JP7411128B2 - Magnetic recording media and magnetic recording/reproducing devices - Google Patents

Description

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

Magnetic recording media are roughly divided into two types: metal thin film type and coating type. A metal thin film magnetic recording medium is a magnetic recording medium having a magnetic layer of a metal thin film formed by vapor deposition or the like. On the other hand, a coated magnetic recording medium (for example, see Patent Document 1) is a magnetic recording medium that has a magnetic layer containing ferromagnetic powder together with a binder. Coating type magnetic recording media have better chemical durability than metal thin film type magnetic recording media, and are therefore useful magnetic recording media as data storage media for storing large amounts of information for long periods of time. In the following, the coated magnetic recording medium will be simply referred to as a magnetic recording medium.

Japanese Patent Application Publication No. 2012-43495

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

In recent years, magnetic recording media used for data storage applications are sometimes used in temperature and humidity controlled data centers. On the other hand, data centers are required to save power in order to reduce costs. In order to save power, it is desirable to be able to relax the current temperature and humidity management conditions in data centers, or to eliminate the need for management. However, if the temperature and humidity management conditions are relaxed or not managed, it is assumed that the magnetic recording medium will be exposed to environmental changes due to weather changes, seasonal changes, and the like.

Regarding the above points, the inventors have found that magnetic recording media with high surface smoothness of the magnetic layer can be used at high temperatures (for example, 30 to 50°C) under low humidity (for example, in an environment with relative humidity of about 0 to 30%) ) to a low temperature (for example, over 0°C to 15°C) (for example, a temperature change of about 15 to 50°C), it has become clear that a phenomenon occurs in which the electromagnetic conversion characteristics deteriorate.

Therefore, an object of the present invention is to suppress deterioration of electromagnetic conversion characteristics caused by a temperature change from high temperature to low temperature under low humidity in a magnetic recording medium whose magnetic layer has a high surface smoothness.

One aspect of the present invention is
A magnetic recording medium having a magnetic layer containing ferromagnetic powder and a binder on a non-magnetic support,
The center line average surface roughness Ra (hereinafter also referred to as "magnetic layer surface roughness Ra") measured on the surface of the magnetic layer is 1.0 nm or more and 1.6 nm or less, and the surface of the magnetic layer The difference between the spacing S _after measured by optical interferometry after washing with ethanol and the spacing S _before measured by optical interferometry before washing with ethanol on the surface of the magnetic layer (S _after −S _before ) (hereinafter referred to as , a magnetic recording medium in which the spacing difference before and after ethanol cleaning (S _after - S _before ) or simply "difference (S _after - S _before )" is greater than 0 nm and less than or equal to 6.0 nm;
Regarding.

In one embodiment, the difference (S _after −S _before ) can be 1.0 nm or more and 6.0 nm or less.

In one embodiment, the difference (S _after −S _before ) can be 2.0 nm or more and 5.0 nm or less.

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

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

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

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

According to one aspect of the present invention, there is provided a magnetic recording medium that has a magnetic layer with high surface smoothness and suppresses deterioration of electromagnetic conversion characteristics caused by temperature change from high temperature to low temperature under low humidity, and this magnetic recording medium. A magnetic recording/reproducing device including a recording medium can be provided.

[Magnetic recording medium]
One aspect of the present invention is a magnetic recording medium having a magnetic layer containing ferromagnetic powder and a binder on a nonmagnetic support, wherein the center line average surface roughness Ra measured on the surface of the magnetic layer is 1. .0 nm or more and 1.6 nm or less, and a spacing S _{after that} is measured by optical interferometry on the surface of the magnetic layer after cleaning with ethanol, and a spacing S after that is measured by optical interferometry on the surface of the magnetic layer before cleaning with ethanol. This relates to a magnetic recording medium in which the difference from the spacing S _before (S _after −S _before ) is more than 0 nm and less than or equal to 6.0 nm.

In the present invention and this specification, "ethanol cleaning" refers to immersing a sample piece cut out from a magnetic recording medium in ethanol (200 g) at a liquid temperature of 20 to 25°C and ultrasonically cleaning it for 100 seconds (ultrasonic output: 40 kHz). ). If the magnetic recording medium to be cleaned is a magnetic tape, a sample piece with a length of 5 cm is cut out and subjected to ethanol cleaning. The width of the magnetic tape and the width of the sample pieces cut from the magnetic tape are typically 1/2 inch (0.0127 meters). For magnetic tapes other than 1/2 inch (0.0127 meter) wide, a sample piece 5 cm long may be cut out and washed with ethanol. If the magnetic recording medium to be cleaned is a magnetic disk, a sample piece measuring 5 cm x 1.27 cm is cut out and subjected to ethanol cleaning. The measurement of spacing after ethanol cleaning, which will be described in detail below, is performed after the sample piece after ethanol cleaning has been left in an environment at a temperature of 23° C. and a relative humidity of 50% for 24 hours.

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

In the present invention and this specification, the spacing measured by optical interferometry on the surface of the magnetic layer of a magnetic recording medium is a value measured by the following method.
A magnetic recording medium (more specifically, the sample piece mentioned above; the same applies hereinafter) and a transparent plate-like member (for example, a glass plate) are stacked so that the surface of the magnetic layer of the magnetic recording medium faces the transparent plate-like member. In this state, a pressing member is pressed with a pressure of 5.05×10 ⁴ N/m (0.5 atm) from the side opposite to the magnetic layer side of the magnetic recording medium. In this state, light is irradiated onto the surface of the magnetic layer of the magnetic recording medium through the transparent plate-like member (irradiation area: 150,000 to 200,000 μm ² ), and the reflected light from the surface of the magnetic layer of the magnetic recording medium and the transparent plate-like member are The magnetic layer surface of the magnetic recording medium and the magnetic recording medium of the transparent plate-like member are determined based on the intensity of the interference light generated due to the optical path difference between the light reflected from the magnetic recording medium side surface of the member (for example, the contrast of the interference fringe image). Find the spacing (distance) between it and the side surface. The light irradiated here is not particularly limited. If the emitted light is light that has emission wavelengths over a relatively wide range of wavelengths, such as white light that includes light of multiple wavelengths, a transparent plate-like member and a light-receiving part that receives reflected light may be used. In between, a member such as an interference filter that has the function of selectively cutting off light of a specific wavelength or light outside of a specific wavelength range is placed, and a member having a function of selectively cutting off light of a specific wavelength or light outside of a specific wavelength range is placed in between. Light is selectively made incident on the light receiving section. If the light to be irradiated is light having a single emission peak (so-called monochromatic light), the above-mentioned member may not be used. The wavelength of the light incident on the light receiving section can be, for example, in the range of 500 to 700 nm. However, the wavelength of the light incident on the light receiving section is not limited to the above range. Further, the transparent plate-like member may be any member as long as it has transparency that allows the irradiated light to pass therethrough to the extent that interference light can be obtained by irradiating the light onto the magnetic recording medium through the member.
The interference fringe image obtained by the above spacing measurement is divided into 300,000 points, and the spacing of each point (distance between the magnetic layer surface of the magnetic recording medium and the magnetic recording medium side surface of the transparent plate-like member) is calculated. This is determined as a histogram, and the mode in this histogram is defined as the spacing. The difference (S _after - S _before ) is the value obtained by subtracting the mode before ethanol washing from the mode after ethanol washing at the 300,000 points.
Two sample pieces are cut out from the same magnetic recording medium, one is not washed with ethanol to find the spacing value S _before , and the other is washed with ethanol and then the spacing value S _after is found to find the difference ( S _after −S _before ) may also be calculated. Alternatively, the difference (S _after - S _before ) may be determined by using a sample piece for which the spacing value was determined before washing with ethanol, and then washing the sample piece with ethanol and then determining the spacing value.
The above measurements can be performed using a commercially available tape spacing analyzer (Tape Spacing Analyzer; TSA) such as Tape Spacing Analyzer manufactured by Micro Physics. Spacing measurements in Examples were performed using Tape Spacing Analyzer manufactured by Micro Physics.

The magnetic recording medium has a center line average surface roughness Ra measured on the surface of the magnetic layer of 1.0 nm or more and 1.6 nm or less. That is, the above magnetic recording medium is a magnetic recording medium having a magnetic layer with high surface smoothness. In such a magnetic recording medium, since the above-mentioned spacing difference (S _after - S _before ) before and after ethanol cleaning is more than 0 nm and less than 6.0 nm, electromagnetic conversion due to temperature change from high temperature to low temperature under low humidity is prevented. Deterioration of characteristics can be suppressed. The inventors' speculations regarding this point are as follows.

Reproduction of information recorded on a magnetic recording medium is usually performed by bringing the surface of the magnetic layer into contact with a magnetic head (hereinafter also simply referred to as "head") and causing them to slide.
On the other hand, under low humidity and high temperature conditions, it is thought that organic components tend to seep out onto the surface of the magnetic layer. It is presumed that when the temperature changes from high temperature to low temperature under low humidity, the organic components that have oozed out onto the surface of the magnetic layer solidify or become highly viscous. It is thought that such solidified or highly viscous organic components adhere to the magnetic head due to sliding between the surface of the magnetic layer and the magnetic head, resulting in head contamination, which causes deterioration of electromagnetic conversion characteristics. In magnetic recording media with highly smooth magnetic layer surfaces, the coefficient of friction during sliding between the magnetic layer surface and the magnetic head tends to increase and the electromagnetic conversion characteristics tend to deteriorate. The inventors believe that this is the reason why electromagnetic conversion characteristics deteriorate when the temperature changes from high to low temperature under low humidity in magnetic recording media with highly smooth magnetic layer surfaces. ing. Therefore, we believe that if we can reduce the amount of organic components that solidify or become highly viscous on the surface of the magnetic layer when the temperature changes from high to low temperatures under low humidity, this will lead to suppressing the deterioration of electromagnetic conversion characteristics. It will be done.

By the way, the surface of the magnetic layer usually has a part (protrusion) that mainly contacts the head when the magnetic layer surface and the head slide (so-called true contact), and a part lower than this part (hereinafter referred to as "base part"). ) exists. The present inventors believe that the spacing described above is a value that is an index of the distance between the head and the base portion when the head slides on the surface of the magnetic layer. However, if some component exists on the surface of the magnetic layer, it is thought that the larger the amount of the component present between the base portion and the head, the narrower the spacing. On the other hand, when this component is removed by ethanol cleaning, the spacing widens, so the value of the spacing S _after after the ethanol cleaning becomes larger than the value of the spacing S _before before the ethanol cleaning. Therefore, it is considered that the difference in spacing (S _after −S _before ) before and after ethanol cleaning can be used as an index of the amount of the component present between the base portion and the head.
Regarding the above points, the present inventors believe that the components removed by ethanol cleaning are organic components that solidify or become highly viscous on the surface of the magnetic layer due to temperature changes from high to low temperatures under low humidity, as described above. thinking. Therefore, reducing the spacing difference (S _after − S _before ) before and after ethanol cleaning, that is, reducing the amount of the above components, is important because the magnetic layer surface is The inventors of the present invention conjecture that this contributes to suppressing the occurrence of head stains due to the organic components adhering to the magnetic head due to the sliding movement between the organic component and the magnetic head. The inventors believe that this makes it possible to suppress the deterioration of electromagnetic conversion characteristics caused by temperature changes from high to low temperatures under low humidity in magnetic recording media with highly smooth magnetic layer surfaces. There is. On the other hand, according to the study by the present inventors, the value of the difference in spacing before and after cleaning with n-hexane described in Patent Document 1, and the value of the difference in spacing under low humidity in a magnetic recording medium with a highly smooth magnetic layer surface. No correlation was found between the decrease in electromagnetic conversion characteristics caused by the temperature change from high temperature to low temperature. This is presumed to be because the above components cannot be removed or cannot be removed sufficiently by n-hexane cleaning.
Details of the above components are not clear. As a matter of speculation, the present inventors believe that the above-mentioned components may originate from organic components and/or binders that are commonly added to magnetic layers as additives (eg, lubricants). As for components derived from the binder, relatively low molecular weight components of the resin used as the binder (which usually has a molecular weight distribution) may easily leach onto the surface of the magnetic layer under low humidity and high temperature. The present inventors speculate that this is the case.
However, the above is speculation by the present inventors, and does not limit the present invention in any way.

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

<Magnetic layer>
(Magnetic layer surface roughness Ra)
The centerline average surface roughness Ra (magnetic layer surface roughness Ra) measured on the magnetic layer surface of the magnetic recording medium is 1.0 nm or more and 1.6 nm or less. The fact that the magnetic layer surface roughness Ra is 1.6 nm or less can contribute to the magnetic recording medium being able to exhibit excellent electromagnetic conversion characteristics. From the viewpoint of further improving electromagnetic conversion characteristics, the surface roughness Ra of the magnetic layer is preferably 1.5 nm or less. However, in a magnetic recording medium having such a magnetic layer with high surface smoothness, if no measures are taken, the electromagnetic conversion characteristics will deteriorate due to temperature change from high temperature to low temperature under low humidity. On the other hand, in the above-mentioned magnetic recording medium in which the spacing difference (S _after − S _before ) before and after ethanol cleaning is within the above range, although it has a magnetic layer with a high surface smoothness, it does not change from high temperature to low temperature under low humidity. It is possible to suppress deterioration of electromagnetic conversion characteristics due to temperature changes. In addition, if the magnetic layer surface roughness Ra is 1.0 nm or more, by setting the spacing difference (S after − S _before ) _before and after ethanol cleaning within the above range, it will be able to withstand temperature changes from high temperature to low temperature under low humidity. It is possible to suppress deterioration of electromagnetic conversion characteristics due to this. From this point of view, the magnetic layer surface roughness Ra of the magnetic recording medium is 1.0 nm or more, preferably 1.1 nm or more.

The centerline average surface roughness Ra measured on the surface of the magnetic layer in the present invention and this specification is measured in an area of 40 μm x 40 μm on the surface of the magnetic layer using an atomic force microscope (AFM). value. Examples of measurement conditions include the following measurement conditions. The magnetic layer surface roughness Ra shown in Examples below is a value determined by measurement under the following measurement conditions.
A region with an area of 40 μm×40 μm on the surface of the magnetic layer of the magnetic tape is measured using AFM (Nanoscope 4 manufactured by Veeco) in tapping mode. As the probe, RTESP-300 manufactured by BRUKER is used, the scanning speed (probe moving speed) is 40 μm/sec, and the resolution is 512 pixels×512 pixels.

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

(Spacing difference before and after ethanol cleaning (S _after - S _before ))
The spacing difference (S _after −S _before ) before and after ethanol cleaning measured on the magnetic layer surface of the magnetic recording medium by optical interferometry is more than 0 nm and less than 6.0 nm. Since the spacing difference (S _after −S _before ) is 6.0 nm or less, electromagnetic conversion due to temperature change from high temperature to low temperature under low humidity is reduced in the above magnetic recording medium in which the surface of the magnetic layer is highly smooth. Deterioration of characteristics can be suppressed. From this point of view, the difference (S _after −S _before ) is 6.0 nm or less, preferably 5.0 nm or less, and more preferably 4.0 nm or less. As will be described in detail later, the difference (S _after −S _before ) can be controlled by surface treatment of the magnetic layer in the manufacturing process of the magnetic recording medium. However, as a result of the studies conducted by the present inventors, if the surface treatment of the magnetic layer is carried out to the extent that the spacing difference (S _after - S _before ) before and after ethanol cleaning becomes 0 nm, the surface of the magnetic layer has a high smoothness. It has also been found that in recording media, it becomes difficult to suppress the deterioration of electromagnetic conversion characteristics caused by temperature changes from high to low temperatures under low humidity. The reason for this is not clear. This is just a guess: If the surface treatment of the magnetic layer is carried out to the extent that the spacing difference (S _after - S _before ) before and after ethanol cleaning is 0 nm, components that contribute to improving running stability (for example, lubricant) will become magnetic. The present inventors believe that one of the reasons is that it is excessively removed from the recording medium. From this point of view, the spacing difference (S _after - S _before ) of the magnetic recording medium before and after ethanol cleaning is more than 0 nm, preferably 1.0 nm or more, and more preferably 2.0 nm or more.

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

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

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

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

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

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

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

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

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

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

When the hexagonal strontium ferrite powder contains rare earth atoms, the rare earth atoms contained may be at least one kind of rare earth atoms. Preferred rare earth atoms from the viewpoint of further suppressing a decrease in reproduction output during repeated reproduction include neodymium atoms, samarium atoms, yttrium atoms, and dysprosium atoms, with neodymium atoms, samarium atoms, and yttrium atoms being more preferred; Atom is more preferred.

In the hexagonal strontium ferrite powder having rare earth atoms unevenly distributed in the surface layer, it is sufficient that the rare earth atoms are unevenly distributed in the surface layer of the particles constituting the hexagonal strontium ferrite powder, and the degree of uneven distribution is not limited. For example, for a hexagonal strontium ferrite powder that has rare earth atoms unevenly distributed in the surface layer, the surface layer content of rare earth atoms determined by partial melting under the melting conditions described below and the rare earth content determined by completely melting under the melting conditions described below. The ratio of atoms to the bulk content, "surface layer content/bulk content" is more than 1.0, and can be 1.5 or more. When the "surface layer content ratio/bulk content ratio" is larger than 1.0, it means that rare earth atoms are unevenly distributed in the surface layer (that is, more present than in the interior) in the particles that make up the hexagonal strontium ferrite powder. do. In addition, the ratio of the surface layer content of rare earth atoms determined by partial dissolution under the dissolution conditions described below and the bulk content of rare earth atoms determined by total dissolution under the dissolution conditions described below, ``surface layer content/ Bulk content" can be, for example, 10.0 or less, 9.0 or less, 8.0 or less, 7.0 or less, 6.0 or less, 5.0 or less, or 4.0 or less. However, in a hexagonal strontium ferrite powder having rare earth atoms unevenly distributed in the surface layer, it is sufficient that the rare earth atoms are unevenly distributed in the surface layer of the particles constituting the hexagonal strontium ferrite powder, and the above "surface layer content/bulk content" is sufficient. The "rate" is not limited to the illustrated upper or lower limits.

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

From the viewpoint of increasing reproduction output when reproducing data recorded on a magnetic recording medium, it is desirable that the mass magnetization σs of the ferromagnetic powder contained in the magnetic recording medium is high. In this regard, a hexagonal strontium ferrite powder that contains rare earth atoms but does not have rare earth atoms unevenly distributed in the surface layer tends to have a significantly lower σs than a hexagonal strontium ferrite powder that does not contain rare earth atoms. On the other hand, a hexagonal strontium ferrite powder having rare earth atoms unevenly distributed in the surface layer is considered to be preferable in order to suppress such a large decrease in σs. In one aspect, σs of the hexagonal strontium ferrite powder can be 45 A·m ² /kg or more, and can also be 47 A·m ² /kg or more. On the other hand, from the viewpoint of noise reduction, σs is preferably 80 A·m ² /kg or less, more preferably 60 A·m ² /kg or less. σs can be measured using a known measuring device capable of measuring magnetic properties, such as a vibrating sample magnetometer. In the present invention and this specification, unless otherwise specified, mass magnetization σs is a value measured at a magnetic field strength of 15 kOe. 1 [kOe]=10 ⁶ /4π [A/m].

Regarding the content of constituent atoms (bulk content) of the hexagonal strontium ferrite powder, the strontium atomic content can be in the range of, for example, 2.0 to 15.0 atomic% with respect to 100 atomic% of iron atoms. . In one embodiment, the hexagonal strontium ferrite powder may contain only strontium atoms as divalent metal atoms. In another aspect, the hexagonal strontium ferrite powder can also contain one or more other divalent metal atoms in addition to strontium atoms. For example, it can contain barium atoms and/or calcium atoms. When divalent metal atoms other than strontium atoms are included, the barium atom content and calcium atom content in the hexagonal strontium ferrite powder are, for example, 0.05 to 5, respectively, relative to 100 at% of iron atoms. It can be in the range of .0 at.%.

As the crystal structure of hexagonal ferrite, magnetoplumbite type (also called "M type"), W type, Y type, and Z type are known. The hexagonal strontium ferrite powder may have any crystal structure. Crystal structure can be confirmed by X-ray diffraction analysis. The hexagonal strontium ferrite powder may have a single crystal structure or two or more types of crystal structures detected by X-ray diffraction analysis. For example, in one embodiment, the hexagonal strontium ferrite powder can have only an M-type crystal structure detected by X-ray diffraction analysis. For example, M-type hexagonal ferrite is represented by the composition formula AFe ₁₂ O ₁₉ . Here, A represents a divalent metal atom, and if the hexagonal strontium ferrite powder is M type, A is only a strontium atom (Sr), or if A contains multiple divalent metal atoms, As mentioned above, strontium atoms (Sr) account for the largest amount on an atomic percent basis. The divalent metal atom content of the hexagonal strontium ferrite powder is usually determined by the type of crystal structure of the hexagonal ferrite, and is not particularly limited. The same applies to the iron atom content and the oxygen atom content. The hexagonal strontium ferrite powder contains at least iron atoms, strontium atoms, and oxygen atoms, and may also contain rare earth atoms. Furthermore, the hexagonal strontium ferrite powder may or may not contain atoms other than these atoms. As an example, the hexagonal strontium ferrite powder may contain aluminum atoms (Al). The content of aluminum atoms can be, for example, 0.5 to 10.0 atomic % with respect to 100 atomic % of iron atoms. From the viewpoint of further suppressing the reduction in playback output during repeated playback, the hexagonal strontium ferrite powder contains iron atoms, strontium atoms, oxygen atoms, and rare earth atoms, and the content of atoms other than these atoms is 100 iron atoms. %, it is preferably 10.0 atom % or less, more preferably in the range of 0 to 5.0 atom %, and may be 0 atom %. 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 % above is the content of each atom (unit: mass %) obtained by completely melting the hexagonal strontium ferrite powder, and is converted to the value expressed in atomic % using the atomic weight of each atom. It can be calculated by converting. Furthermore, in the present invention and this specification, "not containing" an atom means that the content thereof as measured by an ICP analyzer after being completely dissolved is 0% by mass. The detection limit of an ICP analyzer is usually 0.01 ppm (parts per million) or less on a mass basis. The above-mentioned "does not contain" is used to mean that it is contained in an amount below the detection limit of the ICP analyzer. In one embodiment, the hexagonal strontium ferrite powder can be free of bismuth atoms (Bi).

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

ε-Iron Oxide Powder A preferred example of the ferromagnetic powder is ε-iron oxide powder. In the present invention and this specification, "ε-iron oxide powder" refers to a ferromagnetic powder in which an ε-iron oxide type crystal structure is detected as the main phase by X-ray diffraction analysis. For example, if the most intense diffraction peak in an X-ray diffraction spectrum obtained by X-ray diffraction analysis is attributed to the ε-iron oxide type crystal structure, it is assumed that the ε-iron oxide type crystal structure has been detected as the main phase. shall judge. Known methods for producing ε-iron oxide powder include a method for producing it from goethite, a reverse micelle method, and the like. All of the above manufacturing methods are publicly known. Further, regarding a method for producing ε-iron oxide powder in which a portion of Fe is substituted with substitution atoms such as Ga, Co, Ti, Al, Rh, etc., see, for example, J. Jpn. Soc. Powder Metallurgy Vol. 61 Supplement, No. S1, pp. S280-S284, J. Mater. Chem. C, 2013, 1, pp. 5200-5206 etc. can be referred to. However, the method for producing the ε-iron oxide powder that can be used as the ferromagnetic powder in the magnetic layer of the magnetic recording medium is not limited to the method mentioned above.

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

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

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

In the present invention and this specification, unless otherwise specified, the average particle size of various powders such as ferromagnetic powder is a value measured by the following method using a transmission electron microscope.
The powder is photographed using a transmission electron microscope at a magnification of 100,000 times and printed on photographic paper at a total magnification of 500,000 times to obtain a photograph of the particles constituting the powder. Select the target particle from the obtained particle photo, trace the outline of the particle with a digitizer, and measure the size of the particle (primary particle). Primary particles refer to independent particles without agglomeration.
The above measurements are performed on 500 randomly extracted particles. The arithmetic mean of the particle sizes of the 500 particles thus obtained is defined as the average particle size of the powder. As the transmission electron microscope, for example, a transmission electron microscope model H-9000 manufactured by Hitachi may be used. Furthermore, the particle size can be measured using known image analysis software, such as Carl Zeiss image analysis software KS-400. Unless otherwise specified, the average particle size shown in the examples below was measured using a transmission electron microscope H-9000 model manufactured by Hitachi as a transmission electron microscope and image analysis software KS-400 manufactured by Carl Zeiss as image analysis software. It is a value. In the present invention and herein, powder refers to a collection of particles. For example, ferromagnetic powder means a collection of multiple ferromagnetic particles. Furthermore, an aggregate of multiple particles is not limited to an embodiment in which the particles constituting the aggregate are in direct contact with each other, but also includes an embodiment in which a binder, an additive, etc., which will be described later, are interposed between the particles. Ru. The term particle is sometimes used to refer to powder.

As a method for collecting sample powder from a magnetic recording medium for particle size measurement, for example, the method described in paragraph 0015 of JP-A No. 2011-048878 can be adopted.

In the present invention and this specification, unless otherwise specified, the size of the particles constituting the powder (particle size) is as follows:
(1) If the particle is needle-shaped, spindle-shaped, columnar (however, the height is larger than the maximum major axis of the bottom surface), etc., it is expressed by the length of the major axis that makes up the particle, that is, the major axis length,
(2) If it is plate-shaped or columnar (however, the thickness or height is smaller than the maximum major axis of the plate surface or bottom surface), it is expressed by the maximum major axis of the plate surface or bottom surface,
(3) If the particle is spherical, polyhedral, unspecified, etc., and the long axis constituting the particle cannot be determined from the shape, it is expressed by an equivalent circle diameter. The equivalent circle diameter refers to what is determined by the circular projection method.

In addition, the average acicular ratio of the powder can be determined by measuring the short axis length of the particles in the above measurement, determining the value of (major axis length / short axis length) of each particle, and calculating the average acicular ratio of the 500 particles. Refers to the arithmetic mean of the values obtained for the particles. Here, unless otherwise specified, the short axis length refers to the length of the short axis constituting the particle in the case of (1) in the above particle size definition, and the thickness or height in the case of (2). In the case of (3), there is no distinction between the major axis and the minor axis, so (major axis length/minor axis length) is regarded as 1 for convenience.
Unless otherwise specified, when the particle shape is specific, for example, in the case of the above definition (1) of particle size, the average particle size is the average major axis length, and in the case of the same definition (2), the average particle size is This is the average plate diameter. In the case of the same definition (3), the average particle size is the average diameter (also referred to as average particle diameter or average particle diameter).

The content (filling rate) of the ferromagnetic powder in the magnetic layer is preferably in the range of 50 to 90% by mass, more preferably in the range of 60 to 90% by mass. Components other than the ferromagnetic powder of the magnetic layer are at least a binder, and optionally one or more additional additives may be included. It is preferable that the filling rate of ferromagnetic powder in the magnetic layer is high from the viewpoint of improving recording density.

(Binder, hardening agent)
The magnetic recording medium is a coated magnetic recording medium, and includes a binder in the magnetic layer. A binder is one or more resins. As the binder, various resins commonly used as binders for coated magnetic recording media can be used. For example, binders include polyurethane resins, polyester resins, polyamide resins, vinyl chloride resins, acrylic resins copolymerized with styrene, acrylonitrile, methyl methacrylate, etc., cellulose resins such as nitrocellulose, epoxy resins, phenoxy resins, polyvinyl acetals, A resin selected from polyvinyl alkyral resins such as polyvinyl butyral can be used alone, or a plurality of resins can be used in combination. Among these, preferred are polyurethane resins, acrylic resins, cellulose resins, and vinyl chloride resins. These resins may be homopolymers or copolymers. These resins can also be used as a binder in the nonmagnetic layer and/or back coat layer described below.
Regarding the above binders, reference can be made to paragraphs 0028 to 0031 of JP-A No. 2010-24113. The average molecular weight of the resin used as the binder can be, for example, 10,000 or more and 200,000 or less as a weight average molecular weight. The weight average molecular weight in the present invention and this specification is a value determined by gel permeation chromatography (GPC) under the following measurement conditions in terms of polystyrene. The weight average molecular weight of the binder shown in the Examples below is a value determined by converting the value measured under the following measurement conditions into polystyrene.
GPC device: HLC-8120 (manufactured by Tosoh Corporation)
Column: TSK gel Multipore HXL-M (manufactured by Tosoh Corporation, 7.8mm ID (Inner Diameter) x 30.0cm)
Eluent: Tetrahydrofuran (THF)

In one aspect, a binder containing an acidic group can be used as the binder. The acidic groups can serve as adsorption sites on the particle surface of the ferromagnetic powder. In the present invention and this specification, the term "acidic group" is used to include a group that can be dissociated into an anion by releasing H ⁺ in water or a water-containing solvent (aqueous solvent), and a salt form thereof. shall be. Specific examples of acidic groups include sulfonic acid groups, sulfuric acid groups, carboxy groups, phosphoric acid groups, and salt forms thereof. For example, the salt form of a sulfonic acid group (-SO ₃ H) is a group represented by -SO ₃ M, where M represents an atom that can become a cation (for example, an alkali metal atom, etc.) in water or an aqueous solvent. means. This point also applies to the salt forms of the various groups mentioned above. An example of a binder containing an acidic group includes, for example, a resin containing at least one acidic group selected from the group consisting of a sulfonic acid group and a salt thereof (eg, polyurethane resin, vinyl chloride resin, etc.). However, the resin contained in the magnetic layer is not limited to these resins. Further, in the binder containing acidic groups, the acidic group content can be in the range of, for example, 0.03 to 0.50 meq/g. eq is equivalent and is a unit that cannot be converted into SI units. The content of various functional groups such as acidic groups contained in the resin can be determined by known methods depending on the type of functional group. The binder can be used in the composition for forming a magnetic layer in an amount of, for example, 1.0 to 30.0 parts by weight based on 100.0 parts by weight of the ferromagnetic powder. From the viewpoint of reducing the spacing difference (S _after - S _before ) before and after ethanol cleaning, it is preferable to reduce the amount of components derived from the binder seeping onto the surface of the magnetic layer under low humidity and high temperature. From this point of view, reducing the amount of binder used in forming the magnetic layer can be cited as one means of reducing the spacing difference (S _after - S _before ) before and after ethanol cleaning.

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

(Additive)
The magnetic layer contains a ferromagnetic powder and a binder, and may contain one or more additives as necessary. Examples of the additives include the above-mentioned curing agents. In addition, examples of additives included in the magnetic layer include non-magnetic powder (for example, inorganic powder, carbon black, etc.), lubricant, dispersant, dispersion aid, fungicide, antistatic agent, antioxidant, etc. I can do it.

For example, examples of the lubricant include fatty acids, fatty acid esters, and fatty acid amides, and the magnetic layer can be formed using one or more selected from the group consisting of fatty acids, fatty acid esters, and fatty acid amides.
Examples of fatty acids include lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, linoleic acid, linolenic acid, behenic acid, erucic acid, and elaidic acid. is preferred, and stearic acid is more preferred. The fatty acid may be contained in the magnetic layer in the form of a salt such as a metal salt.
Examples of fatty acid esters include esters of lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, linoleic acid, linolenic acid, behenic acid, erucic acid, and elaidic acid. Specific examples include butyl myristate, butyl palmitate, butyl stearate, neopentyl glycol dioleate, sorbitan monostearate, sorbitan distearate, sorbitan tristearate, oleyl oleate, isocetyl stearate, stearin. Examples include isotridecyl stearate, octyl stearate, isooctyl stearate, amyl stearate, butoxyethyl stearate, and the like.
Examples of fatty acid amides include amides of the various fatty acids mentioned above, such as lauric acid amide, myristic acid amide, palmitic acid amide, stearic acid amide, and the like.
The fatty acid content in the composition for forming a magnetic layer is, for example, 0 to 10.0 parts by mass, preferably 0.1 to 10.0 parts by mass, and more preferably It is 1.0 to 7.0 parts by mass. The fatty acid ester content in the composition for forming a magnetic layer is, for example, 0.1 to 10.0 parts by weight, preferably 1.0 to 7.0 parts by weight, per 100.0 parts by weight of the ferromagnetic powder. The fatty acid amide content in the composition for forming a magnetic layer is, for example, 0 to 3.0 parts by mass, preferably 0 to 2.0 parts by mass, and more preferably 0 parts by mass, per 100.0 parts by mass of ferromagnetic powder. ~1.0 part by mass.
Further, when the magnetic recording medium has a nonmagnetic layer between the nonmagnetic support and the magnetic layer, the fatty acid content in the composition for forming a nonmagnetic layer is, for example, 0.0 parts per 100.0 parts by mass of the nonmagnetic powder. ~10.0 parts by weight, preferably 1.0 to 10.0 parts by weight, more preferably 1.0 to 7.0 parts by weight. The fatty acid ester content in the composition for forming a nonmagnetic layer is, for example, 0 to 10.0 parts by weight, preferably 0.1 to 8.0 parts by weight, per 100.0 parts by weight of the nonmagnetic powder. The fatty acid amide content in the composition for forming a nonmagnetic layer is, for example, 0 to 3.0 parts by weight, preferably 0 to 1.0 parts by weight, per 100.0 parts by weight of the nonmagnetic powder.
In the present invention and this specification, unless otherwise specified, one kind of a certain component may be used, or two or more kinds thereof may be used. When two or more types of certain components are used, the content refers to the total content of these two or more types.

In addition, the non-magnetic powder used for forming the magnetic layer includes non-magnetic powder that can function as an abrasive, and non-magnetic powder that can function as a protrusion-forming agent that forms moderately protruding protrusions on the surface of the magnetic layer. Examples include magnetic powder (eg, non-magnetic colloid particles). Note that the average particle size of colloidal silica (silica colloidal particles) shown in the Examples below is a value determined by the method described in paragraph 0015 of JP 2011-048878A as the method for measuring the average particle size. . Examples of additives that can be used in the magnetic layer containing an abrasive include the dispersants described in paragraphs 0012 to 0022 of JP-A No. 2013-131285 as a dispersant for improving the dispersibility of the abrasive. be able to. Regarding the dispersant for improving the dispersibility of the ferromagnetic powder, paragraph 0035 of JP 2017-016721A can be referred to. Further, regarding the dispersant, paragraphs 0061 and 0071 of JP-A-2012-133837 can also be referred to. Regarding additives for the magnetic layer, paragraphs 0035 to 0077 of JP-A No. 2016-51493 can also be referred to.
The dispersant may be included in the nonmagnetic layer. Regarding the dispersant that can be included in the nonmagnetic layer, paragraph 0061 of JP 2012-133837A can be referred to.
Various additives can be used in any amount by appropriately selecting commercially available products or by manufacturing them by known methods depending on the desired properties.

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

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

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

In the present invention and herein, a non-magnetic layer is intended to include a substantially non-magnetic layer containing a small amount of ferromagnetic powder, for example as an impurity or intentionally, along with the non-magnetic powder. Here, a substantially non-magnetic layer means that this layer has a residual magnetic flux density of 10 mT or less, a coercive force of 7.96 kA/m (100 Oe) or less, or a residual magnetic flux density of 10 mT or less. A layer having a coercive force of 7.96 kA/m (100 Oe) or less. Preferably, the nonmagnetic layer has no residual magnetic flux density and no coercive force.

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

<Back coat layer>
The magnetic recording medium may also have a back coat layer containing nonmagnetic powder and a binder on the surface of the nonmagnetic support opposite to the surface having the magnetic layer. The back coat layer preferably contains one or both of carbon black and inorganic powder. Regarding the binder contained in the back coat layer and various additives that may be optionally included, known techniques related to the back coat layer can be applied, and known techniques related to the formulation of the magnetic layer and/or nonmagnetic layer can be applied. You can also do it. For example, the descriptions in paragraphs 0018 to 0020 of JP-A No. 2006-331625 and in column 4, line 65 to column 5, line 38 of US Patent No. 7,029,774 can be referred to regarding the back coat layer.

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

The thickness of the magnetic layer can be optimized depending on the saturation magnetization of the magnetic head used, the head gap length, and the recording signal band, and is, for example, 10 nm to 100 nm, and preferably 20 to 90 nm from the viewpoint of high-density recording. The range is more preferably 30 to 70 nm. It is sufficient that there is at least one magnetic layer, and the magnetic layer may be separated into two or more layers having different magnetic properties, and a structure related to a known multilayer magnetic layer can be applied. The thickness of the magnetic layer when it is separated into two or more layers is the total thickness of these layers.

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

The thickness of the back coat layer is preferably 0.9 μm or less, more preferably in the range of 0.1 to 0.7 μm.

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

<Manufacturing method>
(Preparation of composition for forming each layer)
Compositions for forming the magnetic layer, nonmagnetic layer, and backcoat layer usually contain a solvent in addition to the various components described above. As the solvent, various organic solvents commonly used for manufacturing coated magnetic recording media can be used. The amount of solvent in each layer-forming composition is not particularly limited, and can be the same as in each layer-forming composition of a typical coating-type magnetic recording medium. The process of preparing the composition for forming each layer usually includes at least a kneading process, a dispersion process, and a mixing process provided before and after these processes as necessary. Each individual process may be divided into two or more stages. All raw materials used in the present invention may be added at the beginning or during any step. Further, individual raw materials may be added in divided portions in two or more steps.

Known techniques can be used to prepare each layer-forming composition. In the kneading step, it is preferable to use a kneader with strong kneading power, such as an open kneader, continuous kneader, pressure kneader, or extruder. Details of these kneading treatments are described in JP-A-1-106338 and JP-A-1-79274. Moreover, in order 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 the dispersion medium. As such dispersed beads, zirconia beads, titania beads, and steel beads, which are high specific gravity dispersed beads, are suitable. The particle size (bead diameter) and packing rate of these dispersed beads can be optimized and used. A known disperser can be used. It is believed that strengthening the dispersion treatment in preparing the magnetic layer forming composition leads to promoting adsorption of the binder to the particle surfaces of the ferromagnetic powder. If the adsorption of the binder to the particle surface of the ferromagnetic powder can be promoted, the amount of components derived from the binder that seep onto the magnetic layer surface under low humidity and high temperature can be reduced, and as a result, the spacing before and after ethanol cleaning can be reduced. It is presumed that the difference (S _after - S _before ) can be reduced. Therefore, one way to reduce the spacing difference (S _after - S _before ) before and after ethanol cleaning is to strengthen the dispersion process. Specific aspects of strengthening the dispersion process include, for example, lengthening the dispersion time and reducing the diameter of the dispersion beads used for dispersion. Various dispersion conditions such as dispersion time and diameter of dispersed beads can be set depending on the dispersion machine used. Moreover, each layer-forming composition may be filtered by a known method before being subjected to the coating process. Filtration can be performed, for example, by filter filtration. As the filter used for filtration, for example, a filter with a pore size of 0.01 to 3 μm (eg, a glass fiber filter, a polypropylene filter, etc.) can be used.

(Coating process)
The magnetic layer can be formed by, for example, directly applying the composition for forming a magnetic layer onto a nonmagnetic support, or by applying the composition in a multilayer manner sequentially or simultaneously with the composition for forming a nonmagnetic layer. The back coat layer can be formed by applying a back coat layer forming composition to the side of the nonmagnetic support opposite to the side having the magnetic layer (or where the magnetic layer will be provided later). For details of coating for forming each layer, paragraph 0051 of JP-A No. 2010-24113 can be referred to.

(Other processes)
After the coating step, various treatments such as drying treatment, magnetic layer orientation treatment, surface smoothing treatment (calendar treatment), etc. can be performed. Regarding various steps, paragraphs 0052 to 0057 of JP-A No. 2010-24113 can be referred to.

It is preferable that the coated layer formed by applying the magnetic layer forming composition is subjected to a heat treatment at an arbitrary stage after the step of applying the magnetic layer forming composition. This heat treatment can be performed, by way of example, before and/or after calendering. The heat treatment can be carried out, for example, by placing the support on which the coating layer of the magnetic layer forming composition is formed in a heated atmosphere. The heating atmosphere can be an atmosphere with an ambient temperature of 65 to 90°C, and more preferably an atmosphere with an ambient temperature of 65 to 75°C. This atmosphere can be, for example, an atmospheric atmosphere. The heat treatment in a heated atmosphere can be carried out for, for example, 20 to 50 hours. In one embodiment, this heat treatment allows the curing reaction of the curable functional group of the curing agent to proceed.

(One aspect of a preferred manufacturing method)
A preferred manufacturing method for the magnetic recording medium includes wiping the surface of the magnetic layer, preferably after the heat treatment, with a wiping material soaked with alcohol (hereinafter also referred to as "alcohol wiping treatment"). Here are some methods. As mentioned earlier, components that can be removed by this alcohol wiping process seep out onto the surface of the magnetic layer under low humidity and high temperature, and solidify or become highly viscous on the surface of the magnetic layer as the temperature changes from high to low temperature. This is thought to be the cause of the deterioration of electromagnetic conversion characteristics due to temperature changes from high to low temperatures under low humidity. The alcohol used in the alcohol wiping treatment is preferably an alcohol having 2 to 4 carbon atoms, more preferably ethanol, 1-propanol and 2-propanol, and even more preferably ethanol. The alcohol wiping process can be carried out using a wiping material impregnated with alcohol instead of the wiping material used in the dry wiping process, in accordance with the dry wiping process that is generally performed in the manufacturing process of magnetic recording media. can. For example, for tape-shaped magnetic recording media (magnetic tape), after or before slitting the magnetic tape to a width that accommodates it in a magnetic tape cartridge, the magnetic tape is run between a feed roller and a take-up roller. By pressing a wiping material (for example, cloth (e.g., non-woven fabric) or paper (e.g., tissue paper)) impregnated with alcohol onto the surface of the magnetic layer of the running magnetic tape, the surface of the magnetic layer can be wiped with alcohol. . The running speed of the magnetic tape during the above-mentioned running and the tension applied in the longitudinal direction of the magnetic layer surface (hereinafter simply referred to as "tension") are generally adopted in the dry wiping process commonly carried out in the manufacturing process of magnetic recording media. The processing conditions can be the same as those currently used. For example, the running speed of the magnetic tape in the alcohol wiping process can be about 60 to 600 m/min, and the tension can be about 0.196 to 3.920 N (Newton). Moreover, the alcohol wiping process can be performed at least once. As described above, if the surface treatment of the magnetic layer is carried out so that the spacing difference (S _after − S _before ) before and after alcohol cleaning becomes 0 nm, in a magnetic recording medium with a highly smooth magnetic layer surface, Since it is difficult to suppress the deterioration of electromagnetic conversion characteristics due to temperature changes from high to low temperatures under low humidity, it is preferable to set the processing conditions and number of treatments for alcohol wiping treatment with this point in mind. .
Furthermore, before and/or after the alcohol wiping treatment, the surface of the magnetic layer may be subjected to a polishing treatment and/or a dry wiping treatment (hereinafter referred to as "dry surface treatment") which is generally performed in the manufacturing process of coated magnetic recording media. ) can be performed more than once. According to the dry surface treatment, it is possible to remove foreign matter, such as chips generated by slits, generated during the manufacturing process and attached to the surface of the magnetic layer.
The above description has been made using a tape-shaped magnetic recording medium (magnetic tape) as an example. Various processes can also be performed on a disk-shaped magnetic recording medium (magnetic disk) with reference to the above description.

The magnetic recording medium according to one aspect of the present invention described above can be, for example, a tape-shaped magnetic recording medium (magnetic tape). Magnetic tape is generally distributed and used in magnetic tape cartridges. Information is transferred to the magnetic tape by attaching a magnetic tape cartridge to a magnetic recording/reproducing device, running the magnetic tape within the magnetic recording/reproducing device, and causing the magnetic tape surface (magnetic layer surface) and magnetic head to come into contact and slide. can be recorded and played back. However, the magnetic recording medium according to one embodiment of the present invention is not limited to a magnetic tape. The magnetic recording medium according to one embodiment of the present invention is suitable as various magnetic recording media (magnetic tape, disk-shaped magnetic recording medium (magnetic disk), etc.) used in a sliding type magnetic recording and reproducing device. The above-mentioned sliding type device refers to a device in which the surface of a magnetic layer and a head come into contact and slide when recording information on a magnetic recording medium and/or reproducing recorded information.

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

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

As shown in ECMA (European Computer Manufacturers Association)-319, a timing-based servo system is employed in magnetic tapes (generally referred to as "LTO tapes") that comply with the LTO (Linear Tape-Open) standard. In this timing-based servo system, a servo pattern is composed of a plurality of pairs of magnetic stripes (also called "servo stripes") that are non-parallel to each other and are continuously arranged in the longitudinal direction of the magnetic tape. As mentioned above, the reason why the servo pattern is composed of a pair of non-parallel magnetic stripes is to tell the servo signal reading element passing over the servo pattern its passing position. Specifically, the above-mentioned pair of magnetic stripes are formed so that the interval between them changes continuously along the width direction of the magnetic tape, and the servo signal reading element reads the interval to read the servo pattern. The relative position of the servo signal reading element and the servo signal reading element can be known. This relative position information enables tracking of the data tracks. For this purpose, a plurality of servo tracks are usually set on the servo pattern along the width direction of the magnetic tape.

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

Further, in one aspect, as shown in Japanese Patent Application Laid-Open No. 2004-318983, each servo band includes information indicating the number of the servo band (“servo band ID (identification)” or “UDIM (Unique Data Band Identification)”). (also called "Method information") is embedded. This servo band ID is recorded by shifting a specific one of a plurality of pairs of servo stripes in the servo band so that its position is relatively displaced in the longitudinal direction of the magnetic tape. Specifically, the way in which a specific one of a plurality of pairs of servo stripes is shifted is changed for each servo band. As a result, the recorded servo band ID becomes unique for each servo band, and therefore, by simply reading one servo band with a servo signal reading element, that servo band can be uniquely identified.

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

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

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

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

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

The direction of the magnetic field of the servo pattern to be formed is determined depending on the erase direction. For example, when a magnetic tape is subjected to horizontal DC erasing, the servo pattern is formed such that the direction of the magnetic field is opposite to the erasing direction. Thereby, the output of the servo signal obtained by reading the servo pattern can be increased. As shown in Japanese Patent Application Laid-Open No. 2012-53940, when a magnetic pattern is transferred using the above-mentioned gap to a vertical DC erased magnetic tape, the formed servo pattern is read and obtained. The servo signal has a unipolar pulse shape. On the other hand, when a magnetic pattern is transferred onto a horizontal DC erased magnetic tape using the gap, the servo signal obtained by reading the formed servo pattern has a bipolar pulse shape.

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

In the present invention and this specification, a "magnetic recording and reproducing device" means a device that can perform at least one of recording information on a magnetic recording medium and reproducing information recorded on a magnetic recording medium. do. Such devices are commonly referred to as drives. The magnetic recording and reproducing device may be a sliding type magnetic recording and reproducing device. The magnetic head included in the magnetic recording and reproducing device may be a recording head capable of recording information on a magnetic recording medium, and a reproducing head capable of reproducing information recorded on the magnetic recording medium. It can also be. Further, in one aspect, the magnetic recording and reproducing apparatus described above can include both a recording head and a reproducing head as separate magnetic heads. In another aspect, the magnetic head included in the magnetic recording and reproducing apparatus may have a configuration in which one magnetic head includes both a recording element and a reproducing element. The reproducing head is preferably a magnetic head (MR head) that includes a magnetoresistive (MR) element as a reproducing element, which can read information recorded on a magnetic recording medium with high sensitivity. As the MR head, various known MR heads can be used. Further, a magnetic head that records information and/or reproduces information may include a servo pattern reading element. Alternatively, the magnetic recording and reproducing apparatus may include a magnetic head (servo head) equipped with a servo pattern reading element as a head other than the magnetic head that records and/or reproduces information.

In the above magnetic recording/reproducing device, recording of information on the magnetic recording medium and reproduction of information recorded on the magnetic recording medium are performed by bringing the surface of the magnetic layer of the magnetic recording medium and the magnetic head into contact with each other and sliding the magnetic head. I can do it. The above-mentioned magnetic recording/reproducing device may be any device as long as it includes the magnetic recording medium according to one embodiment of the present invention, and known techniques can be applied to the other components.

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

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

<Formulation of magnetic layer forming composition>
(Magnetic liquid)
Ferromagnetic powder (see Table 1): 100.0 parts Oleic acid: 2.0 parts Vinyl chloride copolymer (MR-104 manufactured by Kaneka): See Table 1 (weight average molecular weight 55,000, OSO ₃ K group (sulfate group) potassium salt): 0.09meq/g)
SO ₃ Na group-containing polyurethane resin: 4.0 parts (weight average molecular weight 70000, SO ₃ Na group (sodium salt of sulfonic acid group): 0.07 meq/g)
Polyalkyleneimine polymer (synthetic product obtained by the method described in paragraphs 0115 to 0123 of JP-A-2016-51493): 6.0 parts Methyl ethyl ketone: 150.0 parts Cyclohexanone: 150.0 parts (abrasive liquid )
α-Alumina (BET (Brunauer-Emmett-Teller) specific surface area 19 m ² /g): 6.0 parts SO ₃ Na group-containing polyurethane resin: 0.6 parts (weight average molecular weight 70000, SO ₃ Na group: 0.1 meq /g)
2,3-dihydroxynaphthalene: 0.6 parts Cyclohexanone: 23.0 parts (projection forming agent liquid)
Colloidal silica (average particle size 120 nm): see Table 1 Methyl ethyl ketone: 8.0 parts (other ingredients)
Stearic acid: 3.0 parts Stearamide: 0.3 parts Butyl stearate: 6.0 parts Methyl ethyl ketone: 110.0 parts Cyclohexanone: 110.0 parts Polyisocyanate (Coronate (registered trademark) L manufactured by Tosoh Corporation): 3 Department

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

<Formulation of composition for forming back coat layer>
Non-magnetic inorganic powder: α-iron oxide (average particle size: 0.15 μm, BET specific surface area 52 m ² /g): 80.0 parts Carbon black (average particle size: 20 nm): 20.0 parts Vinyl chloride copolymer : 13.0 parts Sulfonic acid group-containing polyurethane resin: 6.0 parts Phenylphosphonic acid: 3.0 parts Cyclohexanone: 155.0 parts Methyl ethyl ketone: 155.0 parts Stearic acid: 3.0 parts Butyl stearate: 3.0 parts Part Polyisocyanate: 5.0 parts Cyclohexanone: 200.0 parts

<Preparation of composition for forming magnetic layer>
A composition for forming a magnetic layer was prepared by the following method.
Magnetic liquids were prepared by dispersing (bead dispersion) various components of the above magnetic liquid using a batch-type vertical sand mill for the times shown in Table 1. Zirconia beads with a bead diameter of 0.5 mm were used as the dispersion beads.
For the abrasive liquid, mix the various components of the above abrasive liquid and put it into a horizontal bead mill disperser together with zirconia beads with a bead diameter of 0.3 mm, so that the bead volume / (abrasive liquid volume + bead volume) becomes 80%. The solution was adjusted as follows, subjected to bead mill dispersion treatment for 120 minutes, and the treated liquid was taken out and subjected to ultrasonic dispersion filtration treatment using a flow-type ultrasonic dispersion filtration device. A polishing liquid was thus prepared.
The prepared magnetic liquid and abrasive liquid, the above-mentioned protrusion forming agent liquid, and other components were introduced into a dissolver stirrer and stirred at a circumferential speed of 10 m/sec for 30 minutes, and then mixed with a flow type ultrasonic disperser at a flow rate of 7.5 kg. After processing the mixture for 3 passes at a speed of 1 μm/minute, it was filtered through a filter with a pore size of 1 μm to prepare a magnetic layer forming composition.

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

<Preparation of composition for forming back coat layer>
After kneading and diluting the various components of the above-mentioned composition for forming a back coat layer, excluding the lubricant (stearic acid and butyl stearate), polyisocyanate and 200.0 parts of cyclohexanone, using an open kneader, Using a dispersing machine, zirconia beads with a bead diameter of 1 mm were used for 12 passes of dispersion treatment, with a bead filling rate of 80% by volume, a rotor tip circumferential speed of 10 m/sec, and a residence time of 2 minutes per pass. Thereafter, the remaining components were added and stirred with a dissolver, and the resulting dispersion was filtered using a filter having an average pore size of 1 μm to prepare a composition for forming a back coat layer.

<Preparation of magnetic tape>
After coating the composition for forming a nonmagnetic layer prepared above on the surface of a polyethylene naphthalate support having a thickness of 5.0 μm so that the thickness after drying becomes 400 nm and drying it to form a nonmagnetic layer. The composition for forming a magnetic layer prepared above was applied onto the surface of the nonmagnetic layer so that the thickness after drying was 70 nm to form a coating layer. While the coating layer of this magnetic layer forming composition was in a wet (undried) state, a vertical alignment treatment was performed by applying a magnetic field with a magnetic field strength of 0.3 T in a direction perpendicular to the surface of the coating layer, and the coating layer was dried. . Thereafter, the composition for forming a back coat layer prepared above was applied to the opposite side of the support so that the thickness after drying was 0.4 μm, and the composition was dried. In this way, an original magnetic tape was produced.
The produced magnetic tape material was calendered (surface smoothed) using a calender consisting only of metal rolls at a speed of 100 m/min, a linear pressure of 300 kg/cm (294 kN/m), and a surface temperature of the calender roll of 100°C. treatment), and then heat treatment was performed for 36 hours in an environment with an ambient temperature of 70°C. After the heat treatment, the original magnetic tape was slit using a cutter to obtain a magnetic tape with a width of 1/2 inch (0.0127 meters). While running this magnetic tape between a feed roller and a take-up roller (running speed 120 m/min, tension: see Table 1), the surface of the magnetic layer was subjected to blade polishing, dry wiping treatment, and ethanol wiping treatment as alcohol wiping treatment. It was carried out in this order. Specifically, a sapphire blade, a dry wiping material (Toraysee (registered trademark) manufactured by Toray Industries, Inc.), and a wiping material soaked with ethanol (Toraysee (registered trademark) manufactured by Toray Industries, Inc.) were placed between the two rollers. , a sapphire blade is pressed against the surface of the magnetic layer of the magnetic tape running between the two rollers, and the blade is polished.Then, the surface of the magnetic layer is dry-wiped with the dry wiping material described above, and then the surface of the magnetic layer described above is The surface of the magnetic layer was wiped with ethanol using a wiping material impregnated with ethanol. As described above, blade polishing, dry wiping treatment, and ethanol wiping treatment were each performed once on the surface of the magnetic layer.
In this way, the magnetic tape of Example 1 was obtained.

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

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

In Table 1, "SrFe1" is hexagonal strontium ferrite powder produced by the following method.
Weighed 1707g of _SrCO3 , 687g _{of H3BO3} , 1120g of _Fe2O3 , 45g of Al(OH) ₃ , 24g of _BaCO3 , 13g of _CaCO3 , and 235g _{of Nd2O3} , and mixed them in _a mixer. The mixture was mixed to obtain a raw material mixture.
The obtained raw material mixture was melted in a platinum crucible at a melting temperature of 1390°C, and while stirring the melt, the tap hole provided at the bottom of the platinum crucible was heated, and the melt was tapped in the form of a rod at a rate of about 6 g/sec. . The tapped liquid was rapidly cooled by rolling with water-cooled twin rollers to produce an amorphous body.
280g of the prepared amorphous material was placed in an electric furnace, heated to 635°C (crystallization temperature) at a heating rate of 3.5°C/min, and held at the same temperature for 5 hours to form hexagonal strontium ferrite particles. Precipitated (crystallized).
Next, the crystallized product obtained above containing hexagonal strontium ferrite particles was coarsely ground in a mortar, and 1000 g of zirconia beads with a particle size of 1 mm and 800 ml of acetic acid aqueous solution with a concentration of 1% were added to a glass bottle, and dispersed in a paint shaker for 3 hours. I did it. Thereafter, the resulting dispersion was separated from the beads and placed in a stainless steel beaker. The dispersion was allowed to stand at a liquid temperature of 100°C for 3 hours to dissolve the glass components, then precipitated in a centrifuge, washed by repeated decantation, and then heated in a heating furnace at an internal temperature of 110°C for 6 hours. After drying for several hours, hexagonal strontium ferrite powder was obtained.
The hexagonal strontium ferrite powder obtained above has 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 / It was kg.
12 mg of sample powder was collected from the hexagonal strontium ferrite powder obtained above, and the sample powder was partially dissolved under the dissolution conditions exemplified above. The obtained filtrate was subjected to elemental analysis using an ICP analyzer, and the neodymium atoms were analyzed using an ICP analyzer. The surface layer content was determined.
Separately, 12 mg of sample powder was collected from the hexagonal strontium ferrite powder obtained above, and this sample powder was completely dissolved under the dissolution conditions exemplified above. The elemental analysis of the obtained filtrate was performed using an ICP analyzer. The bulk content of atoms was determined.
The content of neodymium atoms (bulk content) with respect to 100 atom % of iron atoms in the hexagonal strontium ferrite powder obtained above was 2.9 atom %. Further, the content of neodymium atoms in the surface layer was 8.0 at %. The ratio of the surface layer content to the bulk content, "surface layer content/bulk content," was 2.8, and it was confirmed that neodymium atoms were unevenly distributed in the surface layer of the particles.
The fact that the powder obtained above exhibits a hexagonal ferrite crystal structure was confirmed by scanning CuKα rays at a voltage of 45 kV and an intensity of 40 mA and measuring the X-ray diffraction pattern under the following conditions (X-ray diffraction analysis). confirmed. The powder obtained above exhibited a magnetoplumbite type (M type) hexagonal ferrite crystal structure. Moreover, the crystal phase detected by X-ray diffraction analysis was a single phase of magnetoplumbite type.
PANalytical X'Pert Pro diffractometer, PIXcel detector Soller slit for incident and diffracted beams: 0.017 radian Fixed angle of dispersion slit: 1/4 degree Mask: 10 mm
Anti-scatter slit: 1/4 degree Measurement mode: Continuous Measurement time per step: 3 seconds Measurement speed: 0.017 degrees per second Measurement step: 0.05 degrees

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

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

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

[Evaluation of magnetic tape]
(1) Center line average surface roughness Ra measured on the surface of the magnetic layer (magnetic layer surface roughness Ra)
Using an atomic force microscope (AFM, Nanoscope 4 manufactured by Veeco) in tapping mode, a measurement area of 40 μm×40 μm was measured on the surface of the magnetic layer of the magnetic tape, and the center line average surface roughness Ra was determined. As the probe, RTESP-300 manufactured by BRUKER was used, the scanning speed (probe moving speed) was 40 μm/sec, and the resolution was 512 pixels×512 pixels.

(2) Spacing difference before and after ethanol cleaning (S _after -S _before )
Using TSA (Tape Spacing Analyzer (manufactured by Micro Physics)), the spacing difference (S _after - S _before ) before and after ethanol cleaning was determined by the following method.
Two sample pieces with a length of 5 cm were cut out from each of the magnetic tapes of Examples and Comparative Examples, and the spacing (S _before ) of one of the sample pieces was determined by the following method without washing with ethanol. The other sample piece was washed with ethanol by the method described above, and then the spacing ( _Safter ) was determined by the following method.
A glass plate (glass plate manufactured by Thorlabs, Inc. (model number: WG10530)) provided in the TSA was placed on the surface of the magnetic layer of the magnetic tape (more specifically, the above sample piece), and the glass plate provided in the TSA was placed as a pressing member. Using a hemisphere made of urethane, this hemisphere was pressed against the surface of the back coat layer of the magnetic tape at a pressure of 5.05×10 ⁴ N/m (0.5 atm). In this state, white light from a stroboscope installed in the TSA is irradiated onto a certain area (150,000 to 200,000 μm ² ) of the surface of the magnetic layer of the magnetic tape through the glass plate, and the resulting reflected light is filtered through an interference filter (wavelength: 633 nm). By receiving the light with a CCD (Charge-Coupled Device) through a filter that selectively transmits the light, an image of interference fringes caused by the unevenness of this area was obtained.
This image is divided into 300,000 points, and the distance (spacing) from the surface of the glass plate on the magnetic tape side to the surface of the magnetic layer of the magnetic tape at each point is calculated and used as a histogram. The difference (S _{after -} S _before ) was obtained by subtracting the mode S _before of the histogram obtained for the sample piece without ethanol washing from the mode S _after of the histogram.

(3) Spacing difference before and after n-hexane cleaning (S _reference -S _before ) (reference value)
Another sample piece of 5 cm in length was cut out from each of the magnetic tapes of Examples and Comparative Examples, and washed in the same manner as above except that n-hexane was used instead of ethanol, and then washed with n-hexane in the same manner as above. Asked for later spacing. As a reference value, the difference (S _reference - S _before ) between the spacing S _reference obtained here and the spacing S _before obtained for the unwashed sample piece obtained in (2) above was determined.

(4) Electromagnetic conversion characteristics (SNR (Signal-to-Noise-Ratio))
In an environment with an ambient temperature of 23°C and a relative humidity of 50%, the SNR of each of the magnetic tapes of Examples and Comparative Examples was measured using a 1/2 inch (0.0127 meter) reel tester with a fixed head according to the following method. was measured.
The head/tape relative speed was 5.5 m/sec, and a MIG (Metal-in-gap) head (gap length 0.15 μm, track width 1.0 μm, 1.8T) was used as the recording head. The current was set to the optimum recording current for each magnetic tape. Reproduction was performed using a GMR (Giant-magnetoresistive) head (element thickness 15 nm, shield interval 0.1 μm, track width 1.0 μm) as a reproduction head. A signal was recorded at a linear recording density of 325 kfci, and the reproduced signal was measured using a spectrum analyzer manufactured by Shibasoku. The unit kfci is a unit of linear recording density (cannot be converted to the SI unit system). As the signal, a portion where the signal was sufficiently stable after the magnetic tape started running was used. Recording and playback were performed under the above conditions, and the ratio between the output value of the carrier signal and the integrated noise of the entire spectrum band was taken as the SNR, and the SNR (Broadband -SNR) are shown in Table 1. If the SNR obtained here is 0 dB or more, it can be evaluated as having good electromagnetic conversion characteristics.

(5) Decreased SNR due to temperature change from high temperature to low temperature under low humidity After evaluating the electromagnetic conversion characteristics of each magnetic tape of the example and comparative example in (4) above, It was stored for 3 hours in a thermobox maintained at 20% humidity. After that, take out the magnetic tape from the thermo box (outside air temperature is 23 degrees Celsius and relative humidity 50%), put it into the thermo room where the inside temperature is kept at 10 degrees Celsius and 20% relative humidity within 1 minute, and then put it into the thermo room within 30 minutes. Perform 3000 passes of recording and playback in the same manner as in (4) above in the room, calculate the difference between the SNR of the 1st pass and the SNR of the 3000th pass (SNR of the 3000th pass - SNR of the 1st pass), and calculate the SNR reduction. It was a minute. If the SNR reduction determined here is within -1.0 dB, it can be evaluated that the SNR reduction due to temperature change from high temperature to low temperature under low humidity is suppressed.

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

As shown in Table 1, the magnetic tape of the example has a magnetic layer surface roughness Ra of 1.0 nm or more and 1.6 nm or less, and has a magnetic layer with high surface smoothness. From the evaluation results (SNR) of electromagnetic conversion characteristics, it can be confirmed that the magnetic tapes of these Examples have excellent electromagnetic conversion characteristics.
Further, in the magnetic tape of the example, the surface smoothness of the magnetic layer is high as described above, and the spacing difference (S _after −S _before ) before and after ethanol cleaning is more than 0 nm and less than 6.0 nm. As shown in Table 1, the magnetic tapes of these Examples have a smaller SNR drop than the magnetic tapes of Comparative Examples even when exposed to temperature changes from high to low temperatures under low humidity.
Furthermore, as shown in Table 1, there is a correlation between the value of the spacing difference (S _reference −S _before ) before and after cleaning with n-hexane and the value of the spacing difference (S _after −S _before ) before and after cleaning with ethanol. cannot be seen.

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

Claims (7)

A magnetic recording medium having a magnetic layer containing ferromagnetic powder on a non-magnetic support,
A non-magnetic layer containing non-magnetic powder is provided between the non-magnetic support and the magnetic layer,
The thickness of the nonmagnetic layer is 50 nm or more and 800 nm or less,
The center line average surface roughness Ra measured on the surface of the magnetic layer is 1.0 nm or more and 1.6 nm or less, and the spacing _S after is measured on the surface of the magnetic layer by optical interferometry after washing with ethanol. , a difference between the spacing S _before measured by optical interferometry on the surface of the magnetic layer before cleaning with ethanol, S _after −S _before , is greater than 0 nm and less than or equal to 6.0 nm. The magnetic recording medium according to claim 1, wherein the difference, S _after −S _before , is 1.0 nm or more and 6.0 nm or less. 3. The magnetic recording medium according to claim 1, wherein the difference, S _after −S _before , is 2.0 nm or more and 5.0 nm or less. The magnetic recording medium according to claim 1, wherein the nonmagnetic layer has a thickness of 70 nm or more and 500 nm or less. 5. The magnetic recording medium according to claim 1, further comprising a back coat layer containing nonmagnetic powder on a surface side of the nonmagnetic support opposite to a surface side having the magnetic layer. The magnetic recording medium according to any one of claims 1 to 5, which is a magnetic tape. A magnetic recording medium according to any one of claims 1 to 6,
magnetic head,
magnetic recording and reproducing devices including;