JP6877281B2 - Magnetic recording medium - Google Patents

Description

The present invention relates to a magnetic recording medium having excellent output characteristics.

The coating type magnetic recording medium in which a magnetic layer containing a magnetic powder and a binder is formed on a non-magnetic support has further improved the recording density with the transition from the analog method to the digital method. Is required. In particular, this demand is increasing year by year for high-density digital video tapes, computer backup tapes, and the like.

With such an improvement in recording density, the recording wavelength has been shortened, and in order to support this short-wavelength recording, the magnetic powder has been made into fine particles year by year, and currently, ferromagnetism with an average particle diameter of about 20 nm is being attempted. A hexagonal ferrite powder has been realized, and a magnetic recording medium using this magnetic powder has been put into practical use (for example, Patent Document 1).

Then, in order to further improve the recording density of the magnetic recording medium using the ferromagnetic hexagonal ferrite powder, it is necessary to further refine the ferromagnetic hexagonal ferrite powder. However, by further miniaturizing the ferromagnetic hexagonal ferrite powder, there is a problem that the particle volume of the magnetic powder becomes smaller and the magnetic powder is more susceptible to thermal fluctuation. Therefore, it is necessary to suppress thermal fluctuation by using a magnetic material having a high coercive force and a large anisotropic energy even if the particles are made into fine particles.

_{Under these circumstances, research on ε-Fe 2} O ₃ has been conducted in recent years as a new magnetic material for magnetic recording media, and ε has ferromagnetic properties even with an average particle diameter of 15 nm or less, preferably 10 nm or less. An _{iron oxide nanomagnetic particle powder composed of a single phase of −Fe 2} O ₃ has been proposed (for example, Patent Document 2). Further, a magnetic recording medium using ε-Fe ₂ O ₃ as a magnetic powder has also been proposed (for example, Patent Documents 3 and 4).

Further, as prior art documents related to the present invention, there is Patent Document 5 relating to a method for measuring and controlling spacing, which will be described later.

Japanese Unexamined Patent Publication No. 2015-91747 Japanese Unexamined Patent Publication No. 2014-224027 Japanese Unexamined Patent Publication No. 2015-82329 Japanese Unexamined Patent Publication No. 2014-149886 Japanese Unexamined Patent Publication No. 2012-43495

The commonly known _{crystal structure of Fe 2} O ₃ is the gamma (γ) phase or the alpha (α) phase, but the epsilon (ε) phase, which is the crystal structure of _{ε-Fe 2} O _{3, is the γ phase.} It has a crystal structure existing in the middle of the α phase and exhibits magnetic anisotropy based on crystal anisotropy. Therefore, it is characterized in that it exhibits a high coercive force even if the particle size is reduced to 10 nm or less.

However, looking at the differential curve obtained by differentiating the hysteresis curve of ε-Fe ₂ O ₃ , a peak (P1) occurs in the magnetic field range of +500 oersted [Oe] or more, and a magnetic field smaller than +500 oersted [Oe]. It is known that a peak (P2) also occurs in the range of. This is because the ε-Fe ₂ O ₃ magnetic powder contains magnetic powders with different coercive forces, especially low coercive components, in addition to the high coercive component, so the magnetic field range is smaller than +500 Oersted [Oe]. It is thought that a peak will also appear in.

Further, in the above differential curve, in the range of the magnetic field of +4000 Oersted [Oe] or more, a portion where the slope of the descending curve portion beyond the peak (P1) is gentle can be seen. This is a component having a particularly high coercive force of +5000 oersted [Oe] or more (ultra-high coercive force component) in addition to a normal high coercive force component having a coercive force of +500 oersted [Oe] or more and less than +5000 oersted [Oe]. This is probably because the magnetic powder is mixed. When this ultra-high coercive force component is present in the magnetic layer of the magnetic recording medium, even if the magnetic head tries to record the magnetic signal, the strength of the recording magnetic field generated from the magnetic head is insufficient and the magnetic signal may be recorded. Therefore, it is not preferable as a magnetic material used for a magnetic recording medium for high-density recording.

The present invention has been made to solve the above problems, and is an ultra-high coercive force having a coercive force of +5000 Elstead [Oe] or more, which is shown in a differential curve obtained by differentiating the hysteresis curve in the thickness direction of the magnetic layer. By setting the ratio of the magnetic force components in a specific range, a magnetic recording medium having excellent output characteristics is provided.

The magnetic recording medium of the present invention is a magnetic recording medium including a non-magnetic support and a magnetic layer containing a magnetic powder. The magnetic powder is made of ε-iron oxide powder and has an average particle size of the magnetic powder. , 8 nm or more and 20 nm or less, the angular ratio of the magnetic layer in the thickness direction is 0.65 or more, the coercive force in the thickness direction of the magnetic layer is 3000 Elstead [Oe] or more, and the magnetism. In the hysteresis curve in the thickness direction of the layer, a magnetic field in the positive direction is applied to the magnetic layer to reach saturation magnetization in the positive direction, and then a magnetic field in the opposite direction to the positive direction is applied to saturate in the reverse direction. When the point where the magnetization is reached is the point A, and the point where the positive magnetic field is applied from the point A and the point where the saturation magnetization is reached in the positive direction is the point B, the hysteresis curve from the point A to the point B is differentiated. In the obtained differential curve, the integrated value of the differential curve in the range of the magnetic field of 0 Elstead [Oe] or more and +10000 Elstead [Oe] or less is A, and the integrated value of the differential curve is A, +5000 Elstead [Oe] or more and +10000 Elstead [Oe] or less. Assuming that the integrated value of the differential curve in the range is B and the ratio B / A of the integrated value A and the integrated value B is R, the relationship of R ≦ 0.15 is established.

According to the magnetic recording medium of the present invention, the ratio of the ultra-high coercive force component having a coercive force of +5000 Elstead [Oe] or more, which is shown in the differential curve obtained by differentiating the hysteresis curve measured in the thickness direction of the magnetic layer. By setting the above to a specific range, it is possible to provide a magnetic recording medium having excellent output characteristics.

FIG. 1 is a schematic cross-sectional view showing an example of the magnetic recording medium of the embodiment. FIG. 2 is a diagram showing an example of a hysteresis curve in the thickness direction of the magnetic layer of the embodiment. FIG. 3 is a diagram showing a part of the differential curve obtained by differentiating the hysteresis curve from the point A to the point B in FIG. FIG. 4 is a diagram showing a part of the differential curve of the hysteresis curve in the thickness direction obtained in each of Example 1, Comparative Example 1 and Comparative Example 2.

An embodiment of the magnetic recording medium of the present invention will be described. An embodiment of the magnetic recording medium of the present invention includes a non-magnetic support and a magnetic layer containing magnetic powder. The magnetic powder is made of ε-iron oxide powder, the average particle size of the magnetic powder is 8 nm or more and 20 nm or less, and the square ratio of the magnetic layer in the thickness direction is 0.65 or more. The coercive force of the magnetic layer in the thickness direction is 3000 oersted [Oe] or more. Further, in the hysteresis curve in the thickness direction of the magnetic layer, a magnetic field in the positive direction is applied to the magnetic layer to reach saturation magnetization in the positive direction, and then a magnetic field in the opposite direction to the positive direction is applied. When the point where the saturation magnetization in the opposite direction is reached is defined as the point A, and the point where the positive magnetic field is further applied from the point A and the saturation magnetization in the positive direction is reached is the point B, the hysteresis from the point A to the point B. In the differential curve obtained by differentiating the curve, the integrated value of the differential curve in the range of the magnetic field of 0 Elstead [Oe] or more and +10000 Elstead [Oe] or less is A, +5000 Elstead [Oe] or more, +10000 Elstead [Oe]. Assuming that the integrated value of the differential curve in the following magnetic field range is B and the ratio B / A of the integrated value A and the integrated value B is R, the relationship of R ≦ 0.15 is established.

By using ε-iron oxide powder as the magnetic powder, the coercive force of the magnetic powder does not decrease even if the average particle size of the magnetic powder is 8 nm or more and 20 nm or less to support short wavelength recording. Further, the output characteristics can be improved by setting the square ratio of the magnetic layer in the thickness direction to 0.65 or more. Further, by setting the coercive force of the magnetic layer in the thickness direction to 3000 Oersted [Oe] or more, a high reproduction output with little self-demagnetization loss can be obtained even in a short wavelength recording region in a high recording density.

On the other hand, when the average particle size of the ε-iron oxide powder is 8 nm or more and 20 nm or less and the ε-iron oxide powder is made into fine particles, the particle size distribution generally tends to widen, and the proportion of particles having a particle size exceeding 20 nm increases. Here, since the ε-iron oxide powder exhibits magnetocrystalline anisotropy, the coercive force of the ε-iron oxide powder is easily affected by the particle size. Generally, when the particle size is small, the coercive force is small, and when the particle size is large, the coercive force is small. The coercive force tends to be large. Therefore, when the average particle size of the ε-iron oxide powder is 8 nm or more and 20 nm or less, the ultra-high coercive force component having a coercive force of +5000 oersted [Oe] or more increases in the ε-iron oxide powder. Further, when the coercive force in the thickness direction of the magnetic layer is set to 3000 oersted [Oe] or more, the ultra-high coercive force component is further increased. As described above, this ultra-high coercive force component cannot record a magnetic signal, and is therefore not preferable as a magnetic material used as a magnetic recording medium for high-density recording.

Therefore, in the present embodiment, the output characteristics of the magnetic recording medium of the present embodiment are improved by reducing the ratio of the ultra-high coercive force component with R ≦ 0.15. That is, in the above differential curve, the integrated value of the above differential curve in the range of the magnetic field of 0 oersted [Oe] or more and +10000 oersted [Oe] or less is A, +5000 oersted [Oe] or more, and the magnetic field of +10000 oersted [Oe] or less. Assuming that the integrated value of the differential curve in the range is B and the ratio B / A of the integrated value A and the integrated value B is R, the magnetic recording medium of the present embodiment can be obtained by establishing the relationship of R ≦ 0.15. Output characteristics can be improved.

The ratio R is more preferably 0.10 or less, and most preferably 0.07 or less. The lower limit of the ratio R is preferably smaller, but the lower limit of the ratio R is about 0.03 because the ultrahigh coercive force component cannot be completely removed due to the influence of the particle size distribution of the magnetic powder and the like. Is.

In the magnetic recording medium of the present embodiment, the output characteristics of the magnetic layer can be improved by setting the ratio R in a specific range. Therefore, the measurement target of the ratio R is the magnetic layer of the completed magnetic recording medium, and the ε iron oxide powder is not the measurement target. Therefore, even if the measured value of the ratio R of the ε-iron oxide powder and the measured value of the ratio R of the magnetic layer containing the ε-iron oxide powder are different, the ratio R of the magnetic layer is R. It may be within the range of ≦ 0.15.

The method for controlling the ratio R to 0.15 or less is not particularly limited. For example, after attracting a magnetic powder by applying a DC magnetic field of 15,000 oersted [Oe] or more with an electromagnet, air is gradually lowered while attracting the magnetic powder. There is a method of attracting the magnetic powder attracted by the above method and extracting only the components having a coercive force of less than +5000 Oersted [Oe]. Further, as described above, since the ε-iron oxide powder exhibits magnetocrystalline anisotropy, the coercive force of the ε-iron oxide powder is easily affected by the particle size. Generally, when the particle size is small, the coercive force becomes small and the particles. The larger the diameter, the larger the coercive force tends to be. Therefore, ε-iron oxide powder was dispersed in a solvent to prepare a slurry, and the slurry was subjected to sedimentation of large particles using an ultracentrifugation method, and then the supernatant was taken out and the coercive force was less than +5000 Elstead [Oe]. There is a method of extracting only the components. By these methods, the ratio R can be controlled to 0.15 or less by producing a magnetic layer using ε-iron oxide powder having an ultra-high coercive force component of +5000 oersted [Oe] or more reduced. it can.

Further, the ratio R can be controlled by changing the strength of the magnetic field orientation and changing the hysteresis curve at the time of forming the magnetic layer. In particular, by appropriately strengthening the orientation magnetic field and aligning the magnetic powder, the magnetic powder can be oriented. Since the shape of the peak generated in the differential curve of the hysteresis curve can be sharpened, the ratio R can be reduced.

Further, the output characteristics can be improved by setting the square ratio in the thickness direction of the magnetic layer to 0.65 or more, but further by setting the square ratio in the thickness direction of the magnetic layer to 0.75 or more. Output characteristics can be improved. The method of controlling the square ratio to 0.65 or more is not particularly limited, but for example, a method of changing and controlling the strength of the magnetic field orientation can be adopted.

Further, when the spacing of the surface of the magnetic layer after washing the surface of the magnetic layer with n-hexane is measured by TSA (Tape Spacing Analyzer), the value of the spacing is 5 nm or more and 15 nm or less. Is preferable. When the spacing value is less than 5 nm, the surface of the magnetic layer becomes too smooth, the contact area between the magnetic head and the magnetic layer becomes large, the friction coefficient increases, and the durability of the magnetic layer decreases. Tend. On the other hand, if the spacing value exceeds 15 nm, the distance between the magnetic head and the surface of the magnetic layer becomes too large, and the recording / reproducing characteristics tend to deteriorate. The spacing value is more preferably 7 nm or more and 13 nm or less, and most preferably 8 nm or more and 11 nm or less.

The method for measuring the spacing value and the method for controlling the spacing are not particularly limited, but can be performed by, for example, the method described in Japanese Patent Application Laid-Open No. 2012-43495 (Patent Document 5).

It is preferable that the surface of the magnetic layer is provided with a lubricant layer containing a fluorine-based lubricant or a silicone-based lubricant. By providing the lubricant layer, the friction coefficient of the magnetic layer is reduced, and the durability of the magnetic layer is further improved.

The thickness of the magnetic layer is preferably 30 nm or more and 200 nm or less. By setting the thickness of the magnetic layer to 200 nm or less, the short wavelength recording characteristics can be improved, and by setting the thickness of the magnetic layer to 30 nm or more, the servo signal can be recorded. Since the saturated magnetization amount of the ε-iron oxide powder used in this embodiment is 1/2 to 1/3 smaller than the saturation magnetization amount of the conventional ferromagnetic hexagonal ferrite powder, a servo signal having a long recording wavelength is recorded. In this case, the thickness of the magnetic layer needs to be 30 nm or more.

When the servo signal is not recorded on the magnetic layer, the thickness of the magnetic layer is preferably 10 nm or more and 50 nm or less. Even if the thickness of the magnetic layer is 10 nm or more and 50 nm or less, data signals can be recorded and reproduced by using a high-sensitivity magnetic head such as a tunnel-type magnetoresistive head (TMR head).

Hereinafter, this embodiment will be described with reference to the drawings. FIG. 1 is a schematic cross-sectional view showing an example of the magnetic recording medium of the present embodiment.

In FIG. 1, the magnetic recording medium 10 of the present embodiment has a non-magnetic support 11, an undercoat layer 12 formed on one main surface (here, an upper surface) of the non-magnetic support 11, and an undercoat layer. 12 is a magnetic tape having a magnetic layer 13 formed on a main surface (here, an upper surface) opposite to the non-magnetic support 11 side. Further, a back coat layer 14 is formed on the main surface (here, the lower surface) on the side where the undercoat layer 12 of the non-magnetic support 11 is not formed.

<Magnetic layer>
The magnetic layer 13 contains ε-iron oxide powder and a binder.

The ε-iron oxide powder is preferably formed in a single phase represented by _{the general composition formula ε-Fe 2} O _3. This is because the coercive force of the magnetic layer decreases when α-iron oxide or γ-iron oxide is mixed. However, α-iron oxide and γ-iron oxide may be contained as impurities as long as the coercive force of the magnetic layer does not decrease.

The coercive force of the ε-iron oxide powder is preferably 3000 oersted [Oe] or more. Further, when the ε-iron oxide powder represented by the general composition formula ε-Fe ₂ O ₃ contains impurities, the coercive force of the ε-iron oxide powder decreases, so that it is preferable that the ε-iron oxide powder does not contain impurities. However, in the above-mentioned ε-iron oxide powder, a part of Fe sites in the crystal is replaced with a trivalent metal element such as aluminum (Al), gallium (Ga), rhodium (Rh), and indium (In). , Coercive force can be controlled. Therefore, the ε-iron oxide powder may contain a metal element other than iron as an impurity as long as the coercive force can be maintained at 3000 oersted [Oe] or more.

As described above, the average particle size of the ε-iron oxide powder contained in the magnetic layer is set to 8 nm or more and 20 nm or less. If the average particle size of the ε-iron oxide powder exceeds 20 nm, the ultra-high coercive force component increases, making it difficult to record the magnetic signal from the magnetic head, and especially in short-wavelength recording, the noise of the magnetic recording medium increases. High electromagnetic conversion characteristics tend not to be obtained.

In the present embodiment, the average particle size of the ε-iron oxide powder contained in the magnetic layer is determined by using a scanning electron microscope (SEM) “S-4800” manufactured by Hitachi, Ltd. on the surface of the magnetic layer using the ε-iron oxide powder. , Acceleration voltage: 2 kV, magnification: 10000 times, observation conditions: from a photograph taken with U-LA100, using 100 particles of ε-iron oxide powder in one visual field, measurement is performed as follows.

When the particles are needle-shaped, the average major axis diameter of 100 particles is used. When the particles are plate-shaped, the average maximum plate diameter of 100 particles is used. The ratio of the major axis length to the minor axis length of the particles. In the case of a spherical or elliptical shape having a value of 1 to 3.5, the average maximum transfer diameter of 100 particles is calculated and determined.

Further, in the present embodiment, ε-iron oxide and other γ-iron oxide and α-iron oxide can be distinguished by analyzing their crystal structures by X-ray diffraction.

As the binder contained in the magnetic layer 13, conventionally known thermoplastic resins, thermosetting resins and the like can be used. Specific examples of the thermoplastic resin include vinyl chloride resin, vinyl chloride-vinyl acetate copolymer resin, vinyl chloride-vinyl alcohol copolymer resin, vinyl chloride-vinyl acetate-vinyl alcohol copolymer resin, and vinyl chloride. Examples thereof include a vinyl chloride-maleic anhydride copolymer resin, a vinyl chloride-hydroxyl chloride-containing alkyl acrylate copolymer resin, and a polyester polyurethane resin. Specific examples of the thermosetting resin include phenol-based resins, epoxy-based resins, polyurethane-based resins, urea-based resins, melamine-based resins, and alkyd-based resins.

The content of the binder in the magnetic layer 13 is preferably 7 to 50 parts by mass, and more preferably 10 to 35 parts by mass with respect to 100 parts by mass of the ε iron oxide powder.

Further, it is preferable to use a thermosetting cross-linking agent that binds to a functional group or the like contained in the binder to form a cross-linked structure together with the above-mentioned binder. Specific examples of the cross-linking agent include isocyanate compounds such as tolylene diisocyanate, hexamethylene diisocyanate, and isophorone diisocyanate; reaction products of isocyanate compounds and compounds having a plurality of hydroxyl groups such as trimethylolpropane; isocyanate compounds. Various polyisocyanates such as the condensation product of. The content of the cross-linking agent is preferably 10 to 50 parts by mass with respect to 100 parts by mass of the binder.

The magnetic layer 13 may further contain additives such as an abrasive, a lubricant, and a dispersant as long as it contains the above-mentioned ε-iron oxide powder and a binder. In particular, from the viewpoint of durability, abrasives and lubricants are preferably used.

Specific examples of the abrasive include α-alumina, β-alumina, silicon carbide, chromium oxide, cerium oxide, α-iron oxide, corundum, artificial diamond, silicon nitride, silicon carbide, titanium carbide, titanium oxide, and the like. Examples thereof include silicon dioxide and boron nitride, and among these, an abrasive having a Mohs hardness of 6 or more is more preferable. These may be used alone or in combination of two or more. The average particle size of the above-mentioned abrasive is preferably 10 to 200 nm, although it depends on the type of the abrasive used. The content of the abrasive is preferably 5 to 20 parts by mass, and more preferably 8 to 18 parts by mass with respect to 100 parts by mass of the magnetic powder.

Examples of the lubricant include fatty acids, fatty acid esters, and fatty acid amides. The fatty acid may be a linear type, a branched type, or a cis-trans isomer, but a linear type having excellent lubrication performance is preferable. Specific examples of such fatty acids include lauric acid, myristic acid, stearic acid, palmitic acid, behenic acid, oleic acid, linoleic acid and the like. Specific examples of the fatty acid ester include n-butyl oleate, hexyl oleate, n-octyl oleate, 2-ethylhexyl oleate, oleyl oleate, n-butyl laurate, heptyl laurate, and myristin. Examples thereof include n-butyl acid, n-butoxyethyl oleate, trimethylolpropane trioleate, n-butyl stearate, s-butyl stearate, isoamyl stearate, and butyl cellosolve stearate. Specific examples of the fatty acid amide include palmitic acid amide and stearic acid amide. These lubricants may be used alone or in combination of two or more.

Among these, it is preferable to use a fatty acid ester and a fatty acid amide in combination. In particular, 0.2 to 3 parts by mass of fatty acid ester and 0.5 to 5 parts by mass of fatty acid amide may be used with respect to 100 parts by mass of the total amount of the magnetic powder, the abrasive and the like in the magnetic layer 13. preferable. If the content of the fatty acid ester is less than 0.2 parts by mass, the effect of reducing the coefficient of friction is small, and if it exceeds 3.0 parts by mass, side effects such as the magnetic layer 13 sticking to the head may occur. Is. Further, if the content of the fatty acid amide is less than 0.5 parts by mass, the effect of preventing seizure caused by the mutual contact between the magnetic head and the magnetic layer 13 becomes small, and it exceeds 5 parts by mass. This is because there is a risk that the fatty acid amide will bleed out.

Further, the magnetic layer 13 may contain carbon black for the purpose of improving conductivity and surface lubricity. Specific examples of such carbon black include acetylene black, furnace black, and thermal black. The average particle size of carbon black is preferably 0.01 to 0.1 μm. When the average particle size is 0.01 μm or more, the magnetic layer 13 in which carbon black is well dispersed can be formed. On the other hand, when the average particle size is 0.1 μm or less, the magnetic layer 13 having excellent surface smoothness can be formed. Further, if necessary, two or more types of carbon black having different average particle diameters may be used. The content of the carbon black is preferably 0.2 to 5 parts by mass, and more preferably 0.5 to 4 parts by mass with respect to 100 parts by mass of the magnetic powder.

The surface roughness of the magnetic layer 13 is preferably less than 2.0 nm as the center line average roughness Ra defined in JIS B0601. The higher the surface smoothness of the magnetic layer 13, the higher the output can be obtained. However, if the surface of the magnetic layer 13 is too smooth, the friction coefficient becomes high and the running stability is lowered. Therefore, Ra is preferably 1.0 nm or more.

Next, the characteristics of the magnetic layer 13 will be described. FIG. 2 is a diagram showing an example of a hysteresis curve in the thickness direction of the magnetic layer 13. As shown in FIG. 2, when a positive magnetic field is applied to the magnetic layer 13 from a state where the magnetic field is 0, the saturation magnetization Ms in the positive direction is reached. After that, when a magnetic field in the opposite direction to the positive direction is applied, saturation magnetization-Ms in the opposite direction is reached. The end point on the hysteresis curve whose saturation magnetization is −Ms is defined as point A. Further, when a magnetic field in the positive direction is further applied from point A, the saturation magnetization Ms in the positive direction is reached. The end point on the hysteresis curve where the saturation magnetization is Ms is defined as point B. Further, in FIG. 2, Mr indicates the residual magnetization which is the magnetization in the magnetic field 0, and Hc indicates the coercive force which is the strength of the magnetic field in the magnetization 0 when the magnetization is inverted from a negative value to a positive value.

In FIG. 2, the magnetic layer 13 has a square ratio in the thickness direction represented by Mr / Ms set to 0.65 or more.

FIG. 3 shows a part of the differential curve obtained by differentiating the hysteresis curve from the point A to the point B in FIG. In FIG. 3, the integrated value of the differential curve in the magnetic field range of 0 oersted [Oe] or more and +10000 oersted [Oe] or less is A, and the integral value of the differential curve is A, +5000 oersted [Oe] or more and +10000 oersted [Oe] or less magnetic field range. Assuming that the integrated value of the differential curve is B and the ratio B / A of the integrated value A and the integrated value B is R, the relationship of R ≦ 0.15 is established. Here, the integral value A corresponds to the area value surrounded by the X-axis and the above differential curve in the range of the magnetic field of 0 oersted [Oe] or more and +10000 oersted [Oe] or less in FIG. 3, and the integral value B is In FIG. 3, it corresponds to the area value surrounded by the X-axis and the differential curve in the range of the magnetic field of +5000 oersted [Oe] or more and +10000 oersted [Oe] or less.

Further, in FIG. 3, there are two or more peaks, and among the above peaks, the maximum value of the maximum peak in the magnetic field range of +500 oersted [Oe] or more is set to P1, and −500 oersted [Oe] or more, +500. The maximum value of the maximum peak in the range of the magnetic field less than Oersted [Oe] is P2.

The maximum value P1 represents the proportion of magnetic powder having a particle size of 8 nm or more and 20 nm or less, which is a high coercive force component, and the maximum value P2 represents the proportion of magnetic powder having a particle size of less than 8 nm, which is a low coercive force component. it is conceivable that. That is, as described above, since the ε-iron oxide powder exhibits magnetocrystalline anisotropy, the coercive force of the ε-iron oxide powder is easily affected by the particle size. Generally, when the particle size is small, the coercive force is small and the particle size is small. The larger the value, the larger the coercive force. In particular, it is considered that when the particle size of the ε-iron oxide powder becomes smaller, the coercive force decreases sharply and the influence on the coercive force increases.

In FIG. 3, P2 / P1 is preferably set in the range of 0.25 ≦ P2 / P1 ≦ 0.60. In this way, the output is achieved by setting the proportion of the magnetic powder having a particle size of less than 8 nm, which is a low coercive component, low, and setting the proportion of the magnetic powder having a particle size of 8 nm or more and 20 nm or less, which is a high coercive component, high. It is possible to provide a magnetic recording medium having better characteristics.

The method for controlling P2 / P1 to 0.25 or more and 0.60 or less is not particularly limited, but for example, the mixing ratio of the ε-iron oxide powder having a high coercive force component and the ε-iron oxide powder having a low coercive force component can be adjusted. A method of changing, a method of changing the strength of the magnetic field orientation, a method of changing the hysteresis curve in the thickness direction, and the like can be adopted. More specifically, by setting the average particle size of the magnetic powder contained in the magnetic layer 13 to 8 nm or more and 20 nm or less, P2 / P1 can be set in the range of 0.25 ≦ P2 / P1 ≦ 0.60.

<Lubricant layer>
Although not shown in FIG. 1, as described above, in order to reduce the friction coefficient of the magnetic layer 13 and further improve the durability of the magnetic layer 13, a fluorine-based lubricant or silicone is placed on the magnetic layer 13. It is preferable to provide a lubricant layer containing a system lubricant. Examples of the fluorine-based lubricant include trichlorofluoroethylene, perfluoropolyether, perfluoroalkyl polyether, perfluoroalkylcarboxylic acid and the like. Examples of the silicone-based lubricant include silicone oil and modified silicone oil. These lubricants may be used alone or in combination of two or more. More specifically, as the fluorine-based lubricant, for example, "Novec7100" and "Novec1720" (trade name) manufactured by 3M can be used, and as the silicone-based lubricant, for example, Shin-Etsu Chemical Co., Ltd. "KF-96L", "KF-96A", "KF-96", "KF-96H", "KF-99", "KF-50", "KF-54", "KF-965" manufactured by Co., Ltd. , "KF-968", "HIVAC F-4", "HIVAC F-5", "KF-56A", "KF995", "KF-69", "KF-410", "KF-412", "KF-414", "FL" (trade name), "BY16-846", "SF8416", "SH200", "SH203", "SH230", "SF8419", "FS1265" manufactured by Toray Dow Corning Co., Ltd. , "SH510", "SH550", "SH710", "FZ-2110", "FZ-2203" (trade name) can be used.

The thickness of the lubricant layer is not particularly limited, and may be, for example, 3 to 5 nm. The thickness of the lubricant layer is determined by the method using TSA described in JP2012-43495 (Patent Document 5), and the magnetic recording medium and the transparent material before and after washing the lubricant layer with an organic solvent. The thickness of the lubricant layer can be measured from the difference in spacing between the two.

The lubricant layer can be formed by topcoating the magnetic layer 13 with the lubricant. As described above, the magnetic layer 13 is densely packed with a magnetic powder having a high coercive force component having a relatively large particle size and a magnetic powder having a low coercive force component having a relatively small particle size. The lubricant contained in 13 does not easily move to the surface of the magnetic layer 13, but the top coat that applies the lubricant to the surface of the magnetic layer can surely form the lubricant layer on the surface of the magnetic layer 13.

<Undercoat layer>
Under the magnetic layer 13, it is preferable to provide an undercoat layer 12 having a function of holding a lubricant and a function of buffering external stress (for example, pressing by a magnetic head). Further, since the strength of the magnetic recording medium 10 is increased by providing the undercoat layer 12, the calendar treatment can be performed when the magnetic recording medium 10 is formed, and the filling property of the magnetic layer 13 can be improved. The undercoat layer 12 contains a non-magnetic powder, a binder, and a lubricant.

Examples of the non-magnetic powder contained in the undercoat layer 12 include carbon black, titanium oxide, iron oxide, aluminum oxide and the like. Usually, carbon black is used alone, or carbon black and titanium oxide, iron oxide and the like are used. , Aluminum oxide and other non-magnetic powders are mixed and used. In order to form a coating film having less uneven thickness and to form a smooth undercoat layer 12, it is preferable to use a non-magnetic powder having a sharp particle size distribution. The average particle size of the non-magnetic powder is, for example, preferably 10 to 1000 nm, preferably 10 to 500 nm, from the viewpoint of ensuring the uniformity, surface smoothness, rigidity, and conductivity of the undercoat layer 12. Is more preferable.

The particle shape of the non-magnetic powder contained in the undercoat layer 12 may be spherical, plate-shaped, needle-shaped, or spindle-shaped. The average particle size of the needle-shaped or spindle-shaped non-magnetic powder is preferably 10 to 300 nm in terms of average major axis diameter, and preferably 5 to 200 nm in terms of average minor axis diameter. The average particle size of the spherical non-magnetic powder is preferably 5 to 200 nm, more preferably 5 to 100 nm. The average particle size of the plate-shaped non-magnetic powder is preferably 10 to 200 nm, which is the largest plate diameter.

As the binder and lubricant contained in the undercoat layer 12, the same binder and lubricant as those used in the magnetic layer 13 described above can be used. The content of the binder is preferably 7 to 50 parts by mass, and more preferably 10 to 35 parts by mass with respect to 100 parts by mass of the non-magnetic powder. The content of the lubricant is preferably 2 to 6 parts by mass, and more preferably 2.5 to 4 parts by mass with respect to 100 parts by mass of the non-magnetic powder.

The saturated magnetization amount of the ε-iron oxide powder used for the magnetic layer 13 described above is 1/2 to 1/3 smaller than the saturation magnetization amount of the conventional ferromagnetic hexagonal ferrite powder, so that the servo signal has a long recording wavelength. When recording, the undercoat layer 12 contains a magnetic powder. As the magnetic powder, for example, needle-shaped metallic iron-based magnetic powder, plate-shaped hexagonal ferrite magnetic powder, granular iron nitride-based magnetic powder, or the like can be used.

The thickness of the undercoat layer 12 is preferably 0.1 to 3 μm, more preferably 0.3 to 2 μm. Within this thickness range, the lubricant holding function and the external stress buffering function can be maintained without unnecessarily increasing the total thickness of the magnetic recording medium 10.

<Non-magnetic support>
As the non-magnetic support 11, a conventional non-magnetic support for a magnetic recording medium can be used. Specific examples thereof include films made of polyesters such as polyethylene terephthalate and polyethylene naphthalate, polyolefins, cellulose triacetate, polycarbonate, polyamide, polyimide, polyamideimide, polysulfone, and aramid.

The thickness of the non-magnetic support 11 varies depending on the application, but is preferably 1.5 to 11 μm, more preferably 2 to 7 μm. When the thickness of the non-magnetic support 11 is 1.5 μm or more, the film forming property is improved and high strength can be obtained. On the other hand, if the thickness of the non-magnetic support 11 is 11 μm or less, the total thickness does not become unnecessarily thick, and for example, in the case of magnetic tape, the recording capacity per roll can be increased.

The Young's modulus in the longitudinal direction of the non-magnetic support 11 is preferably 5.8 GPa or more, and more preferably 7.1 GPa or more. When the Young's modulus in the longitudinal direction of the non-magnetic support 11 is 5.8 GPa or more, the runnability can be improved. Further, in the magnetic recording medium used in the helicopter scan method, the ratio (MD / TD) of the Young's modulus (MD) in the longitudinal direction to the Young's modulus (TD) in the width direction is preferably 0.6 to 0.8. It is more preferably 0.65 to 0.75, and even more preferably 0.7. Within the range of the above ratio, it is possible to suppress the variation (flatness) of the output between the entry side and the exit side of the track of the magnetic head. In the magnetic recording medium used in the linear recording method, the ratio (MD / TD) of the Young's modulus (MD) in the longitudinal direction to the Young's modulus (TD) in the width direction is preferably 0.7 to 1.3.

<Back coat layer>
It is preferable to provide a back coat layer 14 on the main surface (here, the lower surface) on the side opposite to the main surface on which the undercoat layer 12 of the non-magnetic support 11 is formed, for the purpose of improving runnability and the like. .. The thickness of the backcoat layer 14 is preferably 0.2 to 0.8 μm, more preferably 0.3 to 0.8 μm. If the thickness of the backcoat layer 14 is too thin, the effect of improving the running performance becomes insufficient, and if it is too thick, the total thickness of the magnetic recording medium 10 becomes thick and the recording capacity per one roll of the magnetic tape becomes small.

The backcoat layer 14 preferably contains, for example, carbon black such as acetylene black, furnace black, and thermal black. Usually, small particle size carbon black and large particle size carbon black, which have relatively different particle sizes, are used in combination. The reason for using them together is that the effect of improving the running performance is increased.

Further, the backcoat layer 14 contains a binder, and as the binder, the same binder as that used for the magnetic layer 13 and the undercoat layer 12 can be used. Among these, in order to reduce the friction coefficient and improve the running performance of the magnetic head, it is preferable to use a cellulosic resin and a polyurethane resin in combination.

The backcoat layer 14 preferably further contains iron oxide, alumina, and the like for the purpose of improving the strength.

Next, a method of manufacturing the magnetic recording medium of the present embodiment will be described. In the method for producing a magnetic recording medium of the present embodiment, for example, each layer-forming component and a solvent are mixed to prepare a magnetic layer-forming paint, an undercoat layer-forming paint, and a backcoat layer-forming paint, respectively. An undercoat layer-forming paint is applied to one side of the magnetic support and dried to form an undercoat layer, and then a magnetic layer-forming paint is applied onto the undercoat layer and dried. A magnetic layer is formed, and a backcoat layer-forming paint is applied to the other side of the non-magnetic support and dried to form a backcoat layer. After that, the whole is calendared to obtain a magnetic recording medium.

Further, instead of the above-mentioned sequential layer coating method, after the undercoat layer forming paint is applied to one side of the non-magnetic support, before the undercoat layer forming paint dries, it is placed on the undercoat layer forming paint. It is also possible to adopt a simultaneous multi-layer coating method in which a paint for forming a magnetic layer is applied and dried.

The coating method of each of the above paints is not particularly limited, and for example, gravure coating, roll coating, blade coating, extrusion coating and the like can be used.

Hereinafter, the present invention will be described with reference to Examples, but the present invention is not limited to the following Examples. Further, in the following description, the term "part" means "part by mass".

(Example 1)
[Preparation of magnetic paint]
The magnetic paint component (1) shown in Table 1 was mixed at high speed with a high-speed stirring mixer to prepare a mixture. Next, the obtained mixture was dispersed in a sand mill for 250 minutes, and then the magnetic coating component (2) shown in Table 2 was added to prepare a dispersion. Next, the obtained dispersion liquid and the magnetic paint component (3) shown in Table 3 were stirred using a dispa, and this was filtered through a filter to prepare a magnetic paint.

_{The ε-Fe 2} O ₃ magnetic powder (A) of the magnetic paint component (1) was prepared by removing particles having a large particle size by an ultracentrifugation method as described below. First, cyclohexanone (400 parts) as a solvent and stearic acid (3 parts) as a dispersant are added to ε-Fe ₂ O ₃ magnetic powder (100 parts) and dispersed for 3 hours using a homogenizer, and then a high-pressure type is used. Using a medialess disperser, a three-pass dispersion treatment was performed at a pressure of 110 MPa and a discharge diameter of 0.18 mm to prepare a slurry. Next, the slurry was subjected to ultracentrifugation treatment at 28,500 rpm for 10 minutes using an ultracentrifuge "CP100NX" (product name) manufactured by Hitachi Koki Co., Ltd. to settle large particles, and then the supernatant portion. Was taken out and the solvent was dried and removed to obtain ε-Fe ₂ O ₃ magnetic powder (A).

[Preparation of undercoat paint]
A kneaded product was prepared by kneading the undercoat paint component (1) shown in Table 4 with a batch kneader. Next, the obtained kneaded product and the undercoat coating component (2) shown in Table 5 were stirred using a dispa to prepare a mixed solution. Next, the obtained mixed liquid was dispersed in a sand mill for 100 minutes to prepare a dispersion liquid, and then this dispersion liquid and the undercoat paint component (3) shown in Table 6 were stirred using a dispa, and this was filtered. The undercoat paint was prepared by filtering with.

[Preparation of paint for back coat layer]
A mixed solution containing the coating components for the back coat layer shown in Table 7 was dispersed in a sand mill for 50 minutes to prepare a dispersion. Fifteen parts of polyisocyanate was added to the obtained dispersion, and the mixture was stirred and filtered through a filter to prepare a coating material for a backcoat layer.

[Preparation of magnetic tape for evaluation]
The above-mentioned undercoat paint is applied onto a non-magnetic support (polyethylene naphthalate film, thickness: 5 μm) so that the thickness of the undercoat layer after the calendar treatment is 1.1 μm, and dried at 100 ° C. An undercoat layer was formed. Next, the magnetic paint is applied onto the undercoat layer so that the thickness of the magnetic layer after the calendar treatment is 55 nm, and an orientation magnetic field (450 kA / m) is applied using an NS counter magnet. Then, while performing the vertical alignment treatment, it was dried at 100 ° C. to form a magnetic layer.

Next, the paint for the back coat layer is applied to the surface of the non-magnetic support opposite to the surface on which the undercoat layer and the magnetic layer are formed, and the thickness after the calendar treatment is 0.5 μm. And dried at 100 ° C. to form a backcoat layer.

After that, the raw fabric roll having the undercoat layer and the magnetic layer formed on the upper surface side of the non-magnetic support and the backcoat layer formed on the lower surface side was subjected to a temperature of 100 ° C. in a calendar device having a seven-stage metal roll. Calendar treatment was performed at a linear pressure of 300 kg / cm.

Finally, the obtained raw fabric roll was cured at 60 ° C. for 48 hours to prepare a magnetic sheet. This magnetic sheet was cut to a width of 1/2 inch to prepare a magnetic tape for evaluation.

(Example 2)
In the magnetic paint component (1), the amount of ε-Fe ₂ O ₃ magnetic powder (A) added was changed to 75 parts, and the amount of ε-Fe ₂ O ₃ magnetic powder (B) added was changed to 25 parts. Made an evaluation magnetic tape in the same manner as in Example 1.

(Example 3)
In the magnetic paint component (1), the amount of ε-Fe ₂ O ₃ magnetic powder (A) added was changed to 85 parts, and the amount of ε-Fe ₂ O ₃ magnetic powder (B) added was changed to 15 parts. Made an evaluation magnetic tape in the same manner as in Example 1.

(Example 4)
In the magnetic paint component (1), the amount of ε-Fe ₂ O ₃ magnetic powder (A) added was changed to 85 parts, and the amount of ε-Fe ₂ O ₃ magnetic powder (B) added was changed to 15 parts. An evaluation magnetic tape was produced in the same manner as in Example 1 except that the amount of carbon black added was changed to 2.5 parts.

(Example 5)
In the magnetic paint component (1), the amount of ε-Fe ₂ O ₃ magnetic powder (A) added was changed to 75 parts, and the amount of ε-Fe ₂ O ₃ magnetic powder (B) added was changed to 25 parts. An evaluation magnetic tape was produced in the same manner as in Example 1 except that the alignment magnetic field of the vertical alignment treatment was changed to 250 kA / m.

(Example 6)
In the magnetic paint component (1), the amount of ε-Fe ₂ O ₃ magnetic powder (A) added was changed to 85 parts, and the amount of ε-Fe ₂ O ₃ magnetic powder (B) added was changed to 15 parts. An evaluation magnetic tape was produced in the same manner as in Example 1 except that the thickness of the magnetic layer was changed to 30 nm.

(Example 7)
In the magnetic paint component (1), the amount of ε-Fe ₂ O ₃ magnetic powder (A) added was changed to 85 parts, and the amount of ε-Fe ₂ O ₃ magnetic powder (B) added was changed to 15 parts. An evaluation magnetic tape was produced in the same manner as in Example 1 except that the thickness of the magnetic layer was changed to 200 nm.

(Example 8)
An evaluation magnetic tape was produced in the same manner as in Example 1 except that the amount of carbon black added was changed to 3 parts in the magnetic paint component (1).

(Comparative Example 1)
In the magnetic paint component (1), the amount of ε-Fe ₂ O ₃ magnetic powder (A) added was changed to 50 parts, and the amount of ε-Fe ₂ O ₃ magnetic powder (B) added was changed to 50 parts. Made an evaluation magnetic tape in the same manner as in Example 1.

(Comparative Example 2)
In the magnetic paint component (1), the amount of ε-Fe ₂ O ₃ magnetic powder (A) added was changed to 0 parts, and the amount of ε-Fe ₂ O ₃ magnetic powder (B) added was changed to 100 parts. Made an evaluation magnetic tape in the same manner as in Example 1.

(Comparative Example 3)
In the magnetic paint component (1), the amount of ε-Fe ₂ O ₃ magnetic powder (A) added was changed to 75 parts, and the amount of ε-Fe ₂ O ₃ magnetic powder (B) added was changed to 25 parts. An evaluation magnetic tape was produced in the same manner as in Example 1 except that the alignment magnetic field of the vertical alignment treatment was changed to 100 kA / m.

Next, the following evaluation was performed using the prepared magnetic tape for evaluation.

<Magnetic characteristics>
A hysteresis curve in the thickness direction of the magnetic layer of the evaluation magnetic tape was obtained using a vibrating sample type magnetometer "VSM-P7 type" (product name) manufactured by Toei Kogyo Co., Ltd. Specifically, the evaluation magnetic tape is cut into a circle with a diameter of 8 mm to obtain a cut sample, and 20 pieces of the cut sample are laminated with the thickness direction of the magnetic tape aligned with the application direction of the external magnetic field to form a measurement sample. did. As the plot mode of the data from the vibrating sample magnetometer, the applied magnetic field was set to -16 kOe to 16 kOe, the time constant TC was set to 0.03 sec, the drawing step was set to 6 bits, and the weight time was set to 0.3 sec.

From the hysteresis curve in the thickness direction, the square ratio in the thickness direction of the magnetic layer and the coercive force in the thickness direction of the magnetic layer were obtained.

Further, in the hysteresis curve in the thickness direction, a positive magnetic field is applied to the magnetic layer to reach the positive saturation magnetization, and then a reverse magnetic field is applied to the positive direction to achieve the reverse saturation magnetization. Assuming that the point where the point reached is the point A and the point where the positive magnetic field is further applied from the point A to reach the saturation magnetization in the positive direction is the point B, the hysteresis curve from the point A to the point B is 2759 points. For each measurement point of the measurement data output by dividing into, one measurement point from the 18th point to the 2742th point and 17 points before and after the measurement point, a total of 35 points. In, the linear minimum square approximation was performed, and the slope of the obtained approximation formula was used as the differential value at the measurement point. A differential curve was obtained from the differential values at each measurement point from the 18th point to the 2742th point obtained by this method.

From the obtained differential curve, the integral value A of the above differential curve in the range of the magnetic field of 0 oersted [Oe] or more and +10000 oersted [Oe] and the magnetic field of +5000 oersted [Oe] or more and +10000 oersted [Oe] or less. The integral value B of the differential curve in the range was obtained, and the ratio R = B / A of the integral value A and the integral value B was calculated.

Here, FIG. 4 shows a part of the differential curve of the hysteresis curve in the thickness direction obtained in each of Example 1, Comparative Example 1 and Comparative Example 2.

<Magnetic layer spacing>
Using TSA (Tape Spacing Analyzer) manufactured by Micro Physics, the spacing Sp after washing the surface of the magnetic layer with n-hexane was measured.

Specifically, the pressure for pressing the magnetic layer on a glass plate with urethane hemisphere was ^{0.5atm (5.05 × 10 4 N /} m). In this state, white light is emitted from the stroboscope through a glass plate onto a certain region (2400,000 to 280000 μm ² ) of the surface of the evaluation magnetic tape on the magnetic layer side, and the reflected light from the region is reflected by the IF filter (633 nm) and the IF filter. By receiving light with a CCD through (546 nm), an interference fringe image generated by the unevenness of this region was obtained.

Next, this image is divided into 66000 points, the distance from the glass plate at each point to the surface of the magnetic layer is obtained, and this is used as a histogram (frequency distribution curve), which is further processed into a smooth curve by low-pass filter (LPF) processing. The distance from the glass plate at the peak position to the surface of the magnetic layer was defined as the spacing Sp.

The optical constants (phase, reflectance) of the magnetic layer surface used in the above spacing calculation were measured using a reflection spectroscopic film thickness meter "FE-3000" manufactured by Otsuka Electronics Co., Ltd., and have a wavelength of around 546 nm. Was used.

Cleaning with n-hexane was performed by immersing the evaluation magnetic tape in n-hexane and ultrasonically cleaning at room temperature for 30 minutes.

<Output characteristics>
A loop tester (dynamic TSA device) manufactured by Micro Physics is used, and an inductive / GMR composite head having a write track width of 11 μm and a read track width of 3.8 μm is attached to the loop tester (tape speed 1.5 m / sec, recording wavelength). A 200 nm signal is recorded on an evaluation magnetic tape, the reproduced signal is amplified by a commercially available MR head Read amplifier, and then a spectrum analyzer "N9020A" manufactured by Keysight Technology Co., Ltd. is used to output the fundamental wave component of the signal ( S) and the integrated noise (N) up to twice the frequency were measured. And
The output characteristics were evaluated using the S / N ratio of Comparative Example 1 as a reference (0 dB) and the other S / N ratios as relative values (dB) to the S / N ratio of Comparative Example 1.

The above evaluation results are shown in Table 8. In addition, Table 8 shows the average particle size of the entire magnetic powder used as a value calculated by weighting the average particle size of the magnetic powders (A) and (B) with a blending ratio, and also shows the thickness of the magnetic layer. Indicated.

From Table 8, it can be seen that all of Examples 1 to 8 are superior in output characteristics as compared with Comparative Examples 1 to 3. On the other hand, in Comparative Examples 1 and 2 in which the ratio R of the ultrahigh coercive force component was more than 0.15, and Comparative Example 3 in which the square ratio was less than 0.65, the output characteristics were all inferior.

The magnetic recording medium of the present invention can be used as a magnetic recording medium having excellent output characteristics.

10 Magnetic recording medium (magnetic tape)
11 Non-magnetic support 12 Undercoat layer 13 Magnetic layer 14 Backcoat layer

Claims (5)

A magnetic recording medium including a non-magnetic support and a magnetic layer containing magnetic powder.
The magnetic powder is composed of ε-iron oxide powder.
The average particle size of the magnetic powder is 8 nm or more and 20 nm or less.
The square ratio of the magnetic layer in the thickness direction is 0.65 or more.
The coercive force in the thickness direction of the magnetic layer is 3000 oersted [Oe] or more.
In the hysteresis curve in the thickness direction of the magnetic layer, a magnetic field in the positive direction is applied to the magnetic layer to reach saturation magnetization in the positive direction, and then a magnetic field in the reverse direction is applied to the magnetic layer in the reverse direction. If the point where the saturation magnetization of the above is reached is the point A, and the point where the positive magnetic field is applied from the point A and the saturation magnetization in the positive direction is reached is the point B, the hysteresis curve from the point A to the point B is defined. In the differential curve obtained by differentiating
The integral value of the differential curve in the magnetic field range of 0 oersted [Oe] or more and +10000 oersted [Oe] or less is A, and the integral value of the differential curve in the magnetic field range of +5000 oersted [Oe] or more and +10000 oersted [Oe] or less. A magnetic recording medium characterized in that the relationship of R ≦ 0.15 is established, where B is the value and R is the ratio B / A of the integrated value A and the integrated value B. Claim 1 in which the value of the spacing is 5 nm or more and 15 nm or less when the spacing of the surface of the magnetic layer after washing the surface of the magnetic layer with n-hexane is measured by TSA (Tape Spacing Analyzer). The magnetic recording medium described in. The magnetic recording medium according to claim 1 or 2, wherein R ≦ 0.10. The magnetic recording medium according to any one of claims 1 to 3, further comprising a lubricant layer containing a fluorine-based lubricant or a silicone-based lubricant on the surface of the magnetic layer. The magnetic recording medium according to any one of claims 1 to 4, wherein the thickness of the magnetic layer is 30 nm or more and 200 nm or less.

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