JP6852217B2 - Magnetic recording medium and magnetic recording / playback device - Google Patents

Description

The present invention relates to a magnetic recording medium and a magnetic recording / playback device.

Magnetic recording media are roughly classified into two types, a metal thin film type and a coating type. The metal thin film type magnetic recording medium is a magnetic recording medium having a magnetic layer of a metal thin film formed by vapor deposition or the like. On the other hand, the coating type magnetic recording medium (see, for example, Patent Document 1) is a magnetic recording medium having a magnetic layer containing a ferromagnetic powder together with a binder. The coating type magnetic recording medium is excellent in chemical durability as compared with the metal thin film type magnetic recording medium, and is therefore a useful magnetic recording medium as a data storage medium for storing a large amount of information for a long period of time. In the following, the coating type magnetic recording medium will be simply referred to as a magnetic recording medium.

Japanese Unexamined Patent Publication No. 2012-43495

For the magnetic recording medium, it is desired to improve the surface smoothness of the magnetic layer (see, for example, paragraph 0003 of Patent Document 1). This is because increasing the surface smoothness of the magnetic layer leads to the improvement of the electromagnetic conversion characteristics.

In recent years, magnetic recording media used for data storage applications may be used in temperature and humidity controlled data centers. On the other hand, data centers are required to save power in order to reduce costs. In order to save power, it is desirable that the temperature and humidity control conditions in the data center can be relaxed from the current level, or that management can be eliminated. However, if the temperature and humidity control conditions are not relaxed or controlled, it is assumed that the magnetic recording medium will be exposed to environmental changes caused by weather changes, seasonal changes, and the like.

Regarding the above points, according to the study by the present inventors, in a magnetic recording medium having a high surface smoothness of a magnetic layer, a low temperature (for example, more than 0 ° C.) is used under high humidity (for example, in an environment where the relative humidity is about 70 to 100%). It has been clarified that when a temperature change from (~ 15 ° C.) to a high temperature (for example, 30 to 50 ° C.) occurs (for example, a temperature change of about 15 to 50 ° C.), a phenomenon that the electromagnetic conversion characteristic deteriorates occurs.

Therefore, an object of the present invention is to suppress a decrease in electromagnetic conversion characteristics due to a temperature change from a low temperature to a high temperature under high humidity in a magnetic recording medium having a high surface smoothness of a magnetic layer.

One aspect of the present invention is
A magnetic recording medium having a magnetic layer containing a ferromagnetic powder and a binder on a non-magnetic support.
The center line average surface roughness Ra (hereinafter, also referred to as “magnetic layer surface roughness Ra”) measured on the surface of the magnetic layer is 1.0 nm or more and 1.6 nm or less, and the surface of the magnetic layer. Difference between the spacing S after measured by the optical interferometry _{after the methyl ethyl ketone cleaning and the spacing S before} measured by the optical interferometry on the surface of the magnetic layer before the methyl ethyl ketone cleaning _(Safter- S _before ) (hereinafter _{, "Spacing difference before and after} washing with methyl ethyl ketone (Suffer-S before)" or simply "difference ( _After- S _before )") is a magnetic recording medium of more than 0 nm and 15.0 nm or less.
Regarding.

In one aspect, the difference ( _After- S _before ) can be 2.0 nm or more and 15.0 nm or less.

In one aspect, the difference ( _After- S _before ) can be 3.0 nm or more and 12.0 nm or less.

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

In one aspect, the magnetic recording medium may have a backcoat layer containing a non-magnetic powder and a binder on the surface side of the non-magnetic support opposite to the surface side 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 / reproducing device including the magnetic recording medium and a magnetic head.

According to one aspect of the present invention, a magnetic recording medium having a magnetic layer having high surface smoothness and suppressing a decrease in electromagnetic conversion characteristics due to a temperature change from a low temperature to a high temperature under high humidity, and a magnetic recording medium thereof. A magnetic recording / reproducing device including a magnetic recording medium can be provided.

[Magnetic recording medium]
One aspect of the present invention is a magnetic recording medium having a magnetic layer containing a ferromagnetic powder and a binder on a non-magnetic support, and the centerline average surface roughness Ra measured on the surface of the magnetic layer is 1. and at .0nm than 1.6nm or less, and is measured and the spacing S _after being measured by an optical interference method after methylethylketone washing the surface of the magnetic layer, before methylethylketone washing the surface of the magnetic layer by an optical interference method It relates to a magnetic recording medium in which the difference from the spacing S _{before (After-} S _before ) is more than 0 nm and 15.0 nm or less.

In the present invention and the present specification, "methyl ethyl ketone cleaning" means that a sample piece cut out from a magnetic recording medium is immersed in methyl ethyl ketone (200 g) having a liquid temperature of 20 to 25 ° C. and ultrasonically cleaned for 100 seconds (ultrasonic output: 40 kHz). ). When the magnetic recording medium to be cleaned is a magnetic tape, a sample piece having a length of 5 cm is cut out and subjected to methyl ethyl ketone cleaning. The width of the magnetic tape and the width of the sample piece cut out from the magnetic tape are typically 1/2 inch (0.0127 meters). For magnetic tapes other than 1/2 inch (0.0127 m) wide, a sample piece having a length of 5 cm may be cut out and subjected to methyl ethyl ketone washing. When the magnetic recording medium to be washed is a magnetic disk, a sample piece having a size of 5 cm × 1.27 cm is cut out and subjected to methyl ethyl ketone washing. The measurement of spacing after washing with methyl ethyl ketone, which will be described in detail below, shall be performed after the sample piece after washing with methyl ethyl ketone is left to stand in an environment of a temperature of 23 ° C. and a relative humidity of 50% for 24 hours.

In the present invention and the present specification, the "surface of the magnetic layer" of the magnetic recording medium is synonymous with the surface of the magnetic recording medium on the magnetic layer side.

In the present invention and the present specification, the spacing measured by the optical interferometry on the surface of the magnetic layer of the magnetic recording medium is a value measured by the following method.
A magnetic recording medium (specifically, the above sample piece; the same applies hereinafter) and a transparent plate-shaped member (for example, a glass plate) are superposed so that the magnetic layer surface of the magnetic recording medium faces the transparent plate-shaped member. In this state, the pressing member is pressed with a pressure of ^{5.05 × 10 4} N / m (0.5 atm) from the side opposite to the magnetic layer side of the magnetic recording medium. In this state, the surface of the magnetic layer of the magnetic recording medium is irradiated with light via a transparent plate-shaped member (irradiation region: 150,000 to 200,000 μm ² ), and the reflected light from the surface of the magnetic layer of the magnetic recording medium and the transparent plate shape. Based on the intensity of the interference light generated by the optical path difference from the reflected light from the surface of the member on the magnetic recording medium side (for example, the contrast of the interference fringe image), the magnetic layer surface of the magnetic recording medium and the magnetic recording medium of the transparent plate-shaped member Find the spacing between the side surface. The light emitted here is not particularly limited. When the emitted light is light having an emission wavelength over a relatively wide wavelength range, such as white light including light having a plurality of wavelengths, the transparent plate-shaped member and the light receiving portion that receives the reflected light are used. A member having a function of selectively cutting light of a specific wavelength or light other than the specific wavelength region such as an interference filter is arranged between them, and the light of a part of the reflected light or the light of a part of the wavelength region is arranged. Light is selectively incident on the light receiving part. When the light to be irradiated is light having a single emission peak (so-called monochromatic light), the above member may not be used. As an example, the wavelength of the light incident on the light receiving portion can be in the range of, for example, 500 to 700 nm. However, the wavelength of the light incident on the light receiving portion is not limited to the above range. Further, the transparent plate-shaped member may be a member having transparency that transmits the irradiated light to the extent that light is irradiated to the magnetic recording medium through this member to obtain interference light.
The interference fringe image obtained by the measurement of the spacing is divided into 300,000 points, and the spacing of each point (the distance between the surface of the magnetic layer of the magnetic recording medium and the surface of the transparent plate-like member on the magnetic recording medium side) is determined. Obtain and use this as a histogram, and the most frequent value in this histogram is the spacing. The difference ( _After- S _before ) is defined as a value obtained by subtracting the mode value before the methyl ethyl ketone washing from the mode value after the methyl ethyl ketone washing at the above 300,000 points.
Two sample pieces are cut out from the same magnetic recording medium _{, one is subjected to the above-mentioned spacing value S before} without methyl ethyl ketone washing, and the other is subjected to the methyl ethyl ketone washing, and then the above-mentioned spacing value S _after is obtained. _After- S _before ) may be obtained. _{Alternatively, the difference (After-} S _before ) may be obtained by obtaining the spacing value after subjecting the sample piece to which the above-mentioned spacing value has been obtained before the methyl ethyl ketone washing and then subjecting the sample piece to the methyl ethyl ketone washing.
The above measurement can be performed using, for example, a commercially available tape spacing analyzer (TSA) such as Tape Spacing Analyzer manufactured by Micro Physics. Spacing measurements in the examples were performed using a Tape Spacing Analyzer manufactured by Micro Physics.

The magnetic recording medium has a center line average surface roughness Ra measured on the surface of the magnetic layer of 1.0 nm or more and 1.6 nm or less. That is, the magnetic recording medium is a magnetic recording medium having a magnetic layer having high surface smoothness. In such a magnetic recording medium, the difference in spacing ( _After- S _{before) before and} after washing with methyl ethyl ketone is more than 0 nm and 15.0 nm or less, so that electromagnetic waves caused by a temperature change from a low temperature to a high temperature under high humidity It is possible to suppress a decrease in conversion characteristics. The present inventors' inferences regarding this point are as follows.

The reproduction of the information recorded on the magnetic recording medium is usually performed by bringing the surface of the magnetic layer and the magnetic head (hereinafter, also simply referred to as “head”) into contact with each other and sliding them.
On the other hand, when the temperature changes from low temperature to high temperature under high humidity, it is considered that dew condensation (adhesion of moisture) occurs on the surface of the magnetic layer of the magnetic recording medium. It is presumed that the presence of this moisture causes an increase in the coefficient of friction when the surface of the magnetic layer and the magnetic head slide. In a magnetic recording medium having a high surface smoothness of the magnetic layer, the coefficient of friction when sliding between the surface of the magnetic layer and the magnetic head tends to increase, and the friction coefficient further increases due to the presence of the above-mentioned moisture. However, the present inventors consider that this is the reason why the electromagnetic conversion characteristics deteriorate when the temperature changes from low temperature to high temperature under high humidity in a magnetic recording medium having a high surface smoothness of the magnetic layer. Therefore, if the amount of water adhering to the surface of the magnetic layer can be reduced when the temperature changes from low temperature to high temperature under high humidity, the above-mentioned increase in friction coefficient can be suppressed, and as a result, electromagnetic conversion It is considered that the deterioration of the characteristics can be suppressed.

By the way, on the surface of the magnetic layer, there are usually a portion (projection) that mainly contacts the head (so-called true contact) when the surface of the magnetic layer and the head slide, and a portion lower than this portion (hereinafter, "base portion"). ”) And exist. The present inventors consider that the spacing described above is a value that serves as an index of the distance between the head and the base material when the surface of the magnetic layer and the head slide. However, if some component is present on the surface of the magnetic layer, it is considered that the spacing becomes narrower as the amount of the component interposed between the base portion and the head increases. On the other hand, when this component is removed by washing with methyl ethyl ketone, the spacing spreads, so that _{the value of the spacing S after after} washing with methyl ethyl ketone becomes larger than the value of the _{spacing S before after washing with methyl ethyl ketone.} Therefore, it is considered that the difference in spacing ( _After- S _{before) before and} after washing with methyl ethyl ketone can be used as an index of the amount of the component intervening between the substrate portion and the head.
Regarding the above points, the present inventors have stated that the presence of the component removed by the methyl ethyl ketone washing on the surface of the magnetic layer causes the moisture on the surface of the magnetic layer when the temperature changes from low temperature to high temperature under high humidity. It is thought that it promotes the adhesion of. _{Therefore, reducing the difference in spacing (After-} S _{before) before and} after washing with methyl ethyl ketone, that is, reducing the amount of the component, contributes to suppressing the adhesion of water, and as a result, the friction described above. The present inventors speculate that this will lead to suppression of the increase in the coefficient. The present inventors consider that this makes it possible to suppress a decrease in electromagnetic conversion characteristics due to a temperature change from a low temperature to a high temperature under high humidity in a magnetic recording medium having a highly smooth surface of the magnetic layer. ing. On the other hand, according to the study by the present inventors, the value of the difference in spacing before and after washing with n-hexane described in Patent Document 1 and the high humidity in the magnetic recording medium having high smoothness of the magnetic layer surface. No correlation was found between the decrease in electromagnetic conversion characteristics due to the temperature change from low temperature to high temperature underneath. It is presumed that this is because the above-mentioned components cannot be removed or sufficiently removed by n-hexane washing.
The details of the above components are not clear. As a guess, the present inventors think that the above-mentioned component may have a higher molecular weight than the organic compound usually added as an additive to the magnetic layer. The present inventors infer one aspect of this component as follows. In one aspect, the magnetic layer is cured by applying a composition for forming a magnetic layer containing a curing agent in addition to a ferromagnetic powder and a binder directly or via another layer on a non-magnetic support. It is formed. By the curing treatment here, the binder and the curing agent can be subjected to a curing reaction (crosslinking reaction). However, it is considered that the binder that did not undergo a curing reaction with the curing agent or the binder that did not have a sufficient curing reaction with the curing agent is easily released from the magnetic layer and may be present on the surface of the magnetic layer. The fact that such a binder (for example, the functional group of this binder) easily adsorbs water promotes the adhesion of water to the surface of the magnetic layer when the temperature changes from low temperature to high temperature under high humidity. The present inventors speculate that this may be the cause.
However, the above is the inference of the present inventors and does not limit the present invention in any way.

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

<Magnetic layer>
(Magnetic layer surface roughness Ra)
The center line average surface roughness Ra (magnetic layer surface roughness Ra) measured on the magnetic layer surface of the magnetic recording medium is 1.0 nm or more and 1.6 nm or less. The fact that the magnetic layer surface roughness Ra is 1.6 nm or less can contribute to the ability of the magnetic recording medium to exhibit excellent electromagnetic conversion characteristics. From the viewpoint of further improving the electromagnetic conversion characteristics, the magnetic layer surface roughness Ra is preferably 1.5 nm or less. However, in a magnetic recording medium having such a magnetic layer having high surface smoothness, the electromagnetic conversion characteristics deteriorate due to the temperature change from a low temperature to a high temperature under high humidity unless some measures are taken. On the other hand, in the _{magnetic recording medium in which the spacing difference (After-} S _{before) before and} after washing with methyl ethyl ketone is in the above range, although it has a magnetic layer with high surface smoothness, it has a high humidity and a low temperature to a high temperature. It is possible to suppress the deterioration of the electromagnetic conversion characteristics due to the temperature change to. Further, when the magnetic layer surface roughness Ra is 1.0 nm or more, the temperature change from low temperature to high temperature under high humidity can be set by setting the _{spacing difference (Safter-} S _{before) before and after washing with methyl ethyl ketone within the above range.} It is possible to suppress the deterioration of the electromagnetic conversion characteristics due to the above. From this point of view, the magnetic layer surface roughness Ra of the magnetic recording medium is 1.0 nm or more, preferably 1.1 nm or more.

The center line average surface roughness Ra measured on the surface of the magnetic layer in the present invention and the present specification is measured by an atomic force microscope (AFM) in a region having an area of 40 μm × 40 μm on the surface of the magnetic layer. Let it be a value. The following measurement conditions can be mentioned as an example of the measurement conditions. The magnetic layer surface roughness Ra shown in Examples described later is a value obtained by measurement under the following measurement conditions.
An area of 40 μm × 40 μm on the surface of the magnetic layer of the magnetic tape is measured using AFM (Nanoscope 4 manufactured by Veeco) in the tapping mode. As the probe, RTESS-300 manufactured by BRUKER is used, the scanning speed (probe moving speed) is 40 μm / sec, and the resolution is 512pixel × 512pixel.

The magnetic layer surface roughness Ra can be controlled by a known method. For example, the surface roughness Ra of the magnetic layer may change depending on the size of various powders (for example, ferromagnetic powder, non-magnetic powder that can be optionally contained, etc.) contained in the magnetic layer, manufacturing conditions of the magnetic recording medium, and the like. Therefore, by adjusting these, a magnetic recording medium having a magnetic layer surface roughness Ra of 1.0 nm or more and 1.6 nm or less can be obtained.

_{(Spacing difference before and} after washing with methyl ethyl ketone ( _After- S before))
_{The spacing difference (After-} S _{before) before and} after washing with methyl ethyl ketone measured by the optical interferometry on the surface of the magnetic layer of the magnetic recording medium is more than 0 nm and 15.0 nm or less. By spacing difference _{(S after -S before)} is equal to or less than 15.0 nm, the high the magnetic recording medium smoothness of the magnetic layer surface, electromagnetic from cold under high humidity due to temperature changes to high temperatures It is possible to suppress a decrease in conversion characteristics. From this point, the difference ( _After- S _before ) is 15.0 nm or less, preferably 14.0 nm or less, more preferably 13.0 nm or less, and further preferably 12.0 nm or less. It is preferably 11.0 nm or less, more preferably 10.0 nm or less, and even more preferably 10.0 nm or less. As will be described in detail later, the difference ( _After- S _before ) can be controlled by the surface treatment of the magnetic layer in the manufacturing process of the magnetic recording medium. However, as a result of the studies by the present inventors, if the surface treatment of the magnetic layer is carried out so that the _{spacing difference (Safter-} S _{Before) before and after washing with methyl ethyl ketone becomes 0 nm, the surface of the magnetic layer becomes highly smooth.} It has also been found that in a recording medium, it becomes difficult to suppress a decrease in electromagnetic conversion characteristics due to a temperature change from a low temperature to a high temperature under high humidity. The reason for this is not clear. As a guess, if _{the surface treatment of the magnetic layer is performed so that the spacing difference (After-} S _{before) before and} after washing with methyl ethyl ketone becomes 0 nm, the component (for example, lubricant) that contributes to the improvement of running stability becomes magnetic. The present inventors suspect that excessive removal from the recording medium may be one of the causes. _{From this point, the spacing difference (After-} S _{before) before and} after washing the magnetic recording medium with methyl ethyl ketone is more than 0 nm, preferably 1.0 nm or more, and more preferably 2.0 nm or more. It is more preferably 3.0 nm or more, and even more preferably 4.0 nm or more.

(Ferromagnetic powder)
As the ferromagnetic powder contained in the magnetic layer, one or a combination of two or more types of ferromagnetic powder known as the ferromagnetic powder used in the magnetic layer of various magnetic recording media can be used. It is preferable to use a ferromagnetic powder having a small average particle size from the viewpoint of improving the recording density. From this point, the average particle size of the ferromagnetic powder is preferably 50 nm or less, more preferably 45 nm or less, further preferably 40 nm or less, further preferably 35 nm or less, and more preferably 30 nm or less. It is even more preferably 25 nm or less, and even more preferably 20 nm or less. On the other hand, from the viewpoint of magnetization stability, the average particle size of the ferromagnetic powder is preferably 5 nm or more, more preferably 8 nm or more, further preferably 10 nm or more, and further preferably 15 nm or more. Is even more preferable, and 20 nm or more is even more preferable.

Hexagonal ferrite powder As a preferable specific example of the ferromagnetic powder, hexagonal ferrite powder can be mentioned. For details of the hexagonal ferrite powder, for example, paragraphs 0012 to 0030 of JP2011-225417A, paragraphs 0134 to 0136 of JP2011-216149A, paragraphs 0013 to 0030 of JP2012-204726 and the like. References can be made to paragraphs 0029 to 0084 of JP-A-2015-127985.

In the present invention and the present specification, the "hexagonal ferrite powder" refers to a ferromagnetic powder in which a hexagonal ferrite type crystal structure is detected as the main phase by X-ray diffraction analysis. The main phase refers to a structure to which the highest intensity diffraction peak belongs in the X-ray diffraction spectrum obtained by X-ray diffraction analysis. For example, when the highest intensity diffraction peak in the X-ray diffraction spectrum obtained by X-ray diffraction analysis belongs to the hexagonal ferrite type crystal structure, it is determined that the hexagonal ferrite type crystal structure is detected as the main phase. It shall be. When only a single structure is detected by X-ray diffraction analysis, this detected structure is used as the main phase. The hexagonal ferrite type crystal structure contains at least iron atoms, divalent metal atoms and oxygen atoms as constituent atoms. The divalent metal atom is a metal atom that can be a divalent cation as an ion, and examples thereof include an alkaline earth metal atom such as a strontium atom, a barium atom, and a calcium atom, and a lead atom. In the present invention and the present specification, the hexagonal strontium ferrite powder means that the main divalent metal atom contained in this powder is a strontium atom, and the hexagonal barium ferrite powder is the main contained in this powder. A ferrite atom is a barium atom. The main divalent metal atom is a divalent metal atom that occupies the largest amount on an atomic% basis among the divalent metal atoms contained in this powder. However, rare earth atoms are not included in the above divalent metal atoms. The "rare earth atom" in the present invention and the present specification is selected from the group consisting of a scandium atom (Sc), a yttrium atom (Y), and a lanthanoid atom. The lanthanoid atoms are lanthanum atom (La), cerium atom (Ce), placeodium atom (Pr), neodymium atom (Nd), promethium atom (Pm), samarium atom (Sm), uropyum atom (Eu), gadolinium atom (Gd) ), Terbium atom (Tb), dysprosium atom (Dy), formium atom (Ho), elbium atom (Er), thulium atom (Tm), ytterbium atom (Yb), and lutetium atom (Lu). To.

Hereinafter, the hexagonal strontium ferrite powder, which is one aspect of the hexagonal ferrite powder, will be described in more detail.

The activated volume of the hexagonal strontium ferrite powder is preferably in the range of ^{800 to 1600 nm 3.} The finely divided hexagonal strontium ferrite powder exhibiting an activated volume in the above range is suitable for producing a magnetic recording medium exhibiting excellent electromagnetic conversion characteristics. The activated volume of the hexagonal strontium ferrite powder is preferably 800 nm ³ or more, and can be, for example, 850 nm ³ or more. Further, from the viewpoint of further improvement of the electromagnetic conversion characteristics, activation volume of hexagonal strontium ferrite powder is more preferably 1500 nm ³ or less, further preferably 1400 nm ³ or less, and 1300 nm ³ or less Is even more preferably 1200 nm ³ or less, and even more preferably 1100 nm ³ or less. The same applies to the activated volume of the hexagonal barium ferrite powder.

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

Anisotropy constant Ku can be mentioned as an index for reducing thermal fluctuation, in other words, improving thermal stability. The hexagonal strontium ferrite powder can preferably have ^{a Ku of 1.8 × 10 5} J / m ³ or more, and more preferably a Ku of 2.0 × 10 ⁵ J / m ³ or more. Also, Ku of hexagonal strontium ferrite powder can be, for example, not 2.5 × ¹⁰ 5 J / ^{m 3} or less. However, the higher the Ku, the higher the thermal stability, which is preferable, and therefore, the value is not limited to the above-exemplified value.

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

When the hexagonal strontium ferrite powder contains rare earth atoms, the rare earth atom content (bulk content) is preferably in the range of 0.5 to 5.0 atom% with respect to 100 atom% of iron atoms. The fact that the rare earth atoms are contained in the bulk content in the above range and the rare earth atoms are unevenly distributed on the surface layer of the particles constituting the hexagonal strontium ferrite powder contributes to suppressing the decrease in the regeneration output in the repeated regeneration. Conceivable. This is because the hexagonal strontium ferrite powder contains rare earth atoms at a bulk content in the above range, and the rare earth atoms are unevenly distributed on the surface layer of the particles constituting the hexagonal strontium ferrite powder. It is presumed that this is because it can be enhanced. The higher the value of the anisotropy constant Ku, the more the occurrence of a phenomenon called thermal fluctuation can be suppressed (in other words, the thermal stability can be improved). By suppressing the occurrence of thermal fluctuation, it is possible to suppress a decrease in the reproduction output during repeated reproduction. The uneven distribution of rare earth atoms on the surface layer of the hexagonal strontium ferrite powder contributes to stabilizing the spin of iron (Fe) sites in the crystal lattice of the surface layer, which results in anisotropy constant Ku. It is speculated that it will increase.
In addition, it is presumed that the use of hexagonal strontium ferrite powder, which has uneven distribution on the surface layer of rare earth atoms, as the ferromagnetic powder of the magnetic layer also contributes to suppressing the surface of the magnetic layer from being scraped by sliding with the magnetic head. To. That is, it is presumed that the hexagonal strontium ferrite powder having uneven distribution of the rare earth atomic surface layer can also contribute to the improvement of the running durability of the magnetic recording medium. This is because the uneven distribution of rare earth atoms on the surface of the particles constituting the hexagonal strontium ferrite powder improves the interaction between the particle surface and the organic substances (for example, binder and / or additive) contained in the magnetic layer. As a result, it is presumed that the strength of the magnetic layer is improved.
The rare earth atom content (bulk content) is in the range of 0.5 to 4.5 atomic% from the viewpoint of further suppressing the decrease in the reproduction output in the repeated regeneration and / or further improving the running durability. It is more preferably in the range of 1.0 to 4.5 atomic%, further preferably in the range of 1.5 to 4.5 atomic%.

The bulk content is a content obtained by completely dissolving the hexagonal strontium ferrite powder. Unless otherwise specified, in the present invention and the present specification, the content of atoms means the bulk content obtained by completely dissolving hexagonal strontium ferrite powder. The hexagonal strontium ferrite powder containing a rare earth atom may contain only one kind of rare earth atom as a rare earth atom, or may contain two or more kinds of rare earth atoms. The bulk content when two or more kinds of rare earth atoms are contained is determined for the total of two or more kinds of rare earth atoms. This point is the same for the present invention and other components in the present specification. That is, unless otherwise specified, a certain component may be used alone or in combination of two or more. The content or content rate when two or more types are used shall mean the total of two or more types.

When the hexagonal strontium ferrite powder contains a rare earth atom, the rare earth atom contained may be any one or more of the rare earth atoms. Examples of the rare earth atom preferable from the viewpoint of further suppressing the decrease in the reproduction output in the repeated regeneration include a neodymium atom, a samarium atom, an ythrium atom and a disprosium atom, and a neodymium atom, a samarium atom and an yttrium atom are more preferable, and a neodymium atom is preferable. Atoms are more preferred.

In the hexagonal strontium ferrite powder having uneven distribution on the surface layer of rare earth atoms, the rare earth atoms may be unevenly distributed on the surface layer of the particles constituting the hexagonal strontium ferrite powder, and the degree of uneven distribution is not limited. For example, a hexagonal strontium ferrite powder having uneven distribution on the surface layer of a rare earth atom is partially dissolved under the dissolution conditions described later, and the content of the surface layer of the rare earth atom is completely dissolved under the dissolution conditions described later. The ratio of the atom to the bulk content, "surface layer content / bulk content" is more than 1.0, and can be 1.5 or more. When the "surface layer content / bulk content" is larger than 1.0, it means that the rare earth atoms are unevenly distributed (that is, more than the inside) in the particles constituting the hexagonal strontium ferrite powder. To do. In addition, the ratio of the surface layer content of rare earth atoms obtained by partial dissolution under the dissolution conditions described later to the bulk content of rare earth atoms obtained by total dissolution under the dissolution conditions described later, "Surface layer content / The "bulk content" can be, for example, 10.0 or less, 9.0 or less, 8.0 or less, 7.0 or less, 6.0 or less, 5.0 or less, or 4.0 or less. However, in the hexagonal strontium ferrite powder having uneven distribution on the surface layer of rare earth atoms, the rare earth atoms need only be unevenly distributed on the surface layer of the particles constituting the hexagonal strontium ferrite powder. The "rate" is not limited to the upper or lower limit illustrated.

The partial dissolution and total dissolution of the hexagonal strontium ferrite powder will be described below. For the hexagonal strontium ferrite powder that exists as a powder, the sample powder that is partially or completely dissolved is collected from the same lot of powder. On the other hand, regarding the hexagonal strontium ferrite powder contained in the magnetic layer of the magnetic recording medium, a part of the hexagonal strontium ferrite powder taken out from the magnetic layer is subjected to partial dissolution, and the other part is subjected to total dissolution. .. The hexagonal strontium ferrite powder can be taken out from the magnetic layer by, for example, the method described in paragraph 0032 of JP2015-91747A.
The above-mentioned partial dissolution means that the hexagonal strontium ferrite powder is dissolved in the liquid to the extent that the residue can be visually confirmed at the end of the dissolution. For example, by partial dissolution, 10 to 20% by mass of the particles constituting the hexagonal strontium ferrite powder can be dissolved with the whole particles as 100% by mass. On the other hand, the above-mentioned total dissolution means that the hexagonal strontium ferrite powder is dissolved in the liquid until the residue is not visually confirmed at the end of the dissolution.
The partial dissolution and the measurement of the surface layer content are carried out by, for example, the following methods. However, the following dissolution conditions such as the amount of sample powder are examples, and dissolution conditions capable of partial dissolution and total dissolution can be arbitrarily adopted.
A container (for example, a beaker) containing 12 mg of sample powder and 10 mL of 1 mol / L hydrochloric acid is held on a hot plate at a set temperature of 70 ° C. for 1 hour. The obtained solution is filtered through a 0.1 μm membrane filter. Elemental analysis of the filtrate thus obtained is performed by an inductively coupled plasma (ICP) analyzer. In this way, the surface layer content of rare earth atoms with respect to 100 atomic% of iron atoms can be determined. When a plurality of rare earth atoms are detected by elemental analysis, the total content of all rare earth atoms is defined as the surface layer content. This point is the same in the measurement of bulk content.
On the other hand, the total dissolution and the measurement of the bulk content are carried out by, for example, the following methods.
A container (for example, a beaker) containing 12 mg of sample powder and 10 mL of 4 mol / L hydrochloric acid is held on a hot plate at a set temperature of 80 ° C. for 3 hours. After that, the same procedure as the above-mentioned partial dissolution and measurement of the surface layer content can be performed to determine the bulk content with respect to 100 atomic% of iron atoms.

From the viewpoint of increasing the reproduction output when reproducing the data recorded on the magnetic recording medium, it is desirable that the mass magnetization σs of the ferromagnetic powder contained in the magnetic recording medium is high. In this regard, the hexagonal strontium ferrite powder containing rare earth atoms but not having uneven distribution on the surface layer of rare earth atoms tended to have a significantly lower σs than the hexagonal strontium ferrite powder containing no rare earth atoms. On the other hand, hexagonal strontium ferrite powder having uneven distribution on the surface layer of rare earth atoms is considered to be preferable in order to suppress such a large decrease in σs. In one aspect, the σs of the hexagonal strontium ferrite powder ^{can be 45 A · m 2} / kg or more, and can also be 47 A · m ^{2 / kg or more.} Meanwhile, [sigma] s from the viewpoint of noise reduction, is preferably not more than 80A ^· m 2 / kg, more preferably not more than 60A ^· m 2 / kg. σs can be measured using a known measuring device capable of measuring magnetic characteristics such as a vibrating sample magnetometer. Unless otherwise specified, in the present invention and the present specification, the mass magnetization σs is a value measured at a magnetic field strength of 15 kOe. 1 [koe] = 10 ⁶ / 4π [A / m].

Regarding the content of constituent atoms (bulk content) of the hexagonal strontium ferrite powder, the strontium atom content can be in the range of, for example, 2.0 to 15.0 atom% with respect to 100 atom% of iron atoms. .. In one aspect, the hexagonal strontium ferrite powder can contain only strontium atoms as divalent metal atoms contained in the powder. In another aspect, the hexagonal strontium ferrite powder can also contain one or more other divalent metal atoms in addition to the strontium atom. For example, it can contain barium and / or calcium atoms. When a divalent metal atom other than the strontium atom is contained, the barium atom content and the calcium atom content in the hexagonal strontium ferrite powder are, for example, 0.05 to 5 with respect to 100 atomic% of iron atoms, respectively. It can be in the range of 0.0 atomic%.

As the crystal structure of hexagonal ferrite, a magnetoplumbite type (also referred to as "M type"), a W type, a Y type, and a Z type are known. The hexagonal strontium ferrite powder may have any crystal structure. The crystal structure can be confirmed by X-ray diffraction analysis. The hexagonal strontium ferrite powder can be one in which a single crystal structure or two or more kinds of crystal structures are detected by X-ray diffraction analysis. For example, in one aspect, the hexagonal strontium ferrite powder can be such that only the M-type crystal structure is detected by X-ray diffraction analysis. For example, M-type hexagonal ferrite is represented by the composition formula of _{AFe 12} O _19. Here, A represents a divalent metal atom, and when the hexagonal strontium ferrite powder is M-type, A is only a strontium atom (Sr), or when A contains a plurality of divalent metal atoms. As mentioned above, the strontium atom (Sr) occupies the largest amount on the basis of atomic%. The divalent metal atom content of the hexagonal strontium ferrite powder is usually determined by the type of crystal structure of the hexagonal ferrite, and is not particularly limited. The same applies to the iron atom content and the oxygen atom content. The hexagonal strontium ferrite powder contains at least iron atoms, strontium atoms and oxygen atoms, and can also contain rare earth atoms. Further, the hexagonal strontium ferrite powder may or may not contain atoms other than these atoms. As an example, the hexagonal strontium ferrite powder may contain an aluminum atom (Al). The content of aluminum atoms can be, for example, 0.5 to 10.0 atomic% with respect to 100 atomic% of iron atoms. From the viewpoint of further suppressing the decrease in regeneration output during repeated regeneration, the hexagonal strontium ferrite powder contains iron atoms, strontium atoms, oxygen atoms and rare earth atoms, and the content of atoms other than these atoms is 100 iron atoms. % Is preferably 10.0 atomic% or less, more preferably in the range of 0 to 5.0 atomic%, and may be 0 atomic%. That is, in one aspect, the hexagonal strontium ferrite powder does not have to contain atoms other than iron atoms, strontium atoms, oxygen atoms and rare earth atoms. The content expressed in atomic% above is the content (unit: mass%) of each atom obtained by completely dissolving the hexagonal strontium ferrite powder, and is converted to the value expressed in atomic% using the atomic weight of each atom. Calculated by conversion. Further, in the present invention and the present specification, "not contained" for a certain atom means that the content is completely dissolved and the content measured by an ICP analyzer is 0% by mass. The detection limit of an ICP analyzer is usually 0.01 ppm (parts per million) or less on a mass basis. The above-mentioned "not included" is used in the sense of including the inclusion in an amount less than the detection limit of the ICP analyzer. In one aspect, the hexagonal strontium ferrite powder can be free of bismuth atoms (Bi).

Metal powder A preferred specific example of the ferromagnetic powder may be a ferromagnetic metal powder. For details of the ferromagnetic metal powder, for example, paragraphs 0137 to 0141 of JP2011-216149A and paragraphs 0009 to 0023 of JP2005-251351 can be referred to.

ε-Iron oxide powder A preferable specific example of the ferromagnetic powder is ε-iron oxide powder. In the present invention and the present specification, "ε-iron oxide powder" refers to a ferromagnetic powder in which an ε-iron oxide type crystal structure is detected as a main phase by X-ray diffraction analysis. For example, when the highest intensity diffraction peak in the X-ray diffraction spectrum obtained by X-ray diffraction analysis is assigned to the ε-iron oxide type crystal structure, the ε-iron oxide type crystal structure is detected as the main phase. Judgment shall be made. As a method for producing ε-iron oxide powder, a method for producing ε-iron oxide powder, a reverse micelle method, and the like are known. All of the above manufacturing methods are known. Regarding the method for producing ε-iron oxide powder in which a part of Fe is substituted with a substituted atom such as Ga, Co, Ti, Al, Rh, for example, J.I. Jpn. Soc. Powder Metallurgy Vol. 61 Supplement, No. S1, pp. S280-S284, J. Mol. Mater. Chem. C, 2013, 1, pp. 5200-5206 and the like can be referred to. However, the method for producing ε-iron oxide powder that can be used as the ferromagnetic powder in the magnetic layer of the magnetic recording medium is not limited to the methods described here.

The activated volume of the ε-iron oxide powder is preferably in the range of ^{300 to 1500 nm 3.} The finely divided ε-iron oxide powder exhibiting an activated volume in the above range is suitable for producing a magnetic recording medium exhibiting excellent electromagnetic conversion characteristics. The activated volume of the ε-iron oxide powder is preferably 300 nm ³ or more, and can be, for example, 500 nm ³ or more. Further, from the viewpoint further improvement of the electromagnetic conversion characteristics, .epsilon. activation volume of the iron oxide powder is more preferably 1400 nm ³ or less, further preferably 1300 nm ³ or less, and 1200 nm ³ or less Is even more preferable, and 1100 nm ³ or less is even more preferable.

Anisotropy constant Ku can be mentioned as an index for reducing thermal fluctuation, in other words, improving thermal stability. The ε-iron oxide powder can preferably have ^{a Ku of 3.0 × 10 4} J / m ³ or more, and more preferably 8.0 × 10 ⁴ J / m ³ or more. Further, .epsilon. Ku iron oxide powder can be, for example, not 3.0 × ¹⁰ 5 J / ^{m 3} or less. However, the higher the Ku, the higher the thermal stability, which is preferable, and therefore, the value is not limited to the above-exemplified value.

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

Unless otherwise specified in the present invention and the present specification, the average particle size of various powders such as ferromagnetic powders is a value measured by the following method using a transmission electron microscope.
The powder is photographed using a transmission electron microscope at an imaging magnification of 100,000 times, and printed on photographic paper so as to have a total magnification of 500,000 times to obtain a photograph of the particles constituting the powder. Select the target particle from the obtained photograph of the particle, trace the outline of the particle with a digitizer, and measure the size of the particle (primary particle). Primary particles are independent particles without agglomeration.
The above measurements are performed on 500 randomly sampled particles. The arithmetic mean of the particle sizes of the 500 particles thus obtained is taken as the average particle size of the powder. As the transmission electron microscope, for example, Hitachi's transmission electron microscope H-9000 can be used. Further, the particle size can be measured by using a known image analysis software, for example, an image analysis software KS-400 manufactured by Carl Zeiss. Unless otherwise specified, the average particle size shown in the examples described later was measured using a transmission electron microscope H-9000 manufactured by Hitachi as a transmission electron microscope and a Carl Zeiss image analysis software KS-400 as an image analysis software. The value. In the present invention and the present specification, the powder means an aggregate of a plurality of particles. For example, a ferromagnetic powder means a collection of a plurality of ferromagnetic particles. Further, the set of a plurality of particles is not limited to a mode in which the particles constituting the set are in direct contact with each other, and also includes a mode in which a binder, an additive or the like described later is interposed between the particles. To. The term particle is sometimes used to describe powder.

As a method for collecting sample powder from a magnetic recording medium for particle size measurement, for example, the method described in paragraph 0015 of JP2011-048878A can be adopted.

Unless otherwise specified in the present invention and the present specification, the size (particle size) of the particles constituting the powder is the shape of the particles observed in the above particle photograph.
(1) In the case of needle-shaped, spindle-shaped, columnar (however, the height is larger than the maximum major axis of the bottom surface), it is represented by the length of the major axis constituting the particle, that is, the major axis length.
(2) If it is plate-shaped or columnar (however, the thickness or height is smaller than the maximum major axis of the plate surface or bottom surface), it is represented by the maximum major axis of the plate surface or bottom surface.
(3) If the particle is spherical, polyhedral, unspecified, etc., and the long axis constituting the particle cannot be specified from the shape, it is represented by the diameter equivalent to a circle. The equivalent diameter of a circle is what is obtained by the circular projection method.

Further, for the average needle-like ratio of the powder, the length of the minor axis of the particles, that is, the minor axis length is measured in the above measurement, and the value of (major axis length / minor axis length) of each particle is obtained, and the above 500 particles are obtained. Refers to the arithmetic average of the values obtained for a particle. Here, unless otherwise specified, the minor axis length is the length of the minor axis constituting the particle in the case of (1) in the above definition of the particle size, and the thickness or height in the case of the same (2). In the case of (3), there is no distinction between the major axis and the minor axis, so (major axis length / minor axis length) is regarded as 1 for convenience.
Unless otherwise specified, when the shape of the particles is specific, for example, in the case of the above definition of particle size (1), the average particle size is the average major axis length, and in the case of the same definition (2), the average particle size is Average plate diameter. In the case of the same definition (3), the average particle size is an average diameter (also referred to as an average particle size or an average particle size).

The content (filling rate) of the ferromagnetic 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 of the magnetic layer other than the ferromagnetic powder are at least a binder and may optionally include one or more additional additives. A high filling rate of the ferromagnetic powder in the magnetic layer is preferable from the viewpoint of improving the recording density.

(Binder, hardener)
The magnetic recording medium is a coating type magnetic recording medium, and the magnetic layer contains a binder. A binder is one or more resins. As the binder, various resins usually used as a binder for a coating type magnetic recording medium can be used. For example, as the binder, polyurethane resin, polyester resin, polyamide resin, vinyl chloride resin, styrene, acrylonitrile, acrylic resin obtained by copolymerizing methyl methacrylate and the like, cellulose resin such as nitrocellulose, epoxy resin, phenoxy resin, polyvinyl acetal, etc. A resin selected from a polyvinyl alkyral resin such as polyvinyl butyral can be used alone, or a plurality of resins can be mixed and used. Of these, polyurethane resins, acrylic resins, cellulose resins, and vinyl chloride resins are preferred. These resins may be homopolymers or copolymers. These resins can also be used as a binder in the non-magnetic layer and / or the backcoat layer described later.
For the above binder, paragraphs 0028 to 0031 of JP-A-2010-24113 can be referred to. The average molecular weight of the resin used as the binder can be, for example, 10,000 or more and 200,000 or less as the weight average molecular weight. The weight average molecular weight in the present invention and the present specification is a value obtained by converting a value measured by gel permeation chromatography (GPC) under the following measurement conditions into polystyrene. The weight average molecular weight of the binder shown in Examples described later is a value obtained by converting a value measured under the following measurement conditions into polystyrene.
GPC device: HLC-8120 (manufactured by Tosoh)
Column: TSK gel Multipore HXL-M (manufactured by Tosoh, 7.8 mm ID (Inner Diameter) x 30.0 cm)
Eluent: tetrahydrofuran (THF)

In one aspect, a binder containing an active hydrogen-containing group can be used as the binder. The "active hydrogen-containing group" in the present invention and the present specification refers to a functional group capable of forming a crosslinked structure by a curing reaction of this group with a curable functional group and the desorption of hydrogen atoms contained in this group. Say. Examples of the active hydrogen-containing group include a hydroxy group, an amino group (preferably a primary amino group or a secondary amino group), a mercapto group, a carboxy group and the like, and a hydroxy group, an amino group and a mercapto group are preferable. Is more preferable. In the binder containing an active hydrogen-containing group, the concentration of the active hydrogen-containing group is preferably in the range of 0.10 meq / g to 2.00 meq / g. Note that eq is an equivalent and is a unit that cannot be converted into SI units. The active hydrogen-containing group concentration can also be expressed in units of "mgKOH / g". In one aspect, in the resin containing active hydrogen-containing groups, the concentration of active hydrogen-containing groups is preferably in the range of 1 to 20 mgKOH / g.

In one aspect, a binder containing an acidic group can be used as the binder. The term "acidic group" as used in the present invention and the present specification is used to include the form of a group capable of releasing ^{H + into an anion and a salt thereof by releasing H + in water or a solvent containing water (aqueous solvent).} And. Specific examples of the acidic group include a sulfonic acid group _(-SO 3 H), sulfuric acid group (-OSO ₃ H), carboxyl group, phosphoric acid group, can be exemplified embodiment thereof and the like salts. For example, the salt form of a sulfonic acid group (-SO _{3 H) is a group represented by -SO 3} M, which represents an atom (eg, an alkali metal atom) in which M can be a cation in water or an aqueous solvent. means. This point is the same for the salt forms of the various groups described above. As an example of the binder containing an acidic group, for example, a resin containing at least one acidic group selected from the group consisting of a sulfonic acid group and a salt thereof (for example, polyurethane resin, vinyl chloride resin, etc.) can be mentioned. However, the resin contained in the magnetic layer is not limited to these resins. Further, in the binder containing an acidic group, the acidic group content can be in the range of, for example, 0.03 to 0.50 meq / g. The content of various functional groups such as acidic groups contained in the resin can be determined by a known method depending on the type of functional group. The binder can be used in the composition for forming a magnetic layer in an amount of, for example, 1.0 to 30.0 parts by mass with respect to 100.0 parts by mass of the ferromagnetic powder.

Further, a curing agent can be used together with a resin that can be used as a binder. The curing agent can be a thermosetting compound which is a compound in which a curing reaction (crosslinking reaction) proceeds by heating in one aspect, and a photocuring agent in which a curing reaction (crosslinking reaction) proceeds by light irradiation in another aspect. It can be a sex compound. As the curing reaction proceeds in the process of forming the magnetic layer, at least a part of the curing agent can be contained in the magnetic layer in a state of reacting (crosslinking) with other components such as a binder. This point is the same for the layer formed by using this composition when the composition used for forming another layer contains a curing agent. Preferred curing agents are thermosetting compounds, with polyisocyanates being preferred. For details of the polyisocyanate, refer to paragraphs 0124 to 0125 of JP2011-216149A. 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 the magnetic layer, preferably 50.0 to 80.0 from the viewpoint of improving the strength of the magnetic layer. It can be used in the amount of parts by mass.

(Additive)
The magnetic layer contains a ferromagnetic powder and a binder, and may contain one or more additives, if necessary. Examples of the additive include the above-mentioned curing agent. Examples of the additive contained in the magnetic layer include non-magnetic powder (for example, inorganic powder, carbon black, etc.), lubricant, dispersant, dispersion aid, fungicide, antistatic agent, antioxidant, and the like. Can be done.

For example, examples of the lubricant include fatty acids, fatty acid esters and fatty acid amides, and one or more selected from the group consisting of fatty acids, fatty acid esters and fatty acid amides can be used to form a magnetic layer.
Examples of the fatty acid include lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, linoleic acid, linolenic acid, bechenic acid, erucic acid, ellagic acid and the like, and stearic acid, myristic acid, palmitic acid and the like. Is preferable, and stearic acid is more preferable. The fatty acid may be contained in the magnetic layer in the form of a salt such as a metal salt.
Examples of the fatty acid ester include esters of lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, linoleic acid, linolenic acid, behenic acid, erucic acid, elaidic acid and the like. Specific examples include, for example, butyl myristate, butyl palmitate, butyl stearate, neopentyl glycol dioleate, sorbitan monostearate, sorbitan distearate, sorbitan tristearate, oleyl oleate, isosetyl stearate, stearic acid. Examples thereof include isotridecyl acid acid, octyl stearate, isooctyl stearate, amyl stearate, butoxyethyl stearate and the like.
Examples of the fatty acid amide include amides of the above-mentioned various fatty acids, such as lauric acid amide, myristic acid amide, palmitic acid amide, and stearic acid amide.
The fatty acid content in the composition for forming a magnetic layer is, for example, 0 to 10.0 parts by mass, preferably 0.1 to 10.0 parts by mass, and more preferably 0.1 to 10.0 parts by mass, per 100.0 parts by mass of the ferromagnetic powder. It is 1.0 to 7.0 parts by mass. The fatty acid ester content in the composition for forming a magnetic layer is, for example, 0.1 to 10.0 parts by mass, preferably 1.0 to 7.0 parts by mass, per 100.0 parts by mass of the ferromagnetic powder. The fatty acid amide content in the composition for forming a magnetic layer is, for example, 0 to 3.0 parts by mass, preferably 0 to 2.0 parts by mass, and more preferably 0, per 100.0 parts by mass of the ferromagnetic powder. ~ 1.0 parts by mass.
When the magnetic recording medium has a non-magnetic layer between the non-magnetic support and the magnetic layer, the fatty acid content in the composition for forming the non-magnetic layer is, for example, 0 per 100.0 parts by mass of the non-magnetic powder. It is ~ 10.0 parts by mass, preferably 1.0 to 10.0 parts by mass, and more preferably 1.0 to 7.0 parts by mass. The fatty acid ester content in the composition for forming a non-magnetic layer is, for example, 0 to 10.0 parts by mass, preferably 0.1 to 8.0 parts by mass, per 100.0 parts by mass of the non-magnetic powder. The fatty acid amide content in the composition for forming a non-magnetic layer is, for example, 0 to 3.0 parts by mass, preferably 0 to 1.0 parts by mass, per 100.0 parts by mass of the non-magnetic powder.
Unless otherwise specified in the present invention and the present specification, a certain component may be used alone or in combination of two or more. The content when two or more kinds of a certain component are used means the total content of these two or more kinds.

Further, the non-magnetic powder used for forming the magnetic layer is a non-magnetic powder that can function as a polishing agent, and a non-magnetic powder that can function as a protrusion forming agent that forms protrusions that appropriately protrude on the surface of the magnetic layer. Examples thereof include magnetic powder (for example, non-magnetic colloidal particles). The average particle size of colloidal silica (silica colloidal particles) shown in Examples described later is a value obtained by the method described as a method for measuring the average particle size in paragraph 0015 of JP2011-048878A. .. As an example of an additive that can be used for a magnetic layer containing an abrasive, the dispersants described in paragraphs 0012 to 0022 of JP2013-131285A are mentioned as dispersants for improving the dispersibility of the abrasive. be able to. For the dispersant for improving the dispersibility of the ferromagnetic powder, refer to paragraph 0035 of JP-A-2017-016721. Further, regarding the dispersant, paragraphs 0061 and 0071 of JP2012-133837A can also be referred to. Regarding the additive of the magnetic layer, paragraphs 0035 to 0077 of JP-A-2016-51493 can also be referred to.
The dispersant may be contained in the non-magnetic layer. For the dispersant that can be contained in the non-magnetic layer, paragraph 0061 of JP2012-133387A can be referred to.
Various additives can be used in any amount by appropriately selecting a commercially available product according to desired properties or by producing by a known method.

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

<Non-magnetic layer>
Next, the non-magnetic layer will be described. The magnetic recording medium may have a magnetic layer directly on the non-magnetic support, and may have a non-magnetic layer containing a non-magnetic powder and a binder between the non-magnetic support and the magnetic layer. May be good. The non-magnetic powder used for the non-magnetic layer may be an inorganic substance powder or an organic substance powder. In addition, carbon black or the like can also be used. Examples of the inorganic substance include metals, metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, metal sulfides and the like. These non-magnetic powders are commercially available and can also be produced by known methods. For details thereof, refer to paragraphs 0146 to 0150 of JP2011-216149A. For carbon black that can be used for the non-magnetic layer, paragraphs 0040 to 0041 of JP2010-24113A can also be referred to. The content (filling rate) of the non-magnetic powder in the non-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.

For other details such as binders and additives for the non-magnetic layer, known techniques relating to the non-magnetic layer can be applied. Further, for example, with respect to 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.

In the present invention and the present specification, the non-magnetic layer includes not only the non-magnetic powder but also a substantially non-magnetic layer containing a small amount of ferromagnetic powder, for example, as an impurity or intentionally. Here, the substantially non-magnetic layer means that the residual magnetic flux density of this layer is 10 mT or less, the coercive force is 7.96 kA / m (100 Oe) or less, or the residual magnetic flux density is 10 mT or less. It refers to a layer having a coercive force of 7.96 kA / m (100 Oe) or less. The non-magnetic layer preferably has no residual magnetic flux density and coercive force.

<Non-magnetic support>
Next, the non-magnetic support will be described. Examples of the non-magnetic support (hereinafter, also simply referred to as “support”) include known biaxially stretched polyethylene terephthalates, polyethylene naphthalates, polyamides, polyamideimides, aromatic polyamides and the like. Among these, polyethylene terephthalate, polyethylene naphthalate, and polyamide are preferable. These supports may be subjected to corona discharge, plasma treatment, easy adhesion treatment, heat treatment and the like in advance.

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

<Various thickness>
Regarding the thickness of the non-magnetic support and each layer in the magnetic recording medium, the thickness of the non-magnetic support is, for example, 3.0 to 80.0 μm, preferably in the range of 3.0 to 50.0 μm, more preferably. Is in the range of 3.0 to 10.0 μm.

The thickness of the magnetic layer can be optimized depending on the saturation magnetization amount of the magnetic head used, the head gap length, and the band of the recording signal. For example, it is 10 nm to 100 nm, and is preferably 20 to 90 nm from the viewpoint of high-density recording. The range is more preferably 30 to 70 nm. The magnetic layer may be at least one layer, and the magnetic layer may be separated into two or more layers having different magnetic characteristics, and a known configuration relating to a multi-layer magnetic layer can be applied. The thickness of the magnetic layer when separated into two or more layers is the total thickness of these layers.

The thickness of the non-magnetic 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 non-magnetic 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, more preferably 0.1 to 0.7 μm.

The thickness of each layer of the magnetic recording medium and the non-magnetic support can be determined by a known film thickness measuring method. As an example, for example, a cross section in the thickness direction of a magnetic recording medium is exposed by a known method such as an ion beam or a microtome, and then the cross section is observed with a scanning electron microscope in the exposed cross section. Various thicknesses can be obtained as the arithmetic mean of the thickness obtained at any one place in the cross-sectional observation, or the thickness obtained at two or more randomly selected places, for example, two places. Alternatively, the thickness of each layer may be obtained as a design thickness calculated from the manufacturing conditions.

<Manufacturing method>
(Preparation of composition for forming each layer)
The composition for forming the magnetic layer, the non-magnetic layer and the backcoat layer usually contains a solvent together with the various components described above. As the solvent, various organic solvents generally used for producing a coating type magnetic recording medium can be used. The amount of the solvent in each layer-forming composition is not particularly limited, and can be the same as that of each layer-forming composition of a normal coating type magnetic recording medium. The step of preparing the composition for forming each layer usually includes at least a kneading step, a dispersion step, and a mixing step provided before and after these steps as necessary. Each step may be divided into two or more steps. All raw materials used in the present invention may be added at the beginning or in the middle of any step. Further, each raw material may be added separately in two or more steps.

Known techniques can be used to prepare the composition for forming each layer. In the kneading step, it is preferable to use a kneader having a strong kneading force such as an open kneader, a continuous kneader, a pressurized kneader, and an extruder. Details of these kneading treatments are described in JP-A-1-106338 and JP-A-1-79274. Further, in order to disperse each layer-forming composition, one or more dispersed beads selected from the group consisting of glass beads and other dispersed beads can be used as the dispersion medium. As such dispersed beads, zirconia beads, titania beads, and steel beads, which are dispersed beads having a high specific gravity, are suitable. The particle size (bead diameter) and filling rate of these dispersed beads can be optimized and used. A known disperser can be used. Each layer-forming composition may be filtered by a known method before being subjected to the coating step. Filtration can be performed, for example, by filter filtration. As the filter used for filtration, for example, a filter having a pore size of 0.01 to 3 μm (for example, a glass fiber filter, a polypropylene filter, etc.) can be used.

(Applying process)
The magnetic layer can be formed by, for example, directly applying the composition for forming a magnetic layer onto a non-magnetic support, or by applying multiple layers sequentially or simultaneously with the composition for forming a non-magnetic layer. The backcoat layer can be formed by applying the backcoat layer forming composition to the side opposite to the side having the magnetic layer of the non-magnetic support (or the magnetic layer is provided later). For details of the coating for forming each layer, refer to paragraph 0051 of JP-A-2010-24113.

(Other processes)
After the coating step, various treatments such as a drying treatment, a magnetic layer orientation treatment, and a surface smoothing treatment (calendar treatment) can be performed. For various steps, paragraphs 0052 to 0057 of JP-A-2010-24113 can be referred to.

It is preferable to heat-treat the coating layer formed by coating the composition for forming the magnetic layer at an arbitrary stage after the coating step of the composition for forming the magnetic layer. This heat treatment can be carried out, for example, before and / or after the calendar treatment. The heat treatment can be carried out, for example, by placing the support on which the coating layer of the composition for forming a magnetic layer is formed in a heating atmosphere. The heating atmosphere can be an atmosphere having an atmospheric temperature of 65 to 90 ° C., and more preferably an atmosphere having an atmospheric temperature of 65 to 75 ° C. This atmosphere can be, for example, an atmospheric atmosphere. The heat treatment in a heating atmosphere can be carried out, for example, for 20 to 50 hours. In one aspect, this heat treatment can allow the curing reaction of the curable functional groups of the curing agent to proceed.

(One aspect of a preferable manufacturing method)
A preferred method for producing the magnetic recording medium includes wiping the surface of the magnetic layer with a wiping material impregnated with methyl ethyl ketone (hereinafter, also referred to as “methyl ethyl ketone wiping treatment”), preferably after the heat treatment. The method can be mentioned. As described above, the fact that the components that can be removed by this methyl ethyl ketone wiping treatment are present on the surface of the magnetic layer is that the moisture on the surface of the magnetic layer when the temperature changes from low temperature to high temperature under high humidity It is thought to promote adhesion. The methyl ethyl ketone wiping treatment may be carried out by using a wiping material impregnated with methyl ethyl ketone in place of the wiping material used in the dry wiping treatment, in accordance with the dry wiping treatment generally performed in the manufacturing process of the magnetic recording medium. it can. For example, for a tape-shaped magnetic recording medium (magnetic tape), the magnetic tape is run between a feeding roller and a take-up roller after or before slitting the magnetic tape to a width to be accommodated in the magnetic tape cartridge. By pressing a wiping material (for example, cloth (for example, non-woven fabric) or paper (for example, tissue paper)) impregnated with methyl ethyl ketone on the surface of the magnetic layer of the running magnetic tape, the surface of the magnetic layer can be wiped off. .. The traveling speed of the magnetic tape and the tension applied in the longitudinal direction of the surface of the magnetic layer in the above traveling (hereinafter, simply referred to as "tension") are generally adopted in the dry wiping process generally performed in the manufacturing process of the magnetic recording medium. It can be the same as the processing conditions that have been set. For example, the traveling speed of the magnetic tape in the methyl ethyl ketone wiping process can be about 60 to 600 m / min, and the tension can be about 0.196 to 3.920 N (Newton). In addition, the methyl ethyl ketone wiping treatment can be performed at least once. As described above, if _{the surface treatment of the magnetic layer is performed so that the spacing difference (Safter} − S _{before) before and} after washing with methyl ethyl ketone becomes 0 nm, the surface of the magnetic layer is highly smooth in the magnetic recording medium. Since it is difficult to suppress the deterioration of the electromagnetic conversion characteristics due to the temperature change from low temperature to high temperature under high humidity, it is necessary to set the treatment conditions and the number of treatments for the methyl ethyl ketone wiping treatment in consideration of this point. preferable.
Further, before and / or after the methyl ethyl ketone wiping treatment, the surface of the magnetic layer is subjected to a polishing treatment and / or a dry wiping treatment generally performed in the manufacturing process of the coating type magnetic recording medium (hereinafter, these are referred to as “dry surface treatment”). ) Can be performed more than once. According to the dry surface treatment, it is possible to remove foreign substances generated in the manufacturing process such as chips generated by slits and adhering to the surface of the magnetic layer.
In the above, a tape-shaped magnetic recording medium (magnetic tape) has been described as an example. With respect to the disk-shaped magnetic recording medium (magnetic disk), various processes can be performed with reference to the above description.

The magnetic recording medium according to one aspect of the present invention described above can be, for example, a tape-shaped magnetic recording medium (magnetic tape). Magnetic tapes are usually housed in magnetic tape cartridges, distributed and used. Information to the magnetic tape is obtained by mounting the magnetic tape cartridge on the magnetic recording / playback device, running the magnetic tape in the magnetic recording / playback device, bringing the magnetic tape surface (magnetic layer surface) into contact with the magnetic head, and sliding the magnetic tape. Can be recorded and played back. However, the magnetic recording medium according to one aspect of the present invention is not limited to the magnetic tape. As various magnetic recording media (magnetic tape, disk-shaped magnetic recording medium (magnetic disk), etc.) used in the sliding magnetic recording / reproducing device, the magnetic recording medium according to one aspect of the present invention is suitable. The above-mentioned sliding type device refers to a device in which the surface of the magnetic layer and the head come into contact with each other and slide when recording information on a magnetic recording medium and / or reproducing the recorded information.

In the magnetic recording medium manufactured as described above, a servo pattern is formed by a known method in order to enable tracking control of the magnetic head in the magnetic recording / playback device, control of the traveling speed of the magnetic recording medium, and the like. Can be done. "Formation of servo pattern" can also be referred to as "recording of servo signal". The magnetic recording medium may be a tape-shaped magnetic recording medium (magnetic tape) or a disk-shaped magnetic recording medium (magnetic disk). In the following, the formation of a servo pattern will be described using a magnetic tape as an example.

The servo pattern is usually formed along the longitudinal direction of the magnetic tape. Examples of the control (servo control) method using the servo signal include timing-based servo (TBS), amplifier servo, frequency servo, and the like.

As shown in ECMA (European Computer Manufacturers Association) -319, a timing-based servo system is adopted in a magnetic tape (generally called "LTO tape") conforming to the LTO (Linear Tape-Open) standard. In this timing-based servo system, the servo pattern is composed of a pair of magnetic stripes (also referred to as "servo stripes") that are non-parallel to each other and are continuously arranged in the longitudinal direction of the magnetic tape. As described above, the reason why the servo pattern is composed of a pair of magnetic stripes that are non-parallel to each other is to teach the passing position to the servo signal reading element passing on the servo pattern. Specifically, the pair of magnetic stripes are formed so that their intervals change continuously along the width direction of the magnetic tape, and the servo signal reading element reads the intervals to obtain a servo pattern. And the relative position of the servo signal reading element can be known. This relative position information allows tracking of the data track. Therefore, a plurality of servo tracks are usually set on the servo pattern along the width direction of the magnetic tape.

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

Further, in one aspect, as shown in Japanese Patent Application Laid-Open No. 2004-318983, each servo band has information indicating a servo band number (“servo band ID (identification)” or “UDIM (Unique DataBand Identification)”. Method) information ”) is embedded. The servo band ID is recorded by shifting a specific one of a plurality of pairs of servo stripes in the servo band so that the position thereof is displaced relative to the longitudinal direction of the magnetic tape. Specifically, the method of shifting a specific one of a plurality of pairs of servo stripes is changed for each servo band. As a result, the recorded servo band ID becomes unique for each servo band, so that the servo band can be uniquely identified by simply reading one servo band with the servo signal reading element.

As a method for uniquely specifying the servo band, there is also a method using a staggered method as shown in ECMA-319. In this staggered method, a group of a pair of magnetic stripes (servo stripes) that are continuously arranged in the longitudinal direction of the magnetic tape and are not parallel to each other are recorded so as to be shifted in the longitudinal direction of the magnetic tape for each servo band. To do. Since this combination of shifting methods between adjacent servo bands is unique to the entire magnetic tape, it is possible to uniquely identify the servo band when reading the servo pattern by the two servo signal reading elements. It is possible.

Further, as shown in ECMA-319, information indicating the position of the magnetic tape in the longitudinal direction (also referred to as "LPOS (Longitudinal Position) information") is usually embedded in each servo band. This LPOS information, like the UDIM information, is also recorded by shifting the positions of the pair of servo stripes in the longitudinal direction of the magnetic tape. However, unlike the UDIM information, in this LPOS information, the same signal is recorded in each servo band.

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

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

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

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

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

In the present invention and the present specification, the "magnetic recording / reproducing device" means an apparatus capable of recording information on a magnetic recording medium and reproducing information recorded on the magnetic recording medium. To do. Such a device is commonly referred to as a drive. The magnetic recording / reproducing device can be a sliding type magnetic recording / reproducing device. The magnetic head included in the magnetic recording / reproducing device can be a recording head capable of recording information on a magnetic recording medium, and can reproduce information recorded on the magnetic recording medium. Can also be. Further, in one aspect, the magnetic recording / reproducing device may include both a recording head and a reproducing head as separate magnetic heads. In another aspect, the magnetic head included in the magnetic recording / reproducing device may have a configuration in which both a recording element and a reproducing element are provided in one magnetic head. As the reproduction head, a magnetic head (MR head) including a magnetoresistive (MR) element capable of reading information recorded on a magnetic recording medium with high sensitivity is preferable. As the MR head, various known MR heads can be used. Further, the magnetic head that records information and / or reproduces information may include a servo pattern reading element. Alternatively, the magnetic recording / playback device may include a magnetic head (servo head) provided with a servo pattern reading element as a head separate from the magnetic head that records and / or reproduces information.

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

Hereinafter, the present invention will be described based on examples. However, the present invention is not limited to the embodiments shown in the examples. The indications of "part" and "%" described below indicate "parts by mass" and "% by mass" unless otherwise specified. Further, unless otherwise specified, the steps and evaluations described below were carried out in an environment with an ambient temperature of 23 ° C. ± 1 ° C.

[Example 1]
The formulation of each layer-forming composition is shown below.

<Prescription of composition for forming magnetic layer>
(Magnetic liquid)
Ferromagnetic powder (see Table 1): 100.0 parts Oleic acid: 2.0 parts Vinyl chloride copolymer (MR-104 manufactured by Kaneka): 10.0 parts (weight average molecular weight: 55,000, active hydrogen-containing group (see Table 1) hydroxy _{groups): 0.33meq / g, OSO 3} K group (potassium salt of sulfate group): 0.09meq / g)
SO ₃ Na group-containing polyurethane resin: 4.0 parts
(Weight average molecular weight: 70000, active hydrogen-containing group (hydroxy group): 4 to 6 mgKOH / g, SO ₃ Na group (sodium salt of sulfonic acid group): 0.07 meq / g)
Polyalkyleneimine-based polymer (synthetic product obtained by the method described in paragraphs 0115 to 0123 of JP-A-2016-51493): 6.0 parts Methyl ethyl ketone: 150.0 parts Cyclohexanone: 15.0 parts (abrasive solution) )
α-Alumina (BET (Brunauer-Emmett-Teller) specific surface area 19 m ^{2 /} g): 6.0 parts SO ₃ Na group-containing polyurethane resin: 0.6 parts (weight average molecular weight 70000, SO ₃ Na group: 0.1 meq) / G)
2,3-Dihydroxynaphthalene: 0.6 parts Cyclohexanone: 23.0 parts (protrusion forming agent solution)
Colloidal silica (average particle size 120 nm): see Table 1 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 (Tosoh Coronate (registered trademark) L): 3 Department

<Prescription of non-magnetic layer forming composition>
Non-magnetic inorganic powder α-iron oxide (average particle size 10 nm, BET specific surface area 75 m ^{2 /} g): 100.0 parts carbon black (average particle size: 20 nm): 25.0 parts SO ₃ Na group-containing polyurethane resin (weight) Average molecular weight 70000, SO ₃ Na group content 0.2 meq / g): 18.0 parts Steeric acid: 1.0 parts Cyclohexanone: 300.0 parts Methyl ethyl ketone: 300.0 parts

<Prescription of composition for forming backcoat layer>
Non-magnetic inorganic powder: α-iron oxide (average particle size: 0.15 μm, BET specific surface area 52 m ^{2 /} g): 80.0 parts carbon black (average particle size: 20 nm): 20.0 parts vinyl chloride copolymer 13.0 parts Sulfonic acid base-containing polyurethane resin: 6.0 parts Phenylphosphonic acid: 3.0 parts Cyclohexanone: 155.0 parts Methyl ethyl ketone: 155.0 parts Steric acid: 3.0 parts Butyl stearate: 3.0 parts Part Polyisocyanate: 5.0 parts Cyclohexanone: 200.0 parts

<Preparation of composition for forming magnetic layer>
The composition for forming a magnetic layer was prepared by the following method.
A magnetic liquid was prepared by dispersing various components of the magnetic liquid for 24 hours (bead dispersion) using a batch type vertical sand mill. As the dispersed beads, zirconia beads having a bead diameter of 0.5 mm were used.
The abrasive liquid is a mixture of various components of the above abrasive liquid and placed in a horizontal bead mill disperser together with zirconia beads having a bead diameter of 0.3 mm, so that the bead volume / (abrasive liquid volume + bead volume) becomes 80%. The bead mill dispersion treatment was performed for 120 minutes, the liquid after the treatment was taken out, and the ultrasonic dispersion filtration treatment was performed using a flow-type ultrasonic dispersion filtration device. In this way, the abrasive solution was prepared.
The prepared magnetic liquid and abrasive liquid, as well as the above-mentioned protrusion forming agent liquid and other components were introduced into a dissolver stirrer, stirred at a peripheral speed of 10 m / sec for 30 minutes, and then flow rate was 7.5 kg by a flow type ultrasonic disperser. After 3 passes treatment at / min, the composition for forming a magnetic layer was prepared by filtering with a filter having a pore size of 1 μm.

<Preparation of composition for forming non-magnetic layer>
Various components of the above non-magnetic layer forming composition were dispersed by a batch type vertical sand mill using zirconia beads having a bead diameter of 0.1 mm for 24 hours, and then using a filter having an average pore size of 0.5 μm. A composition for forming a non-magnetic layer was prepared by filtering.

<Preparation of composition for backcoat layer formation>
Of the various components of the backcoat layer forming composition described above, the components excluding the lubricant (stearic acid and butyl stearate), polyisocyanate and 200.0 parts of cyclohexanone are kneaded and diluted with an open kneader, and then a horizontal bead mill is used. Using zirconia beads having a bead diameter of 1 mm by a disperser, the bead filling rate was 80% by volume, the rotor tip peripheral speed was 10 m / sec, and the 1-pass residence time was 2 minutes, and the mixture was subjected to 12-pass dispersion treatment. Then, the remaining components described above were added and stirred with a dissolver, and the obtained dispersion was filtered using a filter having an average pore size of 1 μm to prepare a composition for forming a backcoat layer.

<Making magnetic tape>
After applying and drying the composition for forming a non-magnetic layer prepared above so that the thickness after drying becomes 400 nm on the surface of a support made of polyethylene naphthalate having a thickness of 5.0 μm, a non-magnetic layer is formed. The coating layer was formed by applying the composition for forming a magnetic layer prepared above so that the thickness after drying would be 70 nm on the surface of the non-magnetic layer. While the coating layer of the composition for forming a magnetic layer was in a wet (undried) state, a magnetic field with a magnetic field strength of 0.3T was applied in a direction perpendicular to the surface of the coating layer, and the coating layer was dried. .. Then, the composition for forming a backcoat layer prepared above was applied to the opposite surface of the support so that the thickness after drying was 0.4 μm, and the mixture was dried. In this way, a magnetic tape raw fabric was produced.
The produced magnetic tape raw fabric is subjected to calendar treatment (surface smoothing) at a speed of 100 m / min, a linear pressure of 300 kg / cm (294 kN / m), and a surface temperature of the calendar roll of 100 ° C. by a calendar composed of only metal rolls. Treatment), and then heat treatment was performed in the environment of the atmospheric temperature shown in Table 1 for the time shown in Table 1. After the heat treatment, the original magnetic tape was slit by a cutting machine to obtain a magnetic tape having a width of 1/2 inch (0.0127 m). While running this magnetic tape between the feeding roller and the winding roller (running speed 120 m / min, tension: see Table 1), blade polishing of the magnetic layer surface, dry wiping treatment, and methyl ethyl ketone wiping treatment are performed in this order. did. Specifically, a sapphire blade, a dry wiping material (Toray's Toraysee (registered trademark)) and a wiping material impregnated with methyl ethyl ketone (Toray's Toraysee (registered trademark)) are placed between the above two rollers. The sapphire blade is pressed against the surface of the magnetic layer of the magnetic tape running between the two rollers to polish the blade, and then the surface of the magnetic layer is dry-wiped with the above-mentioned dry wiping material, and then the above-mentioned The surface of the magnetic layer was wiped off with a wiping material impregnated with the methyl ethyl ketone of. As described above, the blade polishing, the dry wiping treatment, and the methyl ethyl ketone wiping treatment were performed once on the surface of the magnetic layer.
In this way, the magnetic tape of Example 1 was obtained.

[Examples 2 to 8 and Comparative Examples 1 to 6]
A magnetic tape was produced in the same manner as in Example 1 except that various conditions were changed as shown in Table 1.
Regarding the surface treatment of the magnetic layer surface after slitting, in Examples 2 to 4, 6 to 8, Comparative Example 4 and Comparative Example 5, blade polishing, dry wiping treatment and methyl ethyl ketone wiping treatment were carried out in the same manner as in Example 1. ..
In Example 5 and Comparative Example 6, blade polishing, dry wiping treatment, and methyl ethyl ketone wiping treatment were carried out in the same manner as in Example 1, except that the tension was changed.
In Comparative Example 1 and Comparative Example 3, the blade polishing and the dry wiping treatment were carried out in the same manner as in Example 1, and the methyl ethyl ketone wiping treatment was not carried out.
In Comparative Example 2, the blade polishing and the dry wiping treatment were repeated three times in the same manner as in Example 1, and the methyl ethyl ketone wiping treatment was not carried out.

In Table 1, "BaFe" is a hexagonal barium ferrite powder having an average particle size (average plate diameter) of 21 nm.

In Table 1, "SrFe1" is a hexagonal strontium ferrite powder produced by the following method.
The SrCO _{3 1707g,} H ₃ BO ₃ and 687 g, _Fe 2 _{O 3} to 1120g, Al _{(OH) 3} to 45 g, the BaCO ₃ 24 g, 13 g of CaCO _3, and _Nd 2 _{O 3} and 235g weighed, with a mixer The mixture was mixed to obtain a raw material mixture.
The obtained raw material mixture was melted in a platinum crucible at a melting temperature of 1390 ° C., and the hot water outlet provided at the bottom of the platinum crucible was heated while stirring the melt, so that the melt was discharged into a rod shape at about 6 g / sec. .. The hot water was rolled and rapidly cooled with a water-cooled twin roller to prepare an amorphous body.
280 g of the produced amorphous body was charged into an electric furnace, the temperature was raised to 635 ° C (crystallization temperature) at a heating rate of 3.5 ° C / min, and the temperature was maintained at the same temperature for 5 hours to obtain hexagonal strontium ferrite particles. It was precipitated (crystallized).
Next, the crystallized product obtained above containing hexagonal strontium ferrite particles was roughly pulverized in a dairy pot, 1000 g of zirconia beads having a particle size of 1 mm and 800 ml of an aqueous acetic acid solution having a concentration of 1% were added to a glass bottle, and dispersion treatment was performed for 3 hours with a paint shaker. Was done. Then, the obtained dispersion was separated from the beads and placed in a stainless beaker. The dispersion is allowed to stand at a liquid temperature of 100 ° C. for 3 hours to dissolve the glass components, then precipitated by a centrifuge and repeatedly decanted for cleaning, and then in a heating furnace having a furnace temperature of 110 ° C. 6 It was dried for a time to obtain a hexagonal strontium ferrite powder.
The average particle size of the hexagonal strontium ferrite powder obtained above is 18 nm, the activated volume is 902 nm ³ , the anisotropic constant Ku is 2.2 × 10 ⁵ J / m ³ , and the mass magnetization σs is 49 A ・ m ² /. It was kg.
12 mg of a sample powder was collected from the hexagonal strontium ferrite powder obtained above, and the sample powder was partially dissolved under the dissolution conditions exemplified above, and the elemental analysis of the obtained filtrate was performed by an ICP analyzer to obtain neodymium atoms. The content of the surface layer was determined.
Separately, 12 mg of sample powder was collected from the hexagonal strontium ferrite powder obtained above, and the sample powder was completely dissolved under the dissolution conditions exemplified above, and the elemental analysis of the obtained filtrate was performed by an ICP analyzer, and neodymium was used. The bulk content of atoms was determined.
The content of neodymium atoms (bulk content) with respect to 100 atomic% of iron atoms of the hexagonal strontium ferrite powder obtained above was 2.9 atomic%. The surface layer content of neodymium atoms was 8.0 atomic%. The ratio of the surface layer content to the bulk content, "surface layer content / bulk content" was 2.8, and it was confirmed that neodymium atoms were unevenly distributed on the surface layer of the particles.
To show the crystal structure of hexagonal ferrite in the powder obtained above, the CuKα ray is scanned under the conditions of a voltage of 45 kV and an intensity of 40 mA, and the X-ray diffraction pattern is measured under the following conditions (X-ray diffraction analysis). confirmed. The powder obtained above showed a crystal structure of a magnetoplumbite type (M type) hexagonal ferrite. Moreover, the crystal phase detected by X-ray diffraction analysis was a single phase of the magnetoplumbite type.
PANalytical X'Pert Pro Diffractometer, PIXcel Detector Singler slit of incident beam and diffracted beam: 0.017 Radian dispersion slit fixed angle: 1/4 degree Mask: 10 mm
Anti-scattering slit: 1/4 degree Measurement mode: Continuous Measurement time per step: 3 seconds Measurement speed: 0.017 degrees per second Measurement step: 0.05 degrees

In Table 1, "SrFe2" is a hexagonal strontium ferrite powder produced by the following method.
The SrCO ₃ 1725 _g, 666 g of H ₃ BO 3, 1332g of _{Fe 2 O 3, Al (OH} ) 3 to 52 g, the CaCO ₃ 34g, the BaCO ₃ was 141g weighed to give a mixing in a mixer starting mixture.
The obtained raw material mixture was melted in a platinum crucible at a melting temperature of 1380 ° C., and the hot water outlet provided at the bottom of the platinum crucible was heated while stirring the melt, and the melt was discharged into a rod shape at about 6 g / sec. .. The hot water was rapidly cooled and rolled with a water-cooled plasmid to prepare an amorphous body.
280 g of the obtained amorphous material was charged into an electric furnace, heated to 645 ° C. (crystallization temperature), and kept at the same temperature for 5 hours to precipitate (crystallize) hexagonal strontium ferrite particles.
Next, the crystallized product obtained above containing hexagonal strontium ferrite particles was roughly pulverized in a dairy pot, 1000 g of zirconia beads having a particle size of 1 mm and 800 ml of an aqueous acetic acid solution having a concentration of 1% were added to a glass bottle, and dispersion treatment was performed for 3 hours with a paint shaker. Was done. Then, the obtained dispersion was separated from the beads and placed in a stainless beaker. The dispersion is allowed to stand at a liquid temperature of 100 ° C. for 3 hours to dissolve the glass components, then precipitated by a centrifuge and repeatedly decanted for cleaning, and then in a heating furnace having a furnace temperature of 110 ° C. 6 It was dried for a time to obtain a hexagonal strontium ferrite powder.
The average particle size of the obtained hexagonal strontium ferrite powder is 19 nm, the activated volume is 1102 nm ³ , the anisotropy constant Ku is 2.0 × 10 ⁵ J / m ³ , and the mass magnetization σs is 50 A · m ² / kg. there were.

In Table 1, "ε-iron oxide" is an ε-iron oxide powder produced by the following method.
In 90 g of pure water, 8.3 g of iron (III) nitrate 9 hydrate, 1.3 g of gallium nitrate (III) octahydrate, 190 mg of cobalt (II) nitrate hexahydrate, 150 mg of titanium (IV) sulfate, and While stirring 1.5 g of polyvinylpyrrolidone (PVP) using a magnetic stirrer, 4.0 g of an aqueous ammonia solution having a concentration of 25% was added in an atmospheric atmosphere under the condition of an atmospheric temperature of 25 ° C. The mixture was stirred for 2 hours under the temperature condition of an atmospheric temperature of 25 ° C. To the obtained solution, a citric acid solution obtained by dissolving 1 g of citric acid in 9 g of pure water was added, and the mixture was stirred for 1 hour. The powder precipitated after stirring was collected by centrifugation, washed with pure water, and dried in a heating furnace having a furnace temperature of 80 ° C.
800 g of pure water was added to the dried powder, and the powder was dispersed again in water to obtain a dispersion liquid. The obtained dispersion was heated to a liquid temperature of 50 ° C., and 40 g of a 25% aqueous ammonia solution was added dropwise with stirring. After stirring for 1 hour while maintaining the temperature of 50 ° C., 14 mL of tetraethoxysilane (TEOS) was added dropwise, and the mixture was stirred for 24 hours. 50 g of ammonium sulfate was added to the obtained reaction solution, and the precipitated powder was collected by centrifugation, washed with pure water, and dried in a heating furnace at a temperature of 80 ° C. for 24 hours to obtain a precursor of the ferromagnetic powder. Obtained.
The obtained precursor of the ferromagnetic powder was loaded into a heating furnace having a furnace temperature of 1000 ° C. under an air atmosphere, and heat-treated for 4 hours.
The precursor of the heat-treated ferromagnetic powder was put into a 4 mol / L aqueous solution of sodium hydroxide (NaOH), and the liquid temperature was maintained at 70 ° C. and stirred for 24 hours to obtain the heat-treated ferromagnetic powder. The silicate compound, which is an impurity, was removed from the precursor.
Then, the ferromagnetic powder from which the silicic acid compound was removed was collected by centrifugation and washed with pure water to obtain a ferromagnetic powder.
The composition of the obtained ferromagnetic powder was confirmed by high frequency inductively coupled plasma emission spectroscopy (ICP-OES; Inductively Coupled Plasma-Optical Mission Spectrometry) and found to be Ga, Co and Ti-substituted ε-iron oxide (ε-Ga _{0). It was .58} Fe _1.42 O ₃ ). Further, X-ray diffraction analysis was performed under the same conditions as those described above for SrFe1, and the ferromagnetic powder obtained from the peak of the X-ray diffraction pattern does not contain the crystal structures of α phase and γ phase. It was confirmed that it had a single-phase crystal structure (ε-iron oxide type crystal structure).
The resulting ε- average particle size of the iron oxide powder is 12 nm, activation volume 746 nm ^3, the anisotropy constant Ku is ^{1.2 × 10 5 J / m 3} , the mass magnetization σs at 16A ^· m 2 / kg there were.

The activated volume and anisotropic constant Ku of the hexagonal strontium ferrite powder and ε-iron oxide powder described above are described above for each ferromagnetic powder using a vibrating sample magnetometer (manufactured by Toei Kogyo Co., Ltd.). It is a value obtained by the method of.
The mass magnetization σs is a value measured with a magnetic field strength of 15 kOe using a vibrating sample magnetometer (manufactured by Toei Kogyo Co., Ltd.).

[Evaluation of magnetic tape]
(1) Centerline average surface roughness Ra measured on the surface of the magnetic layer (magnetic layer surface roughness Ra)
Using an atomic force microscope (AFM, Nanoscope4 manufactured by Veeco) in a tapping mode, the measurement area of 40 μm × 40 μm was measured on the surface of the magnetic layer of the magnetic tape, and the average surface roughness Ra of the center line was determined. As the probe, RTESS-300 manufactured by BRUKER was used, the scanning speed (probe moving speed) was 40 μm / sec, and the resolution was 512pixel × 512pixel.

_{(2) Spacing difference before and} after washing with methyl ethyl ketone ( _After- S before)
Using TSA (Tape Spacing Analyzer (manufactured by Micro Physics)), the spacing difference ( _Suffer- S before) _{before and after washing with methyl ethyl ketone was determined by the following method.}
Two sample pieces having a length of 5 cm were cut out from each of the magnetic tapes of Examples and Comparative Examples, and the spacing ( _Sbefore ) of one sample piece was determined by the following method without washing with methyl ethyl ketone. After about the other sample piece was methylethylketone washing by the method described above, was determined spacing (S _after) by the following method.
A glass plate (Thorlabs, Inc. glass plate (model number: WG10530)) provided for the TSA is provided on the surface of the magnetic layer of the magnetic tape (specifically, the sample piece) and prepared for the TSA as a pressing member. The hemisphere made of urethane was pressed against the surface of the backcoat layer of the magnetic tape at ^{a pressure of 5.05 × 10 4} N / m (0.5 atm). In this state, white light is irradiated from a strobescope provided in the TSA to a certain region (150,000 to 200,000 μm ² ) on the surface of the magnetic layer of the magnetic tape through a glass plate, and the obtained reflected light is applied to an interference filter (wavelength 633 nm). By receiving light with a CCD (Charge-Coupled Device) through a filter that selectively transmits the light of the above, an interference fringe image generated by the unevenness of this region was obtained.
This image is divided into 300,000 points, the distance (spacing) from the surface of the glass plate on the magnetic tape side to the surface of the magnetic layer of the magnetic tape at each point is obtained, and this is used as a histogram to obtain a sample piece after washing with methyl ethyl ketone. The mode (S _After − S _before ) was obtained _{by subtracting the mode S Before of} the histogram obtained for the sample piece without methyl ethyl ketone washing from the mode S _{After of the histogram.}

(3) Spacing difference before and after washing with n-hexane (S _reference- S _before ) (reference value)
A sample piece having a length of 5 cm was further cut out from each of the magnetic tapes of Examples and Comparative Examples, and washed in the same manner as above except that n-hexane was used instead of methyl ethyl ketone, and then n-hexane was washed in the same manner as above. Asked for later spacing. As a reference value, the difference (S _reference − S _{before ) between the spacing S reference obtained here and the spacing S before} obtained for the unwashed sample piece obtained in (2) above was obtained.

(4) Electromagnetic conversion characteristics (SNR (Signal-to-Noise-Ratio))
For each of the magnetic tapes of Examples and Comparative Examples in an environment of an ambient temperature of 23 ° C. and a relative humidity of 50%, a recording head (MIG (Metal-in-gap) head, gap length 0.15 μm, track width 1.0 μm). A 1.8T) and a reproduction head (GMR (Giant Magnetorestive) head, element thickness 15 nm, shield interval 0.1 μm, track width 1.0 μm) were attached to a loop tester to record a signal with a line recording density of 325 kfci. The unit kfci is a unit of line recording density (cannot be converted into the SI unit system). Then, the reproduction output was measured, and the SNR was obtained as the ratio of the reproduction output to the noise. The SNR was determined as a relative value when the SNR of Comparative Example 1 was 0 dB. If the SNR obtained here is 0 dB or more, it can be evaluated as having good electromagnetic conversion characteristics.

(5) SNR decrease due to temperature change from low temperature to high temperature under high humidity After evaluating the electromagnetic conversion characteristics of each of the magnetic tapes of Examples and Comparative Examples in (4) above, the temperature inside is 10 ° C. It was stored for 3 hours in a thermobox maintained at a relative humidity of 80%. After that, the magnetic tape is taken out from the thermo box (outside air has a temperature of 23 ° C and a relative humidity of 50%), and the inside is placed in a thermo room kept at a temperature of 32 ° C and a relative humidity of 80% within 1 minute. Recording and reproduction were performed in the room in the same manner as in (4) above, and the SNR was obtained as a relative value when the SNR of Comparative Example 1 obtained in (4) above was set to 0 dB. For each magnetic tape of Examples and Comparative Examples, the difference between the SNR obtained here and the SNR obtained in (4) above (“SNR obtained in this section”-“SNR obtained in (4) above). ”) Was calculated and used as the SNR decrease. If the SNR decrease obtained here is within −1.0 dB, it can be evaluated that the decrease in SNR due to the temperature change from low temperature to high temperature under high humidity is suppressed.

The above results are shown in Table 1 (Table 1-1, Table 1-2).

As shown in Table 1, the magnetic tape of the example has a magnetic layer having a magnetic layer surface roughness Ra of 1.0 nm or more and 1.6 nm or less, and has a magnetic layer having high surface smoothness. From the evaluation result (SNR) of the electromagnetic conversion characteristics, it can be confirmed that the magnetic tapes of these examples are excellent in the electromagnetic conversion characteristics.
Further, in the magnetic tape of the example, as described above, the surface smoothness of the magnetic layer is high, and the spacing difference ( _After- S _{before) before and} after washing with methyl ethyl ketone is more than 0 nm and 15.0 nm or less. As shown in Table 1, the magnetic tapes of these examples have a smaller SNR reduction than the magnetic tapes of the comparative examples even when exposed to a temperature change from a low temperature to a high temperature under high humidity.
Further, as shown in Table 1, the correlation between the value of n- hexane wash before and after the spacing difference _{(S reference} -S _before) values and methyl ethyl ketone before and after washing of the spacing difference _{(S after -S before)} Can not be seen.

One aspect of the present invention is useful in the technical field of various magnetic recording media for data storage.

Claims (8)

A magnetic recording medium having a magnetic layer containing ferromagnetic powder on a non-magnetic support.
The average particle size of the ferromagnetic powder is 5 nm or more and 25 nm or less.
The center line average surface roughness Ra measured at the surface of the magnetic layer is not more than 1.6nm least 1.0 nm, and said spacing S _after which the surface of the magnetic layer as measured by an optical interference method after methylethylketone washing , A magnetic recording medium having a difference from the _{spacing S before} measured by an optical interferometry on the surface of the magnetic layer before washing with methyl ethyl ketone _{, After-} S _before , which is more than 0 nm and 15.0 nm or less. The magnetic recording medium according to claim 1, wherein the difference, _After- S _before , is 2.0 nm or more and 15.0 nm or less. The _difference, S _{after -S before,} is less 12.0nm than 3.0 nm, the magnetic recording medium according to claim 1 or 2. The magnetic recording medium according to any one of claims 1 to 3, further comprising a non-magnetic layer containing non-magnetic powder between the non-magnetic support and the magnetic layer. The magnetic recording medium according to any one of claims 1 to 4, further comprising a backcoat layer containing a non-magnetic powder on the surface side of the non-magnetic support opposite to the surface side having the magnetic layer. The magnetic recording medium according to any one of claims 1 to 5, which is a magnetic tape. The magnetic recording medium according to any one of claims 1 to 6, wherein the average particle size of the ferromagnetic powder is 5 nm or more and 20 nm or less. The magnetic recording medium according to any one of claims 1 to 7, and the magnetic recording medium.
With a magnetic head
Magnetic recording / playback device including.