JP2017191633A - Magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic recording medium which is excellent in output characteristics and durability.SOLUTION: A magnetic recording medium comprises a non-magnetism support, a magnetic layer containing magnetic powder, and an undercoat layer. The magnetic powder is made from ε-iron oxide powder and an average particle diameter of the magnetic powder is 8 nm or more and 20 nm or less. At least one of the magnetic layer and the undercoat layer includes lubricant. In a differential curve obtained by differentiating a hysteresis curve in a thickness direction of the magnetic layer, there are two or more peaks. In peaks in the same direction among the two or more peaks, when a maximum value of the maximum peak in a range of a magnetic field of +500 oersted (Oe) or more is defined as P1 and a maximum value of the maximum peak in a range of a magnetic field of -500 oersted (Oe) or more and less than +500 oersted (Oe) is defined as P2, the following relation is satisfied: 0.25≤P2/P1≤0.60.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a magnetic recording medium excellent in output characteristics and durability.

  Coating-type magnetic recording media in which a magnetic layer containing magnetic powder and a binder is formed on a non-magnetic support will further improve recording density as the recording / reproducing method shifts from analog to digital. Is required. In particular, this demand is increasing year by year for high-density digital video tapes and computer backup tapes.

  With such an increase in recording density, the recording wavelength has been shortened, and in order to cope with this short wavelength recording, the magnetic powder has been made finer year by year. At present, the ferromagnetic material has an average particle diameter of about 20 nm. 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).

  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, there is a problem that by further miniaturizing the ferromagnetic hexagonal ferrite powder, the particle volume of the magnetic powder becomes small and is easily affected by thermal fluctuations. For this reason, it is necessary to suppress thermal fluctuations using a magnetic material that has a high coercive force and a large anisotropic energy even if it is made fine.

Under such circumstances, in recent years, ε-Fe ₂ O ₃ has been studied as a new magnetic material for magnetic recording media, and ε has a ferromagnetic property even with an average particle size of 15 nm or less, preferably 10 nm or less. -fe ₂ O ₃ of consisting of a single-phase iron oxide magnetic nanoparticles powder has been proposed (e.g., Patent Document 2). 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).

Japanese Patent Laying-Open No. 2015-91747 JP 2014-224027 A Japanese Patent Laying-Open No. 2015-82329 JP 2014-149886 A

The crystal structure of Fe ₂ O ₃ that is generally known is a gamma (γ) phase or an alpha (α) phase, but ε-Fe ₂ O ₃ is a crystal structure that exists between them, and has a different crystal structure. In order to show magnetic anisotropy based on the directionality, it is characterized by high coercive force even when the particle diameter is reduced to 10 nm or less.

However, when looking at the differential curve obtained by differentiating the hysteresis curve of ε-Fe ₂ O ₃ , a peak occurs in the magnetic field range of +500 Oersted [Oe] or more, and in the magnetic field range of less than +500 Oersted [Oe]. It is known that a peak occurs. This is because, in the ε-Fe ₂ O ₃ magnetic powder, in addition to the high coercive force component, different coercive force, particularly magnetic powder having a low coercive force component is mixed, so that the magnetic field range is smaller than +500 Oersted [Oe]. It is thought that a peak also appears. When this low coercive force component is present in the magnetic layer of the magnetic recording medium, the magnetic material cannot be recorded even if the magnetic signal is recorded by the magnetic head. Therefore, the magnetic material used for the magnetic recording medium for high-density recording It is not preferable.

  The present invention has been made to solve such a problem of the prior art, the maximum value of the maximum peak generated in a specific range of the differential curve obtained by differentiating the hysteresis curve in the thickness direction of the magnetic layer, Provided is a magnetic recording medium capable of satisfying both output characteristics and durability by setting the ratio of the next largest peak maximum value to a specific range.

The magnetic recording medium of the present invention is a magnetic recording medium comprising a nonmagnetic support, a magnetic layer containing magnetic powder, and an undercoat layer disposed between the magnetic layer and the nonmagnetic support. The magnetic powder is composed of ε-iron oxide powder, the average particle diameter of the magnetic powder is 8 nm or more and 20 nm or less, and at least one of the magnetic layer and the undercoat layer contains a lubricant, and the magnetic powder In the hysteresis curve in the thickness direction of the layer, a positive magnetic field is applied to the magnetic layer to reach a positive saturation magnetization, and then a reverse magnetic field is applied to the positive direction to reverse saturation. If the point that has reached magnetization is point A, and a point that has reached saturation magnetization in the positive direction by applying a positive magnetic field from point A is point B, the hysteresis curve from point A to point B is differentiated. There are two or more peaks in the differential curve obtained by Of these peaks, the maximum peak maximum value in the magnetic field range of +500 Oersted [Oe] or more is P1, and the maximum peak maximum value in the magnetic field range of −500 Oersted [Oe] or more and less than +500 Oersted [Oe] is set. If P2 is P2, the following relationship is established.
0.25 ≦ P2 / P1 ≦ 0.60

  According to the magnetic recording medium of the present invention, the maximum value of the maximum peak generated in a specific range of the differential curve obtained by differentiating the hysteresis curve measured in the thickness direction of the magnetic layer, and the maximum value of the next largest peak Since the ratio is controlled within a specific range, a magnetic recording medium having excellent output characteristics and durability can be provided.

FIG. 1 is a schematic sectional view showing an example of the magnetic recording medium of the present invention. FIG. 2 is a diagram illustrating an example of a hysteresis curve. FIG. 3 is a diagram showing a part of a differential curve obtained by differentiating the hysteresis curve from point A to point B in FIG. FIG. 4 is a diagram showing a hysteresis curve obtained in Example 1. FIG. FIG. 5 is a diagram illustrating a part of the differential curve of the hysteresis curve obtained in the first embodiment.

The magnetic recording medium of the present invention includes a nonmagnetic support and a magnetic layer containing magnetic powder. The magnetic powder is composed of ε-iron oxide powder, and the squareness ratio of the magnetic layer in the thickness direction is 0.65 or more. Further, in the hysteresis curve measured in the thickness direction of the magnetic layer, a magnetic field in the positive direction is applied to the magnetic layer to reach a saturation magnetization in the positive direction, and then a magnetic field in the opposite direction to the positive direction is applied. Then, when the point reaching the saturation magnetization in the reverse direction is designated as point A, and the point reaching the saturation magnetization in the positive direction by applying a forward magnetic field further from point A is designated as point B, from point A to point B In the differential curve obtained by differentiating the hysteresis curve heading, there are two or more peaks, and among these peaks, the maximum value of the maximum peak in the magnetic field range of +500 Oersted [Oe] or more is P1, and −500 If the maximum value of the maximum peak in the magnetic field range of Oersted [Oe] or more and less than +500 Oersted [Oe] is P2, the following relationship is established.
0.25 ≦ P2 / P1 ≦ 0.60

  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 diameter of the magnetic powder is 8 nm or more and 20 nm or less and it corresponds to short wavelength recording. Further, the output characteristics can be improved by setting the squareness ratio in the thickness direction of the magnetic layer to 0.65 or more, and further the output can be achieved by setting the squareness ratio in the thickness direction of the magnetic layer to 0.75 or more. Can be improved. The method for controlling the squareness ratio to 0.65 or more is not particularly limited. For example, a method for controlling the squareness ratio by changing the strength of the magnetic field orientation can be employed.

  Furthermore, output characteristics and durability can be improved by setting 0.25 ≦ P2 / P1 ≦ 0.60. Specifically, the maximum value P1 represents the ratio of magnetic powder having a particle size of 8 nm to 20 nm, which is a high coercive force component, and the maximum value P2 is a magnetism having a particle size of less than 8 nm, which is a low coercive force component. It is thought to represent the proportion of powder. The reason for this is as follows. That is, 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, the smaller the particle size, the smaller the coercive force and the larger the particle size. And the coercive force increases. In particular, it is considered that when the particle diameter of the ε-iron oxide powder becomes smaller, the coercive force rapidly decreases and the influence on the coercive force increases.

  Thus, when P2 / P1 is 0.25 or more and 0.60 or less, the output characteristics of the magnetic layer are improved by the high coercivity magnetic powder having a large particle diameter of 8 nm or more and 20 nm or less. Furthermore, a low coercivity magnetic powder with a particle size of less than 8 nm and a small coercive force is filled in voids between particles with a large particle size and a high coercivity component with a particle size of 8 nm to 20 nm.

  As described above, even when the gap between large particles having a high coercive force component and a particle size of 8 nm or more and 20 nm or less is filled with particles having a low coercive force component having a particle size of less than 8 nm and a small particle size, as described above. In addition, since the number of magnetic powders having a small particle size and a low coercive force component is smaller than the number of magnetic powders having a high coercive force component, the influence of the low coercive force component on the output characteristics is small. Particles are dominant in exerting output characteristics. Further, due to the filling effect that the low coercive force magnetic powder having a particle size of less than 8 nm is filled in the space between the particles having a large coercive force component having a particle size of 8 nm or more and 20 nm or less. It is considered that the durability of the magnetic layer is improved because the strength of the entire magnetic layer is increased. That is, by setting P2 / P1 to 0.25 or more and 0.60 or less, it is possible to achieve both the output characteristics and durability of the magnetic layer.

  On the other hand, when P2 / P1 is less than 0.25, filling of voids between particles having a high coercive force component and a large particle size is reduced, and particles having a low coercive force component and a small particle size become too small. The increase in the strength of the entire magnetic layer due to cannot be expected, and the durability of the magnetic layer cannot be improved. As a result, it becomes impossible to achieve both the output characteristics and durability improvement of the magnetic layer.

  Further, when P2 / P1 exceeds 0.60, the filling property of the magnetic powder is improved and the durability of the magnetic layer is improved, but the low coercive force component having a small particle diameter does not contribute to magnetic recording in the magnetic powder. Therefore, the output characteristics of the magnetic layer cannot be improved.

  The P2 / P1 is preferably 0.26 or more and 0.58 or less, more preferably 0.27 or more and 0.45 or less, and most preferably 0.30 or more and 0.35 or less.

  The method for controlling P2 / P1 to 0.25 or more and 0.60 or less is not particularly limited. For example, a mixture of ε-iron oxide powder having a high coercive force component and ε-iron oxide powder having a low coercive force component is used. A method of changing the ratio, a method of changing the strength of the magnetic field orientation, and changing the hysteresis curve in the thickness direction can be employed. More specifically, the average particle size of the magnetic powder contained in the magnetic layer is preferably 8 nm or more and 20 nm or less.

  The present invention is characterized in that both the durability and the output characteristics of the magnetic layer are compatible by setting P2 / P1 in a specific range. Therefore, the measurement target of P2 / P1 is the magnetic layer of the completed magnetic recording medium, and ε-iron oxide powder is not the measurement target. Therefore, even if the value obtained by measuring P2 / P1 of the ε-iron oxide powder is different from the value obtained by measuring P2 / P1 of the magnetic layer containing the ε-iron oxide powder, P2 / P1 of the magnetic layer is different. May be in the range of 0.25 ≦ P2 / P1 ≦ 0.60.

  Further, when the spacing of the surface of the magnetic layer after the surface of the magnetic layer is washed with n-hexane is measured by TSA (Tape Spacing Analyzer), the spacing value is 5 nm or more and 15 nm or less. Is preferred. If 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 increases, 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 to 13 nm, and most preferably 8 nm to 11 nm.

  The method for measuring the spacing value and the control method thereof are not particularly limited, and can be performed by, for example, the method described in JP 2012-43495 A.

  It is preferable to provide a lubricant layer containing a fluorine-based lubricant or a silicone-based lubricant on the surface of the magnetic layer. 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. Short wavelength recording characteristics can be improved by setting the thickness of the magnetic layer to 200 nm or less, and servo signals can be recorded by setting the thickness of the magnetic layer to 30 nm or more. Since the saturation magnetization of the ε-iron oxide powder used in the present invention is 1/2 to 1/3 smaller than that 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 to 50 nm. 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 highly sensitive magnetic head such as a tunnel type magnetoresistive head (TMR head).

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

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

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

The ε-iron oxide powder is preferably formed of a single phase represented by the general composition formula ε-Fe ₂ O ₃ . This is because the coercivity of the magnetic layer decreases when α-iron oxide or γ-iron oxide is mixed. However, as long as the coercive force of the magnetic layer does not decrease, α-iron oxide or γ-iron oxide may be included as impurities.

The coercive force of the ε-iron oxide powder is preferably 2500 Oersted [Oe] or more. In addition, when impurities are contained in the ε-iron oxide powder represented by the general composition formula ε-Fe ₂ O ₃ , the coercive force of the ε-iron oxide powder is reduced. However, in the ε-iron oxide powder, a part of Fe site in the crystal is replaced with a trivalent metal element such as aluminum (Al), gallium (Ga), rhodium (Rh), indium (In) or the like. Thus, the coercive force can be controlled. For this reason, the ε-iron oxide powder may contain a metal element other than iron as an impurity as long as the coercive force can maintain 2500 Oersted [Oe] or more.

  As described above, the average particle size of the ε-iron oxide powder contained in the magnetic layer is preferably set to 8 nm or more and 20 nm or less. When the average particle diameter of the ε-iron oxide powder exceeds 20 nm, noise in the magnetic recording medium increases particularly in short wavelength recording, and thus high electromagnetic conversion characteristics tend not to be obtained.

  In the present invention, the average particle diameter of the ε-iron oxide powder contained in the magnetic layer is the same as the surface of the magnetic layer using the ε-iron oxide powder by using a scanning electron microscope (SEM) “S-4800” manufactured by Hitachi, Ltd. Using, acceleration voltage: 2 kV, magnification: 10,000 times, observation condition: from a photograph taken with U-LA100, measurement was performed as follows using 100 particles of ε-iron oxide powder in one field of view.

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

  In the present invention, ε-iron oxide can be distinguished from other γ-iron oxide and α-iron oxide by analyzing their crystal structures by X-ray diffraction.

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

  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.

  Moreover, it is preferable to use together with the said binder the thermosetting crosslinking agent which couple | bonds with the functional group etc. which are contained in a binder, and forms a crosslinked structure. Specific examples of the crosslinking 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 And various polyisocyanates such as condensation products. The content of the crosslinking 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-described ε-iron oxide powder and a binder. In particular, abrasives and lubricants are preferably used from the viewpoint of durability.

  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, Examples thereof include titanium oxide, silicon dioxide, and boron nitride. Among these, an abrasive having a Mohs hardness of 6 or more is more preferable. These may be used alone or in combination. The average particle diameter of the abrasive is preferably 10 to 200 nm, although it depends on the type of 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 any of linear, branched and cis / trans isomers, but is preferably a linear type having excellent lubricating performance. 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 myristic. Examples 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.

  Among these, it is preferable to use a fatty acid ester and a fatty acid amide in combination. In particular, it is possible to use 0.2 to 3 parts by mass of fatty acid ester and 0.5 to 5 parts by mass of fatty acid amide with respect to 100 parts by mass of the total amount of all powders such as magnetic powder and abrasive in 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 friction coefficient is small, and if it exceeds 3.0 parts by mass, side effects such as sticking of the magnetic layer 13 to the head may occur. It 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 mutual contact between the magnetic head and the magnetic layer 13 becomes small, and exceeds 5 parts by mass. This is because the fatty acid amide may bleed out.

  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 diameter 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 diameter is 0.1 μm or less, the magnetic layer 13 having excellent surface smoothness can be formed. Moreover, you may use 2 or more types of carbon black from which an average particle diameter differs as needed. 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 centerline average roughness Ra defined by JIS B0601. As the surface smoothness of the magnetic layer 13 is improved, a higher output is obtained. However, if the surface of the magnetic layer 13 is too smooth, the friction coefficient is increased and the running stability is lowered. For this reason, it is preferable that Ra is 1.0 nm or more.

  Next, the characteristics of the magnetic layer 13 will be described. FIG. 2 is a diagram illustrating 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 zero, the saturation magnetization Ms in the positive direction is reached. Thereafter, when a magnetic field in the reverse direction with respect to the positive direction is applied, the saturation magnetization −Ms in the reverse direction is reached. The end point on the hysteresis curve where the saturation magnetization is −Ms is defined as A point. Further, when a positive magnetic field is further applied from the point A, the saturation magnetization Ms in the positive direction is reached. The end point on the hysteresis curve where the saturation magnetization becomes Ms is defined as B point. In FIG. 2, Mr represents residual magnetization that is magnetization in a magnetic field 0.

  FIG. 3 shows a part of a differential curve obtained by differentiating the hysteresis curve from point A to point B in FIG. In FIG. 3, there are two or more peaks, and among these peaks, the maximum value of the maximum peak in the magnetic field range of +500 oersted [Oe] or more is P1, and −500 oersted [Oe] or more +500 oersted [Oe]. ] Let P2 be the maximum value of the maximum peak in the magnetic field range below.

  In the magnetic layer 13, the squareness ratio in the thickness direction indicated by Mr / Ms in FIGS. 2 and 3 is set to 0.65 or more, and P2 / P1 is 0.25 ≦ P2 / P1 ≦ 0.60. Is set in the range. Thereby, a magnetic recording medium having excellent output characteristics and durability can be provided.

<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 provided 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, and perfluoroalkylcarboxylic acid. Examples of the silicone lubricant include silicone oil and modified silicone oil. These lubricants may be used alone or in combination. More specifically, for example, “Novec7100” and “Novec1720” (trade names) manufactured by 3M can be used as the fluorine-based lubricant, and examples of the silicone-based lubricant include Shin-Etsu Chemical Co., Ltd. “KF-96L”, “KF-96A”, “KF-96”, “KF-96H”, “KF-99”, “KF-50”, “KF-54”, “KF-965” manufactured by Corporation "," 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”, “F” manufactured by Toray Dow Corning Co., Ltd. 1265 "," SH510 "," SH550 "," SH710 "," FZ-2110 "," can be used FZ-2203 "(trade name).

  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 difference in the spacing between the magnetic recording medium and the transparent body before and after the lubricant layer is washed away with an organic solvent by the method using TSA described in JP 2012-43495 A described above. Thus, the thickness of the lubricant layer can be measured.

  The lubricant layer can be formed by top-coating the lubricant on the magnetic layer 13. As described above, the magnetic layer 13 is densely filled with a magnetic powder having a high coercivity component having a relatively large particle size and a magnetic powder having a low coercivity component having a relatively small particle size. Although the lubricant contained in 13 hardly migrates to the surface of the magnetic layer 13, the lubricant layer can be reliably formed on the surface of the magnetic layer 13 by the top coat in which the lubricant is applied to the surface of the magnetic layer.

<Undercoat layer>
Under the magnetic layer 13, it is preferable to provide an undercoat layer 12 having a function of retaining a lubricant and a buffering function of external stress (for example, pressure applied by a magnetic head). In addition, since the strength of the magnetic recording medium 10 is increased by providing the undercoat layer 12, calendering 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 nonmagnetic powder, a binder, and a lubricant.

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

  The particle shape of the nonmagnetic powder contained in the undercoat layer 12 may be any of a spherical shape, a plate shape, a needle shape, and a spindle shape. The average particle diameter of the acicular or spindle-shaped nonmagnetic powder is preferably 10 to 300 nm in terms of the average major axis diameter, and more preferably 5 to 200 nm in terms of the average minor axis diameter. The average particle size of the spherical nonmagnetic powder is preferably 5 to 200 nm, and more preferably 5 to 100 nm. The average particle diameter of the plate-like nonmagnetic powder is preferably 10 to 200 nm with the largest plate diameter. Furthermore, a nonmagnetic powder having a sharp particle size distribution is preferably used in order to form the undercoat layer 12 which is smooth and has little thickness unevenness.

  As the binder and lubricant contained in the undercoat layer 12, the same binder and lubricant as those used for 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 nonmagnetic powder. Moreover, the content of the lubricant is preferably 2 to 6 parts by mass, more preferably 2.5 to 4 parts by mass with respect to 100 parts by mass of the nonmagnetic powder.

  Since the saturation magnetization of the ε-iron oxide powder used for the magnetic layer 13 is 1/2 to 1/3 that of the conventional ferromagnetic hexagonal ferrite powder, the servo with a long recording wavelength is used. When recording a signal, the undercoat layer 12 contains magnetic powder. As the magnetic powder, for example, acicular metal 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. By setting the thickness within this range, the retaining function of the lubricant and the buffering function of the external stress can be maintained without unnecessarily increasing the total thickness of the magnetic recording medium 10.

<Non-magnetic support>
As the nonmagnetic support 11, a conventionally used nonmagnetic support for a magnetic recording medium can be used. Specific examples 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 nonmagnetic support 11 varies depending on the application, but is preferably 1.5 to 11 μm, more preferably 2 to 7 μm. If the thickness of the nonmagnetic support 11 is 1.5 μm or more, the film formability is improved and high strength can be obtained. On the other hand, if the thickness of the nonmagnetic support 11 is 11 μm or less, the total thickness is not unnecessarily increased. For example, in the case of a magnetic tape, the recording capacity per roll can be increased.

  The Young's modulus in the longitudinal direction of the nonmagnetic support 11 is preferably 5.8 GPa or more, more preferably 7.1 GPa or more. If the Young's modulus in the longitudinal direction of the nonmagnetic support 11 is 5.8 GPa or more, the running performance can be improved. In the magnetic recording medium used for the helical scan method, the ratio (MD / TD) of Young's modulus (MD) in the longitudinal direction to Young's modulus (TD) in the width direction is preferably 0.6 to 0.8. More preferably, it is 0.65-0.75, More preferably, it is 0.7. As long as the ratio is within the above range, output variation (flatness) between the input side and the output side of the track of the magnetic head can be suppressed. In the magnetic recording medium used for the linear recording system, the ratio (MD / TD) of Young's modulus (MD) in the longitudinal direction to Young's modulus (TD) in the width direction is preferably 0.7 to 1.3.

<Back coat layer>
A back coat layer 14 is preferably provided on the main surface (here, the lower surface) opposite to the main surface on which the undercoat layer 12 of the nonmagnetic support 11 is formed for the purpose of improving running performance and the like. . The thickness of the backcoat layer 14 is preferably 0.2 to 0.8 μm, and more preferably 0.3 to 0.8 μm. If the thickness of the backcoat layer 14 is too thin, the effect of improving running performance is insufficient, and if it is too thick, the total thickness of the magnetic recording medium 10 is increased, and the recording capacity per magnetic tape roll is reduced.

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

  The back coat layer 14 contains a binder, and the same binder as that used for the magnetic layer 13 and the undercoat layer 12 can be used as the binder. Among these, in order to reduce the coefficient of friction and improve the runnability of the magnetic head, it is preferable to use a cellulose resin and a polyurethane resin in combination.

  The back coat layer 14 preferably further contains iron oxide, alumina or the like for the purpose of improving the strength.

  Next, a method for manufacturing the magnetic recording medium of the present invention will be described. The method for producing a magnetic recording medium of the present invention includes, for example, mixing each layer forming component and a solvent to prepare a magnetic layer forming paint, an undercoat layer forming paint, and a backcoat layer forming paint, respectively. A magnetic layer is formed by a sequential multilayer coating method in which an undercoat layer-forming paint is applied to one side of a support and dried to form an undercoat layer, and then a magnetic layer-forming paint is applied to the undercoat layer and dried. Further, a backcoat layer-forming coating material is applied to the other surface of the nonmagnetic support and dried to form a backcoat layer. Thereafter, the whole is calendered to obtain a magnetic recording medium.

  Also, instead of the sequential multi-layer coating method, after applying the undercoat layer-forming paint on one side of the nonmagnetic support, before the undercoat layer-forming paint is dried, on the undercoat layer-forming paint. It is also possible to employ a simultaneous multilayer coating method in which a magnetic layer forming coating is applied and dried.

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

  EXAMPLES Hereinafter, although an Example demonstrates this invention, this invention is not limited to the following Example. In the following description, “parts” means “parts by mass”.

Example 1
[Preparation of magnetic paint]
A 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 subjected to a dispersion treatment with a sand mill for 250 minutes, and then a magnetic coating component (2) shown in Table 2 was added to prepare a dispersion. Next, the obtained dispersion and the magnetic paint component (3) shown in Table 3 were stirred using a dispaper, and this was filtered with a filter to prepare a magnetic paint.

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

[Preparation of paint for back coat layer]
A mixed liquid in which the coating components for the back coat layer shown in Table 7 were mixed was dispersed by a sand mill for 50 minutes to prepare a dispersion. 15 parts of polyisocyanate was added to the obtained dispersion and stirred, and this was filtered through a filter to prepare a coating material for a backcoat layer.

[Production of magnetic tape for evaluation]
On the non-magnetic support (polyethylene naphthalate film, thickness: 5 μm), the primer coating is applied so that the thickness of the primer layer after the calendering process is 1.1 μm and dried at 100 ° C. An undercoat layer was formed. Next, the magnetic coating was applied on the undercoat layer so that the thickness of the magnetic layer after the calendering treatment was 55 nm, and dried at 100 ° C. to form a magnetic layer. Thereafter, a vertical alignment treatment was performed while applying an alignment magnetic field (450 kA / m) using a solenoid magnet.

  Next, the thickness after the calendering is 0.5 μm on the surface of the non-magnetic support on the side opposite to the surface on which the undercoat layer and the magnetic layer are formed. And then dried at 100 ° C. to form a backcoat layer.

  Thereafter, a raw roll having an undercoat layer and a magnetic layer formed on the upper surface side of the non-magnetic support and a back coat layer formed on the lower surface side is heated at a temperature of 100 ° C. with a calender device having seven metal rolls. Calendar treatment was performed at a linear pressure of 300 kg / cm.

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

(Example 2)
In the magnetic paint component (1), the addition amount of ε-Fe ₂ O ₃ magnetic powder (A) was changed to 70 parts, and the addition amount of ε-Fe ₂ O ₃ magnetic powder (B) was changed to 30 parts. Produced a magnetic tape for evaluation in the same manner as in Example 1.

(Example 3)
In the magnetic paint component (1), the addition amount of ε-Fe ₂ O ₃ magnetic powder (A) was changed to 20 parts, and the addition amount of ε-Fe ₂ O ₃ magnetic powder (B) was changed to 80 parts. Produced a magnetic tape for evaluation in the same manner as in Example 1.

Example 4
In the magnetic coating component (1), the addition amount of ε-Fe ₂ O ₃ magnetic powder (A) was changed to 70 parts, the addition amount of ε-Fe ₂ O ₃ magnetic powder (B) was changed to 30 parts, A magnetic tape for evaluation was produced in the same manner as in Example 1 except that the amount of carbon black added was changed to 1 part.

(Example 5)
In the magnetic coating component (1), the addition amount of ε-Fe ₂ O ₃ magnetic powder (A) was changed to 70 parts, the addition amount of ε-Fe ₂ O ₃ magnetic powder (B) was changed to 30 parts, A magnetic tape for evaluation was produced in the same manner as in Example 1 except that the amount of carbon black added was changed to 1 part. Thereafter, a lubricant layer was top-coated on the surface of the magnetic tape by a method in which a strip-like impregnated coated fabric impregnated with a silicone-based lubricant was slidably contacted with the surface of the magnetic tape on the magnetic layer of the magnetic tape for evaluation. Examples of the silicone lubricant include 62.5% by mass of isopropyl alcohol, 17.5% by mass of a polyol ester, and 20.0% by mass of silicone oil “KF-69” (trade name) manufactured by Shin-Etsu Chemical Co., Ltd. , Shin-Etsu Chemical Co., Ltd. UV curable silicone resin “X-12-2441F” (trade name) 5.0 mass% was used. Moreover, as said polyol ester, butyl stearate (SB) 4.0 mass%, "UNISTAR H-208BRS" (brand name) 7.5 mass% made from NOF Corporation, and "UNISTAR H-445R" ( Product name) A mixture of 6.0% by mass was used.

(Example 6)
In the magnetic coating component (1), the addition amount of ε-Fe ₂ O ₃ magnetic powder (A) was changed to 70 parts, the addition amount of ε-Fe ₂ O ₃ magnetic powder (B) was changed to 30 parts, A magnetic tape for evaluation was produced in the same manner as in Example 1 except that the amount of carbon black added was changed to 1 part. Thereafter, the lubricant layer was top-coated on the surface of the magnetic tape by a method in which a strip-like impregnated coated fabric impregnated with a fluorine-based lubricant was slidably contacted on the surface of the magnetic tape on the magnetic layer of the magnetic tape for evaluation. As the fluorine-based lubricant, 62.5% by mass of isopropyl alcohol, 17.5% by mass of a polyol ester, and 20.0% by mass of a fluorine-based lubricant “Novec7100” (trade name) manufactured by 3M Company were mixed. A thing was used. Moreover, as said polyol ester, butyl stearate (SB) 4.0 mass%, "UNISTAR H-208BRS" (brand name) 7.5 mass% made from NOF Corporation, and "UNISTAR H-445R" ( Product name) A mixture of 6.0% by mass was used.

(Example 7)
In the magnetic coating component (1), the addition amount of ε-Fe ₂ O ₃ magnetic powder (A) was changed to 70 parts, the addition amount of ε-Fe ₂ O ₃ magnetic powder (B) was changed to 30 parts, A magnetic tape for evaluation was produced in the same manner as in Example 1 except that the amount of carbon black added was changed to 1 part. Thereafter, the lubricant layer was top-coated on the surface of the magnetic tape by a method in which a belt-like impregnated coated cloth impregnated with an ester lubricant on the magnetic layer of the evaluation magnetic tape was brought into sliding contact with the surface of the magnetic tape. As the ester lubricant, a mixture of n-hexane 65% by mass and polyol ester 35% by mass was used. Moreover, as said polyol ester, butyl stearate (SB) 8.0 mass%, "UNISTAR H-208BRS" (brand name) 15 mass% made from NOF Corporation, and "UNISTAR H-445R" (brand name) ) A mixture of 12% by mass was used.

(Example 8)
In the magnetic coating component (1), the addition amount of ε-Fe ₂ O ₃ magnetic powder (A) is changed to 20 parts, the addition amount of ε-Fe ₂ O ₃ magnetic powder (B) is changed to 80 parts, A magnetic tape for evaluation 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 9
In the magnetic coating component (1), the addition amount of ε-Fe ₂ O ₃ magnetic powder (A) is changed to 20 parts, the addition amount of ε-Fe ₂ O ₃ magnetic powder (B) is changed to 80 parts, A magnetic tape for evaluation was produced in the same manner as in Example 1 except that the thickness of the magnetic layer was changed to 30 nm.

(Example 10)
In the magnetic coating component (1), the addition amount of ε-Fe ₂ O ₃ magnetic powder (A) is changed to 20 parts, the addition amount of ε-Fe ₂ O ₃ magnetic powder (B) is changed to 80 parts, A magnetic tape for evaluation was produced in the same manner as in Example 1 except that the thickness of the magnetic layer was changed to 200 nm.

(Example 11)
In the magnetic coating component (1), the addition amount of ε-Fe ₂ O ₃ magnetic powder (A) is changed to 20 parts, the addition amount of ε-Fe ₂ O ₃ magnetic powder (B) is changed to 80 parts, A magnetic tape for evaluation was produced in the same manner as in Example 1 except that the amount of carbon black added was changed to 3 parts.

Example 12
In the magnetic coating component (1), the addition amount of ε-Fe ₂ O ₃ magnetic powder (A) is changed to 20 parts, the addition amount of ε-Fe ₂ O ₃ magnetic powder (B) is changed to 80 parts, A magnetic tape for evaluation was produced in the same manner as in Example 1 except that the amount of carbon black added was changed to 0.7 part.

(Example 13)
In the magnetic coating component (1), the addition amount of ε-Fe ₂ O ₃ magnetic powder (A) is changed to 20 parts, the addition amount of ε-Fe ₂ O ₃ magnetic powder (B) is changed to 80 parts, A magnetic tape for evaluation was produced in the same manner as in Example 1 except that the thickness of the magnetic layer was changed to 25 nm.

(Comparative Example 1)
In the magnetic coating component (1), the addition amount of ε-Fe ₂ O ₃ magnetic powder (A) was changed to 100 parts, the addition amount of ε-Fe ₂ O ₃ magnetic powder (B) was changed to 0 parts, A magnetic tape for evaluation was produced in the same manner as in Example 1 except that the thickness of the magnetic layer was changed to 60 nm.

(Comparative Example 2)
In the magnetic coating component (1), the addition amount of ε-Fe ₂ O ₃ magnetic powder (A) was changed to 0 part, the addition amount of ε-Fe ₂ O ₃ magnetic powder (B) was changed to 100 parts, A magnetic tape for evaluation was produced in the same manner as in Example 1 except that the thickness of the magnetic layer was changed to 60 nm.

(Comparative Example 3)
In the magnetic coating component (1), the addition amount of ε-Fe ₂ O ₃ magnetic powder (A) was changed to 70 parts, the addition amount of ε-Fe ₂ O ₃ magnetic powder (B) was changed to 30 parts, A magnetic tape for evaluation was produced in the same manner as in Example 1 except that the amount of carbon black added was changed to 1 part and no orientation magnetic field was applied to the magnetic layer.

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

<Magnetic properties>
Using a vibrating sample magnetometer “VSM-P7 type” (product name) manufactured by Toei Kogyo Co., Ltd., a hysteresis curve of the magnetic tape for evaluation was obtained. Specifically, the evaluation magnetic tape is cut into a circular shape with a diameter of 8 mm to obtain a cut sample, and the cut samples are stacked by aligning the thickness direction of the magnetic tape with the application direction of the external magnetic field and stacking 20 samples. did. As a data plot mode 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 wait time was set to 0.3 sec.

  Also, in this hysteresis curve in the thickness direction, a positive magnetic field is applied to the magnetic layer to reach a positive saturation magnetization, and then a reverse magnetic field is applied to the positive direction to reverse saturation magnetization. When the point that has reached the point A and the point where the magnetic field in the positive direction is further applied from the point A and reaches the saturation magnetization in the positive direction is the point B, 2759 points for the hysteresis curve from the point A to the point B For each measurement point of measurement data divided and output, a total of 35 points including one measurement point from the 18th point to the 2742th point and 17 points around the measurement point The linear least square approximation was performed, and the slope of the obtained approximate expression was used as the differential value at the measurement point. A differential curve was obtained from the differential values at the respective measurement points from the 18th point to the 2742th point obtained by this method.

  From the obtained differential curve, the maximum peak maximum value P1 in a magnetic field range of +500 Oersted [Oe] or more and the maximum peak maximum in a magnetic field range of −500 Oersted [Oe] or more and less than +500 Oersted [Oe]. The value P2 was obtained and the value of P2 / P1 was calculated. Further, the squareness ratio in the thickness direction was determined from the hysteresis curve in the thickness direction.

  Here, FIG. 4 shows the hysteresis curve in the thickness direction obtained in Example 1, and FIG. 5 shows a part of the differential curve of the hysteresis curve in the thickness direction obtained in Example 1.

<Space of magnetic layer>
Spacing Sp after the surface of the magnetic layer was washed with n-hexane was measured using TSA (Tape Spacing Analyzer) manufactured by Micro Physics.

Specifically, the pressure for pressing the magnetic layer against the glass plate with a hemisphere made of urethane was 0.5 atm (5.05 × 10 ⁴ N / m). In this state, white light is irradiated from the stroboscope to a certain area (240000 to 280000 μm ² ) on the magnetic layer side surface of the magnetic tape for evaluation through the glass plate, and the reflected light from the light is reflected to the IF filter (633 nm) and the IF filter. By receiving the light through the CCD through (546 nm), an interference fringe image generated by the unevenness in this region was obtained.

  Next, this image is divided into 66000 points, and the distance from the glass plate of each point to the magnetic layer surface is obtained, and this is used as a histogram (frequency distribution curve), and further as 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 spacing Sp.

  The optical constants (phase, reflectance) of the magnetic layer surface used for the above spacing calculation were measured using a reflection spectral film thickness meter “FE-3000” manufactured by Otsuka Electronics Co., Ltd., and a value near a wavelength of 546 nm. Was used.

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

<Surface characteristics of magnetic layer>
The magnetic layer side of the evaluation magnetic tape is wrapped at 90 ° on a 6 mm diameter stainless steel round bar, and a load of 63.36 g is applied to the tip of the evaluation magnetic tape at a speed of 1200 mm / min for 70 mm sliding. The load during sliding in the 50th pass was detected by a load cell and used as a measured load, and the friction coefficient M was calculated by the following equation.
Friction coefficient M = In [measurement load (g) /63.36 (g)] / 0.5π

  As a result, when the friction coefficient M is less than 0.35, the surface property of the magnetic layer is judged as “good”, and when the friction coefficient M is 0.35 or more and 0.45 or less, the surface property of the magnetic layer is “good”. When the coefficient of friction M exceeds 0.45, the surface characteristics of the magnetic layer were determined to be “poor”.

<Output characteristics>
Using a loop tester (dynamic TSA device) manufactured by Micro Physics, 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 tape at a recording speed of 1.5 m / sec. A 200 nm signal is recorded on a magnetic tape for evaluation, and the reproduced signal is amplified by a commercially available MR head read amplifier, and then the fundamental component of the signal is output using a spectrum analyzer “N9020A” manufactured by Keysight Technologies, Inc. S) and integrated noise (N) up to twice that frequency were measured. Then, the output characteristic was evaluated with the relative value using the S / N value of the IBM LTO6 tape as a reference (0 dB).

<Durability>
The short running durability of the magnetic tape for evaluation was evaluated using an LTO6 drive manufactured by HP. Specifically, in an environment where the temperature is 23 ° C. and the relative humidity is 50%, the tape position is 20 to 50 m, and the speed is 6.0 m / sec. The S / N value before running of the tape (SNR1) and the S / N value after running (SNR2) were measured, and the change amount ΔSNR (SNR1−SNR2) was calculated.

  As a result, when the ΔSNR is less than 1 dB, the durability is judged as “good”, when the ΔSNR is between 1 dB and less than 3 dB, the durability is judged as “good”, and when the ΔSNR is more than 3 dB, the durability is judged. The sex was judged as “bad”.

  The above evaluation results are shown in Table 8. Table 8 also shows a value calculated by weighting the average particle diameter of the whole magnetic powder used by weighting the average particle diameter of the magnetic powders (A) and (B) with the blending ratio.

  From Table 8, it can be seen that Examples 1 to 13 are excellent in all of the surface characteristics, output characteristics and durability. However, in Example 11 in which Sp exceeded 15 nm, the output characteristics and durability were slightly lowered, and in Example 12 in which Sp was less than 5 nm, the surface characteristics and durability were slightly lowered, and the ester lubricant was the top. In the coated Example 7, the surface characteristics are slightly lowered and the thickness of the magnetic layer is less than 30 nm, as compared with Example 5 in which the silicone-based lubricant is top-coated and Example 6 in which the fluorine-based lubricant is top-coated. In Example 13, the output characteristics slightly decreased.

  On the other hand, Comparative Example 1 in which P2 / P1 is less than 0.25 has poor surface characteristics and durability, and Comparative Example 2 in which P2 / P1 is more than 0.60 has inferior output characteristics and a squareness ratio of 0.2. In Comparative Example 3 below 65, the surface characteristics, output characteristics and durability were inferior.

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

10 Magnetic recording media (magnetic tape)
11 Nonmagnetic support 12 Undercoat layer 13 Magnetic layer 14 Backcoat layer

Claims (5)

A magnetic recording medium comprising a nonmagnetic support, a magnetic layer containing magnetic powder, and an undercoat layer disposed between the magnetic layer and the nonmagnetic support,
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,
At least one of the magnetic layer and the undercoat layer contains a lubricant,
In the hysteresis curve in the thickness direction of the magnetic layer, a positive magnetic field is applied to the magnetic layer to reach a positive saturation magnetization, and then a reverse magnetic field is applied to the positive direction. When the point that reached the saturation magnetization of A is point A, and the point that reaches the saturation magnetization in the positive direction by applying a positive magnetic field further from point A is B point, the hysteresis curve from point A to point B is There are two or more peaks in the derivative curve obtained by differentiation,
Among the peaks, the maximum peak maximum value in a magnetic field range of +500 Oersted [Oe] or more is P1, and the maximum peak maximum value in a magnetic field range of −500 Oersted [Oe] or more and less than +500 Oersted [Oe] is set. A magnetic recording medium characterized in that the following relationship is established when P2 is P2.
0.25 ≦ P2 / P1 ≦ 0.60   The magnetic recording medium according to claim 1, wherein a squareness ratio in a thickness direction of the magnetic layer is 0.65 or more.   2. The spacing value 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). Or the magnetic recording medium of 2.   The magnetic recording medium according to claim 1, 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 claim 1, wherein the magnetic layer has a thickness of 30 nm to 200 nm.

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