or less,
The logarithmic attenuation factor determined by the pendulum viscoelasticity test on the surface of the magnetic layer is 0.050 or less, and the oxidation determined from the secondary ion image acquired by irradiating the surface of the magnetic layer with a focused ion beam A magnetic recording medium, wherein the average particle diameter of the abrasive is 0.04 μm or more and 0.08 μm or less. The magnetic recording medium according to claim 1, wherein the perpendicular direction squareness ratio is 0.65 or more and 0.90 or less. The magnetic recording medium according to claim 1, wherein the logarithmic attenuation factor is 0.010 or more and 0.050 or less. The magnetic recording medium according to any one of claims 1 to 3, wherein the oxide abrasive is an alumina powder. The magnetic recording medium according to any one of claims 1 to 4, wherein a nonmagnetic layer containing a nonmagnetic powder and a binder is provided between the nonmagnetic support and the magnetic layer. The magnetic recording according to any one of claims 1 to 5, wherein a back coat layer comprising a nonmagnetic powder and a binder is provided on the surface side opposite to the surface side having the magnetic layer of the nonmagnetic support. Medium. The magnetic recording medium according to any one of claims 1 to 6, which is a magnetic tape. A magnetic recording medium according to any one of claims 1 to 7,
Magnetic head,
Magnetic recording and reproducing apparatus. The magnetic recording and reproducing apparatus according to claim 8, wherein the magnetic head is a magnetic head including a magnetoresistive element.