JP7351824B2 - Magnetic recording media and magnetic recording/reproducing devices - Google Patents

Description

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

Regarding magnetic recording media, a back coat layer is provided on the surface side of a nonmagnetic support opposite to the surface side having the magnetic layer (see, for example, paragraphs 0040 to 0044 of Patent Document 1).

Japanese Patent Application Publication No. 2012-43495

In recent years, magnetic recording media have been stored and used under various environments. For example, magnetic recording media may be stored in temperature-controlled data centers. However, from the viewpoint of reducing air conditioning costs for temperature control, it is desirable to be able to relax the temperature control conditions during storage or eliminate the need for temperature control. In view of the above, the present inventor investigated the performance of magnetic recording media after being stored in a high temperature environment. As a result of this study, it was found that a magnetic recording medium having a back coat layer has a problem in that the reproduction output decreases when reproduction is repeated in a low temperature and high humidity environment after being stored in a high temperature environment.

One aspect of the present invention is to provide a magnetic recording medium having a back coat layer, which exhibits little decrease in playback output even if it is repeatedly played back in a low temperature and high humidity environment after being stored 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 back coat layer containing non-magnetic powder on the other surface side,
Spacing S _0.5 measured under a pressure of 0.5 atm by optical interferometry after washing with n-hexane on the surface of the back coat layer, and by optical interferometry after washing with n-hexane on the surface of the back coat layer. A magnetic recording medium in which the difference (S _0.5 - S _13.5 ) from the spacing S _13.5 measured under a pressure of 13.5 atm is 3 nm or less,
Regarding. In the following, the difference (S _0.5 −S _13.5 ) will also be simply referred to as a “difference”. Moreover, 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 greater than or equal to 20 nm and less than or equal to 90 nm.

In one form, S _13.5 can be greater than or equal to 20 nm and less than or equal to 90 nm.

In one form, the back coat layer can include inorganic oxide particles.

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

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

In one form, the magnetic recording medium may have a nonmagnetic layer containing nonmagnetic powder between the nonmagnetic 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 above magnetic recording medium and a magnetic head.

According to one aspect of the present invention, there is provided a magnetic recording medium having a back coat layer, which exhibits little decrease in playback output even if it is repeatedly played back in a low temperature and high humidity environment after being stored 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 ferromagnetic powder on one surface side of a nonmagnetic support, and a back coat layer containing nonmagnetic powder on the other surface side, Spacing S _0.5 measured under a pressure of 0.5 atm by optical interferometry after washing with n-hexane on the surface of the back coat layer, and by optical interferometry after washing with n-hexane on the surface of the back coat layer. The present invention relates to a magnetic recording medium in which the difference (S _0.5 - S _13.5 ) from the spacing S _13.5 measured under a pressure of 13.5 atm is 3 nm or less.

In the present invention and this specification, "n-hexane cleaning" refers to ultrasonic cleaning for 10 seconds by immersing a sample piece cut out from a magnetic recording medium in fresh n-hexane (200 g) at a liquid temperature of 20 to 25 °C. (Ultrasonic output: 40kHz). If the magnetic recording medium to be cleaned is a magnetic tape, a sample piece of 5 cm in length is cut out and subjected to n-hexane cleaning. The width of the magnetic tape and the width of the sample pieces cut from the magnetic tape are typically 1/2 inch. 1 inch = 0.0254 meter. For magnetic tapes other than 1/2 inch wide, a sample piece of 5 cm in length may be cut out and washed with n-hexane. If the magnetic recording medium to be cleaned is a magnetic disk, a sample piece with a size of 5 cm x 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 washed with n-hexane is left in an environment at a temperature of 23° C. and a relative humidity of 50% for 24 hours.

In the present invention and this specification, the "surface of the backcoat layer" of the magnetic recording medium has the same meaning as the surface of the magnetic recording medium on the backcoat layer side.

In the present invention and this specification, the spacing measured by optical interferometry on the surface of the back coat layer of a magnetic recording medium is a value measured by the following method.
A magnetic recording medium (more specifically, the sample piece mentioned above; the same applies hereinafter) and a transparent plate-like member (for example, a glass plate) are stacked so that the surface of the back coat layer of the magnetic recording medium faces the transparent plate-like member. In this state, a 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, light is irradiated onto the surface of the back coat layer of the magnetic recording medium through the transparent plate member (irradiation area: 150,000 to 200,000 μm ² ), and the reflected light from the surface of the back coat layer of the magnetic recording medium and the transparent Based on the intensity of the interference light generated due to the optical path difference between the light reflected from the surface of the plate-shaped member on the magnetic recording medium side (for example, the contrast of the interference fringe image), the difference between the back coat layer surface of the magnetic recording medium and the transparent plate-shaped member is determined. Find the spacing (distance) from the magnetic recording medium side surface. The light irradiated here is not particularly limited. If the emitted light is light that has emission wavelengths over a relatively wide range of wavelengths, such as white light that includes light of multiple wavelengths, a transparent plate-like member and a light-receiving part that receives reflected light may be used. In between, a member such as an interference filter that has the function of selectively cutting off light of a specific wavelength or light outside of a specific wavelength range is placed, and a member having a function of selectively cutting off light of a specific wavelength or light outside of a specific wavelength range is placed in between. Light is selectively made incident on the light receiving section. If the light to be irradiated is light having a single emission peak (so-called monochromatic light), the above-mentioned member may not be used. The wavelength of the light incident on the light receiving section can be, for example, in the range of 500 to 700 nm. However, the wavelength of the light incident on the light receiving section is not limited to the above range. Further, the transparent plate member may be any member as long as it is transparent enough to allow the irradiated light to pass therethrough to the extent that the magnetic recording medium is irradiated with light through the member and interference light is obtained.
The interference fringe image obtained by the above spacing measurement is divided into 300,000 points and the spacing of each point (distance between the back coat layer surface of the magnetic recording medium and the magnetic recording medium side surface of the transparent plate-like member) , and use this as a histogram, and the mode in this histogram as the spacing.
Five sample pieces were cut out from the same magnetic recording medium, and after cleaning each sample piece with n-hexane, the spacing was determined by pressing a pressing member at a pressure of 0.5 atm, and further pressing the pressing member at a pressure of 13.5 atm. Ask for spacing. The arithmetic mean of the spacings obtained under a pressure of 0.5 atm after cleaning with n-hexane for the five sample pieces is taken as spacing S _0.5 , and the pressure of 13.5 atm after cleaning with n-hexane for the five sample pieces is taken as spacing S 0.5. Let the arithmetic mean of the spacings determined below be the spacing S _13.5 . The difference (S _0.5 - S _13.5 ) between S 0.5 and S _13.5 thus obtained is defined as the difference ( _{S 0.5 -} S _13.5 ) for the magnetic recording medium.
The above measurements can be performed using a commercially available tape spacing analyzer (TSA) such as Tape Spacing Analyzer manufactured by Micro Physics. Spacing measurements in Examples were performed using Tape Spacing Analyzer manufactured by Micro Physics.

The above magnetic recording medium has a spacing S measured under a pressure of _0.5 atm by optical interferometry after washing with n-hexane on the surface of the back coat layer, and a spacing S measured under a pressure of 13.5 atm. The difference from S _13.5 (S _0.5 - S _13.5 ) is 3 nm or less. Thereby, according to the magnetic recording medium, it is possible to suppress a decrease in reproduction output during repeated reproduction in a low temperature and high humidity environment after storage in a high temperature environment. The inventor's speculations regarding this point are as follows.

It is presumed that the decrease in playback output during repeated playback occurs because components that can contribute to running stability (such as lubricants, which will be described later) are reduced from the magnetic layer during storage before the start of playback. . For example, if the surface of the magnetic layer and the surface of the back coat layer are in contact with each other during storage, it is thought that the above components will be transferred from the surface of the magnetic layer to the back coat layer side and will be reduced from the magnetic layer. . When stored in a high-temperature environment, for example, a magnetic recording medium (such as a magnetic tape) wound on a reel and housed in a cartridge may be wound tightly, causing the back coat layer surface to receive high pressure from the magnetic layer surface. It is assumed that this is why the amount of the above-mentioned components transferred from the surface of the magnetic layer to the back coat layer side increases. Furthermore, running stability tends to deteriorate in low-temperature, high-humidity environments, so if data recorded on a magnetic layer in which the above components have been reduced during storage is repeatedly reproduced in low-temperature, high-humidity environments, the reproduction output will decrease. It is inferred that a decrease is likely to occur. High temperature can be, for example, a temperature of about 30 to 50°C, low temperature can be, for example, a temperature of about 0 to 15°C, and high humidity can be, for example, a relative humidity of about 70 to 100%. Something can happen.
Regarding the above points, the present inventor has repeatedly studied and found that it is possible to suppress the deformation of the back coat layer surface when subjected to high pressure. We thought that this might contribute to suppressing the transfer of a large amount of the above components to. The details are as follows.
On the surface of the backcoat layer, protrusions formed from nonmagnetic powder contained in the backcoat layer may normally exist in a state embedded in the base portion of the backcoat layer. It is presumed that if these protrusions sink deeply into the backcoat layer when subjected to high pressure, minute gaps will be created between the protrusions and the base portion of the backcoat layer. It is thought that the entry of the above-mentioned components into these minute gaps by capillary action increases the amount of the above-mentioned components transferred to the surface of the back coat layer. Therefore, suppressing the deformation of the backcoat layer surface when subjected to high pressure prevents components that can contribute to the running stability of the magnetic layer from being transferred to the backcoat layer side during storage in high-temperature environments. The present inventor thought that this would lead to suppressing. Regarding such knowledge, JP-A-2012-43495 (Patent Document 1) does not describe anything. As a result of further studies based on this knowledge, the present inventors determined that by setting the difference (S _0.5 - S _13.5 ) measured on the surface of the back coat layer to 3 nm or less, We have newly discovered that it is possible to suppress the decline in playback output during repeated playback in low temperature, high humidity environments after storage. The fact that the difference (S _0.5 - S _13.5 ) is as small as 3 nm or less indicates that the protrusions on the surface of the back coat layer are difficult to sink into the inside of the back coat layer when high pressure is applied as described above. It is thought that there are.
However, the present invention is not limited to the above speculation. Furthermore, the present invention is not limited to other speculations made by the inventors described in this specification. Regarding the pressing pressures of 13.5 atm and 0.5 atm in the spacing measurement for determining the above-mentioned difference, in the present invention, an exemplary 13.5 atm is adopted as the value, and 0.5 atm is adopted as an exemplary value of the pressure that can be applied to the surface of the back coat layer when it comes into contact with the surface of the magnetic layer in other conditions. The pressure that can be applied to the magnetic recording medium at certain times is not limited to these pressures.

The above magnetic recording medium will be explained in more detail below.

<Difference (S _0.5 - S _13.5 )>
The difference (S _0.5 - _{S 13.5} ) measured on the surface of the back coat layer of the above magnetic recording medium is 3 nm or less, which indicates that the difference (S 0.5 - S 13.5 ) during repeated playback in a low temperature and high humidity environment after storage in a high temperature environment From the viewpoint of further suppressing a decrease in reproduction output, the thickness is preferably 2 nm or less. Moreover, the above-mentioned difference can be 0 nm or more, can also be more than 0 nm, and can also 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 each be, for example, 20 nm or more, 22 nm or more, or 25 nm or more. Further, S _0.5 and S _13.5 can each be, for example, 90 nm or less, 85 nm or less, or 80 nm or less.

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

Next, the magnetic layer, backcoat layer, nonmagnetic support, and optionally included nonmagnetic layer of the magnetic recording medium will be further explained.

<Magnetic layer>
(Ferromagnetic powder)
As the ferromagnetic powder contained in the magnetic layer, one type or a combination of two or more types of ferromagnetic powders known as ferromagnetic powders used in the magnetic layers of various magnetic recording media can be used. It is preferable to use ferromagnetic powder having a small average particle size from the viewpoint of improving recording density. From this point of view, the average particle size of the ferromagnetic powder is preferably 50 nm or less, more preferably 45 nm or less, even more preferably 40 nm or less, even more preferably 35 nm or less, and 30 nm or less. It is even more preferable that it is, even more preferably that it is 25 nm or less, and even more preferably that it is 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, even more preferably 10 nm or more, and still more preferably 15 nm or more. is more preferable, and even more preferably 20 nm or more.

Hexagonal ferrite powder A preferred example of the ferromagnetic powder is hexagonal ferrite powder. For details on hexagonal ferrite powder, see, for example, paragraphs 0012 to 0030 of JP2011-225417A, paragraphs 0134 to 0136 of JP2011-216149A, paragraphs 0013 to 0030 of JP2012-204726A, and Paragraphs 0029 to 0084 of Japanese Patent Application Publication No. 2015-127985 can be referred to.

In the present invention and this specification, "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 most intense diffraction peak belongs in an X-ray diffraction spectrum obtained by X-ray diffraction analysis. For example, if the most intense diffraction peak in an X-ray diffraction spectrum obtained by X-ray diffraction analysis is attributed to a hexagonal ferrite crystal structure, it is determined that a hexagonal ferrite crystal structure has been detected as the main phase. shall be taken as a thing. When only a single structure is detected by X-ray diffraction analysis, this detected structure is taken as the main phase. The hexagonal ferrite type crystal structure includes at least iron atoms, divalent metal atoms, and oxygen atoms as constituent atoms. The divalent metal atom is a metal atom that can become a divalent cation as an ion, and includes alkaline earth metal atoms such as strontium atom, barium atom, and calcium atom, lead atom, and the like. In the present invention and this specification, hexagonal strontium ferrite powder refers to powder in which the main divalent metal atoms contained are strontium atoms, and hexagonal barium ferrite powder refers to powder in which the main divalent metal atoms contained in this powder are strontium atoms. The divalent metal atom is a barium atom. The main divalent metal atoms are the divalent metal atoms that account for the largest percentage on an atomic percent basis among the divalent metal atoms contained in this powder. However, the divalent metal atoms mentioned above do not include rare earth atoms. A "rare earth atom" in the present invention and herein is selected from the group consisting of scandium atom (Sc), yttrium atom (Y), and lanthanide atom. Lanthanoid atoms include lanthanum atom (La), cerium atom (Ce), praseodymium atom (Pr), neodymium atom (Nd), promethium atom (Pm), samarium atom (Sm), europium atom (Eu), and gadolinium atom (Gd). ), terbium atom (Tb), dysprosium atom (Dy), holmium atom (Ho), erbium atom (Er), thulium atom (Tm), ytterbium atom (Yb), and lutetium atom (Lu). Ru.

Below, hexagonal strontium ferrite powder, which is one form of hexagonal ferrite powder, will be explained in more detail.

The activation volume of the hexagonal strontium ferrite powder is preferably in the range of 800 to 1600 nm ³ . Finely divided hexagonal strontium ferrite powder exhibiting an activation volume within the above range is suitable for producing a magnetic recording medium exhibiting excellent electromagnetic conversion characteristics. The activation volume of the hexagonal strontium ferrite powder is preferably 800 nm ³ or more, and can also be, for example, 850 nm ³ or more. In addition, from the viewpoint of further improving electromagnetic conversion characteristics, the activated volume of the hexagonal strontium ferrite powder is more preferably 1500 nm ³ or less, even more preferably 1400 nm ³ or less, and 1300 nm ³ or less. is more preferable, it is even more preferable that it is 1200 nm ³ or less, and even more preferably that it is 1100 nm ³ or less. The same applies to the activation volume of hexagonal barium ferrite powder.

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

The anisotropy constant Ku can be cited as an index of reduction in thermal fluctuation, in other words, improvement in thermal stability. The hexagonal strontium ferrite powder can preferably have a Ku of 1.8×10 ⁵ J/m ³ or more, more preferably 2.0×10 ⁵ J/m ³ or more. Moreover, Ku of the hexagonal strontium ferrite powder can be, for example, 2.5×10 ⁵ J/m ³ or less. However, since a higher Ku means higher thermal stability and is preferable, it is not limited to the values exemplified above.

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 rare earth atoms are contained at a content (bulk content) of 0.5 to 5.0 at % based on 100 at % iron atoms. In one form, the hexagonal strontium ferrite powder containing rare earth atoms can have rare earth atoms unevenly distributed in the surface layer. In the present invention and this specification, "rare earth atom uneven distribution in the surface layer" refers to the rare earth atom content (hereinafter, "Rare earth atom surface content" or simply "surface layer content" for rare earth atoms) is 100 at.% of iron atoms in the solution obtained by completely dissolving hexagonal strontium ferrite powder with acid. Rare earth atom content (hereinafter referred to as "rare earth atom bulk content" or simply "bulk content" with respect to rare earth atoms),
Rare earth atom surface layer content/rare earth atom bulk content>1.0
This means that the ratio of The rare earth atom content of the hexagonal strontium ferrite powder described below 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, so the rare earth atom content in the solution obtained by partial dissolution is This is the rare earth atom content in the surface layer of the particles. The fact that the rare earth atom content in the surface layer satisfies the ratio of "rare earth atom surface layer content/rare earth atom bulk content >1.0" means that rare earth atoms are present in the surface layer in the particles constituting the hexagonal strontium ferrite powder. It means that it is omnipresent (that is, it is present more than inside). In the present invention and the present specification, the surface layer portion refers to a partial region from the surface of the particle constituting the hexagonal strontium ferrite powder toward the inside.

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 % based on 100 atom % of iron atoms. It is presumed that the uneven distribution of rare earth atoms in the surface layer of the particles constituting the hexagonal strontium ferrite powder that contains rare earth atoms at the bulk content in the above range contributes to increasing the anisotropy constant Ku. . The higher the value of the anisotropy constant Ku, the more the phenomenon called thermal fluctuation can be suppressed (in other words, the thermal stability can be improved). The uneven distribution of rare earth atoms in the particle surface of hexagonal strontium ferrite powder contributes to stabilizing the spin of iron (Fe) sites in the crystal lattice in the surface layer, which increases the anisotropy constant Ku. It is speculated that this will increase.
In addition, it is assumed that using hexagonal strontium ferrite powder, which has rare earth atoms unevenly distributed in the surface layer, as the ferromagnetic powder for the magnetic layer also contributes to suppressing the surface of the magnetic layer from being scraped by sliding with the magnetic head. Ru. That is, it is surmised that the hexagonal strontium ferrite powder having rare earth atoms unevenly distributed in the surface layer may also contribute to improving the running durability of the magnetic recording medium. This is due to the uneven distribution of rare earth atoms on the surface of the particles constituting the hexagonal strontium ferrite powder, which improves the interaction between the particle surface and the organic substances (e.g. binders and/or additives) contained in the magnetic layer. It is presumed that this is because the magnetic layer contributes to the strength of the magnetic layer, and as a result, the strength of the magnetic layer is improved.
From the perspective of further improving the anisotropy constant Ku and/or further improving running durability, the rare earth atom content (bulk content) is preferably in the range of 0.5 to 4.5 at%. It is more preferably in the range of 1.0 to 4.5 atom %, even more preferably in the range of 1.5 to 4.5 atom %.

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

When the hexagonal strontium ferrite powder contains rare earth atoms, the rare earth atoms contained may be at least one kind of rare earth atoms. Preferable rare earth atoms from the viewpoint of further improving the anisotropy constant Ku include neodymium atoms, samarium atoms, yttrium atoms, and dysprosium atoms, with neodymium atoms, samarium atoms, and yttrium atoms being more preferable, and neodymium atoms being more preferable. More preferred.

In the hexagonal strontium ferrite powder having rare earth atoms unevenly distributed in the surface layer, it is sufficient that the rare earth atoms are unevenly distributed in the surface layer of the particles constituting the hexagonal strontium ferrite powder, and the degree of uneven distribution is not limited. For example, for a hexagonal strontium ferrite powder that has rare earth atoms unevenly distributed in the surface layer, the surface layer content of rare earth atoms determined by partial melting under the melting conditions described below and the rare earth content determined by completely melting under the melting conditions described below. The ratio of atoms 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 ratio/bulk content ratio" is larger than 1.0, it means that rare earth atoms are unevenly distributed in the surface layer (that is, more present than in the interior) in the particles that make up the hexagonal strontium ferrite powder. do. In addition, the ratio of the surface layer content of rare earth atoms determined by partial dissolution under the dissolution conditions described below and the bulk content of rare earth atoms determined by total dissolution under the dissolution conditions described below, ``surface layer content/ 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 a hexagonal strontium ferrite powder having rare earth atoms unevenly distributed in the surface layer, it is sufficient that the rare earth atoms are unevenly distributed in the surface layer of the particles constituting the hexagonal strontium ferrite powder, and the above "surface layer content/bulk content" is sufficient. The "rate" is not limited to the illustrated upper or lower limits.

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

From the viewpoint of increasing reproduction output when reproducing data recorded on a 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, a hexagonal strontium ferrite powder that contains rare earth atoms but does not have rare earth atoms unevenly distributed in the surface layer tends to have a significantly lower σs than a hexagonal strontium ferrite powder that does not contain rare earth atoms. On the other hand, a hexagonal strontium ferrite powder having rare earth atoms unevenly distributed in the surface layer 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. On the other hand, from the viewpoint of noise reduction, σs is preferably 80 A·m ² /kg or less, more preferably 60 A·m ² /kg or less. σs can be measured using a known measuring device capable of measuring magnetic properties, such as a vibrating sample magnetometer. In the present invention and this specification, unless otherwise specified, 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 atomic content can be in the range of, for example, 2.0 to 15.0 atomic% with respect to 100 atomic% of iron atoms. . In one form, the hexagonal strontium ferrite powder can have strontium atoms as the only divalent metal atoms contained in the powder. In another form, the hexagonal strontium ferrite powder can also contain one or more other divalent metal atoms in addition to strontium atoms. For example, it can contain barium atoms and/or calcium atoms. When divalent metal atoms other than strontium atoms are included, the barium atom content and calcium atom content in the hexagonal strontium ferrite powder are, for example, 0.05 to 5, respectively, relative to 100 at% of iron atoms. It can be in the range of .0 at.%.

As the crystal structure of hexagonal ferrite, magnetoplumbite type (also called "M type"), W type, Y type, and Z type are known. The hexagonal strontium ferrite powder may have any crystal structure. Crystal structure can be confirmed by X-ray diffraction analysis. The hexagonal strontium ferrite powder may have a single crystal structure or two or more types of crystal structures detected by X-ray diffraction analysis. For example, in one form, the hexagonal strontium ferrite powder can have only an M-type crystal structure detected by X-ray diffraction analysis. For example, M-type hexagonal ferrite is represented by the composition formula AFe ₁₂ O ₁₉ . Here, A represents a divalent metal atom, and if the hexagonal strontium ferrite powder is M type, A is only a strontium atom (Sr), or if A contains multiple divalent metal atoms, As mentioned above, strontium atoms (Sr) account for the largest amount on an atomic percent basis. The divalent metal atom content of the hexagonal strontium ferrite powder is usually determined by the type of crystal structure of the hexagonal ferrite, and is not particularly limited. The same applies to the iron atom content and the oxygen atom content. The hexagonal strontium ferrite powder contains at least iron atoms, strontium atoms, and oxygen atoms, and may also contain rare earth atoms. Furthermore, the hexagonal strontium ferrite powder may or may not contain atoms other than these atoms. As an example, the hexagonal strontium ferrite powder may contain aluminum atoms (Al). The content of aluminum atoms can be, for example, 0.5 to 10.0 atomic % with respect to 100 atomic % of iron atoms. From the viewpoint of further improving the anisotropy 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 at % of iron atoms. 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 may not contain atoms other than iron atoms, strontium atoms, oxygen atoms, and rare earth atoms. The content expressed in atomic % above is the content of each atom (unit: mass %) obtained by completely melting the hexagonal strontium ferrite powder, and is converted to the value expressed in atomic % using the atomic weight of each atom. It can be calculated by converting. Furthermore, in the present invention and this specification, "not containing" an atom means that the content thereof as measured by an ICP analyzer after being completely dissolved 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 "does not contain" is used to mean that it is contained in an amount below the detection limit of the ICP analyzer. In one form, the hexagonal strontium ferrite powder can be free of bismuth atoms (Bi).

Metal Powder A preferable specific example of the ferromagnetic powder is a ferromagnetic metal powder. For details of the ferromagnetic metal powder, reference can be made to, for example, paragraphs 0137 to 0141 of JP-A No. 2011-216149 and paragraphs 0009 to 0023 of JP-A No. 2005-251351.

ε-Iron Oxide Powder A preferred example of the ferromagnetic powder is ε-iron oxide powder. In the present invention and this specification, "ε-iron oxide powder" refers to a ferromagnetic powder in which an ε-iron oxide type crystal structure is detected as the main phase by X-ray diffraction analysis. For example, if the most intense diffraction peak in an X-ray diffraction spectrum obtained by X-ray diffraction analysis is attributed to the ε-iron oxide type crystal structure, it is assumed that the ε-iron oxide type crystal structure has been detected as the main phase. shall judge. Known methods for producing ε-iron oxide powder include a method for producing it from goethite, a reverse micelle method, and the like. All of the above manufacturing methods are publicly known. Further, regarding a method for producing ε-iron oxide powder in which a portion of Fe is substituted with substitution atoms such as Ga, Co, Ti, Al, Rh, etc., see, for example, J. Jpn. Soc. Powder Metallurgy Vol. 61 Supplement, No. S1, pp. S280-S284, J. Mater. Chem. C, 2013, 1, pp. 5200-5206 etc. can be referred to. However, the method for producing the 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 method mentioned above.

The activated volume of the ε-iron oxide powder is preferably in the range of 300 to 1500 nm ³ . Finely divided ε-iron oxide powder exhibiting an activation volume within 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 also be, for example, 500 nm ³ or more. Further, from the viewpoint of further improving electromagnetic conversion characteristics, the activated volume of the ε-iron oxide powder is more preferably 1400 nm ³ or less, even more preferably 1300 nm ³ or less, and 1200 nm ³ or less. is more preferable, and even more preferably 1100 nm ³ or less.

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

From the viewpoint of increasing reproduction output when reproducing data recorded on a 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 greater than or equal to 8 A·m ² /kg, and may also be greater than or equal to 12 A·m ² /kg. On the other hand, from the viewpoint of noise reduction, σs of the ε-iron oxide powder is preferably 40 A·m ² /kg or less, more preferably 35 A·m ² /kg or less.

In the present invention and this specification, unless otherwise specified, the average particle size of various powders such as ferromagnetic powder is a value measured by the following method using a transmission electron microscope.
The powder is photographed using a transmission electron microscope at a magnification of 100,000 times and printed on photographic paper at a total magnification of 500,000 times to obtain a photograph of the particles constituting the powder. Select the target particle from the obtained particle photo, trace the outline of the particle with a digitizer, and measure the size of the particle (primary particle). Primary particles refer to independent particles without agglomeration.
The above measurements are performed on 500 randomly extracted particles. The arithmetic mean of the particle sizes of the 500 particles thus obtained is defined as the average particle size of the powder. As the transmission electron microscope, for example, a transmission electron microscope model H-9000 manufactured by Hitachi may be used. Furthermore, the particle size can be measured using known image analysis software, such as Carl Zeiss image analysis software KS-400. Unless otherwise specified, the average particle size shown in the examples below was measured using a transmission electron microscope H-9000 model manufactured by Hitachi as a transmission electron microscope and image analysis software KS-400 manufactured by Carl Zeiss as image analysis software. It is a value. In the present invention and herein, powder refers to a collection of particles. For example, ferromagnetic powder means a collection of multiple ferromagnetic particles. Furthermore, an aggregate of multiple particles is not limited to a form in which the particles constituting the aggregate are in direct contact with each other, but also includes a form in which a binder, an additive, etc., which will be described later, are interposed between the particles. Ru. The term particle is sometimes used to refer to 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 JP-A No. 2011-048878 can be adopted.

In the present invention and this specification, unless otherwise specified, the size of the particles constituting the powder (particle size) is as follows:
(1) If the particle is needle-shaped, spindle-shaped, columnar (however, the height is larger than the maximum major axis of the bottom surface), etc., it is expressed by the length of the major axis that makes up 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 expressed 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 identified from the shape, it is expressed by an equivalent circle diameter. The equivalent circle diameter refers to what is determined by the circular projection method.

In addition, the average acicular ratio of the powder can be determined by measuring the short axis length of the particles in the above measurement, determining the value of (major axis length / short axis length) of each particle, and calculating the average acicular ratio of the 500 particles. Refers to the arithmetic mean of the values obtained for the particles. Here, unless otherwise specified, the short axis length refers to the length of the short axis constituting the particle in the case of (1) in the above particle size definition, and the thickness or height in the case of (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 particle shape is specific, for example, in the case of the above definition (1) of particle size, the average particle size is the average major axis length, and in the case of the same definition (2), the average particle size is This is the average plate diameter. In the case of the same definition (3), the average particle size is the average diameter (also referred to as average particle diameter or average particle diameter).

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

(Binder)
The magnetic recording medium may be a coated magnetic recording medium, and the magnetic layer may contain a binder. A binder is one or more resins. As the binder, various resins commonly used as binders for coated magnetic recording media can be used. For example, binders include polyurethane resins, polyester resins, polyamide resins, vinyl chloride resins, acrylic resins copolymerized with styrene, acrylonitrile, methyl methacrylate, etc., cellulose resins such as nitrocellulose, epoxy resins, phenoxy resins, polyvinyl acetals, A resin selected from polyvinyl alkyral resins such as polyvinyl butyral can be used alone, or a plurality of resins can be used in combination. Among these, preferred are polyurethane resins, acrylic resins, cellulose resins, and vinyl chloride resins. These resins may be homopolymers or copolymers. These resins can also be used as a binder in the back coat layer and/or the nonmagnetic layer described below.
Regarding the above binders, reference can be made to paragraphs 0028 to 0031 of JP-A No. 2010-24113. 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 a weight average molecular weight. The weight average molecular weight in the present invention and this specification is a value determined by gel permeation chromatography (GPC) under the following measurement conditions in terms of polystyrene. The weight average molecular weight of the binder shown in the Examples below is a value determined by converting the 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 weight based on 100.0 parts by weight of the ferromagnetic powder.
GPC device: HLC-8120 (manufactured by Tosoh Corporation)
Column: TSK gel Multipore HXL-M (manufactured by Tosoh Corporation, 7.8mm ID (Inner Diameter) x 30.0cm)
Eluent: Tetrahydrofuran (THF)

A curing agent can also be used in conjunction with a resin that can be used as a binder. In one form, the curing agent can be a thermosetting compound, which is a compound that undergoes a curing reaction (crosslinking reaction) by heating, and in another form, it can be a photocuring compound, in which the curing reaction (crosslinking reaction) proceeds by light irradiation. It can be a chemical compound. As a curing reaction progresses during the magnetic layer forming process, at least a portion of the curing agent may be contained in the magnetic layer in a reacted (crosslinked) state with other components such as a binder. This point also applies to layers formed using the composition when the composition used to form the other layer contains a curing agent. Preferred curing agents are thermosetting compounds, with polyisocyanates being preferred. For details of the polyisocyanate, paragraphs 0124 to 0125 of JP-A No. 2011-216149 can be referred to. The curing agent is contained in the magnetic layer forming composition in an amount of, for example, 0 to 80.0 parts by mass based on 100.0 parts by mass of the binder, and preferably 50.0 to 80.0 parts by mass from the viewpoint of improving the strength of the magnetic layer. It can be used in amounts of parts by weight.

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

(Additive)
The magnetic layer may contain one or more additives as necessary. Examples of the additives include the above-mentioned curing agents. Further, examples of additives contained in the magnetic layer include nonmagnetic powder, lubricant, dispersant, dispersion aid, fungicide, antistatic agent, antioxidant, and the like. Examples of the non-magnetic powder include non-magnetic powder that can function as an abrasive, and non-magnetic powder that can function as a protrusion-forming agent that forms protrusions that appropriately protrude on the surface of the magnetic layer (for example, non-magnetic colloidal particles). etc. Note that the average particle size of colloidal silica (silica colloidal particles) shown in the Examples below is a value determined by the method described in paragraph 0015 of JP 2011-048878A as the method for measuring the average particle size. . Examples of additives that can be used in the magnetic layer containing an abrasive include the dispersants described in paragraphs 0012 to 0022 of JP-A No. 2013-131285 as a dispersant for improving the dispersibility of the abrasive. be able to. Regarding the dispersant, paragraphs 0061 and 0071 of JP 2012-133837A can be referred to. Further, regarding the dispersant, paragraphs 0061 and 0071 of JP2012-133837A and paragraph 0035 of JP2017-016721A can also be referred to. Regarding additives for the magnetic layer, paragraphs 0035 to 0077 of JP-A No. 2016-51493 can also be referred to.
The dispersant may be included in the nonmagnetic layer. Regarding the dispersant that can be included in the nonmagnetic layer, paragraph 0061 of JP 2012-133837A can be referred to.
Various additives can be used in any amount by appropriately selecting commercially available products or by manufacturing them by known methods depending on the desired properties.

In one form, the magnetic recording medium can include a lubricant in a portion of the nonmagnetic support on the magnetic layer side. The part on the magnetic layer side on the non-magnetic support is the magnetic layer in a magnetic recording medium that has a magnetic layer directly on the non-magnetic support, and the non-magnetic layer and the magnetic layer are placed on the non-magnetic support in this order. In a magnetic recording medium having a nonmagnetic layer, the magnetic layer is a nonmagnetic layer and/or a magnetic layer. A lubricant is an example of a component that can contribute to the running stability described above. If it is possible to reduce the amount of lubricant contained in the magnetic layer side of the non-magnetic support transferred from the magnetic layer surface to the back coat layer side during storage in a high temperature environment, it will be possible to It is thought that it becomes possible to suppress a decrease in reproduction output during repeated reproduction. It is presumed that the fact that the difference is 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 fatty acids include lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, linoleic acid, linolenic acid, behenic acid, erucic acid, and elaidic acid. is preferred, and stearic acid is more preferred. The fatty acid may be contained in the magnetic layer in the form of a salt such as a metal salt.
Examples of fatty acid esters include esters of lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, linoleic acid, linolenic acid, behenic acid, erucic acid, and elaidic acid. Specific examples include butyl myristate, butyl palmitate, butyl stearate (butyl stearate), neopentyl glycol dioleate, sorbitan monostearate, sorbitan distearate, sorbitan tristearate, oleyl oleate, Examples include isocetyl stearate, isotridecyl stearate, octyl stearate, isooctyl stearate, amyl stearate, butoxyethyl stearate, and the like.
Examples of fatty acid amides include amides of the various fatty acids mentioned above, such as lauric acid amide, myristic acid amide, palmitic acid amide, stearic acid amide, and the like.
Regarding fatty acids and fatty acid derivatives (amides, esters, etc.), the fatty acid-derived moiety of the fatty acid derivative preferably has the same or similar structure to the fatty acid used in combination. For example, when using stearic acid 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 composition for forming a 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 ferromagnetic powder. Parts by weight, more preferably 1.0 to 7.0 parts by weight. 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 parts by mass, per 100.0 parts by mass of 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 ferromagnetic powder. Part by mass.
In addition, when the magnetic recording medium has a nonmagnetic layer between the nonmagnetic support and the magnetic layer, the fatty acid content of the nonmagnetic layer (or composition for forming a nonmagnetic layer; the same shall apply hereinafter) is Per 100.0 parts by weight, it is, for example, 0 to 10.0 parts by weight, preferably 1.0 to 10.0 parts by weight, and more preferably 1.0 to 7.0 parts by weight. The fatty acid ester content of the nonmagnetic layer is, for example, 0 to 15.0 parts by weight, preferably 0.1 to 10.0 parts by weight, per 100.0 parts by weight of the nonmagnetic powder. The fatty acid amide content of the nonmagnetic layer is, for example, 0 to 3.0 parts by mass, and preferably 0 to 1.0 parts by mass, per 100.0 parts by mass of the nonmagnetic powder. The lubricant contained in the non-magnetic layer can migrate to the magnetic layer and contribute to running stability.

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

<Nonmagnetic layer>
Next, the nonmagnetic layer will be explained. The magnetic recording medium may have a magnetic layer directly on the non-magnetic support, or may have a non-magnetic layer containing non-magnetic powder and a binder between the non-magnetic support and the magnetic layer. Good too. The nonmagnetic powder used in the nonmagnetic layer may be an inorganic powder or an organic powder. Further, 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 nonmagnetic powders are available as commercial products, and can also be produced by known methods. For details thereof, paragraphs 0146 to 0150 of JP-A No. 2011-216149 can be referred to. Regarding carbon black that can be used in the nonmagnetic layer, paragraphs 0040 to 0041 of JP-A No. 2010-24113 can also be referred to. The content (filling rate) of the nonmagnetic powder in the nonmagnetic layer is preferably in the range of 50 to 90% by mass, more preferably in the range of 60 to 90% by mass.

The non-magnetic layer can include a binder and can also include additives. For other details such as the binder and additives of the nonmagnetic layer, known techniques regarding nonmagnetic layers can be applied. Further, for example, regarding the type and content of the binder, the type and content of the additive, and the like, known techniques regarding the magnetic layer can also be applied.

In the present invention and herein, a non-magnetic layer is intended to include a substantially non-magnetic layer containing a small amount of ferromagnetic powder, for example as an impurity or intentionally, along with the non-magnetic powder. Here, a substantially non-magnetic layer means that this layer has a residual magnetic flux density of 10 mT or less, a coercive force of 100 Oe or less, or a residual magnetic flux density of 10 mT or less and a coercive force of 100 Oe. It refers to the following layers. Preferably, the nonmagnetic layer has no residual magnetic flux density and no coercive force.

<Nonmagnetic support>
Next, the nonmagnetic support will be explained. Examples of the nonmagnetic support (hereinafter also simply referred to as "support") include known ones such as biaxially stretched polyethylene terephthalate, polyethylene naphthalate, polyamide, polyamideimide, aromatic polyamide, and the like. Among these, polyethylene terephthalate, polyethylene naphthalate, and polyamide are preferred. These supports may be subjected to corona discharge, plasma treatment, adhesive treatment, heat treatment, etc. in advance.

<Back coat layer>
The magnetic recording medium has a back coat layer containing nonmagnetic powder on the surface side of the nonmagnetic support opposite to the surface side having the magnetic layer. The non-magnetic powder contained in the back coat layer can preferably include a protrusion-forming agent and one or more other non-magnetic powders.

(Protrusion forming agent)
As the projection forming agent, particles of an inorganic substance, particles of an organic substance, or composite particles of an inorganic substance and an organic substance can also be used. Examples of the inorganic substance include inorganic oxides such as metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, and metal sulfides, with inorganic oxides being preferred. In one form, the protrusion-forming agent can be an inorganic oxide-based particle. Here, "system" is used to mean "including". One type of inorganic oxide particles are particles made of an inorganic oxide. Another form of inorganic oxide particles is a composite particle of an inorganic oxide and an organic substance, and a specific example is 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 an inorganic oxide particle.

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. _S13.5 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 perfect sphere, the smaller the indentation resistance that acts when a high pressure is applied, so the particles are more likely to be pushed into the back coat layer, and _S13.5 tends to be smaller. On the other hand, if the particle shape deviates from a true sphere, for example, a so-called irregular shape, a large indentation resistance tends to occur when high pressure is applied, making it difficult to be pushed into the back coat layer. , S _13.5 tends to be large. In addition, particles with non-uniform particle surfaces and low surface smoothness tend to have a large indentation resistance when high pressure is applied, making it difficult to be pushed into the back coat layer, resulting in a large _S13.5 . Cheap. By controlling S _0.5 and S _13.5 , the difference (S _0.5 - S _13.5 ) can be made 3 nm or less.

In addition, from the viewpoint that the protrusion-forming agent can better exhibit its function, the content of the protrusion-forming agent in the back coat layer is preferably set to 100.0 parts by mass of the other non-magnetic powder. , 1.0 to 4.0 parts by weight, more preferably 1.2 to 3.5 parts by weight. In one form, the content of the composite particles of inorganic oxide and polymer in the back coat layer is 1.0 to 4.0 parts by mass based on 100.0 parts by mass of other nonmagnetic powder. It is preferably 1.2 to 3.5 parts by mass, and more preferably 1.2 to 3.5 parts by mass.

(Other non-magnetic powder)
Regarding other non-magnetic powders included in the back coat layer, the above description regarding the non-magnetic powders in the non-magnetic layer can be referred to. As the other non-magnetic powder, preferably, one or both of carbon black and non-magnetic powder other than carbon black can be used. Examples of nonmagnetic powders other than carbon black include nonmagnetic inorganic powders. Specific examples of non-magnetic inorganic powders include iron oxides such as α-iron oxide, titanium oxides 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, goethite, corundum, silicon nitride, titanium carbide, magnesium oxide, boron nitride, molybdenum disulfide, copper oxide, MgCO ₃ , CaCO ₃ , BaCO ₃ , SrCO ₃ , BaSO ₄ , silicon carbide, titanium carbide, and other non-magnetic inorganic powders. Preferred non-magnetic inorganic powders are non-magnetic inorganic oxide powders, more preferably α-iron oxide and titanium oxide, and still more preferably α-iron oxide.

The shape of the non-magnetic powder other than carbon black may be acicular, spherical, polyhedral, or plate-like. The average particle size of these non-magnetic powders is preferably in the range of 5 nm to 2.00 μm, more preferably in the range of 10 nm to 200 nm. Further, the BET specific surface area of 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 still more preferably 10 to 65 m ² /g. is within 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. Further, the carbon black content can be in the range of 10.0 to 100.0 parts by mass, for example, based on 100.0 parts by mass of the other non-magnetic powder. The entire amount of other non-magnetic powder may be carbon black. Alternatively, the entire amount of other non-magnetic powder may be a non-magnetic powder other than carbon black. The values of S _0.5 and S _13.5 tend to increase as the proportion of carbon black in other non-magnetic powders increases. Regarding the content (filling rate) of other nonmagnetic powders in the back coat layer, the above description regarding the nonmagnetic powders in the nonmagnetic layer can be referred to.

The backcoat layer can include a binder and can also include additives. Regarding the binder and additives for the backcoat layer, known techniques regarding the backcoat layer can be applied, and known techniques regarding the formulation of the magnetic layer and/or the nonmagnetic layer can also be applied. For example, the descriptions in paragraphs 0018 to 0020 of JP-A No. 2006-331625 and in column 4, line 65 to column 5, line 38 of US Patent No. 7,029,774 can be referred to regarding the back coat layer.

An example of an additive that may be included in the backcoat layer is a lubricant. Regarding the lubricant, the above description regarding the lubricant that can be included in the magnetic layer etc. can be referred to. The fatty acid content of the back coat layer (or composition for forming a back coat layer; the same shall apply hereinafter) is, for example, 0 to 10.0 parts by mass per 100.0 parts by mass of the nonmagnetic powder contained in the back coat layer, and is preferably is 0.1 to 10.0 parts by weight, more preferably 1.0 to 7.0 parts by weight. The fatty acid ester content of the back coat layer forming composition is, for example, 0.1 to 10.0 parts by mass, preferably 1.0 to 5 parts by mass, per 100.0 parts by mass of the nonmagnetic powder contained in the back coat layer. .0 part by mass. The fatty acid amide content of the back coat layer is, for example, 0 to 3.0 parts by mass, preferably 0 to 2.0 parts by mass, and more preferably Preferably it is 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 lubricant exists on the surface of the back coat layer that is pressed during spacing measurement, the measured spacing becomes narrower by the thickness of this liquid film. On the other hand, it is presumed that the lubricant that may exist as a liquid film during pressing can be removed by cleaning with n-hexane. Therefore, it is considered that by measuring the spacing after cleaning with n-hexane, a measured value of the spacing can be obtained as a value that corresponds well to the height of the protrusions on the surface of the back coat layer in a pressed state.

<Various thicknesses>
Regarding the thickness of the nonmagnetic support and each layer in the magnetic recording medium, the thickness of the nonmagnetic support is, for example, 3.0 to 80.0 μm, preferably 3.0 to 50.0 μm, and more preferably 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 of the magnetic head used, the head gap length, and the recording signal band, and is, for example, 10 nm to 100 nm, and preferably 20 to 90 nm from the viewpoint of high-density recording. The range is more preferably 30 to 70 nm. It is sufficient that there is at least one magnetic layer, and the magnetic layer may be separated into two or more layers having different magnetic properties, and a structure related to a known multilayer magnetic layer can be applied. The thickness of the magnetic layer when it is separated into two or more layers is the total thickness of these layers.

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

The thickness of the back coat layer can be, for example, 0.1 μm or more. Further, the thickness of the back coat 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 nonmagnetic support can be determined by a known film thickness measurement method. As an example, a cross section in the thickness direction of a magnetic recording medium is exposed using a known method such as an ion beam or a microtome, and then the exposed cross section is observed using a scanning electron microscope. Various thicknesses can be determined as the arithmetic mean of the thickness determined at any one location during cross-sectional observation, or the thickness determined at two or more randomly extracted locations, for example, two locations. Alternatively, the thickness of each layer may be determined as a design thickness calculated from manufacturing conditions.

<Manufacturing method>
A composition for forming a magnetic layer, a nonmagnetic layer, or a backcoat layer usually contains a solvent in addition to the various components described above. As the solvent, various organic solvents commonly used for manufacturing coated magnetic recording media can be used. Among them, from the viewpoint of solubility of the binder normally used in coated magnetic recording media, each layer forming composition contains ketone solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, cyclohexanone, isophorone, and tetrahydrofuran. It is preferable that one or more of these are included. The amount of solvent in each layer-forming composition is not particularly limited, and can be the same as in each layer-forming composition of a typical coating-type magnetic recording medium. Moreover, the process of preparing each layer-forming composition can usually include at least a kneading process, a dispersion process, and a mixing process provided before and after these processes as necessary. Each individual process may be divided into two or more stages. The components used to prepare each layer-forming composition may be added at the beginning or middle of any step. Further, the individual components may be added in divided portions in two or more steps. For example, the binder may be added in portions during the kneading step, the dispersion step, and the mixing step for adjusting the viscosity after dispersion. In the manufacturing process of the magnetic recording medium, conventional and well-known manufacturing techniques can be used in some or all of the steps. In the kneading step, it is preferable to use a kneader with strong kneading power, such as an open kneader, continuous kneader, pressure kneader, or extruder. Details of these kneading treatments are described in JP-A-1-106338 and JP-A-1-79274. Furthermore, glass beads and/or other beads can be used to disperse the composition for forming each layer. As such dispersed beads, zirconia beads, titania beads, and steel beads, which are high specific gravity dispersed beads, are suitable. It is preferable to use these dispersed beads by optimizing the particle size (bead diameter) and packing rate. A known disperser can be used. Each layer-forming composition may be filtered by a known method before being subjected to the coating process. Filtration can be performed, for example, by filter filtration. As the filter used for filtration, for example, a filter with a pore size of 0.01 to 3 μm (eg, a glass fiber filter, a polypropylene filter, etc.) can be used.

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

The magnetic layer can be formed by, for example, directly applying the composition for forming a magnetic layer onto a nonmagnetic support, or by applying the composition in a multilayer manner sequentially or simultaneously with the composition for forming a nonmagnetic layer. In the embodiment in which the alignment treatment is performed, the coating layer of the magnetic layer forming composition is subjected to the alignment treatment in the alignment zone while the coating layer is in a wet state. Regarding the alignment treatment, various known techniques such as the one described in paragraph 0052 of JP-A No. 2010-24113 can be applied. For example, the vertical alignment process can be performed by a known method such as a method using magnets with different polarities facing each other. In the orientation zone, the drying speed of the coating layer can be controlled by the temperature and volume of the drying air and/or the conveyance speed in the orientation zone. Furthermore, the coating layer may be pre-dried before being transported to the orientation zone.
The back coat layer can be formed by applying a back coat layer forming composition to the side of the nonmagnetic support opposite to the side having the magnetic layer (or where the magnetic layer will be provided later). For details of coating for forming each layer, reference can be made to paragraph 0066 of JP-A No. 2010-231843.
Regarding various other processes for manufacturing magnetic recording media, paragraphs 0067 to 0070 of JP-A-2010-231843 can be referred to.

A servo pattern is formed on the magnetic recording medium manufactured as described above by a known method in order to enable tracking control of the magnetic head in a magnetic recording/reproducing device, control of the running speed of the magnetic recording medium, etc. I can do it. "Formation of a servo pattern" can also be referred to as "recording of a 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. Below, formation of a servo pattern will be explained using a magnetic tape as an example.

Servo patterns are usually formed along the longitudinal direction of the magnetic tape. Examples of control methods (servo control) using servo signals include timing-based servo (TBS), amplitude servo, and frequency servo.

As shown in ECMA (European Computer Manufacturers Association)-319, a timing-based servo system is employed in magnetic tapes (generally referred to as "LTO tapes") that comply with the LTO (Linear Tape-Open) standard. In this timing-based servo system, a servo pattern is composed of a plurality of pairs of magnetic stripes (also called "servo stripes") that are non-parallel to each other and are continuously arranged in the longitudinal direction of the magnetic tape. As mentioned above, the reason why the servo pattern is composed of a pair of non-parallel magnetic stripes is to tell the servo signal reading element passing over the servo pattern its passing position. Specifically, the above-mentioned pair of magnetic stripes are formed so that the interval between them changes continuously along the width direction of the magnetic tape, and the servo signal reading element reads the interval to read the servo pattern. The relative position of the servo signal reading element and the servo signal reading element can be known. This relative position information enables tracking of the data tracks. For this purpose, 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 that are continuous in the longitudinal direction of the magnetic tape. A plurality of servo bands are usually provided on a magnetic tape. For example, in an 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 includes information indicating the number of the servo band (“servo band ID (identification)” or “UDIM (Unique Data Band Identification)”). (also called "Method information") is embedded. This servo band ID is recorded by shifting a specific one of a plurality of pairs of servo stripes in the servo band so that its position is relatively displaced in the longitudinal direction of the magnetic tape. Specifically, the way in which a specific one of a plurality of pairs of servo stripes is shifted is changed for each servo band. As a result, the recorded servo band ID becomes unique for each servo band, and therefore, by simply reading one servo band with a servo signal reading element, that servo band can be uniquely identified.

Note that as a method for uniquely specifying a servo band, there is also a method using a staggered method as shown in ECMA-319. In this staggered method, a plurality of groups of non-parallel magnetic stripes (servo stripes) arranged consecutively in the longitudinal direction of the magnetic tape are recorded so as to be shifted in the longitudinal direction of the magnetic tape for each servo band. do. This combination of shifting between adjacent servo bands is unique on the entire magnetic tape, so it is possible to uniquely identify a servo band when reading a servo pattern with two servo signal reading elements. It is possible.

Furthermore, as shown in ECMA-319, each servo band is usually embedded with information indicating the position in the longitudinal direction of the magnetic tape (also called "LPOS (Longitudinal Position) information"). Like the UDIM information, this LPOS information is also recorded by shifting the positions of a pair of servo stripes in the longitudinal direction of the magnetic tape. However, unlike the UDIM information, 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-mentioned UDIM information and LPOS information into 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, it is also possible to employ methods other than those described above. For example, a predetermined code may be recorded by thinning out a predetermined pair from a group of pairs of servo stripes.

The servo pattern forming head is called a servo write head. The servo write head has a pair of gaps corresponding to the pair of magnetic stripes, equal to the number of servo bands. Usually, a core and a coil are connected to each pair of gaps, and by supplying current pulses to the coils, the magnetic field generated in the core can cause a leakage magnetic field in the pair of gaps. When forming a servo pattern, by inputting a current pulse while running the magnetic tape over the servo write head, a magnetic pattern corresponding to a pair of gaps is transferred onto the magnetic tape, thereby forming a servo pattern. I can do it. The width of each gap can be appropriately set depending on the density of the servo pattern to be formed. The width of each gap can be set to, for example, 1 μm or less, 1 to 10 μm, or 10 μm or more.

Before forming servo patterns on a magnetic tape, the magnetic tape is usually subjected to demagnetization (erase) processing. This erasing process can be performed by applying a uniform magnetic field to the magnetic tape using a DC magnet or an AC magnet. Erase processing includes DC (Direct Current) erase and AC (Alternating Current) erase. AC erase is performed by gradually lowering 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 length of the magnetic tape. The second method is vertical DC erase, which applies a unidirectional magnetic field along the thickness of the magnetic tape. The erase process may be performed on the entire magnetic tape, or may be performed on each servo band of the magnetic tape.

The direction of the magnetic field of the servo pattern to be formed is determined depending on the erase direction. For example, when a magnetic tape is subjected to horizontal DC erasing, the servo pattern is formed such that the direction of the magnetic field is opposite to the erasing direction. Thereby, 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 a magnetic pattern is transferred using the above-mentioned gap to a vertical DC erased magnetic tape, the formed servo pattern is read and obtained. The servo signal has a unipolar pulse shape. On the other hand, when a magnetic pattern is transferred onto a horizontal DC erased magnetic tape using the gap, the servo signal obtained by reading the formed servo pattern has a bipolar pulse shape.

Magnetic tape is usually housed in a magnetic tape cartridge, and the magnetic tape cartridge is loaded into a magnetic recording/reproducing device.

In a magnetic tape cartridge, a magnetic tape is generally housed inside a cartridge main body in a state where it is wound onto a reel. The reel is rotatably provided inside the cartridge body. As magnetic tape cartridges, single-reel type magnetic tape cartridges having one reel inside the cartridge body and twin-reel type magnetic tape cartridges having two reels inside the cartridge body are widely used. When a single-reel magnetic tape cartridge is inserted into a magnetic recording/playback device to record and/or play back data on the magnetic tape, the magnetic tape is pulled out from the magnetic tape cartridge and reeled onto the magnetic recording/playback device. It is wound up. A magnetic head is arranged on the magnetic tape transport path from the magnetic tape cartridge to the take-up reel. The magnetic tape is fed and wound between a reel (supply reel) on the magnetic tape cartridge side and a reel (take-up reel) on the magnetic recording and reproducing device side. During this time, the magnetic head and the surface of the magnetic layer of the magnetic tape come into contact and slide, thereby recording and/or reproducing data. On the other hand, in a 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 twin reel type magnetic tape cartridge. For other details of the magnetic tape cartridge, known techniques can be applied.

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

In the present invention and this specification, a "magnetic recording and reproducing device" means a device that can perform at least one of recording data on a magnetic recording medium and reproducing data recorded on a magnetic recording medium. do. Such devices are commonly referred to as drives. The magnetic recording and reproducing device may be a sliding type magnetic recording and reproducing device. A sliding type magnetic recording/reproducing device is a device in which a magnetic layer surface and a magnetic head come into contact and slide when recording data on a magnetic recording medium and/or reproducing recorded data.

The magnetic head included in the magnetic recording and reproducing device may be a recording head capable of recording data on a magnetic recording medium, and a reproducing head capable of reproducing data recorded on the magnetic recording medium. It can also be. Further, in one form, the magnetic recording and reproducing apparatus described above can include both a recording head and a reproducing head as separate magnetic heads. In another form, the magnetic head included in the magnetic recording and reproducing device includes both an element for recording data (recording element) and an element for reproducing data (reproducing element) in one magnetic head. It is also possible to have a different configuration. In the following, an element for recording data and an element for reproducing data will be collectively referred to as a "data element." As the reproducing head, a magnetic head (MR head) including a magnetoresistive (MR) element as a reproducing element, which can read data recorded on a magnetic tape with high sensitivity, is preferable. As the MR head, various known MR heads such as an AMR (Anisotropic Magnetoresistive) head, a GMR (Giant Magnetoresistive) head, a TMR (Tunnel Magnetoresistive) head, etc. can be used. Furthermore, a magnetic head that records data and/or reproduces data may include a servo signal reading element. Alternatively, the magnetic recording and reproducing apparatus may include a magnetic head (servo head) equipped 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 (hereinafter also referred to as a "recording/reproducing head") that records data and/or reproduces recorded data can include two servo signal reading elements. can read two adjacent servo bands simultaneously. One or more data elements can be placed between the two servo signal reading elements.

In the above magnetic recording/reproducing device, data is recorded on the magnetic recording medium and/or data recorded on the magnetic recording medium is reproduced by bringing the magnetic layer surface of the magnetic recording medium into contact with the magnetic head and sliding the magnetic head. It can be carried out. The above-mentioned magnetic recording/reproducing device may be any device as long as it includes the magnetic recording medium according to one embodiment of the present invention, and known techniques can be applied to the other components.

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

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

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

<Formulation of magnetic layer forming composition>
(Magnetic liquid)
Ferromagnetic powder (see Table 1): 100.0 parts Oleic acid: 2.0 parts Vinyl chloride copolymer (MR-104 manufactured by Kaneka Corporation): 10.0 parts SO ₃ Na group-containing polyurethane resin: 4.0 parts (Weight average molecular weight 70000, SO ₃ Na group: 0.07meq/g)
Polyalkyleneimine 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 liquid )
α-Alumina (BET (Brunauer-Emmett-Teller) specific surface area 19 m ² /g): 6.0 parts SO ₃ Na group-containing polyurethane resin: 0.6 parts (weight average molecular weight 70000, SO ₃ Na group: 0.1 meq /g)
2,3-dihydroxynaphthalene: 0.6 parts cyclohexanone: 23.0 parts (silica sol)
Colloidal silica (average particle size 120 nm): 2.0 parts Methyl ethyl ketone: 8.0 parts (other ingredients)
Stearic acid: 3.0 parts Stearamide: 0.3 parts Butyl stearate: 6.0 parts Methyl ethyl ketone: 110.0 parts Cyclohexanone: 110.0 parts Polyisocyanate (Coronate (registered trademark) L manufactured by Tosoh Corporation): 3 .0 copies

<Formulation of composition for forming non-magnetic layer>
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 Stearic acid: 1.0 parts Cyclohexanone: 300.0 parts Methyl ethyl ketone: 300.0 parts

<Formulation of composition for forming back coat 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 ingredients)
α-Iron oxide: See Table 1 Average particle size (average major axis length): 150 nm
Average acicular ratio: 7
BET specific surface area: 52m ² /g
Carbon black: See Table 1 Average particle size 20nm
Vinyl chloride copolymer (MR-104 manufactured by Kaneka Corporation): 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 Butyl stearate: 3.0 parts Polyisocyanate: 5.0 parts Cyclohexanone: 200.0 parts

<Preparation of composition for forming magnetic layer>
A composition for forming a magnetic layer was prepared by the following method.
A magnetic liquid was prepared by dispersing (bead dispersion) various components of the above magnetic liquid for 24 hours using a batch type vertical sand mill. Zirconia beads with a bead diameter of 0.5 mm were used as the dispersion beads.
For the abrasive liquid, mix the various components of the above abrasive liquid and put it into a horizontal bead mill disperser together with zirconia beads with a bead diameter of 0.3 mm, so that the bead volume / (abrasive liquid volume + bead volume) becomes 80%. The solution was adjusted as follows, subjected to bead mill dispersion treatment for 120 minutes, and the treated liquid was taken out and subjected to ultrasonic dispersion filtration treatment using a flow type ultrasonic dispersion filtration device. A polishing liquid was thus prepared.
The prepared magnetic liquid and abrasive liquid, as well as the above-mentioned silica sol and other components were introduced into a dissolver stirrer and stirred at a circumferential speed of 10 m/sec for 30 minutes, followed by a flow rate of 7.5 kg/min using a flow type ultrasonic disperser. After three passes of treatment, the mixture was filtered through a filter with a pore size of 1 μm to prepare a composition for forming a magnetic layer.

<Preparation of composition for forming non-magnetic layer>
The various components of the composition for forming a non-magnetic layer described above are dispersed for 24 hours using a batch-type vertical sand mill using zirconia beads with a bead diameter of 0.1 mm, and then filtered using a filter with a pore diameter of 0.5 μm. A composition for forming a nonmagnetic layer was prepared.

<Preparation of composition for forming back coat layer>
The protrusion-forming agent liquid is a dispersion obtained by mixing the components of the protrusion-forming agent liquid and then performing ultrasonic treatment (dispersion treatment) using a horn-type ultrasonic dispersion machine at an ultrasonic output of 500 watts per 200 cc for 60 minutes. was prepared by filtering it through a filter with a pore size of 0.5 μm.
The above projection forming agent liquid and the other components of the above back coat layer forming composition excluding lubricant (stearic acid and butyl stearate), polyisocyanate and 200.0 parts of cyclohexanone, After kneading and diluting with an open kneader, using zirconia beads with a bead diameter of 1 mm using a horizontal bead mill dispersion machine, one pass residence time was 2 minutes at a bead filling rate of 80 volume %, a rotor tip peripheral speed of 10 m/sec, and 12 passes. It was subjected to dispersion treatment. A lubricant (stearic acid and butyl stearate), polyisocyanate, and 200.0 parts of cyclohexanone were added to the resulting dispersion, and the mixture was stirred with a dissolver. A composition for forming a back coat layer was prepared by filtering the thus obtained dispersion using a filter with a pore size of 1 μm.

<Preparation of magnetic tape>
The composition for forming a nonmagnetic layer prepared above was coated on the surface of a support made of biaxially oriented polyethylene naphthalate having a thickness of 5.0 μm so that the thickness after drying was 400 nm, and the composition was dried to form a nonmagnetic layer. After the formation, the composition for forming a magnetic layer prepared above was applied onto the surface of the nonmagnetic layer so that the thickness after drying was 70 nm to form a coating layer. While the coating layer of the magnetic layer forming composition was in a wet (undried) state, a vertical alignment treatment was performed by applying a magnetic field with a magnetic field strength of 0.3 T in a direction perpendicular to the surface of the coating layer, and the coating layer was dried. . Thereafter, the composition for forming a back coat layer prepared above was applied to the opposite side of the support so that the thickness after drying was 0.2 μm, and the composition was dried. In this way, an original magnetic tape was produced.
The produced magnetic tape material was subjected to calendering treatment (surface smoothing treatment), and then heat treatment was performed at an ambient temperature of 70° C. for 36 hours. After heat treatment, the original magnetic tape is slit into 1/2-inch width pieces using a cutting machine, and the slit products are sent out and attached to a device with a winding device so that the nonwoven fabric and razor blade press against the surface of the magnetic layer. The surface of the magnetic layer was cleaned using a cleaning device to obtain a magnetic tape.

[Examples 2 to 9, Comparative Examples 1 to 5]
Example 1 except that the type of ferromagnetic powder, the type of protrusion forming agent contained in the back coat 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 above. In Comparative Example 1, a back coat layer forming composition was prepared without using a protrusion forming agent liquid.

[Protrusion forming agent]
The protrusion forming agents used in preparing the back coat layer forming composition for producing the magnetic tapes of Examples and Comparative Examples are as follows. Protrusion-forming agent A and protrusion-forming agent C are particles with low surface smoothness. The particle shape of protrusion forming agent B is cocoon-like. The particle shape of the protrusion-forming agent D is so-called amorphous. The particle shape of the protrusion-forming agent E is close to a perfect sphere.
Protrusion forming agent A: Cabot ATLAS (composite particles of silica and polymer), average particle size 100 nm
Protrusion forming agent B: Cabot TGC6020N (silica particles), average particle size 140 nm
Protrusion-forming agent C: Cataloid manufactured by JGC Catalysts & Chemicals (water-dispersed sol of silica particles; as a protrusion-forming agent for preparing a composition for forming a back coat layer, obtained by heating the above water-dispersed sol to remove the solvent) (Used dry matter), average particle size 120 nm
Protrusion forming agent D: Asahi #52 (carbon black) manufactured by Asahi Carbon Co., Ltd., average particle size 60 nm
Protrusion forming agent E: Quattron PL-10L manufactured by Fuso Chemical Industry Co., Ltd. (water-dispersed sol of silica particles; as a protrusion-forming agent for preparing a composition for forming a back coat layer, the above water-dispersed sol is heated to remove the solvent. (Use the dried product obtained by

[Ferromagnetic powder]
In Table 1, "BaFe" indicates hexagonal barium ferrite powder with 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 activation volume and anisotropy constant Ku of the various ferromagnetic powders described below were determined for each ferromagnetic powder by the method described above using a vibrating sample magnetometer (manufactured by Toei Kogyo Co., Ltd.). This is the value.
Moreover, the mass magnetization σs is a value measured at a magnetic field strength of 15 kOe using a vibrating sample magnetometer (manufactured by Toei Kogyo Co., Ltd.).

<Production method 1 of hexagonal strontium ferrite powder>
"SrFe1" shown in Table 1 is a hexagonal strontium ferrite powder produced by the following method.
Weighed 1707g _{of SrCO3, 687g of H3BO3, 1120g of Fe2O3, 45g of Al(OH)3 , 24g of BaCO3 , 13g} of _CaCO3 , and 235g _{of Nd2O3} , and mixed them in 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 while stirring the melt, the tap hole provided at the bottom of the platinum crucible was heated, and the melt was tapped in the form of a rod at a rate of about 6 g/sec. . The tapped liquid was rapidly cooled by rolling with water-cooled twin rollers to produce an amorphous body.
280g of the prepared amorphous material was placed in an electric furnace, heated to 635°C (crystallization temperature) at a heating rate of 3.5°C/min, and held at the same temperature for 5 hours to form hexagonal strontium ferrite particles. Precipitated (crystallized).
Next, the crystallized product obtained above containing hexagonal strontium ferrite particles was coarsely ground in a mortar, 1000 g of zirconia beads with a particle size of 1 mm and 800 mL of an acetic acid aqueous solution with a concentration of 1% were added to a glass bottle, and dispersed for 3 hours in a paint shaker. I did it. Thereafter, the resulting dispersion was separated from the beads and placed in a stainless steel beaker. The dispersion was allowed to stand at a liquid temperature of 100°C for 3 hours to dissolve the glass components, then precipitated in a centrifuge, washed by repeated decantation, and then heated in a heating furnace at an internal temperature of 110°C for 6 hours. After drying for several hours, hexagonal strontium ferrite powder was obtained.
The average particle size of the hexagonal strontium ferrite powder ("SrFe1" in Table 1) obtained above is 18 nm, the activation volume is 902 nm ³ , the anisotropy constant Ku is 2.2 × 10 ⁵ J/m ³ , Mass magnetization σs was 49 A·m ² /kg.
12 mg of 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. The obtained filtrate was subjected to elemental analysis using an ICP analyzer, and the neodymium atoms were analyzed using an ICP analyzer. The surface layer content was determined.
Separately, 12 mg of sample powder was collected from the hexagonal strontium ferrite powder obtained above, and this sample powder was completely dissolved under the dissolution conditions exemplified above. The elemental analysis of the obtained filtrate was performed using an ICP analyzer. The bulk content of atoms was determined.
The content of neodymium atoms (bulk content) with respect to 100 atom % of iron atoms in the hexagonal strontium ferrite powder obtained above was 2.9 atom %. Further, the content of neodymium atoms in the surface layer was 8.0 at %. 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 in the surface layer of the particles.

The fact that the powder obtained above exhibits a hexagonal ferrite crystal structure was confirmed by scanning CuKα rays at a voltage of 45 kV and an intensity of 40 mA and measuring the X-ray diffraction pattern under the following conditions (X-ray diffraction analysis). confirmed. The powder obtained above exhibited a magnetoplumbite type (M type) hexagonal ferrite crystal structure. Moreover, the crystal phase detected by X-ray diffraction analysis was a single phase of magnetoplumbite type.
PANalytical X'Pert Pro diffractometer, PIXcel detector Soller slit for incident and diffracted beams: 0.017 radian Fixed angle of dispersion slit: 1/4 degree Mask: 10 mm
Anti-scatter 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

<Production method 2 of hexagonal strontium ferrite powder>
"SrFe2" shown in Table 1 is a hexagonal strontium ferrite powder produced by the following method.
1725 g of SrCO ₃ , 666 g of H ₃ BO ₃ , 1332 g of Fe ₂ O ₃ , 52 g of Al(OH) ₃ , 34 g of CaCO ₃ and 141 g of BaCO ₃ were weighed and mixed in a mixer to obtain a raw material mixture.
The obtained raw material mixture was melted in a platinum crucible at a melting temperature of 1380°C, and while stirring the melt, the tap hole provided at the bottom of the platinum crucible was heated, and the melt was tapped in the form of a rod at a rate of about 6 g/sec. . The tapped liquid was rapidly cooled and rolled using water-cooled twin rolls to produce an amorphous body.
280 g of the obtained amorphous material was placed in an electric furnace, heated to 645° C. (crystallization temperature), and held at the same temperature for 5 hours to precipitate (crystallize) hexagonal strontium ferrite particles.
Next, the crystallized product obtained above containing hexagonal strontium ferrite particles was coarsely ground in a mortar, 1000 g of zirconia beads with a particle size of 1 mm and 800 mL of an acetic acid aqueous solution with a concentration of 1% were added to a glass bottle, and dispersed for 3 hours in a paint shaker. I did it. Thereafter, the resulting dispersion was separated from the beads and placed in a stainless steel beaker. The dispersion was allowed to stand at a liquid temperature of 100°C for 3 hours to dissolve the glass components, then precipitated in a centrifuge, washed by repeated decantation, and then heated in a heating furnace at an internal temperature of 110°C for 6 hours. After drying for several hours, hexagonal strontium ferrite powder was obtained.
The average particle size of the obtained hexagonal strontium ferrite powder ("SrFe2" in Table 1) was 19 nm, the activation volume was 1102 nm ³ , the anisotropy constant Ku was 2.0×10 ⁵ J/m ³ , and the mass magnetization was σs was 50 A·m ² /kg.

<Method for producing ε-iron oxide powder>
The "ε-iron oxide" shown in Table 1 is an ε-iron oxide powder produced by the following method.
90 g of pure water, 8.3 g of iron (III) nitrate nonahydrate, 1.3 g of gallium (III) nitrate octahydrate, 190 mg of cobalt (II) nitrate hexahydrate, 150 mg of titanium (IV) sulfate, and To a solution of 1.5 g of polyvinylpyrrolidone (PVP), while stirring using a magnetic stirrer, 4.0 g of an ammonia aqueous solution with a concentration of 25% was added in the atmosphere at an ambient temperature of 25°C. The mixture was stirred for 2 hours while maintaining the ambient temperature at 25°C. A citric acid aqueous solution obtained by dissolving 1 g of citric acid in 9 g of pure water was added to the obtained solution, and the mixture was stirred for 1 hour. The powder precipitated after stirring was collected by centrifugation, washed with pure water, and dried in a heating furnace with an internal temperature of 80°C.
800 g of pure water was added to the dried powder and the powder was again dispersed in water to obtain a dispersion. The temperature of the resulting dispersion was raised to 50° C., and 40 g of a 25% ammonia aqueous solution was added dropwise while stirring. After stirring for 1 hour while maintaining the temperature at 50° C., 14 mL of tetraethoxysilane (TEOS) was added dropwise, and the mixture was stirred for 24 hours. 50 g of ammonium sulfate was added to the resulting reaction solution, and the precipitated powder was collected by centrifugation, washed with pure water, and dried in a heating furnace at an internal temperature of 80°C for 24 hours to obtain a ferromagnetic powder precursor. Obtained.
The obtained ferromagnetic powder precursor was loaded into a heating furnace with an internal temperature of 1000° C. under an air atmosphere, and heat-treated for 4 hours.
The heat-treated ferromagnetic powder precursor was poured into a 4 mol/L sodium hydroxide (NaOH) aqueous solution, and stirred for 24 hours while maintaining the liquid temperature at 70°C. The impurity silicic acid compound was removed from the precursor.
Thereafter, the ferromagnetic powder from which the silicic acid compound had been removed was collected by centrifugation and washed with pure water to obtain ferromagnetic powder.
The composition of the obtained ferromagnetic powder was confirmed by high-frequency inductively coupled plasma-optical emission spectrometry (ICP-OES), and it was found that it contained Ga, Co, and Ti substituted ε-iron oxide (ε-Ga _{0 .28} Co _0.05 Ti _0.05 Fe _1.62 O ₃ ). In addition, X-ray diffraction analysis was performed under the same conditions as previously described for production method 1 of hexagonal strontium ferrite powder, and from the peaks of the X-ray diffraction pattern, the obtained ferromagnetic powder was found to have α phase and γ phase. It was confirmed that the material had a single-phase ε-phase crystal structure (ε-iron oxide type crystal structure), which did not include the ε-phase crystal structure.
The average particle size of the obtained ε-iron oxide powder ("ε-iron oxide" in Table 1) was 12 nm, the activation volume was 746 nm ³ , and the anisotropy constant Ku was 1.2×10 ⁵ J/m ³ , mass magnetization σs was 16 A·m ² /kg.

[Evaluation method]
(1) Difference (S _0.5 - S _13.5 )
Using TSA (Tape Spacing Analyzer (manufactured by Micro Physics)), the spacings S _0.5 and S _13.5 after n-hexane cleaning were measured by the following method, and the difference (S _0.5 - S _13.5 ) was calculated.
Five sample pieces with 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 _13.5 was calculated.
A glass plate (glass plate manufactured by Thorlabs, Inc. (model number: WG10530)) provided in the TSA was placed on the surface of the back coat layer of the magnetic tape (i.e., the above sample piece), and the glass plate provided in the TSA was placed as a pressing member. Using a hemisphere made of urethane, this hemisphere 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 a stroboscope equipped on the TSA to a certain area (150,000 to 200,000 μm ² ) of the surface of the back coat layer of the magnetic tape through the glass plate, and the resulting reflected light is filtered through an interference filter (wavelength By receiving the light with a CCD (Charge-Coupled Device) through a filter that selectively transmits light of 633 nm, an image of interference fringes caused by the unevenness of this area was obtained.
Divide this image into 300,000 points, find the distance (spacing) from the surface of the glass plate on the magnetic tape side to the surface of the back coat layer of the magnetic tape at each point, make this a histogram, and use the mode of the histogram as the spacing. I asked for it as.
The same sample piece was further pressed and the spacing was determined by the same method as above under a pressure of 13.5 atm.
The arithmetic mean of the spacings determined for the five sample pieces under a pressure of 0.5 atm after cleaning with n-hexane is defined as spacing S _0.5 , and the arithmetic mean of the spacings determined under a pressure of 0.5 atm after cleaning with n-hexane for the five sample pieces is taken as the spacing S 0.5. The arithmetic mean of the spacings obtained was defined as spacing S _13.5 .

(2) Decrease in playback output The magnetic tapes of the examples and comparative examples were stored in a single-reel magnetic tape cartridge for 24 hours in a thermobox maintained at an internal temperature of 50°C and relative humidity of 80%. did. Thereafter, the magnetic tape cartridge was taken out of the thermo box (outside air temperature was 23° C. and relative humidity was 50%) and placed within one minute into a thermo room whose inside temperature was maintained at 10° C. and relative humidity of 85%. Thereafter, within 30 minutes, recording and reproduction were performed in a thermo room using a 1/2 inch reel tester to which a magnetic head was fixed according to the following method.
The relative speed between the magnetic head and the magnetic tape was 5.5 m/sec, and an MIG (Metal-In-Gap) head (gap length 0.15 μm, track width 1.0 μm) was used for recording. The optimum recording current for magnetic tape was set. For reproduction, a GMR (Giant-Magnetoresistive) head with an element thickness of 15 nm, a shield interval of 0.1 μm, and a lead width of 0.5 μm was used. A signal was recorded at a linear recording density of 300 kfci, and the reproduced signal was measured using a spectrum analyzer manufactured by Shibasoku. The signal used was a portion where the signal was sufficiently stable after the magnetic tape started running. The unit kfci is a unit of linear recording density (cannot be converted to the SI unit system).
Under the above conditions, reproduction was repeated for 3000 passes at 1000 m per pass. Obtain the output value of the carrier signal of the 1st pass and the output value of the carrier signal of the 3000th pass, and calculate the difference "(output value of the 3000th pass) - (output value of the 1st pass)" as the playback output reduction. 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 back coat layer was 3 nm or less were different from the magnetic tapes of Comparative Examples 1 to 5. In comparison, it can be confirmed that there is little decrease in playback output even if the playback is repeated in a low temperature and high humidity environment after being stored in a high temperature environment.

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

Claims (10)

A magnetic recording medium having a magnetic layer containing ferromagnetic powder on one surface side of a non-magnetic support and a back coat layer containing non-magnetic powder on the other surface side,
A non-magnetic layer containing non-magnetic powder is provided between the non-magnetic support and the magnetic layer,
The thickness of the nonmagnetic layer is 50 nm or more and 800 nm or less,
Spacing S 0.5 measured under a pressure of 0.5 atm by optical interferometry after washing with n-hexane on the surface of the back coat layer, and spacing S _0.5 measured by optical interference method after washing with n-hexane on the surface of the back coat layer. A magnetic recording medium in which the difference between the spacing S _13.5 measured under a 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 the 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 the S _13.5 is 20 nm or more and 90 nm or less. The magnetic recording medium according to claim 1, wherein the back coat layer contains inorganic oxide particles. 6. The magnetic recording medium according to claim 5, wherein the inorganic oxide particles are composite particles of an inorganic oxide and a polymer. The magnetic recording medium according to claim 1, wherein the back coat layer has a thickness of 0.5 μm or less. The magnetic recording medium according to claim 1, wherein the nonmagnetic layer has a thickness of 50 nm or more and 500 nm or less. The magnetic recording medium according to any one of claims 1 to 8, which is a magnetic tape. A magnetic recording medium according to any one of claims 1 to 9,
magnetic head,
magnetic recording and reproducing devices including;