JP2021047975A - Magnetic recording medium and magnetic recording and reproducing device - Google Patents

Abstract

To provide a magnetic recording medium including a back coating layer, in which a decrease in reproduction output is small even though reproduction is repeated under an environment of a low temperature and a high humidity after storage under an environment of a high temperature.SOLUTION: A magnetic recording medium includes: a magnetic layer that includes a ferromagnetic powder on one surface side of a non-magnetic support; and a back coating layer that includes a non-magnetic powder on the other surface side; in which a difference (S0.5-S13.5) between a spacing S0.5 measured on a surface of the back coating layer by optical interferometry under a pressure of 0.5 atm after n-hexane cleaning and a spacing S13.5 measured on the surface of the back coating layer by optical interferometry under a pressure of 13.5 atm after n-hexane cleaning is equal to or less than 3 nm. A magnetic recording and reproducing device includes the magnetic recording medium.SELECTED DRAWING: None

Description

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

With respect to the magnetic recording medium, a backcoat layer is provided on the surface side opposite to the surface side having the magnetic layer of the non-magnetic support (see, for example, paragraphs 0040 to 0044 of Patent Document 1).

Japanese Unexamined Patent Publication No. 2012-43495

In recent years, magnetic recording media have been stored and used in various environments. For example, magnetic recording media may be stored in temperature controlled data centers. However, from the viewpoint of reducing the air conditioning cost for temperature control, it is desirable that the temperature control conditions at the time of storage can be relaxed from the present or the control can be eliminated. In view of the above, the present inventor has examined the performance of the magnetic recording medium after being stored in a high temperature environment. As a result of this study, it has been found that the magnetic recording medium having the backcoat layer has a problem that the reproduction output is lowered when the reproduction is repeated in the low temperature and high humidity environment after the storage in the high temperature environment.

One aspect of the present invention is to provide a magnetic recording medium having a backcoat layer, wherein the reproduction output does not decrease even if reproduction is repeated in a low temperature and high humidity environment after storage in a high temperature environment. With the goal.

One aspect of the present invention is
A magnetic recording medium having a magnetic layer containing ferromagnetic powder on one surface side of a non-magnetic support and a backcoat layer containing non-magnetic powder on the other surface side.
_{Spacing S 0.5} measured under pressure of 0.5 atm by optical interferometry after n-hexane cleaning on the surface of the backcoat layer, and optical interferometry after n-hexane cleaning on the surface of the backcoat layer. A magnetic recording medium having a difference (S _0.5- S _13.5 ) from the spacing S _13.5 measured under pressure of 13.5 atm of 3 nm or less.
Regarding. In the following, the above difference (S _0.5- S _13.5 ) is also simply referred to as “difference”. Further, 1 atm = 101325 Pa (Pascal).

In one form, the difference can be 1 nm or more and 3 nm or less.

In one form, S _0.5 can be 20 nm or more and 90 nm or less.

In one form, S _13.5 can be 20 nm or more and 90 nm or less.

In one form, the backcoat layer can contain inorganic oxide particles.

In one form, the inorganic oxide-based particles can be composite particles of an inorganic oxide and a polymer.

In one form, the thickness of the backcoat layer can be 0.5 μm or less.

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

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

One 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, there is provided a magnetic recording medium having a backcoat layer, wherein the reproduction output does not decrease even if reproduction is repeated in a low temperature and high humidity environment after storage in a high temperature environment. can do. Further, according to one aspect of the present invention, it is possible to provide a magnetic recording / reproducing device including such a magnetic recording medium.

[Magnetic recording medium]
One aspect of the present invention is a magnetic recording medium having a magnetic layer containing a ferromagnetic powder on one surface side of a non-magnetic support and a backcoat layer containing a non-magnetic powder on the other surface side. _{Spacing S 0.5} measured under a pressure of 0.5 atm by the optical interference method after n-hexane cleaning on the surface of the backcoat layer, and by the optical interference method after n-hexane cleaning on the surface of the backcoat layer. It relates to a magnetic recording medium having a difference (S _0.5- S _{13.5 ) from the spacing S 13.5} measured under a pressure of 13.5 atm of 3 nm or less.

In the present invention and the present specification, "n-hexane cleaning" means that a sample piece cut out from a magnetic recording medium is immersed in fresh n-hexane (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 n-hexane cleaning. The width of the magnetic tape and the width of the sample piece cut out from the magnetic tape are usually 1/2 inch. 1 inch = 0.0254 meters. For magnetic tapes other than 1/2 inch width, a sample piece having a length of 5 cm may be cut out and subjected to n-hexane washing. When the magnetic recording medium to be cleaned is a magnetic disk, a sample piece having a size of 5 cm × 1.27 cm is cut out and subjected to n-hexane cleaning. The spacing measurement described in detail below shall be performed after the sample piece after washing with n-hexane 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 backcoat layer" of the magnetic recording medium is synonymous with the surface of the magnetic recording medium on the backcoat layer side.

In the present invention and the present specification, the spacing measured by the optical interferometry on the surface of the backcoat 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 surface of the backcoat layer of the magnetic recording medium faces the transparent plate-shaped member. In this state, the pressing member is pressed with a pressure of 0.5 atm or 13.5 atm from the side opposite to the back coat layer side of the magnetic recording medium. In this state, the surface of the backcoat layer of the magnetic recording medium is irradiated with light via a transparent plate-shaped member (irradiation area: 150,000 to 200,000 μm ² ), and the reflected light from the surface of the backcoat layer of the magnetic recording medium is transparent. Based on the intensity of the interference light generated by the optical path difference from the reflected light from the surface of the plate-shaped member on the magnetic recording medium side (for example, the contrast of the interference fringe image), the surface of the backcoat layer of the magnetic recording medium and the transparent plate-shaped member Obtain the spacing (distance) from the surface on the magnetic recording medium side. 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 through which the irradiated light is transmitted to such an extent that the magnetic recording medium is irradiated with light 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 (distance between the surface of the backcoat layer of the magnetic recording medium and the surface of the transparent plate-like member on the magnetic recording medium side). Is obtained, this is used as a histogram, and the mode value in this histogram is used as spacing.
Five sample pieces were cut out from the same magnetic recording medium, and after washing with n-hexane, the pressing member was pressed against the pressing member at a pressure of 0.5 atm to obtain spacing, and the pressing member was further pressed at a pressure of 13.5 atm. Ask for spacing. Then, the arithmetic mean of the spacing obtained for the five sample pieces under a pressure of 0.5 atm after washing with n-hexane was set to spacing S _0.5, and the pressure of 13.5 atm after washing with n-hexane for the five sample pieces was taken. Let the arithmetic mean of the spacing obtained below be the spacing S _13.5 . The difference _{(S 0.5 -S 13.5)} of the thus the _{S 0.5} and _{S 13.5} found, and the difference _{(S 0.5 -S 13.5)} of the magnetic recording medium.
The above measurement can be performed using, for example, a commercially available tape spacing analyzer (TSA; Tape Spacing Analyzer) 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, the spacing is measured by the pressing of a spacing S _0.5 and 13.5atm measured under pressing of 0.5atm by optical interferometry after n- hexane washing the surface of the backcoat layer The difference from S _{13.5 (S 0.5-} S _13.5 ) is 3 nm or less. As a result, according to the magnetic recording medium, it is possible to suppress a decrease in the reproduction output during repeated reproduction in a low temperature and high humidity environment after storage in a high temperature environment. The inventor's inference in this regard is as follows.

It is presumed that the decrease in the reproduction output in the repeated reproduction is caused by the decrease of the components (for example, the lubricant described later) that can contribute to the running stability from the magnetic layer during the storage before the start of the reproduction. .. For example, if the surface of the magnetic layer and the surface of the backcoat layer are in contact with each other during storage, it is considered that the above components are transferred from the surface of the magnetic layer to the backcoat layer side and are reduced from the magnetic layer. .. In storage in a high temperature environment, for example, the backcoat layer surface exerts a high pressure from the magnetic layer surface due to winding tightening in a magnetic recording medium (for example, magnetic tape) wound on a reel and contained in a cartridge. It is considered that the component is easily received, and it is presumed that the amount of the above component transferred from the surface of the magnetic layer to the backcoat layer side increases. Further, since the running stability tends to decrease in a low temperature and high humidity environment, when the data recorded on the magnetic layer in which the above components are reduced during storage is repeatedly reproduced in the low temperature and high humidity environment, the reproduction output is output. It is presumed that the decline is likely to occur. The high temperature can be, for example, about 30 to 50 ° C., the low temperature can be, for example, about 0 to 15 ° C., and the high humidity can be, for example, about 70 to 100% relative humidity. There can be.
With respect to the above points, the present inventor has been studying repeatedly, and it is possible to suppress the deformation of the backcoat layer surface under high pressure from the magnetic layer surface to the backcoat layer surface during storage in a high temperature environment. It was thought that it might contribute to suppressing the transfer of a large amount of the above-mentioned components to. The details are as follows.
On the surface of the backcoat layer, protrusions formed by the non-magnetic powder contained in the backcoat layer can be present in a state of being embedded in the base portion of the backcoat layer. If these protrusions sink deeply into the backcoat layer when a high pressure is applied, it is presumed that a fine gap is formed between the protrusions and the base portion of the backcoat layer. It is considered that the inclusion of the above-mentioned components into the fine gaps due to the capillary phenomenon increases the amount of the above-mentioned components transferred to the surface of the backcoat layer. Therefore, suppressing the deformation of the backcoat layer surface under high pressure means that the components that can contribute to the running stability of the magnetic layer are transferred to the backcoat layer side during storage in a high temperature environment. The present inventor thought that it would lead to suppression of. Nothing is described about such findings in Japanese Patent Application Laid-Open No. 2012-43495 (Patent Document 1). Then, as a result of further studies based on such findings, the present inventors set the difference (S _0.5- S _13.5 ) measured on the surface of the backcoat layer to 3 nm or less under a high temperature environment. It has been newly found that it is possible to suppress a decrease in the reproduction output during repeated reproduction in a low temperature and high humidity environment after storage in. The small difference (S _0.5- S _13.5 ) of 3 nm or less indicates that the protrusions on the surface of the backcoat layer are unlikely to sink into the backcoat layer when a high pressure is applied as described above. It is thought that there is.
However, the present invention is not limited to the above inference. In addition, the present invention is not limited to other inventors of the present inventor described in the present specification. Regarding the pressures of 13.5 atm and 0.5 atm at the time of pressing in the measurement of spacing for obtaining the above difference, in the present invention, it is exemplary that it is possible to correspond to the state where a high pressure is applied to the surface of the backcoat layer as described above. A value of 13.5 atm is adopted, and 0.5 atm is adopted as an exemplary value of the pressure that can be applied to the surface of the backcoat layer when it comes into contact with the surface of the magnetic layer in other states. The pressure that can be applied to the magnetic recording medium at times is not limited to these pressures.

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

<Difference (S _0.5- S _13.5 )>
_{The difference (S 0.5-} S _13.5 ) measured for the surface of the backcoat layer of the magnetic recording medium is 3 nm or less, and is used in repeated reproduction in a low temperature and high humidity environment after storage in a high temperature environment. From the viewpoint of further suppressing the decrease in the reproduction output, it is preferably 2 nm or less. Further, the above difference can be 0 nm or more, can be more than 0 nm, or can be 1 nm or more.

S _0.5 and S _13.5 are not particularly limited as long as the above difference is 3 nm or less. S _0.5 and S _13.5 can be, for example, 20 nm or more, 22 nm or more, or 25 nm or more, respectively. Further, S _0.5 and S _13.5 can be, for example, 90 nm or less, 85 nm or less, or 80 nm or less, respectively.

The above differences, S _0.5 and S _13.5 , are non-magnetic powders contained in the back coat layer that can mainly contribute to the formation of protrusions on the surface of the back coat layer (hereinafter, “protrusions”). It can be controlled by the type of "forming agent"), the manufacturing conditions of the magnetic recording medium, and the like. Details of this point will be described later.

Next, the magnetic layer, the backcoat layer, the non-magnetic support, and the non-magnetic layer that can be optionally included in the magnetic recording medium will be further described.

<Magnetic layer>
(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 JP2012-204726A, and 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 a form 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 form, the hexagonal strontium ferrite powder containing rare earth atoms can have uneven distribution on the surface layer of rare earth atoms. The "uneven distribution of the surface layer of rare earth atoms" 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. “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. It is presumed that 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 increasing the anisotropy constant Ku. .. 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). 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.
From the viewpoint of further improving the anisotropy constant Ku and / or further improving the running durability, the rare earth atomic content (bulk content) may be in the range of 0.5 to 4.5 atomic%. 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. Preferred rare earth atoms from the viewpoint of further improving the anisotropic constant Ku include neodymium atom, samarium atom, yttrium atom and dysprosium atom, and neodymium atom, samarium atom and yttrium atom are more preferable, and neodymium atom is preferable. 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 form, 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 form, the hexagonal strontium ferrite powder can contain only strontium atoms as divalent metal atoms. In another embodiment, 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 form, 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 improving the anisotropic constant Ku, the hexagonal strontium ferrite powder contains iron atoms, strontium atoms, oxygen atoms and rare earth atoms, and the content of atoms other than these atoms is 100 atomic% of iron atoms. On the other hand, it is preferably 10.0 atomic% or less, more preferably 0 to 5.0 atomic%, and may be 0 atomic%. That is, in one form, 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. The hexagonal strontium ferrite powder can, in one form, 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 form, 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 form in which the particles constituting the set are in direct contact with each other, and also includes a form in which a binder, an additive, etc., which will be described later, are 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, amorphous, 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. A high filling rate of the ferromagnetic powder in the magnetic layer is preferable from the viewpoint of improving the recording density.

(Binder)
The magnetic recording medium can be a coating type magnetic recording medium, and the magnetic layer can contain 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 binders in the backcoat layer and / or the non-magnetic layer described below.
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. The binder can be used 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.
GPC device: HLC-8120 (manufactured by Tosoh)
Column: TSK gel Multipore HXL-M (manufactured by Tosoh Corporation, 7.8 mm ID (Inner Diameter) x 30.0 cm)
Eluent: tetrahydrofuran (THF)

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 form, and a photocuring agent in which a curing reaction (crosslinking reaction) proceeds by light irradiation in another form. 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.

The above description of the binder and the curing agent can also be applied to the backcoat layer and / or the non-magnetic layer. In that case, the above description regarding the content can be applied by replacing the ferromagnetic powder with a non-magnetic powder.

(Additive)
The magnetic layer 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, lubricant, dispersant, dispersion aid, fungicide, antistatic agent, antioxidant and the like. The non-magnetic powder includes a non-magnetic powder that can function as an abrasive, and a non-magnetic powder that can function as a protrusion-forming agent that forms protrusions that appropriately project on the surface of the magnetic layer (for example, non-magnetic colloidal particles). And so on. 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, paragraphs 0061 and 0071 of JP2012-133387A can be referred to. Further, regarding the dispersant, paragraphs 0061 and 0071 of JP2012-133387A and paragraphs 0035 of JP2017-016721A 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 Japanese Patent Application Laid-Open No. 2012-1333837 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.

In one form, the magnetic recording medium may contain a lubricant in a portion on the non-magnetic support on the magnetic layer side. The portion on the non-magnetic support on the magnetic layer side is a magnetic layer in a magnetic recording medium having a magnetic layer directly on the non-magnetic support, and the non-magnetic layer and the magnetic layer are arranged on the non-magnetic support in this order. The magnetic recording medium to have is a non-magnetic layer and / or a magnetic layer. The lubricant is an example of the components that can contribute to the running stability described above. If the amount of the lubricant contained in the portion on the non-magnetic support on the magnetic layer side during storage in a high temperature environment can be reduced from the surface of the magnetic layer to the backcoat layer side, then in a low temperature and high humidity environment. It is considered that it becomes possible to suppress a decrease in the reproduction output during repeated reproduction. It is presumed that the difference of 3 nm or less can contribute to this.

Examples of the lubricant include one or more lubricants selected from the group consisting of fatty acids, fatty acid esters and fatty acid amides.
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 (butyl stearate), neopentyl glycol dioleate, sorbitan monostearate, sorbitan distearate, sorbitan tristearate, oleyl oleate, Examples thereof include isocetyl stearate, isotridecyl stearate, 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.
For fatty acids and fatty acid derivatives (amides, esters, etc.), the fatty acid-derived site of the fatty acid derivative preferably has a structure similar to or similar to that of the fatty acid used in combination. For example, when stearic acid is used as the fatty acid, it is preferable to use stearic acid ester and / or stearic acid amide.
The fatty acid content of the magnetic layer (or the composition for forming the magnetic layer; the same applies hereinafter) is, for example, 0 to 10.0 parts by mass, preferably 0.1 to 10.0 parts by mass per 100.0 parts by mass of the ferromagnetic powder. It is by mass, more preferably 1.0 to 7.0 parts by mass. The fatty acid ester content of the magnetic layer is, for example, 0 to 10.0 parts by mass, preferably 0.1 to 10.0 parts by mass, and more preferably 1.0, per 100.0 parts by mass of the ferromagnetic powder. ~ 7.0 parts by mass. The fatty acid amide content of the magnetic layer is, for example, 0 to 3.0 parts by mass, preferably 0 to 2.0 parts by mass, and more preferably 0 to 1.0 parts by mass per 100.0 parts by mass of the ferromagnetic powder. It is a mass part.
When the magnetic recording medium has a non-magnetic layer between the non-magnetic support and the magnetic layer, the fatty acid content of the non-magnetic layer (or composition for forming the non-magnetic layer; the same applies hereinafter) is a non-magnetic powder. It is, for example, 0 to 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 per 100.0 parts by mass. The fatty acid ester content of the non-magnetic layer is, for example, 0 to 15.0 parts by mass, preferably 0.1 to 10.0 parts by mass, per 100.0 parts by mass of the non-magnetic powder. The fatty acid amide content of the 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. The lubricant contained in the non-magnetic layer can be transferred to the magnetic layer, and can be transferred to contribute to running stability.

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 powder or an organic powder. In addition, carbon black or the like can also be used. Examples of the inorganic powder include powders of metals, metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, metal sulfides and the like. These 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.

The non-magnetic layer can contain a binder and can also contain an additive. 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 100 Oe or less, or the residual magnetic flux density is 10 mT or less and the coercive force is 100 Oe. It refers to the following layers. 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 has a backcoat layer containing non-magnetic powder on the surface side opposite to the surface side having the magnetic layer of the non-magnetic support. The non-magnetic powder contained in the backcoat layer can preferably be a protrusion forming agent and one or more of other non-magnetic powders.

(Protrusion forming agent)
As the protrusion forming agent, particles of an inorganic substance can be used, particles of an organic substance can be used, or composite particles of an inorganic substance and an organic substance can be used. Examples of the inorganic substance include inorganic oxides such as metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, metal sulfides and the like, and inorganic oxides are preferable. In one form, the protrusion-forming agent can be inorganic oxide-based particles. Here, "system" is used to mean "including". One form of inorganic oxide-based particles is particles composed of inorganic oxides. Further, another form of the inorganic oxide-based particles is a composite particle of an inorganic oxide and an organic substance, and specific examples thereof include a composite particle of an inorganic oxide and a polymer. Examples of such particles include particles in which a polymer is bonded to the surface of particles of an inorganic oxide.

The above S _0.5 can be controlled mainly by the particle size of the protrusion forming agent. The average particle size of the protrusion forming agent can be, for example, 30 to 300 nm, preferably 40 to 200 nm. S _13.5 can be controlled by the shape of the protrusion forming agent in addition to the particle size of the protrusion forming agent. The closer the particle shape is to a true sphere, the smaller the pushing resistance that acts when a high pressure is applied, so that the particles are more likely to be pushed into the backcoat layer, and S _13.5 tends to be smaller. On the other hand, if the shape of the particles is distant from the true sphere, for example, a so-called irregular shape, a large pushing resistance is likely to work when a high pressure is applied, so that it is difficult to be pushed into the backcoat layer. , S _13.5 tends to be large. Further, even particles having an inhomogeneous particle surface and low surface smoothness are likely to have a large pushing resistance when a high pressure is applied, so that they are difficult to be pushed into the backcoat layer, and S _13.5 becomes large. Cheap. By controlling S _0.5 and S _13.5 , the difference (S _0.5- S _13.5 ) can be reduced to 3 nm or less.

Further, from the viewpoint that the protrusion-forming agent can exert its function better, the content of the protrusion-forming agent in the backcoat layer is preferably 100.0 parts by mass with respect to other non-magnetic powders. , 1.0 to 4.0 parts by mass, more preferably 1.2 to 3.5 parts by mass. In one form, the content of the composite particles of the inorganic oxide and the polymer in the backcoat layer is 1.0 to 4.0 parts by mass with respect to 100.0 parts by mass of the other non-magnetic powder. It is preferably 1.2 to 3.5 parts by mass, more preferably 1.2 to 3.5 parts by mass.

(Other non-magnetic powder)
For other non-magnetic powders contained in the backcoat layer, the above description regarding the non-magnetic powder of the non-magnetic layer can be referred to. As the other non-magnetic powder, preferably, either one or both of carbon black and non-magnetic powder other than carbon black can be used. Examples of the non-magnetic powder other than carbon black include non-magnetic inorganic powder. Specific examples of the non-magnetic inorganic powder include iron oxide such as α-iron oxide, titanium oxide such as titanium dioxide, cerium oxide, tin oxide, tungsten oxide, ZnO, ZrO ₂ , SiO ₂ , Cr ₂ O ₃ , α. -Alumina, β-alumina, γ-alumina, Gecite, corundum, silicon nitride, titanium carbide, magnesium oxide, boron nitride, molybdenum disulfide, copper oxide, MgCO ₃ , CaCO ₃ , BaCO ₃ , SrCO ₃ , BaSO ₄ , Non-magnetic inorganic powders such as silicon carbide and titanium carbide. The preferred non-magnetic inorganic powder is a non-magnetic inorganic oxide powder, more preferably α-iron oxide, titanium oxide, and further preferably α-iron oxide.

The shape of the non-magnetic powder other than carbon black may be needle-shaped, spherical, polyhedral, or plate-shaped. The average particle size of these non-magnetic powders is preferably in the range of 5 nm to 2.00 μm, and more preferably in the range of 10 nm to 200 nm. The BET specific surface area of the non-magnetic powder other than carbon black ^{is preferably in the range of 1 to 100 m 2} / g, more preferably 5 to 70 m ² / g, and further preferably 10 to 65 m ² / g. Is the range of. On the other hand, the average particle size of carbon black is, for example, in the range of 5 to 80 nm, preferably in the range of 10 to 50 nm, and more preferably in the range of 10 to 40 nm. The carbon black content with respect to 100.0 parts by mass of the total amount of the other non-magnetic powder can be, for example, in the range of 10.0 to 100.0 parts by mass. The entire amount of the other non-magnetic powder may be carbon black. Alternatively, the entire amount of the other non-magnetic powder may be a non-magnetic powder other than carbon black. The higher the proportion of carbon black in the other non-magnetic powder, the higher _{the values of S 0.5} and S _13.5 tend to be. For the content (filling rate) of the other non-magnetic powder in the backcoat layer, the above description regarding the non-magnetic powder in the non-magnetic layer can be referred to.

The backcoat layer can contain binders and can also contain additives. For the binders and additives of the backcoat layer, known techniques relating to the backcoat layer can be applied, and known techniques relating to the formulation of magnetic layers and / or non-magnetic layers can also be applied. 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.

An example of an additive that can be contained in the backcoat layer is a lubricant. As for the lubricant, the description regarding the lubricant that can be contained in the magnetic layer or the like can be referred to. The fatty acid content of the backcoat layer (or the composition for forming the backcoat layer; the same applies hereinafter) is preferably 0 to 10.0 parts by mass per 100.0 parts by mass of the non-magnetic powder contained in the backcoat layer. Is 0.1 to 10.0 parts by mass, more preferably 1.0 to 7.0 parts by mass. The fatty acid ester content of the composition for forming the backcoat layer is, for example, 0.1 to 10.0 parts by mass, preferably 1.0 to 5 parts by mass, based on 100.0 parts by mass of the non-magnetic powder contained in the backcoat layer. It is 0.0 parts by mass. The fatty acid amide content of the backcoat layer is, for example, 0 to 3.0 parts by mass, preferably 0 to 2.0 parts by mass, based on 100.0 parts by mass of the non-magnetic powder contained in the backcoat layer. It is preferably 0 to 1.0 parts by mass.

The above S _0.5 and S _13.5 are values measured after washing with n-hexane. If a liquid film of the lubricant is present on the surface of the backcoat layer that is pressed during the spacing measurement, the measured spacing becomes narrower by the thickness of the liquid film. On the other hand, it is presumed that the lubricant that may exist as a liquid film at the time of pressing can be removed by washing with n-hexane. Therefore, by measuring the spacing after washing with n-hexane, it is considered that the measured value of the spacing can be obtained as a value that corresponds well with the height of the protrusions on the surface of the backcoat layer in the pressed state.

<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. 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 can be, for example, 0.1 μm or more. The thickness of the backcoat layer is preferably 0.9 μm or less, more preferably 0.7 μm or less, and even more preferably 0.5 μm or less.

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>
The composition for forming the magnetic layer, the non-magnetic layer or 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. Above all, from the viewpoint of solubility of a binder usually used for a coating type magnetic recording medium, each layer-forming composition contains a ketone solvent such as acetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, cyclohexanone, isophorone, and tetrahydrofuran. It is preferable that one or more of the above is contained. 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. In addition, the step of preparing each layer-forming composition can usually include 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. The components used in the preparation of each layer-forming composition may be added at the beginning or in the middle of any step. Further, each component may be added separately in two or more steps. For example, the binder may be divided and added in a kneading step, a dispersion step, and a mixing step for adjusting the viscosity after dispersion. In the manufacturing process of the magnetic recording medium, conventionally known manufacturing techniques can be used in some or all of the steps. In the kneading step, it is preferable to use a kneader having a strong kneading force such as an open kneader, a continuous kneader, a pressurized kneader, and an extruder. Details of these kneading treatments are described in JP-A-1-106338 and JP-A-1-79274. In addition, glass beads and / or other beads can be used to disperse each layer-forming composition. As such dispersed beads, zirconia beads, titania beads, and steel beads, which are dispersed beads having a high specific gravity, are suitable. It is preferable to use these dispersed beads by optimizing the particle size (bead diameter) and the filling rate. 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.

In one aspect, in the step of preparing the composition for forming a backcoat layer, after preparing a dispersion liquid containing a protrusion-forming agent (hereinafter, referred to as "protrusion-forming agent liquid"), this protrusion-forming agent liquid is used. It can be mixed with one or more of the other components of the backcoat layer forming composition. For example, the protrusion forming agent solution can be prepared by a known dispersion treatment such as ultrasonic treatment. The ultrasonic treatment can be performed for about 1 to 300 minutes with an ultrasonic output of about 10 to 2000 watts per 200 cc (1 cc = 1 cm ^{3), for example.} Further, filtration may be performed after the dispersion treatment. The above description can be referred to for the filter used for filtration.

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. In the form of the alignment treatment, the coating layer is oriented in the alignment zone while the coating layer of the composition for forming the magnetic layer is in a wet state. For the orientation treatment, various known techniques such as the description in paragraph 0052 of JP-A-2010-24113 can be applied. For example, the vertical alignment treatment can be performed by a known method such as a method using a magnet opposite to the opposite pole. In the alignment zone, the drying rate of the coating layer can be controlled by the temperature of the drying air, the air volume and / or the transport rate in the alignment zone. Alternatively, the coating layer may be pre-dried before being transported to the alignment zone.
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 0066 of Japanese Patent Application Laid-Open No. 2010-231843.
For other various steps for producing a magnetic recording medium, paragraphs 0067 to 0070 of JP2010-231843A can be referred to.

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 can be a tape-shaped magnetic recording medium (magnetic tape) in one form, and can be a disk-shaped magnetic recording medium (magnetic disk) in another form. 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 form, 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.

The magnetic tape is usually housed in a magnetic tape cartridge, and the magnetic tape cartridge is mounted in a magnetic recording / playback device.

In a magnetic tape cartridge, generally, the magnetic tape is housed inside the cartridge body in a state of being wound on a reel. The reel is rotatably provided inside the cartridge body. As the magnetic tape cartridge, a single reel type magnetic tape cartridge having one reel inside the cartridge main body and a twin reel type magnetic tape cartridge having two reels inside the cartridge main body are widely used. When the single reel type magnetic tape cartridge is attached to the magnetic recording / playback device for recording and / or playing back data on the magnetic tape, the magnetic tape is pulled out from the magnetic tape cartridge and the reel on the magnetic recording / playback device side. It is taken up by. A magnetic head is arranged in the magnetic tape transport path from the magnetic tape cartridge to the take-up reel. The magnetic tape is sent out and wound between the reel (supply reel) on the magnetic tape cartridge side and the reel (winding reel) on the magnetic recording / playback device side. During this time, the magnetic head and the surface of the magnetic layer of the magnetic tape come into contact with each other and slide to record and / or reproduce the data. On the other hand, in the twin reel type magnetic tape cartridge, both a supply reel and a take-up reel are provided inside the magnetic tape cartridge. The magnetic tape cartridge may be either a single reel type or a double reel type magnetic tape cartridge. Known techniques can be applied to other details of magnetic tape cartridges.

[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 data on a magnetic recording medium and reproducing data 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 sliding type magnetic recording / reproducing device means a device in which the surface of the magnetic layer and the magnetic head slide in contact with each other when recording data on a magnetic recording medium and / or reproducing the recorded data.

The magnetic head included in the magnetic recording / reproducing device can be a recording head capable of recording data on a magnetic recording medium, and can reproduce data recorded on the magnetic recording medium. Can also be. Further, in one form, the magnetic recording / reproducing device can include both a recording head and a reproducing head as separate magnetic heads. In another embodiment, the magnetic head included in the magnetic recording / reproducing device includes both an element for recording data (recording element) and an element for reproducing data (reproduction element) in one magnetic head. It is also possible to have a configuration. Hereinafter, the element for recording data and the element for reproducing data are collectively referred to as "data element". As the reproduction head, a magnetic head (MR head) including a magnetoresistive (MR) element capable of reading data recorded on a magnetic tape with high sensitivity as a reproduction element is preferable. As the MR head, various known MR heads such as an AMR (Anisotropic Magnetoresistive) head, a GMR (Giant Magnetoresistive) head, and a TMR (Tunnel Magnetoresistive) head can be used. Further, the magnetic head that records data and / or reproduces data may include a servo signal reading element. Alternatively, the magnetic recording / playback device may include a magnetic head (servo head) provided with a servo signal reading element as a head separate from the magnetic head that records data and / or reproduces data. For example, a magnetic head that records data and / or reproduces recorded data (hereinafter, also referred to as “recording / reproducing head”) can include two servo signal reading elements, and two servo signal reading elements. Each of the two adjacent servo bands can be read at the same time. One or more data elements can be arranged between the two servo signal reading elements.

In the magnetic recording / reproducing device, recording of data on a magnetic recording medium and / or reproduction of data recorded on a magnetic recording medium is 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. It can be carried out. 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.

For example, when recording data on a magnetic recording medium on which a servo pattern is formed and / or reproducing the recorded data, first, tracking is performed using a servo signal obtained by reading the servo pattern. That is, by making the servo signal reading element follow a predetermined servo track, the data element is controlled to pass on the target data track. The movement of the data track is performed by changing the servo track read by the servo signal reading element in the tape width direction.
The recording / playback head can also record and / or play back to other data bands. In that case, the servo signal reading element may be moved to a predetermined servo band by using the UDIM information described above, and tracking for the servo band may be started.

Hereinafter, the present invention will be described based on examples. However, the present invention is not limited to the embodiments shown in the examples. Unless otherwise specified, the indications of "part" and "%" described below indicate "parts by mass" and "% by mass". eq is an equivalent and is a unit that cannot be converted into SI units. 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 SO ₃ Na group-containing polyurethane resin: 4.0 parts (Weight average molecular weight 70000, SO ₃ Na 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: 150.0 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 70,000, SO ₃ Na group: 0.1 meq) / G)
2,3-Dihydroxynaphthalene: 0.6 parts Cyclohexanone: 23.0 parts (silica sol)
Colloidal silica (average particle size 120 nm): 2.0 parts 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 .0 copies

<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: 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>
(Protrusion forming agent liquid)
Protrusion forming agent (see Table 1): 1.3 parts
Methyl ethyl ketone: 9.0 parts Cyclohexanone: 6.0 parts (other components)
α-Iron oxide: See Table 1. Average particle size (average major axis length): 150 nm
Average needle-like ratio: 7
BET specific surface area: 52m ² / g
Carbon black: see Table 1 Average particle size 20 nm
Vinyl chloride copolymer (MR-104 manufactured by Kaneka): 12.0 parts SO ₃ Na group-containing polyurethane resin: 6.0 parts Phenylphosphonic acid: 3.0 parts Cyclohexanone: 155.0 parts Methyl ethyl ketone: 155.0 parts Stearic acid: 3.0 parts Phenyl stearate: 3.0 parts 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 silica sol and other components were introduced into a dissolver stirrer, stirred at a peripheral speed of 10 m / sec for 30 minutes, and then flowed by a flow type ultrasonic disperser at a flow rate of 7.5 kg / min. After 3 passes of treatment with, 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 are dispersed by a batch type vertical sand mill using zirconia beads having a bead diameter of 0.1 mm for 24 hours, and then filtered using a filter having a pore size of 0.5 μm. To prepare a composition for forming a non-magnetic layer.

<Preparation of composition for backcoat layer formation>
The protrusion-forming agent liquid is a dispersion obtained by mixing the components of the protrusion-forming agent liquid and then ultrasonically treating (dispersing) the protrusion-forming agent liquid with an ultrasonic output of 500 watts per 200 cc for 60 minutes using a horn-type ultrasonic disperser. Was prepared by filtering with a filter having a pore size of 0.5 μm.
The above-mentioned protrusion-forming agent solution and the other components of the above-mentioned backcoat layer-forming composition, excluding the lubricant (stearic acid and butyl stearate), polyisocyanate, and 200.0 parts of cyclohexanone, were added. After kneading and diluting with an open kneader, using zirconia beads with a bead diameter of 1 mm using a horizontal bead mill disperser, a bead filling rate of 80% by volume, a rotor tip peripheral speed of 10 m / sec, and a 1-pass residence time of 2 minutes for 12 passes. It was subjected to dispersion processing. To the obtained dispersion, a lubricant (stearic acid and butyl stearate), polyisocyanate and 200.0 parts of cyclohexanone were added, and the mixture was stirred with a dissolver. The dispersion liquid thus obtained was filtered using a filter having a pore size of 1 μm to prepare a composition for forming a backcoat layer.

<Making magnetic tape>
On the surface of a support made of biaxially stretched polyethylene naphthalate having a thickness of 5.0 μm, the composition for forming a non-magnetic layer prepared above so as to have a thickness of 400 nm after drying is applied and dried to form a non-magnetic layer. After the formation, the coating layer was formed by applying the magnetic layer forming composition 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 to perform a vertical alignment treatment and 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 0.2 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) using a 7-stage calendar roll composed of only metal rolls at a calendar speed of 100 m / min, a linear pressure of 294 kN / m, and a surface temperature of the calendar roll of 100 ° C. (Smoothing treatment), and then heat treatment was performed in an environment with an ambient temperature of 70 ° C. for 36 hours. After the heat treatment, the magnetic tape raw fabric is slit to a width of 1/2 inch by a cutting machine, and the tape is attached so that the non-woven fabric and the razor blade are pressed against the surface of the magnetic layer on a device having a slit product feeding and winding device. The surface of the magnetic layer was cleaned with a cleaning device to obtain a magnetic tape.

[Examples 2-9, Comparative Examples 1-5]
Example 1 except that the type of ferromagnetic powder, the type of protrusion-forming agent contained in the backcoat layer forming composition, the amount of α-iron oxide and / or the amount of carbon black were changed as shown in Table 1. A magnetic tape was obtained in the same manner as in the above. In Comparative Example 1, a composition for forming a backcoat layer was prepared without a protrusion-forming agent solution.

[Protrusion forming agent]
The protrusion-forming agents used in the preparation of the backcoat layer-forming composition for the preparation of the magnetic tapes of Examples or Comparative Examples are as follows. The protrusion forming agent A and the protrusion forming agent C are particles having low surface smoothness on the particle surface. The particle shape of the protrusion forming agent B is a cocoon-like shape. The particle shape of the protrusion forming agent D is a so-called amorphous shape. The particle shape of the protrusion forming agent E is close to a true sphere.
Protrusion forming agent A: ATLAS (composite particle of silica and polymer) manufactured by Cabot Corporation, average particle size 100 nm
Protrusion forming agent B: TGC6020N (silica particles) manufactured by Cabot Corporation, average particle size 140 nm
Protrusion forming agent C: Cataloid manufactured by JGC Catalysts and Chemicals Co., Ltd. (Aqueous dispersion sol of silica particles; as a projection forming agent for preparing a composition for forming a backcoat layer, obtained by heating the above aqueous dispersion sol to remove a solvent. Uses dry material), average particle size 120 nm
Protrusion forming agent D: Asahi Carbon Co., Ltd. Asahi # 52 (carbon black), average particle size 60 nm
Protrusion forming agent E: Quattron PL-10L manufactured by Fuso Chemical Industry Co., Ltd. (Aqueous dispersion sol of silica particles; as a projection forming agent for preparing a composition for forming a backcoat layer, the aqueous dispersion sol is heated to remove the solvent. Using the obtained dry material), average particle size 130 nm

[Ferromagnetic powder]
In Table 1, "BaFe" indicates a hexagonal barium ferrite powder having an average particle size (average plate diameter) of 21 nm. “SrFe1” and “SrFe2” indicate hexagonal strontium ferrite powder, and “ε-iron oxide” indicates ε-iron oxide powder.
The activated volume and anisotropy constant Ku of the various ferromagnetic powders described below can be determined for each ferromagnetic powder by the method described above using a vibrating sample magnetometer (manufactured by Toei Kogyo Co., Ltd.). Value.
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.).

<Method 1 for producing hexagonal strontium ferrite powder>
“SrFe1” shown in Table 1 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 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 hexagonal strontium ferrite powder (“SrFe1” in Table 1) obtained above has an average particle size of 18 nm, an activated volume of 902 nm ³ , and an anisotropy constant Ku of 2.2 × 10 ⁵ J / m ³ . The mass magnetization σs was 49 A · m ² / 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

<Method 2 for producing hexagonal strontium ferrite powder>
“SrFe2” shown in Table 1 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, so that 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 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.
(In Table 1, "SrFe2") resulting hexagonal strontium ferrite powder having an average particle size of 19 nm, activation volume 1102Nm ^3, the anisotropy constant Ku is ^{2.0 × 10 5 J / m 3} , the mass magnetization σs was 50 A · m ² / kg.

<Method of producing ε-iron oxide powder>
“Ε-Iron oxide” shown in Table 1 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, an aqueous 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 .28} Co _0.05 Ti _0.05 Fe _1.62 O ₃ ). Further, X-ray diffraction analysis was performed under the same conditions as those described for the method 1 for producing hexagonal strontium ferrite powder, and the ferromagnetic powder obtained from the peak of the X-ray diffraction pattern was the α phase and the γ phase. It was confirmed that it had a ε-phase single-phase crystal structure (ε-iron oxide type crystal structure) that did not include the crystal structure of.
(In Table 1, "ε- iron oxide") obtained ε- iron oxide powder having an average particle size of 12 nm, activation volume 746 nm ^3, the anisotropy constant Ku is 1.2 × ¹⁰ 5 J / ^{m 3} The mass magnetization σs was 16 A · m ² / kg.

[Evaluation methods]
(1) Difference (S _0.5- S _13.5 )
Using TSA (Tape Spacing Analyzer (manufactured by Micro Physics)), the spacing S _0.5 and S _13.5 after washing with n-hexane were measured by the following method, and the difference (S) from the measured values was measured. _0.5- S _13.5 ) was calculated.
Five sample pieces having a length of 5 cm were cut out from each of the magnetic tapes of Examples and Comparative Examples, and each sample piece was washed with n-hexane by the method described above, and then S _{0.5 and S were carried out by the following method. 13.5} was calculated.
A glass plate provided for TSA (Thorlabs, Inc. glass plate (model number: WG10530)) is provided on the surface of the backcoat layer of the magnetic tape (that is, the sample piece) and prepared for TSA as a pressing member. The hemisphere made of urethane was pressed against the surface of the magnetic layer of the magnetic tape at a pressure of 0.5 atm. In this state, white light is irradiated from the strobescope provided in the TSA to a certain region (150,000 to 200,000 μm ² ) on the surface of the backcoat layer of the magnetic tape through a glass plate, and the obtained reflected light is applied to an interference filter (wavelength). By receiving light with a CCD (Charge-Coupled Device) through a filter through which light of 633 nm is selectively transmitted), 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 of the glass plate to the surface of the backcoat layer of the magnetic tape is obtained, and this is used as a histogram, and the mode of the histogram is the spacing. Asked as.
The same sample piece was further pressed, and spacing was determined by the same method as above under a pressure of 13.5 atm.
The arithmetic mean of the spacing obtained for the five sample pieces under a pressure of 0.5 atm after _{washing with n-hexane was defined as spacing S 0.5} , and the five sample pieces were washed with n-hexane under a pressure of 13.5 atm. The arithmetic mean of the spacing obtained in (1) was defined as the spacing S _13.5 .

(2) Reduced playback output Each of the magnetic tapes of Examples and Comparative Examples is stored in a thermobox whose temperature is 50 ° C and relative humidity is 80% for 24 hours while being housed in a single reel type magnetic tape cartridge. did. Then, the magnetic tape cartridge was taken out from the thermobox (the outside air had a temperature of 23 ° C. and a relative humidity of 50%), and the inside was placed in a thermoroom kept at a temperature of 10 ° C. and a relative humidity of 85% within 1 minute. Then, within 30 minutes, recording and reproduction were performed in the thermoroom by the following method using a 1/2 inch reel tester with a fixed magnetic head.
The relative speed between the magnetic head and the magnetic tape is 5.5 m / sec, a MIG (Metal-In-Gap) head (gap length 0.15 μm, track width 1.0 μm) is used for recording, and the recording current is each. The optimum recording current of the magnetic tape was set. A GMR (Giant-Magnetoristive) head having an element thickness of 15 nm, a shield spacing of 0.1 μm, and a lead width of 0.5 μm was used for reproduction. A signal having a line recording density of 300 kfci was recorded, and the reproduced signal was measured with a spectrum analyzer manufactured by ShibaSoku Co., Ltd. As the signal, the part where the signal was sufficiently stable after the start of running of the magnetic tape was used. The unit kfci is a unit of line recording density (cannot be converted into the SI unit system).
Under the above conditions, reproduction was repeated 3000 passes at 1000 m per pass. The output value of the carrier signal of the 1st pass and the output value of the carrier signal of the 3000th pass are obtained respectively, and the difference "(output value of the 3000th pass)-(output value of the 1st pass)" is used as the reduction in the reproduction output. It is shown in Table 1.

The above results are shown in Table 1.

From the results shown in Table 1, _{the magnetic tapes of Examples 1 to 9 in which the difference (S 0.5-} S _13.5 ) measured on the surface of the backcoat layer was 3 nm or less were the same as those of Comparative Examples 1 to 5. In comparison, it can be confirmed that the reduction in the reproduction output is small even if the reproduction is repeated in the low temperature and high humidity environment after the storage in the high temperature environment.

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

Claims (10)

A magnetic recording medium having a magnetic layer containing ferromagnetic powder on one surface side of a non-magnetic support and a backcoat layer containing non-magnetic powder on the other surface side.
_{Spacing S 0.5} measured under pressure of 0.5 atm by optical interferometry after n-hexane cleaning on the surface of the backcoat layer, and optical interferometry after n-hexane cleaning on the surface of the backcoat layer. A magnetic recording medium in which the difference from the _{spacing S 13.5} measured under pressure of _{13.5 atm, S 0.5-} S _13.5 , is 3 nm or less. The magnetic recording medium according to claim 1, wherein the difference is 1 nm or more and 3 nm or less. The magnetic recording medium according to claim 1 or 2, wherein S _{0.5 is 20 nm or more and 90 nm or less.} The magnetic recording medium according to any one of claims 1 to 3, wherein S _{13.5 is 20 nm or more and 90 nm or less.} The magnetic recording medium according to any one of claims 1 to 4, wherein the backcoat layer contains inorganic oxide-based particles. The magnetic recording medium according to claim 5, wherein the inorganic oxide-based particles are composite particles of an inorganic oxide and a polymer. The magnetic recording medium according to any one of claims 1 to 6, wherein the backcoat layer has a thickness of 0.5 μm or less. The magnetic recording medium according to any one of claims 1 to 7, 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 8, which is a magnetic tape. The magnetic recording medium according to any one of claims 1 to 9,
With a magnetic head
Magnetic recording / playback device including.