JP7473732B2 - Magnetic recording medium and magnetic recording/reproducing device - Google Patents

Description

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

In recent years, magnetic recording media have come into widespread use as data storage media for recording and storing various types of data (see, for example, Patent Document 1).

JP 2019-008849 A

Data recorded on a magnetic recording medium is usually reproduced by reading the data recorded on the magnetic layer by sliding a magnetic head over the surface of the magnetic layer while the magnetic recording medium is running inside a magnetic recording and reproduction device. However, if the magnetic recording medium has poor running stability, off-track will result in a decrease in the playback output. For this reason, it is desirable for magnetic recording media to have excellent running stability.

Data recorded on various recording media such as magnetic recording media is called hot data, warm data, or cold data depending on its access frequency (reproduction frequency). The access frequency decreases in the order of hot data, warm data, and cold data, and recording and storing infrequently accessed data (e.g., cold data) for long periods of time is called archiving. With the dramatic increase in the amount of information in recent years and the digitization of various types of information, the amount of data recorded and stored on recording media for archiving is increasing, and so there is growing interest in recording and reproduction systems suitable for archiving.

Magnetic recording media that can exhibit excellent running stability when playing back data after long-term storage as described above are suitable as archival recording media.

In view of the above, one aspect of the present invention aims to provide a magnetic recording medium that has excellent running stability even after long-term storage.

One aspect of the present invention is
A magnetic recording medium having a non-magnetic support and a magnetic layer containing a ferromagnetic powder,
a magnetic recording medium in which the isoelectric point of the surface zeta potential of the magnetic layer after the magnetic layer is pressed at a pressure of 70 atm (hereinafter also referred to as the "isoelectric point of the surface zeta potential of the magnetic layer after pressing" or the "isoelectric point after pressing") is 5.5 or more;
In terms of units, 1 atm = 101325 Pa (Pascal) = 101325 N (Newton)/ ^m2 .

In one embodiment, the isoelectric point can be 5.5 or more and 7.0 or less.

In one embodiment, the magnetic layer can include inorganic oxide particles.

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

In one embodiment, the magnetic layer may include a binder having an acidic group.

In one embodiment, the acidic group can be at least one acidic group selected from the group consisting of sulfonic acid groups and salts thereof.

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

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

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

One aspect of the present invention relates to a magnetic recording and reproducing device including the above-mentioned magnetic recording medium and a magnetic head.

According to one aspect of the present invention, it is possible to provide a magnetic recording medium that has excellent running stability even after long-term storage. Also, according to one aspect of the present invention, it is possible to provide a magnetic recording and reproducing device that includes the above-mentioned magnetic recording medium.

[Magnetic Recording Medium]
One aspect of the present invention relates to a magnetic recording medium having a non-magnetic support and a magnetic layer containing a ferromagnetic powder, in which the isoelectric point of the surface zeta potential of the magnetic layer after the magnetic layer is pressed with a pressure of 70 atm is 5.5 or more.

<Isoelectric point of surface zeta potential of magnetic layer after pressing>
The pressure of 70 atm applied to the magnetic layer is the surface pressure applied to the surface of the magnetic layer by pressing. The magnetic recording medium is passed between two rolls while running at a speed of 20 m/min, and a surface pressure of 70 atm is applied to the surface of the magnetic layer. A tension of 0.5 N/m is applied to the magnetic recording medium in the running direction while it is running. For example, for a tape-shaped magnetic recording medium (i.e., a magnetic tape), a tension of 0.5 N/m is applied in the longitudinal direction of the magnetic tape while it is running. The pressing is performed by passing the magnetic recording medium between two rolls a total of six times, and applying a surface pressure of 70 atm each time it passes between the rolls. Metal rolls are used as the rolls, and the rolls are not heated. The pressing is performed in an environment with an atmospheric temperature of 20 to 25°C and a relative humidity of 40 to 60%. The magnetic recording medium to which the pressing is performed is a magnetic recording medium that has not been stored for a long period of 10 years or more in a room temperature environment with a relative humidity of 40 to 60%, nor has it been stored equivalent to such long-term storage or subjected to an accelerated test equivalent to such long-term storage. This also applies to various physical properties relating to the magnetic recording medium described in the present invention and this specification, unless otherwise specified.
The above pressing can be performed, for example, by using a calender used for manufacturing magnetic recording media. For example, the magnetic tape stored in a magnetic tape cartridge can be taken out and passed between calender rolls in the calender, whereby the magnetic tape can be pressed at a pressure of 70 atm.

The present inventors, in the course of intensive research to provide a magnetic recording medium with excellent running stability after long-term storage, have come to the conclusion that, as an example of an archive, it is appropriate to press the magnetic layer with a pressure of 70 atm. This point will be explained further below.
For example, a magnetic tape is usually stored in a magnetic tape cartridge in a state where it is wound on a reel. Therefore, the magnetic tape is also stored for a long period of time in a magnetic tape cartridge after data with a low access frequency is recorded. In the magnetic tape wound on a reel, the magnetic layer surface is in contact with the backcoat layer (if the magnetic tape has a backcoat layer) or the surface of the non-magnetic support opposite to the magnetic layer side (if the magnetic tape does not have a backcoat layer), so the magnetic layer is in a pressed state in the magnetic tape cartridge. As a result of various simulations, the inventors have come to determine that it is appropriate to press the magnetic layer with a pressure of 70 atm as an accelerated test equivalent to long-term storage of about 10 years (an example of an archive) in an environment with an atmospheric temperature of 20 to 25°C and a relative humidity of 40 to 60% (an example of an archive storage environment). Therefore, the present inventors conducted a running stability test after pressing the magnetic layer at 70 atm, and as a result of extensive investigations based on the results of this test, it was found that a magnetic recording medium having an isoelectric point of 5.5 or more after pressing has excellent running stability after pressing the magnetic layer at 70 atm, i.e., excellent running stability in a state equivalent to the above-mentioned long-term storage. In this respect, the inventors obtained new knowledge that was not disclosed in the above-mentioned JP 2019-008849 A (Patent Document 1) and was not previously known.

The method for measuring the post-pressure isoelectric point is described below. In the present invention and this specification, the term "surface of the magnetic layer" is synonymous with the magnetic layer side surface of the magnetic recording medium.
In the present invention and this specification, the isoelectric point of the surface zeta potential of the magnetic layer refers to the pH value at which the surface zeta potential measured by the streaming potential method (also called the streaming current method) becomes zero. A sample is cut out from the magnetic recording medium to be measured, and the sample is placed in a measurement cell so that the surface of the magnetic layer is in contact with the electrolyte. The surface zeta potential is calculated by the following formula after flowing the electrolyte into the measurement cell at various pressures and measuring the streaming potential at each pressure.

The pressure is changed in the range of 0 to 400,000 Pa (0 to 400 mbar). The electrolyte is passed through the measurement cell to measure the flow potential and calculate the surface zeta potential using electrolytes with different pH values (from pH 9 to pH 3 in increments of about 0.5). There are a total of 13 measurement points, starting with the measurement point of pH 9 and ending with the 13th measurement point of pH 3. In this way, the surface zeta potential is calculated for each pH measurement point. Since the value of the surface zeta potential decreases as the pH decreases, two measurement points may appear where the polarity of the surface zeta potential changes (from a positive value to a negative value) as the pH decreases from 9 to 3. When two such measurement points appear, the pH at which the value of the surface zeta potential becomes zero is determined by interpolation using a straight line (linear function) showing the relationship between the surface zeta potential and pH at those two measurement points. On the other hand, if the surface zeta potentials obtained while the pH is decreasing from 9 to 3 are all positive values, the pH at which the surface zeta potential becomes zero is determined by extrapolation using a straight line (linear function) showing the relationship between the surface zeta potential and pH at the 13th measurement point (pH 3) which is the final measurement point and the 12th measurement point. On the other hand, if the surface zeta potentials obtained while the pH is decreasing from 9 to 3 are all negative values, the pH at which the surface zeta potential becomes zero is determined by extrapolation using a straight line (linear function) showing the relationship between the surface zeta potential and pH at the first measurement point (pH 9) which is the initial measurement point and the 12th measurement point. In this way, the pH at which the surface zeta potential of the magnetic layer measured by the streaming potential method becomes zero is determined.
The above measurements are carried out three times in total at room temperature using different samples cut out from the magnetic recording medium after the above pressing, and the pH is determined when the surface zeta potential value becomes zero in each measurement. The viscosity and relative dielectric constant of the electrolyte are measured at room temperature. In the present invention and this specification, "room temperature" is defined as a range of 20 to 27°C. The arithmetic average of the three pH values thus determined for the magnetic layer is taken as the isoelectric point of the surface zeta potential of the magnetic layer after pressing of the magnetic recording medium to be measured. As an electrolyte with pH 9, a 1 mmol/L KCl aqueous solution adjusted to pH 9 using a 0.1 mol/L KOH aqueous solution is used. As electrolytes with other pH values, an electrolyte with pH 9 thus adjusted is used with a 0.1 mol/L HCl aqueous solution. The isoelectric point of the surface zeta potential measured by the above method is the isoelectric point determined for the surface of the magnetic layer after pressing.

The isoelectric point of the surface zeta potential of the magnetic layer of the magnetic recording medium after pressing is 5.5 or more from the viewpoint of improving running stability after long-term storage. When reproducing data recorded on the magnetic recording medium after long-term storage, if the contact state between the magnetic layer surface and the magnetic head becomes unstable, the running stability will decrease. In contrast, in a magnetic recording medium in which the isoelectric point of the surface zeta potential of the magnetic layer after pressing (i.e., the isoelectric point of the surface zeta potential of the magnetic layer placed in a state equivalent to long-term storage) is 5.5 or more, i.e., in a pH range of approximately neutral to basic, it is presumed that the magnetic layer surface and the magnetic head are unlikely to electrochemically react, and/or that the shavings generated by the magnetic layer surface being scraped by contacting the magnetic layer surface with the magnetic head are unlikely to adhere to the magnetic head, which contribute to stabilizing the contact state between the magnetic layer surface and the magnetic head. However, the present invention is not limited in any way to the above presumption and other presumptions described in this specification.
From the viewpoint of further improving running stability after long-term storage, the isoelectric point of the surface zeta potential of the magnetic layer after pressing of the magnetic recording medium is preferably 5.7 or more, more preferably 6.0 or more. The isoelectric point of the surface zeta potential of the magnetic layer can be controlled by the type of components used for forming the magnetic layer, the forming process of the magnetic layer, etc., as described in detail below. From the viewpoint of ease of control, etc., the isoelectric point of the surface zeta potential of the magnetic layer is preferably 7.0 or less, more preferably 6.7 or less, and even more preferably 6.5 or less.
The above magnetic recording medium will now be described in more detail.

<Magnetic Layer>
(Ferromagnetic powder)
As the ferromagnetic powder contained in the magnetic layer, one or more of the ferromagnetic powders known as ferromagnetic powders used in the magnetic layers of various magnetic recording media can be used in combination. It is preferable to use ferromagnetic powders with a small average particle size from the viewpoint of improving recording density. From this viewpoint, 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, even more preferably 30 nm or less, even more preferably 25 nm or less, and even more preferably 20 nm or less. On the other hand, from the viewpoint of magnetization stability, the average particle size of the ferromagnetic powder is preferably 5 nm or more, more preferably 8 nm or more, even more preferably 10 nm or more, even more preferably 15 nm or more, and even more preferably 20 nm or more.

Hexagonal ferrite powder A preferred specific example of the ferromagnetic powder is hexagonal ferrite powder. For details of the hexagonal ferrite powder, see, for example, JP 2011-225417 A, paragraphs 0012 to 0030, JP 2011-216149 A, paragraphs 0134 to 0136, JP 2012-204726 A, paragraphs 0013 to 0030, and JP 2015-127985 A, paragraphs 0029 to 0084.

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

Hereinafter, we will explain in more detail about hexagonal strontium ferrite powder, which is one form of hexagonal ferrite powder.

The activation volume of the hexagonal strontium ferrite powder is preferably in the range of 800 to 1600 ^nm3 . The finely divided hexagonal strontium ferrite powder exhibiting an activation volume in 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 ^nm3 or more, and can be, for example, 850 ^nm3 or more. In addition, from the viewpoint of further improving the electromagnetic conversion characteristics, the activation volume of the hexagonal strontium ferrite powder is more preferably 1500 nm3 or less, even more preferably ^{1400 nm3} or less, even more preferably ¹³⁰⁰ nm3 or less, even more preferably 1200 ^nm3 or less, and even more preferably 1100 ^nm3 or less. The same applies to the activation volume of the hexagonal barium ferrite powder.

"Activation volume" is a unit of magnetization reversal and is an index showing the magnetic size of a particle. The activation volume described in this invention and this specification and the anisotropy constant Ku described later are values obtained by measuring the coercive force Hc measurement unit at magnetic field sweep speeds of 3 minutes and 30 minutes (measurement temperature: 23°C ± 1°C) using a vibrating sample magnetometer and calculating from the following relationship between Hc and activation volume V. The unit of the anisotropy constant Ku is 1 erg/cc = 1.0 x ^10-1 J/ ^m3 .
Hc=2Ku/Ms{1−[(kT/KuV)ln(At/0.693)] ^1/2 }
[In the above formula, Ku is anisotropy constant (unit: J/m ³ ), Ms is saturation magnetization (unit: kA/m), k is Boltzmann's constant, T is absolute temperature (unit: K), V is activation volume (unit: cm ³ ), A is spin precession frequency (unit: s ⁻¹ ), t is magnetic field reversal time (unit: s)]

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

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 atomic % relative to 100 atomic % of iron atoms. In one embodiment, the hexagonal strontium ferrite powder containing rare earth atoms can have uneven distribution of rare earth atoms in the surface layer portion. In the present invention and this specification, the term "surface layer distribution of rare earth atoms" refers to the rare earth atom content relative to 100 atomic % of iron atoms in a solution obtained by partially dissolving a hexagonal strontium ferrite powder in an acid (hereinafter referred to as "surface layer content of rare earth atoms" or simply "surface content" for rare earth atoms) being greater than the rare earth atom content relative to 100 atomic % of iron atoms in a solution obtained by completely dissolving a hexagonal strontium ferrite powder in an acid (hereinafter referred to as "bulk content of rare earth atoms" or simply "bulk content" for rare earth atoms).
Rare earth atom surface content/rare earth atom bulk content>1.0
The rare earth atom content of the hexagonal strontium ferrite powder described later is synonymous with the rare earth atom bulk content. In contrast, 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 the rare earth atom content in the surface layer of the particles constituting the hexagonal strontium ferrite powder. The rare earth atom surface layer content satisfying the ratio of "rare earth atom surface layer content / rare earth atom bulk content >1.0" means that the rare earth atoms are unevenly distributed (i.e., more present in the surface layer) in the particles constituting the hexagonal strontium ferrite powder. In the present invention and this specification, the surface layer refers to a part of the region from the surface of the particles 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 atomic % relative to 100 atomic % of iron atoms. It is believed that the rare earth atoms contained in the bulk content in the above range and the rare earth atoms unevenly distributed in the surface layer of the particles constituting the hexagonal strontium ferrite powder contribute to suppressing the decrease in the reproduction output during repeated reproduction. This is presumed to be because the hexagonal strontium ferrite powder contains rare earth atoms in the bulk content in the above range and the rare earth atoms unevenly distributed in the surface layer of the particles constituting the hexagonal strontium ferrite powder can increase the anisotropy constant Ku. The higher the value of the anisotropy constant Ku, the more the occurrence of the phenomenon called thermal fluctuation can be suppressed (in other words, the thermal stability can be improved). By suppressing the occurrence of thermal fluctuation, the decrease in the reproduction output during repeated reproduction can be suppressed. It is speculated that the uneven distribution of rare earth atoms in the surface layer of the hexagonal strontium ferrite powder particles contributes to stabilizing the spin of the iron (Fe) sites in the crystal lattice of the surface layer, thereby increasing the anisotropy constant Ku.
In addition, it is inferred that the use of the hexagonal strontium ferrite powder having rare earth atom uneven distribution on the surface as the ferromagnetic powder of the magnetic layer also contributes to suppressing the wear of the magnetic layer surface caused by sliding with the magnetic head.That is, it is inferred that the hexagonal strontium ferrite powder having rare earth atom uneven distribution on the surface can also contribute to improving the running durability of the magnetic recording medium.This is inferred to be because the uneven distribution of rare earth atoms on the surface of the particles constituting the hexagonal strontium ferrite powder contributes to improving the interaction between the particle surface and the organic substance (e.g., binder and/or additive) contained in the magnetic layer, and as a result, the strength of the magnetic layer is improved.
From the viewpoint of further suppressing the decrease in the reproduction output during repeated reproduction and/or from the viewpoint of further improving the running durability, the rare earth atom content (bulk content) is more preferably in the range of 0.5 to 4.5 atomic %, even more preferably in the range of 1.0 to 4.5 atomic %, and even more preferably in the range of 1.5 to 4.5 atomic %.

The bulk content is the content obtained by completely dissolving the hexagonal strontium ferrite powder. In the present invention and this specification, unless otherwise specified, the content of an atom refers to the bulk content obtained by completely dissolving the hexagonal strontium ferrite powder. The hexagonal strontium ferrite powder containing rare earth atoms may contain only one rare earth atom as the rare earth atom, or may contain two or more rare earth atoms. When two or more rare earth atoms are contained, the bulk content is obtained for the total of the two or more rare earth atoms. This point is the same for other components in the present invention and this specification. In other words, unless otherwise specified, a certain component may be used in only one type, or in two or more types. When two or more types are used, the content or content 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 therein may be any one or more of rare earth atoms. From the viewpoint of further suppressing the decrease in the playback output during repeated playback, preferred rare earth atoms include neodymium atoms, samarium atoms, yttrium atoms, and dysprosium atoms, with neodymium atoms, samarium atoms, and yttrium atoms being more preferred, and neodymium atoms being even more preferred.

In the hexagonal strontium ferrite powder having rare earth atom uneven distribution in the surface layer, the rare earth atoms may be 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 the hexagonal strontium ferrite powder having rare earth atom uneven distribution in the surface layer, the ratio of the surface content of rare earth atoms obtained by partial dissolution under the dissolution conditions described below to the bulk content of rare earth atoms obtained by complete dissolution under the dissolution conditions described below, "surface content/bulk content", is greater than 1.0 and can be 1.5 or more. A "surface content/bulk content" greater than 1.0 means that rare earth atoms are unevenly distributed in the surface layer (i.e., more present than in the interior) in the particles constituting the hexagonal strontium ferrite powder. In addition, the ratio of the surface content of rare earth atoms obtained by partial dissolution under the dissolution conditions described below to the bulk content of rare earth atoms obtained by complete dissolution under the dissolution conditions described below, "surface 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 atom surface distribution unevenness, 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 content/bulk content" is not limited to the upper or lower limit exemplified.

The partial and total dissolution of hexagonal strontium ferrite powder will be described below. For hexagonal strontium ferrite powder present as a powder, the sample powders to be partially and completely dissolved 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 dissolution, and the other part is subjected to total dissolution. The hexagonal strontium ferrite powder can be taken out from the magnetic layer by, for example, the method described in paragraph 0032 of JP 2015-91747 A.
The above-mentioned partial dissolution means that the hexagonal strontium ferrite powder is dissolved to such an extent that the remaining powder can be visually confirmed in the liquid at the end of dissolution. For example, the partial dissolution can dissolve a region of 10 to 20 mass% of the particles constituting the hexagonal strontium ferrite powder, with the whole particles being 100 mass%. On the other hand, the above-mentioned complete dissolution means that the hexagonal strontium ferrite powder is dissolved to such an extent that the remaining powder cannot be visually confirmed in the liquid at the end of dissolution.
The partial dissolution and the surface layer content are measured, for example, by the following method. Note that the dissolution conditions such as the amount of sample powder are merely examples, and any dissolution conditions that allow partial or complete dissolution can be used.
A container (e.g., a beaker) containing 12 mg of sample powder and 10 mL of 1 mol/L hydrochloric acid is held on a hot plate set at 70° C. for 1 hour. The resulting solution is filtered through a 0.1 μm membrane filter. The elemental analysis of the filtrate thus obtained is performed using an inductively coupled plasma (ICP) analyzer. In this way, the surface layer content of rare earth atoms relative to 100 atomic % of iron atoms can be obtained. When multiple types of rare earth atoms are detected by elemental analysis, the total content of all rare earth atoms is taken as the surface layer content. This is also true for the measurement of bulk content.
On the other hand, the total dissolved and bulk contents are measured, for example, by the following method.
A container (e.g., a beaker) containing 12 mg of sample powder and 10 mL of 4 mol/L hydrochloric acid is held for 3 hours on a hot plate set at 80° C. Thereafter, the same procedures as in the measurement of partial dissolution and surface layer content described above are carried out to determine the bulk content relative to 100 atomic % of iron atoms.

From the viewpoint of increasing the 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 containing rare earth atoms but not having rare earth atom surface uneven distribution has a tendency to have a large decrease in σs compared to a hexagonal strontium ferrite powder not containing rare earth atoms. In contrast, a hexagonal strontium ferrite powder having rare earth atom surface uneven distribution is considered preferable in terms of suppressing such a large decrease in σs. In one embodiment, 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, and more preferably 60 A·m ² /kg or less. σs can be measured using a known measuring device capable of measuring magnetic properties such as a vibrating sample magnetometer. In the present invention and this specification, unless otherwise specified, the mass magnetization σs is a value measured at a magnetic field strength of 15 kOe, where 1 [kOe]=10 ⁶ /4π [A/m].

Regarding the content (bulk content) of the constituent atoms of the hexagonal strontium ferrite powder, the strontium atom content can be, for example, in the range of 2.0 to 15.0 atomic % relative to 100 atomic % of iron atoms. In one embodiment, the divalent metal atoms contained in the hexagonal strontium ferrite powder can be only strontium atoms. In another embodiment, 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 contained, the barium atom content and calcium atom content in the hexagonal strontium ferrite powder can each be, for example, in the range of 0.05 to 5.0 atomic % relative to 100 atomic % of iron atoms.

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. The crystal structure can be confirmed by X-ray diffraction analysis. The hexagonal strontium ferrite powder can be one in which a single crystal structure or two or more types of crystal structures are detected by X-ray diffraction analysis. For example, in one embodiment, the hexagonal strontium ferrite powder can be one in which only the M-type crystal structure is detected by X-ray diffraction analysis. For example, M-type hexagonal ferrite is represented by the composition formula AFe ₁₂ O _19. Here, A represents a divalent metal atom, and when the hexagonal strontium ferrite powder is M-type, A is only strontium atom (Sr), or when multiple divalent metal atoms are included as A, strontium atom (Sr) accounts for the most on an atomic % basis as described above. 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 further 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 aluminum atom content can be, for example, 0.5 to 10.0 atomic % with respect to 100 atomic % of iron atoms. From the viewpoint of further suppressing the decrease in regeneration output during repeated regeneration, the hexagonal strontium ferrite powder contains iron atoms, strontium atoms, oxygen atoms, and rare earth atoms, and the content of atoms other than these atoms is preferably 10.0 atomic % or less, more preferably in the range of 0 to 5.0 atomic %, relative to 100 atomic % of iron atoms, and may be 0 atomic %. That is, in one embodiment, 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 % is obtained by converting the content (unit: mass %) of each atom obtained by completely dissolving the hexagonal strontium ferrite powder into a value expressed in atomic % using the atomic weight of each atom. In addition, in the present invention and this specification, "not containing" a certain atom means that the content measured by an ICP analyzer after completely dissolving is 0 mass %. The detection limit of an ICP analyzer is usually 0.01 ppm (parts per million) or less on a mass basis. The above "not containing" is used to mean that it is contained in an amount less than the detection limit of the ICP analyzer. In one form, the hexagonal strontium ferrite powder can be one that does not contain bismuth atoms (Bi).

Metal Powder A preferred 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 JP2011-216149A and paragraphs [0009] to [0023] of JP2005-251351A.

ε-Iron Oxide Powder A preferred specific example of the ferromagnetic powder is ε-iron oxide powder. In the present invention and this specification, the term "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, when the highest intensity diffraction peak in the X-ray diffraction spectrum obtained by X-ray diffraction analysis is assigned to the ε-iron oxide type crystal structure, it is determined that the ε-iron oxide type crystal structure is detected as the main phase. Known methods for producing ε-iron oxide powder include a method of producing it from goethite and a reverse micelle method. All of the above production methods are publicly known. In addition, a method for producing ε-iron oxide powder in which part of Fe is replaced by a substitution atom such as Ga, Co, Ti, Al, or Rh is described, for example, in J. Jpn. Soc. Powder Metallurgy Vol. 61 Supplement, No. S1, pp. S280-S284 and J. Mater. Chem. C, 2013, 1, pp. 5200-5206, etc. can be referenced. 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 here.

The activation volume of the ε-iron oxide powder is preferably in the range of 300 to 1500 ^nm3 . Microparticulated ε-iron oxide powder exhibiting an activation volume in the above range is suitable for producing a magnetic recording medium exhibiting excellent electromagnetic conversion characteristics. The activation volume of the ε-iron oxide powder is preferably ³⁰⁰ nm3 or more, and can be, for example, 500 nm3 ^or more. From the viewpoint of further improving the electromagnetic conversion characteristics, the activation volume of the ε-iron oxide powder is more preferably ¹⁴⁰⁰ nm3 or less, even more preferably 1300 ^nm3 or less, even more preferably ¹²⁰⁰ nm3 or less, and even more preferably 1100 ^nm3 or less.

An anisotropy constant Ku can be mentioned as an index of reduction in thermal fluctuation, in other words improvement in thermal stability. The ε-iron oxide powder can have a Ku of preferably 3.0×10 ⁴ J/m ³ or more, and more preferably 8.0×10 ⁴ J/m ³ or more. The 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 value means higher thermal stability and is preferable, the Ku value is not limited to the above-mentioned values.

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

In the present invention and this specification, unless otherwise specified, the average particle size of various powders such as ferromagnetic powders is a value measured by the following method using a transmission electron microscope.
The powder is photographed at a magnification of 100,000 times using a transmission electron microscope, and printed on photographic paper at a total magnification of 500,000 times to obtain a photograph of the particles that make up the powder. From the obtained particle photograph, a target particle is selected, and the particle outline is traced with a digitizer to measure the size of the particle (primary particle). Primary particles are independent particles that are not aggregated.
The above measurements are carried out for 500 randomly selected particles. The arithmetic mean of the particle sizes of the 500 particles thus obtained is taken as the average particle size of the powder. As the transmission electron microscope, for example, a Hitachi transmission electron microscope H-9000 can be used. The particle size can be measured using known image analysis software, for example, Carl Zeiss image analysis software KS-400. The average particle size shown in the examples described below is a value measured using a Hitachi transmission electron microscope H-9000 as the transmission electron microscope and Carl Zeiss image analysis software KS-400 as the image analysis software, unless otherwise specified. In the present invention and this specification, powder means an assembly of multiple particles. For example, ferromagnetic powder means an assembly of multiple ferromagnetic particles. In addition, the assembly of multiple particles is not limited to a form in which the particles constituting the assembly are in direct contact with each other, and also includes a form in which a binder, additive, etc., described later, are interposed between the particles. 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, the method described in paragraph 0015 of JP 2011-048878 A can be used, for example.

In the present invention and this specification, unless otherwise specified, the size of the particles constituting the powder (particle size) is determined by the shape of the particles observed in the particle photograph.
(1) In the case of needle-like, spindle-like, columnar (where the height is greater than the maximum major axis of the base), etc., it is expressed by the length of the major axis that constitutes the particle, i.e., the major axis length.
(2) In the case of a plate-like or columnar shape (where the thickness or height is smaller than the maximum long diameter of the plate surface or base surface), it shall be expressed by the maximum long diameter of the plate surface or base surface.
(3) In the case of a particle having a spherical, polyhedral, or amorphous shape, and the major axis of the particle cannot be determined from its shape, the particle is expressed as a circle-equivalent diameter, which is determined by a circle projection method.

The average acicular ratio of the powder refers to the arithmetic average of the short axis length of the particle, that is, the short axis length, the value of (long axis length/short axis length) of each particle, and the values obtained for the 500 particles. Here, unless otherwise specified, the short axis length refers to the length of the short axis constituting the particle in the above definition of particle size (1), and to the thickness or height in the above definition of particle size (2), and in the above definition of particle size (3), since there is no distinction between the long axis and the short axis, (long axis length/short axis length) is regarded as 1 for convenience.
Unless otherwise specified, when the particle shape is specific, for example, in the case of the above definition of particle size (1), the average particle size is the average major axis length, in the case of definition (2), the average particle size is the average plate diameter, and in the case of definition (3), the average particle size is the average diameter (also called the average particle diameter or average particle size).

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

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

In one embodiment, a binder having an acidic group can be used as the binder. In the present invention and this specification, the term "acidic group" is used to include a group capable of releasing H ⁺ and dissociating into an anion in water or a solvent containing water (aqueous solvent) and the salt form thereof. Specific examples of the acidic group include a sulfonic acid group, a sulfate group, a carboxy group, a phosphoric acid group, and the salt forms thereof. For example, the salt form of a sulfonic acid group (-SO ₃ H) means a group represented by -SO ₃ M, where M represents an atom (e.g., an alkali metal atom) that can become a cation in water or an aqueous solvent. This point also applies to the salt forms of the above various groups. An example of a binder having an acidic group can be, for example, a resin having at least one acidic group selected from the group consisting of a sulfonic acid group and its salts (e.g., a polyurethane resin, a vinyl chloride resin, etc.). However, the resin contained in the magnetic layer is not limited to these resins. In addition, in a binder having an acidic group, the acidic group content can be, for example, in the range of 0.03 to 0.50 meq/g. "eq" is an equivalent unit that cannot be converted to SI units. The content of various functional groups, such as acidic groups, contained in the resin can be determined by a known method depending on the type of functional group. The binder can be used in the magnetic layer forming composition in an amount of, for example, 1.0 to 30.0 parts by mass per 100.0 parts by mass of the ferromagnetic powder.

With regard to the control of the isoelectric point of the surface zeta potential of the magnetic layer, it is presumed that forming the magnetic layer so as to reduce the amount of acidic components present in the surface portion of the magnetic layer contributes to increasing the value of the isoelectric point. It is also presumed that increasing the amount of basic components present in the surface portion of the magnetic layer also contributes to increasing the value of the isoelectric point. However, even if the magnetic layer is formed in this way, it is considered that the value of the isoelectric point of the surface zeta potential of the magnetic layer will be reduced if the surface of the magnetic layer is pressed during long-term storage, causing the acidic components to move from the inside of the magnetic layer to the surface portion and/or the basic components to move from the surface portion of the magnetic layer to the inside. In response to this, for example, by using a protrusion-forming agent described later, the isoelectric point of the surface zeta potential of the magnetic layer after the above-mentioned pressing (i.e., the isoelectric point of the surface zeta potential of the magnetic layer placed in a state equivalent to long-term storage) can be made 5.5 or more. The inventors presume that this makes it possible to improve the running stability after long-term storage. The term "acidic component" is used to include components that can release H ⁺ and dissociate into anions in water or an aqueous solvent, and the salt forms thereof. The term "basic component" is used to include components capable of dissociating into cations by releasing ^OH- in water or an aqueous solvent, and salt forms thereof. For example, when an acidic component is used, first, a process is performed to cause the acidic component to be unevenly distributed in the surface layer of the coating layer of the magnetic layer-forming composition, and then a process is performed to reduce the amount of the acidic component in this surface layer, which is thought to lead to increasing the isoelectric point value of the surface zeta potential of the magnetic layer and controlling it to 5.5 or more. For example, in the process of coating the magnetic layer-forming composition on a non-magnetic support directly or via a non-magnetic layer, applying an AC magnetic field and coating in an AC magnetic field is thought to lead to uneven distribution of the acidic component in the surface layer of the coating layer of the magnetic layer-forming composition. Furthermore, it is presumed that performing a burnishing process thereafter contributes to removing at least a part of the unevenly distributed acidic component. The burnishing process is a process of rubbing the surface of the treatment target with a member (for example, an abrasive tape, or a grinding tool such as a grinding blade or a grinding wheel). The magnetic layer formation process including the burnishing process will be described in detail below. For example, the acidic component may be a binder having an acidic group.

A curing agent can also be used together with the resin usable as a binder. In one embodiment, the curing agent can be a thermosetting compound, which is a compound in which a curing reaction (crosslinking reaction) proceeds when heated, and in another embodiment, the curing agent can be a photocuring compound in which a curing reaction (crosslinking reaction) proceeds when irradiated with light. As the curing reaction proceeds during the magnetic layer formation process, at least a part of the curing agent can be included in the magnetic layer in a state of reacting (crosslinking) with other components such as the binder. This point is also true for layers formed using a composition used to form other layers that contains a curing agent. A preferred curing agent is a thermosetting compound, and polyisocyanate is suitable. For details on polyisocyanate, see paragraphs 0124 to 0125 of JP 2011-216149 A. The curing agent can be used in an amount of, for example, 0 to 80.0 parts by mass relative to 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.

(Additive)
The magnetic layer may contain one or more additives as necessary in addition to the various components described above. The additives may be selected from commercially available products according to the desired properties. Alternatively, a compound synthesized by a known method may be used as the additive. An example of the additive is the above-mentioned hardener. In addition, examples of additives that may be contained in the magnetic layer include non-magnetic fillers, lubricants, dispersants, dispersion aids, antifungal agents, antistatic agents, antioxidants, and the like. The non-magnetic filler is synonymous with non-magnetic particles or non-magnetic powder. Examples of the non-magnetic filler include non-magnetic fillers that can function as protrusion-forming agents and non-magnetic fillers that can function as abrasives. In addition, known additives such as various polymers described in paragraphs 0030 to 0080 of JP-A-2016-051493 can also be used as the additive.

Protrusion-Forming Agent As a protrusion-forming agent, which is one form of a non-magnetic filler, particles of an inorganic substance can be used, particles of an organic substance can be used, and composite particles of an inorganic substance and an organic substance can also be used. Examples of inorganic substances include inorganic oxides such as metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, metal sulfides, etc., and inorganic oxides are preferred. In one form, the protrusion-forming agent can be inorganic oxide-based particles. Here, "based" is used to mean "including." One form of inorganic oxide-based particles is particles made of inorganic oxide. Another form of inorganic oxide-based particles is composite particles 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 average particle size of the protrusion forming agent is, for example, 30 to 300 nm, preferably 40 to 200 nm. The closer the particle shape is to a perfect sphere, the smaller the resistance to intrusion when a large pressure is applied, and the easier it is to be pressed into the magnetic layer. In contrast, if the particle shape is far from a perfect sphere, for example, a shape that is so-called an irregular shape, it is easy to have a large resistance to intrusion when a large pressure is applied, and it is difficult to be pressed into the magnetic layer. In addition, particles with a non-uniform particle surface and low surface smoothness are also easy to have a large resistance to intrusion when a large pressure is applied, and are difficult to be pressed into the magnetic layer. When particles that are easy to be pressed into the magnetic layer are included in the magnetic layer, it is thought that the basic component moves from the surface part of the magnetic layer to the inside, and/or the acidic component moves from the inside of the magnetic layer to the surface part, due to the particles being pressed into the magnetic layer by pressure. In contrast, if the particles of the protrusion-forming agent are difficult to press into the magnetic layer, it is believed that it is possible to prevent basic components from migrating from the surface layer of the magnetic layer to the inside, and/or prevent acidic components from migrating from the inside of the magnetic layer to the surface layer. In other words, it is believed that using a protrusion-forming agent that is difficult to press into the magnetic layer contributes to controlling the isoelectric point of the surface zeta potential of the magnetic layer after pressing to 5.5 or more.

Abrasives The abrasives, which are another form of non-magnetic filler, are preferably non-magnetic powders with a Mohs hardness of more than 8, and more preferably non-magnetic powders with a Mohs hardness of 9 or more. In contrast, the Mohs hardness of the protrusion-forming agent can be, for example, 8 or less or 7 or less. The maximum value of the Mohs hardness is 10, which is that of diamond. Specifically, powders such as alumina (Al ₂ O ₃ ), silicon carbide, boron carbide (B ₄ C), SiO ₂ , TiC, chromium oxide (Cr ₂ O ₃ ), cerium oxide, zirconium oxide (ZrO ₂ ), iron oxide, and diamond can be mentioned, and among them, alumina powders such as α-alumina and silicon carbide powders are preferred. The average particle size of the abrasives is, for example, in the range of 30 to 300 nm, and preferably in the range of 50 to 200 nm.

In order for the protrusion-forming agent and the abrasive to perform their functions better, the content of the protrusion-forming agent in the magnetic layer is preferably 1.0 to 4.0 parts by mass, and more preferably 1.2 to 3.5 parts by mass, per 100.0 parts by mass of the ferromagnetic powder. On the other hand, the content of the abrasive in the magnetic layer is preferably 1.0 to 20.0 parts by mass, and more preferably 3.0 to 15.0 parts by mass, and even more preferably 4.0 to 10.0 parts by mass, per 100.0 parts by mass of the ferromagnetic powder.

As an example of an additive that can be used in a magnetic layer containing an abrasive, the dispersant described in paragraphs 0012 to 0022 of JP 2013-131285 A can be cited as a dispersant for improving the dispersibility of the abrasive in the magnetic layer forming composition. For the dispersant, refer to paragraphs 0061 and 0071 of JP 2012-133837 A. The dispersant may be contained in the non-magnetic layer. For the dispersant that can be contained in the non-magnetic layer, refer to paragraph 0061 of JP 2012-133837 A. For the lubricant, refer to paragraphs 0030 to 0033, 0035 and 0036 of JP 2016-126817 A. The non-magnetic layer may contain a lubricant. For information about lubricants that may be contained in the nonmagnetic layer, see paragraphs 0030, 0031, 0034, 0035, and 0036 of JP 2016-126817 A.

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

<Nonmagnetic Layer>
Next, the non-magnetic layer will be described. The magnetic recording medium may have a magnetic layer directly on the non-magnetic support, or may have a non-magnetic layer containing a non-magnetic powder between the non-magnetic support and the magnetic layer. The non-magnetic powder used in the non-magnetic layer may be a powder of an inorganic substance or a powder of an organic substance. Carbon black or the like may also be used. Examples of inorganic substances include metals, metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, and metal sulfides. These non-magnetic powders are commercially available, and can also be produced by known methods. For details, refer to paragraphs 0146 to 0150 of JP-A-2011-216149. For carbon black that can be used in the non-magnetic layer, refer to paragraphs 0040 to 0041 of JP-A-2010-24113. The content (filling rate) of the non-magnetic powder in the non-magnetic layer is preferably in the range of 50 to 90% by mass, and more preferably in the range of 60 to 90% by mass.

The non-magnetic layer may contain a binder and may also contain additives. For other details of the binder, additives, etc. of the non-magnetic layer, known techniques related to non-magnetic layers may be applied. In addition, for example, for the type and content of the binder, and the type and content of the additive, known techniques related to magnetic layers may also be applied.

In the present invention and this specification, the nonmagnetic layer also includes a substantially nonmagnetic layer that contains a small amount of ferromagnetic powder, for example as an impurity or intentionally, along with the nonmagnetic powder. A substantially nonmagnetic layer here refers to a layer whose residual magnetic flux density is 10 mT or less, whose coercive force is 100 Oe or less, or whose residual magnetic flux density is 10 mT or less and whose coercive force is 100 Oe or less. It is preferable that the nonmagnetic layer has no residual magnetic flux density or coercive force.

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

<Backcoat layer>
The magnetic recording medium may or may not have a backcoat layer containing a nonmagnetic powder on the surface side opposite to the surface side having the magnetic layer of the nonmagnetic support. The backcoat layer preferably contains one or both of carbon black and inorganic powder. The backcoat layer may contain a binder and may also contain additives. For the binder and additives of the backcoat layer, known techniques related to the backcoat layer may be applied, and known techniques related to the formulation of the magnetic layer and/or the nonmagnetic layer may also be applied. For example, the descriptions of paragraphs 0018 to 0020 of JP-A-2006-331625 and US Pat. No. 7,029,774, column 4, line 65 to column 5, line 38, may be referred to for the backcoat layer.

<Various thicknesses>
The thickness of the non-magnetic support is preferably 3.00 to 20.00 μm, more preferably 3.00 to 10.00 μm, and even more preferably 3.00 to 6.00 μm.

The thickness of the magnetic layer can be optimized according to the saturation magnetization of the magnetic head used, the head gap length, the bandwidth of the recording signal, etc. The thickness of the magnetic layer is generally 0.01 μm to 0.15 μm, and from the viewpoint of high density recording, it is preferably 0.02 μm to 0.12 μm, and more preferably 0.03 μm to 0.10 μm. There should be at least one magnetic layer, and the magnetic layer may be separated into two or more layers having different magnetic properties, and known configurations related to multilayer magnetic layers can be applied. When the magnetic layer is separated into two or more layers, the thickness of the magnetic layer is the total thickness of these layers.

The thickness of the non-magnetic layer is, for example, 0.10 to 1.50 μm, and preferably 0.10 to 1.00 μm.

The thickness of the backcoat layer is preferably 0.90 μm or less, and more preferably in the range of 0.10 to 0.70 μm.

The thickness of each layer of the magnetic recording medium and the non-magnetic support can be determined by known film thickness measurement methods. For example, a cross section of the magnetic recording medium in the thickness direction is exposed by known methods 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 average of the thickness determined at one location in the cross-sectional observation, or the thickness determined at two or more randomly selected locations, for example, two locations. Alternatively, the thickness of each layer may be determined as a design thickness calculated from the manufacturing conditions.

<Manufacturing process>
(Preparation of compositions for forming each layer)
The composition for forming the magnetic layer, non-magnetic layer or backcoat layer usually contains a solvent in addition to the various components described above. As the solvent, various organic solvents generally used for manufacturing coating-type magnetic recording media can be used. The amount of solvent in each layer-forming composition is not particularly limited, and can be the same as that of each layer-forming composition of a normal coating-type magnetic recording medium. The process for preparing the composition for forming each layer can usually include at least a kneading process, a dispersing process, and a mixing process provided before or after these processes as necessary. Each individual process may be divided into two or more stages. The components used in the preparation of each layer-forming composition may be added at the beginning or during any process. Also, each component may be added in two or more steps in separate steps.

To prepare each layer-forming composition, known techniques can be used. In the kneading process, 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 processes are described in JP-A-1-106338 and JP-A-1-79274. In addition, to disperse each layer-forming composition, one or more types of dispersion beads selected from the group consisting of glass beads and other dispersion beads can be used as a dispersion medium. As such dispersion beads, zirconia beads, titania beads, and steel beads, which are dispersion beads with high specific gravity, are suitable. The particle size (bead diameter) and packing rate of these dispersion beads can be optimized. A known dispersing machine 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 (for example, a glass fiber filter, a polypropylene filter, etc.) can be used.

In one embodiment, in the step of preparing the magnetic layer forming composition, a dispersion containing a protrusion-forming agent (hereinafter referred to as "protrusion-forming agent liquid") is prepared, and then this protrusion-forming agent liquid can be mixed with one or more other components of the magnetic layer forming composition. For example, a protrusion-forming agent liquid, a dispersion containing an abrasive (hereinafter referred to as "abrasive liquid"), and a dispersion containing a ferromagnetic powder (hereinafter referred to as "magnetic liquid") can be separately prepared, and then mixed and dispersed to prepare the magnetic layer forming composition. Separately preparing various dispersions in this manner is preferable for improving the dispersibility of the ferromagnetic powder, protrusion-forming agent, and abrasive in the magnetic layer forming composition. For example, the protrusion-forming agent liquid can be prepared by a known dispersion treatment such as ultrasonic treatment. The ultrasonic treatment can be performed, for example, with an ultrasonic output of about 10 to 2000 watts per 200 cc (1 cc = 1 cm ³ ) for about 1 to 300 minutes. In addition, filtration may be performed after the dispersion treatment. The above description can be referred to for the filter used for filtration.

The magnetic layer can be formed by applying the magnetic layer-forming composition directly onto the surface of the non-magnetic support, or by applying it in layers successively or simultaneously with the non-magnetic layer-forming composition. The backcoat layer can be formed by applying the backcoat layer-forming composition onto the surface of the non-magnetic support on which the magnetic layer is formed, or onto the surface opposite to the surface on which the magnetic layer is subsequently formed. For details on the application for forming each layer, see paragraph 0066 of JP 2010-231843 A.

By applying the magnetic layer-forming composition in an alternating magnetic field, the acidic components (e.g., binders having acidic groups) tend to be unevenly distributed on the surface of the applied layer of the magnetic layer-forming composition, and it is presumed that drying this applied layer makes it possible to unevenly distribute the acidic components on the surface of the magnetic layer. Furthermore, it is considered that carrying out a burnishing treatment thereafter contributes to removing at least a portion of the unevenly distributed acidic components and controlling the isoelectric point of the surface zeta potential of the magnetic layer to 5.5 or more.
The application of the AC magnetic field can be performed by placing a magnet in the coating device so that the AC magnetic field is applied perpendicular to the surface of the coating layer of the magnetic layer-forming composition. The magnetic field strength of the AC magnetic field can be, for example, about 0.05 to 3.00 T. However, it is not limited to this range. In the present invention and this specification, "perpendicular" does not necessarily mean only perpendicular in the strict sense, but includes the range of error allowed in the technical field to which the present invention belongs. The range of error can mean, for example, a range of less than ±10° from the strict perpendicular.

Burnishing is a process in which the surface of the object to be treated is rubbed with a member (for example, an abrasive tape, or a grinding tool such as a grinding blade or a grinding wheel), and can be performed in the same manner as known burnishing for the production of coated magnetic recording media. Burnishing can be performed by either or both of rubbing (polishing) the surface of the coating layer to be treated with an abrasive tape and rubbing (grinding) the surface of the coating layer to be treated with a grinding tool. As the abrasive tape, a commercially available product may be used, or an abrasive tape prepared by a known method may be used. As the grinding tool, known grinding blades and grinding wheels such as fixed blades, diamond wheels, and rotary blades may be used. In addition, a wiping process may be performed in which the coating layer surface rubbed with the abrasive tape and/or grinding tool is wiped off with a wiping material. For details of the preferred abrasive tape, grinding tool, burnishing, and wiping, refer to paragraphs 0034 to 0048, FIG. 1, and the examples in the same publication in JP-A-6-52544. It is believed that the stronger the burnishing treatment, the more the acidic components unevenly distributed in the surface layer of the coating layer of the magnetic layer forming composition can be removed by coating in an alternating magnetic field. The burnishing treatment can be strengthened by using a harder abrasive as the abrasive contained in the polishing tape, and the more the amount of abrasive in the polishing tape is increased. In addition, the burnishing treatment can be strengthened by using a harder grinding tool as the grinding tool. Regarding the burnishing treatment conditions, the burnishing treatment can be strengthened by increasing the sliding speed between the coating layer surface to be treated and the member (e.g., the polishing tape or grinding tool). The above sliding speed can be increased by increasing one or both of the speed at which the member is moved and the speed at which the magnetic tape to be treated is moved. In addition, although the reason is not clear, there are also cases where the isoelectric point of the surface zeta potential of the magnetic layer after the burnishing treatment tends to be higher as the amount of binder having an acidic group in the coating layer of the magnetic layer forming composition increases.

When the magnetic layer forming composition contains a curing agent, it is preferable to perform a curing treatment at any stage of the process for forming the magnetic layer. It is preferable to perform the burnishing treatment at least before the curing treatment. A further burnishing treatment may be performed after the curing treatment. It is considered preferable to perform the burnishing treatment before the curing treatment in order to increase the efficiency of removing acidic components from the surface portion of the coating layer of the magnetic layer forming composition. The curing treatment can be performed by heating, light irradiation, or other treatment depending on the type of curing agent contained in the magnetic layer forming composition. The curing treatment conditions are not particularly limited and may be appropriately set depending on the formulation of the magnetic layer forming composition, the type of curing agent, the thickness of the coating layer, and the like. For example, when a coating layer is formed using a magnetic layer forming composition containing a polyisocyanate as a curing agent, the curing treatment is preferably a heat treatment.

Preferably, a surface smoothing treatment can be performed before the above-mentioned hardening treatment. The surface smoothing treatment is a treatment performed to increase the smoothness of the surface of the magnetic recording medium, and is preferably performed by calendaring. For details of the calendaring treatment, see, for example, paragraph 0026 of JP 2010-231843 A.

For other various processes for manufacturing a magnetic recording medium, known techniques can be applied. For various processes, see, for example, paragraphs 0067 to 0070 of JP-A-2010-231843. It is preferable to perform an orientation treatment on the coating layer of the composition for forming a magnetic layer while the coating layer is in a wet (undried) state. For the orientation treatment, various known techniques, including those described in paragraph 0067 of JP-A-2010-231843, can be applied. For example, the vertical orientation treatment can be performed by known methods such as a method using magnets with opposite poles 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 transport speed of the magnetic tape in the orientation zone. In addition, the coating layer may be pre-dried before being transported to the orientation zone. When performing the orientation treatment, it is preferable to apply a magnetic field (e.g., a direct current magnetic field) to the coating layer of the composition for forming a magnetic layer that has been applied in an alternating current magnetic field in order to orient the ferromagnetic powder.

In the magnetic recording medium manufactured as described above, a servo pattern can be formed by a known method to enable tracking control of the magnetic head in a magnetic recording and reproducing device, control of the running speed of the magnetic recording medium, etc. "Formation of a servo pattern" can also be called "recording of a servo signal." In one form, the magnetic recording medium can be a tape-shaped magnetic recording medium (magnetic tape), and in another form, a disk-shaped magnetic recording medium (magnetic disk). Below, the formation of a servo pattern will be explained using magnetic tape as an example.

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

As shown in ECMA (European Computer Manufacturers Association)-319, magnetic tapes (commonly called "LTO tapes") that comply with the LTO (Linear Tape-Open) standard use a timing-based servo system. In this timing-based servo system, the servo pattern is composed of a pair of non-parallel magnetic stripes (also called "servo stripes") that are continuously arranged in the longitudinal direction of the magnetic tape. As described above, the reason why the servo pattern is composed of a pair of non-parallel magnetic stripes is to inform the servo signal reading element that passes over the servo pattern of its passing position. Specifically, the pair of magnetic stripes is formed so that the interval between them changes continuously along the width direction of the magnetic tape, and the servo signal reading element can know the relative position between the servo pattern and the servo signal reading element by reading the interval. This relative position information enables tracking of the data track. For this reason, multiple servo tracks are typically set on the servo pattern along the width of the magnetic tape.

A servo band consists of a continuous servo signal in the longitudinal direction of the magnetic tape. Typically, multiple servo bands are provided on a magnetic tape. For example, there are five servo bands on an LTO tape. The area between two adjacent servo bands is called a data band. A data band consists of multiple data tracks, and each data track corresponds to a servo track.

In one embodiment, as shown in JP 2004-318983 A, information indicating the servo band number (also called "servo band ID (identification)" or "UDIM (Unique Data Band Identification Method) information") is embedded in each servo band. This servo band ID is recorded by shifting a specific one of the multiple pairs of servo stripes in the servo band so that its position is displaced relatively in the longitudinal direction of the magnetic tape. Specifically, the way in which a specific one of the multiple pairs of servo stripes is shifted is changed for each servo band. As a result, the recorded servo band ID is unique for each servo band, so that a servo band can be uniquely identified simply by reading one servo band with a servo signal reading element.

The method of uniquely identifying servo bands also uses the staggered method as specified in ECMA-319. In this staggered method, a group of non-parallel pairs of magnetic stripes (servo stripes) arranged continuously in the longitudinal direction of the magnetic tape are recorded so that they are shifted in the longitudinal direction of the magnetic tape for each servo band. The combination of the shift between adjacent servo bands is unique across the entire magnetic tape, so it is possible to uniquely identify servo bands when reading the servo pattern with two servo signal reading elements.

As specified in ECMA-319, each servo band also typically has embedded therein information indicating the longitudinal position of the magnetic tape (also called "LPOS (Longitudinal Position) information"). Like 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 UDIM information, this LPOS information records the same signal in each servo band.

It is also possible to embed information other than the above-mentioned UDIM information and LPOS information in the servo bands. 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.
Also, methods other than those described above can be used to embed information in the servo bands. For example, a predetermined code may be recorded by thinning out a predetermined pair from a group of pairs of servo stripes.

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

Before forming a servo pattern on the magnetic tape, the magnetic tape is usually subjected to a demagnetization (erase) process. This erase process can be performed by applying a uniform magnetic field to the magnetic tape using a direct current magnet or an alternating current magnet. There are two types of erase processes: DC (Direct Current) erase and AC (Alternating Current) erase. AC erase is performed by gradually reducing the strength of the magnetic field applied to the magnetic tape while reversing the direction of the magnetic field. On the other hand, DC erase is performed by applying a unidirectional magnetic field to the magnetic tape. There are two more methods of DC erase. The first method is horizontal DC erase, in which a unidirectional magnetic field is applied along the length of the magnetic tape. The second method is vertical DC erase, in which a unidirectional magnetic field is applied along the thickness direction of the magnetic tape. The erase process may be performed on the entire magnetic tape, or on each servo band of the magnetic tape.

The direction of the magnetic field of the servo pattern formed is determined by the direction of the erase. For example, when a horizontal DC erase is applied to a magnetic tape, the servo pattern is formed so that the direction of the magnetic field is opposite to the direction of the erase. This makes it possible to increase the output of the servo signal obtained by reading the servo pattern. As shown in JP 2012-53940 A, when a magnetic pattern is transferred to a magnetic tape that has been vertically DC erased using the above gap, the servo signal obtained by reading the formed servo pattern has a unipolar pulse shape. On the other hand, when a magnetic pattern is transferred to a magnetic tape that has been horizontally DC erased using the above gap, the servo signal obtained by reading the formed servo pattern has a bipolar pulse shape.

Magnetic tapes are usually housed in magnetic tape cartridges, which are then inserted into magnetic recording and playback devices.

In a magnetic tape cartridge, the magnetic tape is generally stored inside the cartridge body in a state where it is wound around a reel. The reel is provided inside the cartridge body so that it can rotate. As magnetic tape cartridges, single-reel type magnetic tape cartridges with one reel inside the cartridge body and dual-reel type magnetic tape cartridges with two reels inside the cartridge body are widely used. When a single-reel type magnetic tape cartridge is loaded into a magnetic recording and playback device for recording and/or playing back data on the magnetic tape, the magnetic tape is pulled out from the magnetic tape cartridge and wound around the reel on the magnetic recording and playback device side. A magnetic head is disposed on the magnetic tape transport path from the magnetic tape cartridge to the take-up reel. The magnetic tape is fed and wound between the reel (supply reel) on the magnetic tape cartridge side and the reel (take-up reel) on the magnetic recording and playback device side. During this time, the magnetic head comes into contact with and slides against the magnetic layer surface of the magnetic tape, thereby recording and/or playing back data. In contrast, a dual-reel magnetic tape cartridge has both a supply reel and a take-up reel inside the magnetic tape cartridge. The magnetic tape cartridge may be either a single-reel type or a dual-reel type magnetic tape cartridge. For other details of the magnetic tape cartridge, known technology may be applied.

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

In this invention and this specification, the term "magnetic recording and reproducing device" refers to a device that can perform at least one of recording data to a magnetic recording medium and reproducing data recorded on the magnetic recording medium. Such devices are generally called drives. The magnetic recording and reproducing device can be a sliding type magnetic recording and reproducing device. A sliding type magnetic recording and reproducing device refers to a device in which the magnetic layer surface and the magnetic head come into contact and slide when recording data to the magnetic recording medium and/or reproducing the recorded data.

The magnetic head included in the magnetic recording and reproducing device can be a recording head capable of recording data on a magnetic recording medium, and can also be a reproducing head capable of reproducing data recorded on the magnetic recording medium. In one embodiment, the magnetic recording and reproducing device can include both a recording head and a reproducing head as separate magnetic heads. In another embodiment, the magnetic head included in the magnetic recording and reproducing device can have a configuration in which both an element for recording data (recording element) and an element for reproducing data (reproducing element) are provided in one magnetic head. Hereinafter, the element for recording data and the element for reproducing data are collectively referred to as "data element". As the reproducing head, a magnetic head (MR head) including a magnetoresistive (MR) element capable of sensitively reading data recorded on a magnetic tape as a reproducing element 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. In addition, the magnetic head that records and/or reproduces data may include a servo signal reading element. Alternatively, the magnetic recording and reproducing device may include a magnetic head (servo head) equipped with a servo signal reading element as a head separate from the magnetic head that records and/or reproduces data. For example, the magnetic head that records and/or reproduces recorded data (hereinafter also referred to as a "recording and reproducing head") may include two servo signal reading elements, and each of the two servo signal reading elements can simultaneously read two adjacent servo bands. One or more data elements may be arranged between the two servo signal reading elements.

In the magnetic recording and reproducing device, data can be recorded on the magnetic recording medium and/or data recorded on the magnetic recording medium can be reproduced by contacting and sliding the magnetic head against the magnetic layer surface of the magnetic recording medium. The magnetic recording and reproducing device may include the magnetic recording medium according to one aspect of the present invention, and other known techniques may be applied.

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, the servo signal reading element is made to follow a specific servo track, so that the data element passes 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.
The recording/reproducing head can also record and/or reproduce data on other data bands by using the UDIM information described above to move the servo signal reading element to a specific servo band and start tracking on that servo band.

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

[Example 1]
(1) Preparation of alumina dispersion 100.0 parts of alumina powder (HIT-80 manufactured by Sumitomo Chemical Co., Ltd.) having an alpha ratio of about 65% and a BET (Brunauer-Emmett-Teller) specific surface area of 20 m ² /g were mixed with 3.0 parts of 2,3-dihydroxynaphthalene (manufactured by Tokyo Chemical Industry Co., Ltd.), 31.3 parts of a 32% solution (solvent is a mixed solvent of methyl ethyl ketone and toluene) of SO ₃ Na group-containing polyester polyurethane resin (UR-4800 manufactured by Toyobo Co., Ltd. (SO ₃ Na group: 0.08 meq/g)), and 570.0 parts of a mixed solvent of methyl ethyl ketone and cyclohexanone at 1:1 (mass ratio) as a solvent, and dispersed for 5 hours in the presence of zirconia beads using a paint shaker. After dispersion, the dispersion liquid and the beads were separated using a mesh to obtain an alumina dispersion.

(2) Magnetic layer forming composition formulation (magnetic liquid)
Ferromagnetic powder (type: see Table 1) 100.0 parts Binder (type: see Table 1) See Table 1 Cyclohexanone 150.0 parts Methyl ethyl ketone 150.0 parts (abrasive liquid)
6.0 parts of the alumina dispersion prepared in (1) above (protrusion-forming agent liquid)
Protrusion forming agent (type: see Table 1) 1.3 parts
Methyl ethyl ketone 9.0 parts Cyclohexanone 6.0 parts (other components)
Stearic acid 2.0 parts Stearic acid amide 0.2 parts Butyl stearate 2.0 parts Polyisocyanate (Tosoh Corporation, Coronate (registered trademark)) 2.5 parts (finishing additive solvent)
Cyclohexanone 200.0 parts Methyl ethyl ketone 200.0 parts

(3) Composition formulation for forming nonmagnetic layer Nonmagnetic inorganic powder: α-iron oxide 100.0 parts Average particle size (average major axis length): 0.15 μm
Average needle ratio: 7
BET specific surface area: 52 m ² /g
Carbon black 20.0 parts Average particle size: 20 nm
Binder A 18.0 parts Stearic acid 2.0 parts Stearic acid amide 0.2 parts Butyl stearate 2.0 parts Cyclohexanone 300.0 parts Methyl ethyl ketone 300.0 parts

(4) Composition formulation for forming backcoat layer Nonmagnetic inorganic powder: α-iron oxide 80.0 parts Average particle size (average major axis length): 0.15 μm
Average needle ratio: 7
BET specific surface area: 52 m ² /g
Carbon black 20.0 parts Average particle size: 20 nm
Vinyl chloride copolymer 13.0 parts Sulfonate group-containing polyurethane resin 6.0 parts Phenylphosphonic acid 3.0 parts Methyl ethyl ketone 155.0 parts Polyisocyanate 5.0 parts Cyclohexanone 355.0 parts

(5) Preparation of Compositions for Forming Each Layer A composition for forming a magnetic layer was prepared by the following method.
The magnetic liquid was prepared by dispersing each component for 24 hours (bead dispersion) using a batch-type vertical sand mill. Zirconia beads with a bead diameter of 0.5 mm were used as the dispersion beads.
The protrusion-forming agent liquid was prepared by mixing the components of the protrusion-forming agent liquid, subjecting the mixture to ultrasonic treatment (dispersion treatment) for 60 minutes at an ultrasonic output of 500 watts per 200 cc using a horn-type ultrasonic disperser, and filtering the resulting dispersion through a filter having a pore size of 0.5 μm.
The magnetic liquid, the abrasive liquid, the protrusion-forming agent liquid, and other components (other components and finishing additive solvent) were mixed using the sand mill and dispersed with beads for 5 minutes, and then treated (ultrasonic dispersion) for 0.5 minutes with a batch-type ultrasonic device (20 kHz, 300 W). Thereafter, filtration was performed using a filter having a pore size of 0.5 μm to prepare a magnetic layer-forming composition.
A composition for forming a nonmagnetic layer was prepared by the following method.
Each component, except for the lubricant (stearic acid, stearic acid amide, and butyl stearate), cyclohexanone, and methyl ethyl ketone, was dispersed for 24 hours using a batch-type vertical sand mill to obtain a dispersion. Zirconia beads with a bead diameter of 0.5 mm were used as the dispersion beads. The remaining components were then added to the obtained dispersion and stirred with a dissolver. The dispersion thus obtained was filtered using a filter with a pore size of 0.5 μm to prepare a composition for forming a nonmagnetic layer.
A composition for forming a backcoat layer was prepared by the following method.
Each component except polyisocyanate and cyclohexanone is kneaded and diluted by open kneader, and then, by horizontal bead mill disperser, zirconia beads with a bead diameter of 1 mm are used, bead filling rate is 80 volume %, rotor tip peripheral speed is 10 m/sec, 1 pass residence time is 2 minutes, and 12 passes are dispersed.Then, the remaining components are added to the obtained dispersion liquid, and stirred by dissolver.The dispersion liquid thus obtained is filtered by a filter with a pore size of 1 μm to prepare a composition for forming a backcoat layer.

(6) Method for Producing Magnetic Tape The nonmagnetic layer-forming composition prepared in (5) above was applied onto the surface of a biaxially oriented polyethylene naphthalate support having a thickness of 5.00 μm, and then dried to give a thickness of 1.00 μm after drying, thereby forming a nonmagnetic layer.
Next, in a coating device in which a magnet for applying an AC magnetic field was arranged, the magnetic layer forming composition prepared in (5) above was applied to the surface of the non-magnetic layer so that the thickness after drying was 0.10 μm, while applying an AC magnetic field (magnetic field strength: 0.15 T) to form a coating layer. The AC magnetic field was applied so that the AC magnetic field was applied perpendicular to the surface of the coating layer. Thereafter, while the coating layer of the magnetic layer forming composition was in a wet (undried) state, a DC magnetic field with a magnetic field strength of 0.30 T was applied perpendicular to the surface of the coating layer to perform a vertical orientation treatment. Thereafter, the coating layer was dried to form a magnetic layer.
Thereafter, the backcoat layer forming composition prepared in (5) above was applied to the surface of the polyethylene naphthalate support opposite to the surface on which the nonmagnetic layer and magnetic layer were formed, and then dried to a thickness of 0.50 μm after drying, thereby forming a backcoat layer.
The magnetic tape thus obtained was slit to a width of 1/2 inch (0.0127 meters), and then burnishing and wiping of the surface of the coating layer of the magnetic layer-forming composition were performed. Burnishing and wiping were performed in a processing device having a configuration shown in FIG. 1 of JP-A-6-52544, using a commercially available polishing tape (manufactured by Fujifilm Corporation under the trade name MA22000, abrasive: diamond/Cr ₂ O ₃ /red iron oxide) as the polishing tape, a commercially available sapphire blade (manufactured by Kyocera Corporation, width 5 mm, length 35 mm, tip angle 60 degrees) as the grinding blade, and a commercially available wiping material (manufactured by Kuraray Co., Ltd. under the trade name WRP736) as the wiping material. The processing conditions employed were those in Example 12 of JP-A-6-52544.
After the burnishing and wiping treatments, the sheet was subjected to a calendering (surface smoothing) treatment using a calender roll consisting of only a metal roll at a speed of 80 m/min, a linear pressure of 294 kN/m (300 kg/cm), and a calender temperature (surface temperature of the calender roll) of 100°C.
Thereafter, a heat treatment (curing treatment) was carried out in an atmosphere at 70° C. for 36 hours to prepare a magnetic tape.
In a state where the magnetic layer of the produced magnetic tape was demagnetized, a servo pattern having a layout and shape according to the LTO Ultrium format was formed on the magnetic layer by a servo write head mounted on a servo writer. This resulted in a magnetic tape having a data band, a servo band, and a guide band in a layout according to the LTO Ultrium format on the magnetic layer, and a servo pattern on the servo band in a layout and shape according to the LTO Ultrium format.

[Examples 2 to 21, Comparative Examples 1 to 13]
A magnetic tape was produced in the same manner as in Example 1, except that the various items shown in Table 1 were changed as shown in Table 1.
As shown in Table 1, in Examples 2 to 21 and Comparative Examples 3 to 10, the magnetic tapes were produced in the same manner as in Example 1. That is, as in Example 1, an AC magnetic field was applied during the coating of the magnetic layer-forming composition, and the coating layer of the magnetic layer-forming composition was subjected to a burnishing treatment and a wiping treatment.
In contrast, in Comparative Examples 1, 2, and 11 to 13, a magnetic tape was produced in the same manner as in Example 1, except that no AC magnetic field was applied during application of the magnetic layer forming composition, and no burnishing or wiping treatment was performed on the coating layer of the magnetic layer forming composition.

[Protrusion-forming agent]
The protrusion-forming agents used in the manufacture of the magnetic tapes of the Examples and Comparative Examples are as follows. Protrusion-Forming Agent 1 and Protrusion-Forming Agent 3 are particles with low surface smoothness. Protrusion-Forming Agent 2 has a cocoon-like particle shape. Protrusion-Forming Agent 4 has a so-called amorphous particle shape. Protrusion-Forming Agent 5 has a nearly spherical particle shape.
Protrusion-forming agent 1: ATLAS (composite particles of silica and polymer) manufactured by Cabot Corporation, average particle size 100 nm
Protrusion-forming agent 2: Cabot TGC6020N (silica particles), average particle size 140 nm
Protrusion-forming agent 3: Cataloid manufactured by JGC Catalysts and Chemicals (water-dispersed sol of silica particles; the water-dispersed sol was heated to remove the solvent and the resulting dried product was used as the protrusion-forming agent for preparing the protrusion-forming agent liquid), average particle size 120 nm
Protrusion-forming agent 4: Asahi #50 (carbon black) manufactured by Asahi Carbon Co., Ltd., average particle size 300 nm
Protrusion-forming agent 5: Quattron PL-10L (water-dispersed sol of silica particles; the protrusion-forming agent used in preparing the protrusion-forming agent solution was a dried product obtained by heating the water-dispersed sol to remove the solvent, and the average particle size was 130 nm.

[Binding Agent]
In Table 1, "binder A" is a SO ₃ Na group-containing polyurethane resin (weight average molecular weight: 70,000, SO ₃ Na group: 0.20 meq/g).
In Table 1, "binder B" is a vinyl chloride copolymer manufactured by Kaneka Corporation (product name: MR110, SO ₃ K group-containing vinyl chloride copolymer, SO ₃ K group: 0.07 meq/g).

[Ferromagnetic powder]
In Table 1, "BaFe" represents hexagonal barium ferrite powder having an average particle size (average plate diameter) of 21 nm, "SrFe1" and "SrFe2" represent hexagonal strontium ferrite powder, and "ε-iron oxide" represents ε-iron oxide powder.
The activation volume and anisotropy constant Ku of each of the 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.).
The mass magnetization σs is a value measured at a magnetic field strength of 15 kOe using a vibrating sample magnetometer (manufactured by Toei Industry Co., Ltd.).

<Method 1 for producing hexagonal strontium ferrite powder>
"SrFe1" shown in Table 1 is a hexagonal strontium ferrite powder produced by the following method.
₁₇₀₇ g of SrCO3, ₆₈₇ g of _H3BO3 , 1120 _g of _Fe2O3 , 45 g of Al(OH) ₃ , 24 g of _BaCO3 , ₁₃ g of CaCO3, and 235 _g of _Nd2O3 were weighed and mixed in a mixer to obtain a raw material mixture.
The obtained raw material mixture was melted in a platinum crucible at a melting temperature of 1390° C., and the melt was stirred while the tapping hole at the bottom of the platinum crucible was heated to tap the melt in a rod shape at about 6 g/sec. The tapped liquid was rolled and quenched with a water-cooled twin roller to produce an amorphous body.
280 g of the produced amorphous body was placed in an electric furnace, heated to 635° C. (crystallization temperature) at a heating rate of 3.5° C./min, and held at that temperature for 5 hours to precipitate (crystallize) hexagonal strontium ferrite particles.
Next, the above-obtained crystallized product containing hexagonal strontium ferrite particles was coarsely crushed in a mortar, and 1000 g of zirconia beads with a particle size of 1 mm and 800 mL of 1% aqueous acetic acid solution were added to a glass bottle and dispersed for 3 hours using a paint shaker. The resulting dispersion was then separated from the beads and placed in a stainless steel beaker. The dispersion was left to stand at a liquid temperature of 100°C for 3 hours to dissolve the glass components, and then precipitated in a centrifuge and washed by repeated decantation, and dried for 6 hours in a heating furnace with an internal temperature of 110°C to obtain hexagonal strontium ferrite powder.
The hexagonal strontium ferrite powder obtained above ("SrFe1" in Table 1) had an average particle size of 18 nm, an activation volume of 902 nm ³ , an anisotropy constant Ku of 2.2×10 ⁵ J/m ³ , and a mass magnetization σs of 49 A·m ² /kg.
12 mg of sample powder was taken from the hexagonal strontium ferrite powder obtained above, and this sample powder was partially dissolved under the dissolution conditions exemplified above. The elemental analysis of the filtrate obtained was performed using an ICP analyzer to determine the surface layer content of neodymium atoms.
Separately, 12 mg of sample powder was taken 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 filtrate obtained was performed using an ICP analyzer to determine the bulk content of neodymium atoms.
The neodymium atom content (bulk content) relative to 100 atomic % of iron atoms in the hexagonal strontium ferrite powder obtained above was 2.9 atomic %. The surface layer content of neodymium atoms was 8.0 atomic %. The ratio of the surface layer content to the bulk content, "surface layer content/bulk content," was 2.8, confirming that neodymium atoms were unevenly distributed in the surface layers of the particles.

The powder obtained above was confirmed to have a hexagonal ferrite crystal structure by scanning with CuKα radiation 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). The powder obtained above exhibited a magnetoplumbite (M-type) hexagonal ferrite crystal structure. The crystal phase detected by the X-ray diffraction analysis was a single magnetoplumbite phase.
PANalytical X'Pert Pro diffractometer, PIXcel detector Soller slits for incident and diffracted beams: 0.017 radians Fixed angle of dispersion slits: 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

<Method 2 for producing hexagonal strontium ferrite powder>
"SrFe2" shown in Table 1 is a hexagonal strontium ferrite powder produced by the following method.
₁₇₂₅ g of SrCO3, ₆₆₆ g of _H3BO3 , 1332 _g of _Fe2O3 , 52 g of Al(OH) ₃ , ₃₄ g of CaCO3, and 141 g of _BaCO3 were weighed and mixed in a mixer to obtain a raw material mixture.
The obtained raw material mixture was melted in a platinum crucible at a melting temperature of 1380° C., and the melt was poured into a rod shape at about 6 g/sec by heating the tapping hole at the bottom of the platinum crucible while stirring the melt. The tapped liquid was quenched and rolled with a water-cooled twin roll to produce an amorphous body.
280 g of the obtained amorphous body was placed in an electric furnace, heated to 645° C. (crystallization temperature), and held at that temperature for 5 hours to precipitate (crystallize) hexagonal strontium ferrite particles.
Next, the above-obtained crystallized product containing hexagonal strontium ferrite particles was coarsely crushed in a mortar, and 1000 g of zirconia beads with a particle size of 1 mm and 800 mL of 1% aqueous acetic acid solution were added to a glass bottle and dispersed for 3 hours using a paint shaker. The resulting dispersion was then separated from the beads and placed in a stainless steel beaker. The dispersion was left to stand at a liquid temperature of 100°C for 3 hours to dissolve the glass components, and then precipitated in a centrifuge and washed by repeated decantation, and dried for 6 hours in a heating furnace with an internal temperature of 110°C to obtain hexagonal strontium ferrite powder.
The obtained hexagonal strontium ferrite powder ("SrFe2" in Table 1) had an average particle size of 19 nm, an activation volume of 1102 nm ³ , an anisotropy constant Ku of 2.0×10 ⁵ J/m ³ , and a mass magnetization σs of 50 A·m ² /kg.

<Method for Producing ε-Iron Oxide Powder>
The "ε-iron oxide" shown in Table 1 is ε-iron oxide powder prepared by the following method.
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 1.5 g of polyvinylpyrrolidone (PVP) were dissolved in 90 g of pure water, and 4.0 g of ammonia solution with a concentration of 25% was added under the condition of an atmospheric temperature of 25 ° C. while stirring with a magnetic stirrer in the air atmosphere, and the mixture was stirred for 2 hours at an atmospheric temperature of 25 ° C. To the obtained solution, an aqueous citric acid solution obtained by dissolving 1 g of citric acid in 9 g of pure water was added, and the mixture was stirred for 1 hour. The powder precipitated after stirring was collected by centrifugation, washed with pure water, and dried in a heating furnace with an internal temperature of 80 ° C.
800 g of pure water was added to the dried powder, and the powder was dispersed in water again to obtain a dispersion. The obtained dispersion was heated to 50° C., and 40 g of 25% aqueous ammonia 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 stirred for 24 hours. 50 g of ammonium sulfate was added to the obtained reaction solution, and the precipitated powder was collected by centrifugation, washed with pure water, and dried for 24 hours in a heating furnace with an internal temperature of 80° C. to obtain a precursor of a ferromagnetic powder.
The obtained precursor of the ferromagnetic powder was placed in a heating furnace at an internal temperature of 1000° C. in an air atmosphere and subjected to a heat treatment for 4 hours.
The heat-treated ferromagnetic powder precursor was placed in a 4 mol/L aqueous solution of sodium hydroxide (NaOH), and the liquid temperature was maintained at 70° C. while stirring for 24 hours, thereby removing the impurity silicic acid compound from the heat-treated ferromagnetic powder precursor.
Thereafter, the silicic acid compound was removed by centrifugation, and the ferromagnetic powder was collected and washed with pure water to obtain a ferromagnetic powder.
The composition of the obtained ferromagnetic powder was confirmed by inductively coupled plasma-optical emission spectrometry (ICP-OES), which revealed that the powder was Ga _, Co and Ti _substituted ε _- iron oxide (ε- _{Ga0.28Co0.05Ti0.05Fe1.62O3} ). In addition, X-ray diffraction analysis was performed under the same conditions as those described above for the hexagonal strontium ferrite powder preparation method 1, and it was confirmed from the peaks of the X-ray diffraction pattern that the obtained _{ferromagnetic} powder had a single-phase ε-phase crystal structure (ε-iron oxide type crystal structure) that did not contain α-phase and γ-phase crystal structures.
The resulting ε-iron oxide powder (in Table 1, "ε-iron oxide") had an average particle size of 12 nm, an activation volume of 746 nm ³ , an anisotropy constant Ku of 1.2×10 ⁵ J/m ³ , and a mass magnetization σs of 16 A·m ² /kg.

[Evaluation method]
(1) Isoelectric point of surface zeta potential of magnetic layer after pressing For each of the magnetic tapes in the Examples and Comparative Examples, a calendering machine equipped with seven stages of calender rolls consisting only of metal rolls was used in an environment with an atmospheric temperature of 20 to 25°C and a relative humidity of 40 to 60%, and the magnetic tape was run in the longitudinal direction at a speed of 20 m/min with a tension of 0.5 N/m applied, while passing between two rolls (without heating the rolls) a total of six times, thereby applying a surface pressure of 70 atm to the surface of the magnetic layer each time it passed between the rolls.
Six samples for isoelectric point measurement were cut out from each magnetic tape of the Examples and Comparative Examples after the above pressing, and two samples were placed in the measurement cell in one measurement. In the measurement cell, the sample mounting surface and the backcoat layer surface of the sample were attached to the upper and lower sample tables of the measurement cell (each sample mounting surface size is 1 cm x 2 cm) with double-sided tape. When the electrolyte is poured into the measurement cell after the two samples are placed in this way, the magnetic layer surfaces of the two samples attached to the upper and lower sample tables of the measurement cell come into contact with the electrolyte, so that the surface zeta potential of the magnetic layer can be measured. In this way, two samples were used in one measurement, and a total of three measurements were performed to determine the isoelectric point of the surface zeta potential of the magnetic layer. The arithmetic average of the three values obtained by the three measurements is shown in Table 1 as the isoelectric point of the surface zeta potential of the magnetic layer after pressing each magnetic tape. As the surface zeta potential measuring device, SurPASS manufactured by Anton Paar was used. The measurement conditions were as follows. Other details of the method for determining the isoelectric point are as described above.
Measurement cell: variable gap cell (20 mm x 10 mm)
Measurement mode: Streaming Current
Gap: about 200 μm
Measurement temperature: room temperature Ramp Target Pressure/Time: 400,000 Pa (400 mbar)/60 seconds Electrolyte: 1 mmol/L KCl aqueous solution (adjusted to pH 9)
pH adjustment solution: 0.1 mol/L HCl aqueous solution or 0.1 mol/L KOH aqueous solution Measured pH: pH 9 → pH 3 (measured at a total of 13 measurement points at intervals of about 0.5)

(2) Evaluation of Running Stability After Pressing at a Pressure of 70 atm For each of the magnetic tapes of the Examples and Comparative Examples, after pressing in the above (1), the PES (Position Error Signal) was determined by the following method.
The servo pattern was read by a verify head on the servo writer used to form the servo pattern. The verify head is a reading magnetic head for checking the quality of the servo pattern formed on the magnetic tape, and has a reading element disposed at a position corresponding to the position of the servo pattern (position in the width direction of the magnetic tape) in the same manner as the magnetic head of a known magnetic tape device (drive).
The verify head is connected to a known PES calculation circuit that calculates the head positioning accuracy in the servo system as PES from the electric signal obtained by reading the servo pattern with the verify head. The PES calculation circuit calculates the displacement in the width direction of the magnetic tape from the input electric signal (pulse signal) at any time, and calculates the PES by applying a high-pass filter (cutoff: 500 cycles/m) to the time change information (signal) of this displacement. The PES can be used as an index of running stability, and if the calculated PES is 18 nm or less, it can be evaluated as having excellent running stability.

The above results are shown in Table 1 (Table 1-1 to Table 1-5).

From the results shown in Table 1, it can be confirmed that all of the magnetic tapes of the examples showed excellent running stability after being pressed at a pressure of 70 atm, i.e., in a state equivalent to long-term storage. Such magnetic tapes can run stably in a magnetic recording and playback device even after being stored in a magnetic tape cartridge in a reeled state for a long period of time after infrequently accessed information is recorded on them, making them suitable as archival recording media.

One aspect of the present invention is useful in data storage applications.

Claims (10)

A magnetic recording medium having a non-magnetic support and a magnetic layer containing a ferromagnetic powder,
The average particle size of the ferromagnetic powder is 5 nm or more and 25 nm or less,
A magnetic recording medium, wherein the isoelectric point of the surface zeta potential of the magnetic layer after the magnetic layer is pressed with a pressure of 70 atm is 5.5 or more and 7.0 or less. The magnetic recording medium according to claim 1, wherein the average particle size of the ferromagnetic powder is 5 nm or more and 20 nm or less. The magnetic recording medium according to claim 1 or 2, wherein the magnetic layer contains inorganic oxide particles. The magnetic recording medium according to claim 3, wherein the inorganic oxide particles are composite particles of an inorganic oxide and a polymer. The magnetic recording medium according to any one of claims 1 to 4, wherein the magnetic layer contains a binder having an acidic group. The magnetic recording medium according to claim 5, wherein the acidic group is at least one acidic group selected from the group consisting of sulfonic acid groups and salts thereof. The magnetic recording medium according to any one of claims 1 to 6, having a non-magnetic layer containing non-magnetic powder between the non-magnetic support and the magnetic layer. The magnetic recording medium according to any one of claims 1 to 7, which has a backcoat layer containing nonmagnetic powder on the surface side of the nonmagnetic support opposite to the surface side having the magnetic layer. 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,
A magnetic head;
A magnetic recording and reproducing apparatus comprising: