JP2018206463A - Magnetic recording medium for high recording density, and recording and reproducing mechanism thereof - Google Patents

Abstract

To provide a magnetic recording medium excellent in electromagnetic conversion characteristics.SOLUTION: A magnetic recording medium 10 includes a non-magnetic support 11, an undercoat layer 12, a magnetic layer 13 containing magnetic particles, and a back coat layer 14, in which an average particle diameter of the magnetic particles is 17 nm or less and linearly polarized light is irradiated on a surface of the magnetic layer 13 from the longitudinal direction of the magnetic layer 13 at an irradiation angle of 70° to obtain a refractive index nL and an extinction coefficient kL of the magnetic layer 13 and to obtain a vertical reflectance RL at the time of perpendicular incidence of the linearly polarized light in the longitudinal direction from nL and kL, and the linearly polarized light is irradiated on the surface of the magnetic layer 13 from the width direction of the magnetic layer 13 at an irradiation angle of 70° to obtain a refractive index nT and an extinction coefficient kT of the magnetic layer 13 and to obtain a vertical reflectance RT at the time of vertical incidence of the linearly polarized light in the width direction from nT and kT, and, when a variation rate A (%) between RL and RT is defined as A=|RL/RT-1|×100, a relationship of A≤10% is established.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a magnetic recording medium for high recording density excellent in electromagnetic conversion characteristics and a recording / reproducing mechanism for the magnetic recording medium for high recording density.

  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).

  Further, as prior art documents related to the present invention, there are Patent Document 5 related to a spacing measuring method, Patent Document 6 related to a layer thickness measuring method, and Patent Document 7 related to an optical property evaluation method for a magnetic layer surface. .

Japanese Patent Laying-Open No. 2015-91747 JP 2014-224027 A JP 2014-149886 A Japanese Patent Laying-Open No. 2015-82329 JP 2012-43495 A JP 2008-128672 A JP 2009-53103 A

  As the recording density for increasing the capacity of such magnetic recording media increases, the track density of the magnetic layer is increasing. However, there is a problem that the track width is reduced with the increase in the track density, and as a result, the output characteristics are lowered and the electromagnetic conversion characteristics are also lowered.

  In addition, multi-channel heads with multiple head elements are used in computer backup tapes, but as the capacity increases over generations, the number of channels increases to improve data access speed. In LTO7, the latest generation of the LTO format, there are 32 channels. Under such circumstances, as the track becomes narrower, the variation in output characteristics between the channels of the magnetic head increases. Therefore, in order to obtain a reliable high-capacity computer backup tape, this head- It is necessary to reduce the deviation of output characteristics between channels.

  The present invention has been made to solve such a problem, and provides a magnetic recording medium for high recording density which has excellent electromagnetic conversion characteristics even when the track density is increased with the increase in recording density.

  The magnetic recording medium for high recording density of the present invention is a magnetic recording medium for high recording density comprising a nonmagnetic support and a magnetic layer containing magnetic particles, and the average particle diameter of the magnetic particles is 17 nm. The surface of the magnetic layer is irradiated with linearly polarized light from the longitudinal direction of the magnetic layer at an irradiation angle of 70 ° to obtain the refractive index nL and the extinction coefficient kL of the magnetic layer. The vertical reflectivity RL at the time of vertical incidence of the linearly polarized light in the longitudinal direction is obtained, and the surface of the magnetic layer is irradiated with linearly polarized light from the width direction of the magnetic layer at an irradiation angle of 70 °. nT and the extinction coefficient kT are obtained, the vertical reflectance RT at the time of perpendicular incidence of the linearly polarized light in the width direction is obtained from nT and kT, and the variation rate A (%) between RL and RT is calculated as A = | RL / RT When −1 | × 100, the relationship of A ≦ 10% is established.

  The recording / reproducing mechanism of the high recording density magnetic recording medium of the present invention includes the above-described high recording density magnetic recording medium of the present invention and a TMR head.

  According to the present invention, it is possible to provide a magnetic recording medium for high recording density that is excellent in electromagnetic conversion characteristics even when the track width is reduced due to the increase in track density accompanying the increase in recording density for increasing the capacity.

FIG. 1 is a schematic cross-sectional view showing an example of a magnetic recording medium. FIG. 2 is a schematic perspective view showing reflection of linearly polarized light incident in the longitudinal direction of the magnetic layer. FIG. 3 is a schematic perspective view showing reflection of linearly polarized light incident in the width direction of the magnetic layer.

(Magnetic recording medium for high recording density)
An embodiment of the magnetic recording medium for high recording density of the present invention will be described.

  The magnetic recording medium for high recording density of the present embodiment (hereinafter also simply referred to as a magnetic recording medium) includes a nonmagnetic support and a magnetic layer containing magnetic particles, and the magnetic particles have an average particle diameter. 17 nm or less. Further, the surface of the magnetic layer is irradiated with linearly polarized light from the longitudinal direction of the magnetic layer at an irradiation angle of 70 ° to obtain the refractive index nL and extinction coefficient kL of the magnetic layer, and the longitudinal direction is obtained from nL and kL. The vertical reflectance RL at the time of perpendicular incidence of the linearly polarized light in the direction is obtained, and further, the surface of the magnetic layer is irradiated with linearly polarized light from the width direction of the magnetic layer at an irradiation angle of 70 °, and the refractive index nT of the magnetic layer is obtained. And the extinction coefficient kT, the vertical reflectance RT at the time of vertical incidence of the linearly polarized light in the width direction is obtained from nT and kT, and the variation rate A (%) between RL and RT is calculated as A = | RL / RT− In the case of 1 | × 100, the relationship of A ≦ 10% is established.

  In the magnetic recording medium of the present embodiment, by setting the average particle diameter of the magnetic particles to 17 nm or less and the variation rate A to 10% or less, a sufficient number of fine particles can be obtained in the longitudinal direction and the width direction of the magnetic layer. It is possible to secure the number of magnetic particles and to obtain a uniform distribution state of the magnetic particles. As a result, even if the track width is reduced due to the increase in the track density accompanying the increase in the recording density for increasing the capacity, it is possible to obtain good electromagnetic conversion characteristics (SN characteristics).

  In particular, in the magnetic recording medium of this embodiment, a sufficient number of fine magnetic particles and a uniform distribution state of each magnetic particle can be obtained in the width direction of the magnetic layer, so that a narrow track width of 1 μm or less is obtained. Even when a multi-channel head is used for the magnetic layer, the deviation of the output characteristics between the head and the channel can be reduced.

  That is, until now, in storage drives in magnetic recording media (for example, magnetic tapes), the track width was 10 times or more longer than the recording bit length (1/2 recording wavelength). Therefore, it is necessary to reduce the track width. In this case, the track width is 1 μm or less, and the track width is less than 10 times the recording bit length. When the track width is reduced in this way, a minute disturbance of the magnetic field becomes noise. Therefore, in order to maintain good SN characteristics, a sufficient number of fine magnetic particles are added in the longitudinal direction and the width direction of the magnetic layer. It is necessary to distribute it uniformly. In the present embodiment, a sufficient number of fine magnetic particles in the longitudinal direction and the width direction of the magnetic layer are obtained by setting the average particle diameter of the magnetic particles to 17 nm or less and the variation rate A to 10% or less. Can be uniformly distributed.

  The magnetic recording medium of the present embodiment has a tunneling magnetoresistive effect when the magnetization length of the signal recorded in the magnetic layer is 1 μm or less in the width direction of the magnetic layer. It is preferably reproduced by a mold head (TMR head). Even when the magnetization length is set to 1 μm or less in order to increase the track density of the magnetic layer, a high S / N ratio can be obtained by reproduction with a highly sensitive TMR head.

The length of the magnetization can be measured as follows, for example. That is, as a magnetic force microscope, “Nano Scope III” (product name) manufactured by Digital Instruments is used, and the length of magnetization in the width direction of the magnetic layer on which the signal is recorded is measured by a frequency detection method. A probe having a cobalt alloy coat (tip radius of curvature: 25 to 40 nm, coercive force: about 400 [Oe], magnetic moment: about 1 × 10 ⁻¹³ emu) is used as the measurement probe, and the scanning range is 5 μm square, scanning The speed is 5 μm / sec.

  Further, when the residual magnetic flux density in the thickness direction of the magnetic layer is Mr and the average thickness of the magnetic layer is t, it is preferable that 0.0013 μT · m <Mr · t <0.0032 μT · m. The squareness ratio in the thickness direction of the magnetic layer is preferably 0.65 or more. Thereby, since the resolution of the recording magnetization is improved, even better electromagnetic conversion characteristics (SN characteristics) can be obtained even if the track width is 1 μm or less. Further, 0.0020 μT · m <Mr · t <0.0030 μT · m is more preferable, and the squareness ratio is more preferably 0.75 or more.

  Further, the magnetic particles are preferably made of ε iron oxide. By using ε iron oxide particles as the magnetic particles, the track width is set to 1 μm or less, and therefore the coercive force of the magnetic particles does not decrease even if the average particle size of the magnetic particles is set to 17 nm or less.

  The ε iron oxide is usually composed of spherical particles, but is not limited to spherical particles, and may be substantially spherical or elliptical.

  The average particle diameter of the magnetic particles made of ε iron oxide is preferably 15 nm or less in order to cope with short wavelength recording. Furthermore, the average particle diameter of the magnetic particles made of the ε iron oxide is more preferably 12 nm or less. The lower limit of the average particle diameter of the magnetic particles made of the ε iron oxide is usually about 8 nm. This is because ε iron oxide having an average particle size of less than 8 nm is not easy to produce.

  The coercive force in the thickness direction of the magnetic layer is preferably 3000 oersted [Oe] or more. This is because by setting the coercive force to 3000 oersted [Oe] or higher, a high reproduction output can be obtained with little self-demagnetization loss even in a short wavelength recording region at high recording density.

  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 12 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, when the spacing value exceeds 12 nm, the distance between the magnetic head and the magnetic layer surface becomes too large, and the recording / reproducing characteristics tend to be deteriorated. The spacing value is more preferably 7 nm to 12 nm, and most preferably 8 nm to 11 nm.

  The method for measuring the spacing value and the method for controlling the spacing value are not particularly limited. For example, the spacing value can be measured by the method described in JP 2012-43495 A (Patent Document 5).

  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. When ε iron oxide particles are used as the magnetic particles of the present embodiment, the saturation magnetization of the ε iron oxide particles is 1/2 to 1/3 compared to the saturation magnetization of the conventional ferromagnetic hexagonal ferrite particles. Therefore, when recording a servo signal having a long recording wavelength, 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 TMR head.

  The method for measuring the average thickness of the magnetic layer is not particularly limited. For example, it can be performed by the method described in JP-A-2008-128672 (Patent Document 6).

  Hereinafter, the magnetic recording medium of 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, a magnetic recording medium 10 of this embodiment 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 is a magnetic tape having 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 magnetic particles and a binder. As the magnetic particles, ε iron oxide particles are preferable.

The ε iron oxide particles are 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.

  In the present embodiment, ε iron oxide and other γ iron oxides and α iron oxides can be identified by analyzing their crystal structures by X-ray diffraction.

The coercive force of the ε iron oxide particles is preferably 3000 oersted [Oe] or more. Thereby, the coercive force in the thickness direction of the magnetic layer can be 3000 oersted [Oe] or more. Moreover, since the coercive force of the ε iron oxide particles is reduced when the ε iron oxide particles represented by the general composition formula ε-Fe ₂ O ₃ contain impurities, it is preferable that no impurities are included. However, the ε iron oxide particles are obtained by substituting a part of Fe sites in the crystal with a trivalent metal element such as aluminum (Al), gallium (Ga), rhodium (Rh), indium (In) or the like. The coercive force can be controlled. For this reason, the ε iron oxide particles may contain a metal element other than iron as an impurity as long as the coercive force can maintain 3000 oersted [Oe] or more.

  The upper limit of the coercive force of the ε iron oxide particles is not particularly limited, but is preferably about 4000 Oersted [Oe] when recording a servo signal having a long recording wavelength.

  As described above, the average particle diameter of the ε iron oxide particles contained in the magnetic layer is set to 17 nm or less. When the average particle diameter of the ε iron oxide particles exceeds 17 nm, noise in the magnetic recording medium increases particularly in short wavelength recording, and thus there is a tendency that high electromagnetic conversion characteristics cannot be obtained. The lower limit of the average particle diameter of the ε iron oxide particles is preferably smaller from the viewpoint of short wavelength recording, but if the average particle size is too small, it is difficult to disperse the ε iron oxide particles to the primary particles. Therefore, the lower limit of the average particle diameter is preferably about 5 nm.

  In the present embodiment, the average particle diameter of the magnetic particles 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, acceleration voltage: 2 kV, and magnification: 10000 times (10k times), Observation condition: From the photograph taken with U-LA100, using 100 magnetic particles in one field of view, it is determined as follows.

  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.

  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 magnetic particles.

  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 the magnetic layer 13 includes the above-described magnetic particles 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, titanium oxide, Examples thereof include 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 particles.

  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, 0.2 to 3 parts by mass of a fatty acid ester and 0.5 to 5 parts by mass of a fatty acid amide are used with respect to 100 parts by mass of the solid content of magnetic particles and abrasives in the magnetic layer 13. Is preferred. 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 particles.

  The surface roughness of the magnetic layer 13 is preferably less than 2.0 nm as the centerline average roughness Ra defined by Japanese Industrial Standard (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 surface state of the magnetic layer 13 will be described. The present inventors have found that by evaluating the optical characteristics of the surface of the magnetic layer, it is possible to evaluate the orientation and degree of dispersion of the acicular magnetic particles in the magnetic layer, and JP 2009-53103 A ( It proposed to patent document 7) as an evaluation method of a coating-film layer. Furthermore, as a result of intensive investigation by evaluating the optical characteristics of the surface of the magnetic layer using not only acicular but also spherical and plate-like fine magnetic particles, the longitudinal direction of the magnetic layer is formed on the surface of the magnetic layer. Then, the linearly polarized light is irradiated at an irradiation angle of 70 °, the refractive index nL and the extinction coefficient kL of the magnetic layer are obtained, and the vertical reflectance RL at the time of perpendicular incidence of the linearly polarized light in the longitudinal direction is obtained from nL and kL. Further, the surface of the magnetic layer is irradiated with linearly polarized light from the width direction of the magnetic layer at an irradiation angle of 70 ° to obtain the refractive index nT and extinction coefficient kT of the magnetic layer, and the width of the magnetic layer is determined from nT and kT. When the vertical reflectance RT at the time of normal incidence of linearly polarized light in the direction is obtained and the variation rate A (%) between RL and RT is A = | RL / RT-1 | × 100, the magnetic particles in the magnetic layer Dispersion due to shape variation such as sphericity, orientation, and aggregation of magnetic particles There was found to affect the above variation rate A. As a result of further investigation based on this, it was found that a uniform distribution state of the magnetic particles can be obtained by setting the variation rate A to 10% or less.

  In the variation rate A (%), RL / RT indicates a ratio of the vertical linear reflectance RL of the longitudinal linearly polarized light to the vertical reflectance RT of the linearly polarized light in the width direction. The closer this ratio is to 1, the more the vertical reflectance of linearly polarized light in the width direction and the vertical reflectance of linearly polarized light in the longitudinal direction are equal. This means that variations in shape such as the sphericity of magnetic particles in the magnetic layer, orientation, dispersibility due to aggregation of magnetic particles, etc. are small in the longitudinal direction and width direction of the magnetic layer. To do. Therefore, it is preferable that the value of A = | RL / RT-1 | × 100 is closer to 0%, which means that the fluctuation of the surface state in the longitudinal direction and the width direction of the magnetic layer is smaller. However, technically, it is difficult to set the variation rate A to 0%, and the lower limit value of the variation rate A is about 0.5%.

In order to calculate the vertical reflectivity R at normal incidence from the refractive indexes nL and nT and extinction coefficients kL and kT of the magnetic layer, assuming that the refractive index is n and the extinction coefficient is k, Can be sought.
R = [(n−1) ² + k ² ] / [(n + 1) ² + k ² ]

  FIG. 2 is a schematic perspective view showing the reflection of linearly polarized light incident in the longitudinal direction of the magnetic layer, and FIG. 3 is a schematic perspective view showing the reflection of linearly polarized light incident in the width direction of the magnetic layer. FIG. 2 shows reflected light 32 when the surface of the magnetic layer 20 is irradiated with linearly polarized light 31 in the longitudinal direction 30 of the magnetic layer 20 at an irradiation angle θ: 70 °. FIG. 3 shows reflected light 42 when the surface of the magnetic layer 20 is irradiated with linearly polarized light 41 in the width direction 40 of the magnetic layer 20 at an irradiation angle θ: 70 °. 2 and 3, by setting the variation rate A to 10% or less, a uniform distribution state of magnetic particles can be obtained. More specifically, by setting the average particle diameter of the magnetic particles to 17 nm or less and the variation rate A to 10% or less, a sufficient number of fine magnetic materials in the longitudinal direction and the width direction of the magnetic layer. The number of particles can be secured and a uniform distribution state of each magnetic particle can be obtained. As a result, even if the track width is reduced due to the increase in the track density accompanying the increase in the recording density for increasing the capacity, it is possible to obtain good electromagnetic conversion characteristics (SN characteristics).

  A specific method for setting the variation rate A to 10% or less in the magnetic layer 13 will be described in detail in the description of the manufacturing method of the magnetic recording medium of the present embodiment to be described later.

<Lubricant layer>
Although not shown in FIG. 1, in order to reduce the friction coefficient of the magnetic layer 13 and further improve the durability of the magnetic layer 13, the magnetic layer 13 contains a fluorine-based lubricant or a silicone-based lubricant. It is preferable to provide a lubricant layer. 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 method using TSA described in JP 2012-43495 A (Patent Document 5), before and after washing the lubricant layer with an organic solvent, and the transparent medium. The thickness of the lubricant layer can be measured from the difference in spacing.

  The lubricant layer can be formed by top-coating the lubricant on the magnetic layer 13. As described above, since the magnetic layer 13 is uniformly filled with fine magnetic particles, the lubricant contained in the magnetic layer 13 is difficult to move to the surface of the magnetic layer 13, but the lubricant is magnetic. The lubricant layer can be reliably formed on the surface of the magnetic layer 13 by the top coat applied to the surface of the 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.

  When ε iron oxide particles are used as the magnetic particles of the magnetic layer 13 described above, the saturation magnetization of the ε iron oxide particles is 1/2 to 1/1 compared to the saturation magnetization of the conventional ferromagnetic hexagonal ferrite particles. When the servo signal having a long recording wavelength is recorded, the undercoat layer 12 contains magnetic particles. Examples of the magnetic particles to be contained in the undercoat layer 12 include acicular metal iron-based magnetic particles, plate-shaped hexagonal ferrite magnetic particles, and granular iron nitride-based magnetic particles.

  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 for example, 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 friction coefficient and improve the running performance 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 this embodiment will be described. The method of manufacturing the magnetic recording medium of the present embodiment 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. Apply the undercoat layer-forming paint on one side of the magnetic support and dry it to form the undercoat layer, then apply the magnetic layer-forming paint on the undercoat layer and dry it. Then, 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.

  In the magnetic layer described above, when the surface of the magnetic layer was irradiated with linearly polarized light at an irradiation angle of 70 °, the vertical reflectance at normal incidence obtained from the refractive index n and extinction coefficient k of the magnetic layer obtained was obtained. The vertical reflectance of the linearly polarized light in the longitudinal direction of the magnetic layer is RL, the vertical reflectance of the linearly polarized light in the width direction of the magnetic layer is RT, and the variation rate A (%) between RL and RT is A. == RL / RT-1 | × 100, the following method can be used to establish the relationship of A ≦ 10%. The following method can be carried out independently, and the following method Can be implemented in combination.

  (1) There is a method of reducing agglomeration of magnetic powder by dispersing a continuous kneader connected in series after a batch kneader and performing kneading. For example, first, the solid content concentration is kneaded in a batch-type kneading apparatus at 70 to 90% by mass, and after completion of the kneading, the solid content concentration is diluted to 50 to 69% by mass and the kneaded product is taken out, and then the solid matter is taken out. A kneaded product obtained by kneading using a continuous kneader with a partial concentration in the range of 25 to 50% by mass is dispersed with a sand mill. In this method, since the solid content concentration when taking out the kneaded product can be kept high after the kneading is completed, the agglomeration of the magnetic powder can be suppressed in the dilution step in which the solvent is added after kneading with a high shearing force. A high magnetic layer is obtained.

  (2) In a method of kneading with a batch kneader and dispersing with a sand mill, there is a method of reducing the aggregation of magnetic powder by performing a pressure pre-dispersion treatment step before kneading. For example, a mixed solution preparation step for preparing a mixed solution containing a magnetic powder, a binder, and an organic solvent and having a solid content concentration of 15% by mass or less, and using the high pressure spray collision type disperser Pressure predispersion treatment step of spraying from a nozzle in a pressurized state, a concentration step of concentrating the obtained predispersion, and the resulting concentrate and binder in a state where the solid content concentration is 80% by mass or more. Preparation having a kneading step for kneading, a diluting step for diluting the obtained kneaded product with a diluting component, and a dispersion treatment step for dispersing a slurry before dispersion having a solid content concentration of 10 to 50% by mass with a dispersion medium Process.

  In the kneading step, it is preferable that the preliminary dispersion is concentrated so that the solid content concentration is 80% by mass or more, and the share during kneading is 70 N · m or more.

  Moreover, as a high-pressure spray collision type disperser used in the pressure pre-dispersion treatment step, a disperser having a chamber for pressurizing the above mixed liquid with a high-pressure flanger pump and discharging the mixed liquid from a small-diameter nozzle, Examples include a disperser having a chamber in which a mixed liquid is sprayed from a plurality of nozzles at a high speed and a high pressure to cause the mixed liquids to face each other. Specific examples include an optimizer, a homogenizer, and a nanomizer. The pressure applied when spraying the mixed solution is preferably 50 MPa or more, and more preferably 100 MPa or more. The number of treatments is preferably performed twice or more in consideration of the viscosity difference before and after dispersion, the particle size distribution of the material to be dispersed, the prevention of a short pass of the mixed solution, and the like. Media-type dispersers used in the dispersion treatment process include a disperser in which a disc (including holes, notches, grooves, etc.), pins, and rings is provided on a stirring shaft, and a rotor rotating disperser (for example, a nanomill) , Pico mill, sand mill, dyno mill, etc.) can be used. The dispersion time varies depending on the components and use of the magnetic paint, but the residence time is preferably 30 to 90 minutes.

  (3) There is a method of reducing the agglomeration of magnetic powder by kneading magnetic powder with a high shear force in a continuous kneader and then kneading for a long time with an appropriate shear force in a batch kneader. For example, a kneading step of kneading a magnetic powder and a binder resin at a first solid content concentration in a continuous kneading device, and applying a high shearing force to the magnetic powder to obtain a magnetic kneaded material as much as possible, A continuous kneading device and a batch kneading device arranged in series with the kneading agent are kneaded at a second solid content concentration equal to or lower than the first solid content concentration, and a shearing force is applied to the surface of the loosened magnetic powder. And a re-kneading step for adsorbing and covering as much as possible in a state where the binder resin as a binder is spread. At this time, it is preferable that the first solid content concentration is in the range of 80 to 90 mass%, the second solid content concentration is in the range of 65 to 90 mass%, and the kneading time is 30 to 240 minutes.

  With this method, magnetic powder can be kneaded with a high shearing force in the continuous kneading apparatus, and the magnetic kneaded material can be kneaded well over time with the batch kneading apparatus. Adsorption of the binder resin to the powder is sufficiently performed, and even in the dispersion step, the dispersibility of the magnetic powder of the magnetic coating material is improved, and a magnetic layer having a high magnetic powder density is obtained.

  (4) Since the dispersion time in the sand mill can be shortened by carrying out the above methods (1) to (3) to increase the degree of dispersion in the kneading process, the occurrence of contamination due to mixing of wear powder of beads, etc. And a magnetic layer having a higher magnetic powder density can be obtained.

  In addition, since a magnetic powder having a predetermined particle size or more can be removed by using a centrifugal separation process after the sand mill dispersion and dilution process, aggregates and undispersed materials are removed, and a uniform magnetic coating is obtained. Is obtained. In the centrifugation step, it is preferable to carry out at an acceleration of 1000 to 20000 G.

  In addition, a magnetic coating with more stable dispersibility can be obtained by including a redispersion step of further dispersing by a collision type disperser after the dispersion step. As the collision type disperser, those that can be used at a high pressure of 50 to 250 MPa are preferable.

  (5) There is a method of obtaining a dense magnetic layer by slowly drying in the coating / drying step. In particular, when a thin magnetic coating film is applied and dried by successive multilayer coating, the magnetic layer is rapidly dried and the magnetic layer is likely to become dense due to drying of the solvent. Therefore, a magnetic layer having a high magnetic powder density can be obtained by increasing the solvent concentration of the air in the drying area, slowing the drying speed, and drying slowly.

  Specifically, in the coating / drying process of the magnetic layer, a preheating process for heating the magnetic coating film until the surface temperature of the magnetic coating film stops rising and reaches a substantially constant temperature, and after the preheating process, the magnetic coating is performed. The constant temperature drying process in which the surface temperature of the coating film is kept substantially constant, and the surface temperature of the magnetic coating film that is performed after the constant rate drying process is higher than the temperature during the constant rate drying process. The rate-decreasing drying process for solidifying the magnetic coating film is performed, and the constant-rate drying period is preferably 0.2 seconds or more.

  (6) As a method other than the above, by applying a solid content concentration S / S at the time of applying a magnetic paint, the amount of solvent that evaporates during drying is reduced, and a denser magnetic layer is formed. be able to.

(Recording / reproducing mechanism of magnetic recording medium for high recording density)
Next, an embodiment of the recording / reproducing mechanism of the magnetic recording medium for high recording density of the present invention will be described.

  The recording / reproducing mechanism for the high recording density magnetic recording medium of the present embodiment includes the high recording density magnetic recording medium of the above-described embodiment and a TMR head. As described above, the magnetic recording medium for high recording density can improve the electromagnetic conversion characteristics. However, when combined with a more sensitive TMR head, it is possible to obtain even better electromagnetic conversion characteristics (SN characteristics).

  In addition, the TMR head has high sensitivity and can obtain a high S / N ratio by combining with fine ε iron oxide. However, since the TMR head has high sensitivity, if the spacing with the magnetic layer is small, the TMR head Thermal asperity noise (cooling noise), which is generated due to the instantaneous cooling of the TMR element due to heat dissipation due to the contact between the head and the protrusions of the magnetic layer, is likely to occur. When the spacing of the surface of the magnetic layer after the surface of the layer is washed with n-hexane is measured by TSA (Tape Spacing Analyzer), the value of the spacing is set to 5 nm or more and 12 nm or less. Noise can be reduced.

  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, on the undercoat layer, a coater tension of 4.5 N / inch was applied using a die coater so that the thickness of the magnetic layer after calendering the magnetic coating was 55 nm, at 100 ° C. A magnetic layer was formed by drying. During the drying process, vertical alignment treatment was performed while applying an orientation magnetic field of 450 kA / m in the vertical direction using an NS counter 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 to a width of 1/2 inch to produce a magnetic tape for evaluation of Example 1.

(Example 2)
A magnetic tape for evaluation of Example 2 was produced in the same manner as in Example 1 except that the magnetic coating material produced in Example 1 was applied so that the thickness of the magnetic layer after calendaring was 135 nm.

Example 3
The magnetic coating material produced in Example 1 was applied so that the thickness of the magnetic layer after the calendar treatment was 85 nm, and dried at 100 ° C. to form a magnetic layer. A magnetic tape for evaluation of Example 3 was produced in the same manner as in Example 1 except that vertical alignment treatment was performed while applying an orientation magnetic field (900 kA / m) during the drying step.

(Example 4)
A magnetic tape for evaluation of Example 4 was produced in the same manner as in Example 1 except that the average particle diameter of the ε-Fe ₂ O ₃ magnetic powder of the magnetic coating component (1) shown in Table 1 was changed to 16 nm. .

(Example 5)
The magnetic tape for evaluation of Example 5 is the same as Example 1 except that the coercive force of the ε-Fe ₂ O ₃ magnetic powder of the magnetic coating component (1) shown in Table 1 is changed to 2800 [Oe]. Produced.

(Example 6)
The magnetic coating material prepared in Example 1 was applied so that the thickness of the magnetic layer after the calendering process was 85 nm, an undercoat layer and a magnetic layer were formed on the upper surface side of the nonmagnetic support, and a back coat was formed on the lower surface side. For evaluation of Example 6 except that the raw roll with the layer formed thereon was calendered at a temperature of 90 ° C. and a linear pressure of 300 kg / cm with a calender device having a seven-stage metal roll. A magnetic tape was prepared.

(Example 7)
A magnetic tape for evaluation of Example 7 was produced in the same manner as in Example 1 except that the magnetic coating material produced in Example 1 was applied so that the thickness of the magnetic layer after calendering was 43 nm.

(Example 8)
A magnetic tape for evaluation of Example 8 was produced in the same manner as in Example 1 except that the magnetic coating material produced in Example 1 was applied so that the thickness of the magnetic layer after calendaring was 145 nm.

Example 9
The magnetic coating material produced in Example 1 was applied so that the thickness of the magnetic layer after the calendar treatment was 85 nm, and dried at 100 ° C. to form a magnetic layer. A magnetic tape for evaluation of Example 9 was produced in the same manner as in Example 1 except that the vertical alignment treatment was performed while applying an alignment magnetic field (100 kA / m) during the drying step.

(Comparative Example 1)
A magnetic tape for evaluation of Comparative Example 1 was produced in the same manner as in Example 1 except that the average particle size of the ε-Fe ₂ O ₃ magnetic powder of the magnetic coating component (1) shown in Table 1 was changed to 18 nm. .

(Comparative Example 2)
The amount of cyclohexanone, methyl ethyl ketone, and toluene, which are solvent components of the magnetic coating component (2), is changed to 100 parts each, the solid content concentration S / S of the magnetic coating is 20%, and the coater tension of the die coater is 2.5N. A magnetic tape for evaluation of Comparative Example 2 was produced in the same manner as in Example 1 except that the magnetic coating was applied as / inch.

(Comparative Example 3)
The same as in Example 1 except that the magnetic paint component (1) ε-Fe ₂ O ₃ magnetic powder shown in Table 1 was changed to an iron nitride magnetic powder having an average particle diameter of 17 nm and a coercive force of 3000 [Oe]. Thus, a magnetic tape for evaluation of Comparative Example 3 was produced.

  Next, the following characteristics were measured using the produced magnetic tape for evaluation.

<Fluctuation rate of reflectance>
Using an automatic ellipsometer “DHA-XAVW / S6” (product name) manufactured by Mizoji Optical Co., Ltd., the surface of the magnetic layer of the magnetic tape for evaluation is irradiated with linearly polarized light having a wavelength of 546 nm from the longitudinal direction and the width direction of the magnetic layer. Irradiated at an angle of 70 °, the refractive index n and extinction coefficient k of the magnetic layer were determined in each of the longitudinal direction and the width direction, and the vertical reflectance R at each normal incidence was calculated by the following formula.
R = [(n−1) ² + k ² ] / [(n + 1) ² + k ² ]

In the vertical reflectivity obtained from the above formula, the vertical reflectivity of linearly polarized light in the longitudinal direction of the magnetic layer is RL, and the vertical reflectivity of linearly polarized light in the width direction of the magnetic layer is RT. The rate A (%) was calculated.
A = | RL / RT-1 | × 100

  In the measurement of the variation rate A, the beam diameter of the irradiated light was about 2.81 mm in the major axis direction and about 0.96 mm in the minor axis direction at an irradiation angle of 70 °. For example, in the case of an LTO6 head, there are 16 channels (CH), and 16 head elements are arranged at an interval of 167 μm and a width of about 2.5 mm. Therefore, when irradiated in the width direction of the magnetic tape, the beam diameter of the light in the width direction is about 2.81 mm, and the reflectance in an area substantially the same as the length of the area where the head elements are arranged is about 2.5 mm. It will be evaluated.

<Mr · t in the thickness direction of the magnetic layer, coercive force and squareness ratio>
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. From the hysteresis curve, Mr · t, coercive force and squareness ratio in the thickness direction of the magnetic layer were determined. 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 plot mode for 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 wait time was set to 0.3 sec.

<Space of magnetic layer>
Spacing 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.

  Further, 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” (product name) manufactured by Otsuka Electronics Co., Ltd. A value around 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.

<Output characteristics>
An induction / GMR composite magnetic head having a write track width of 5 μm and a read track width of 2.3 μm is attached to a linear tape electromagnetic conversion characteristic measuring device manufactured by modifying an LTO drive, and a tape speed of 1.5 m / In seconds, a signal having a recording wavelength of 200 nm (G7 × 1.05 times linear recording density) was recorded on an evaluation magnetic tape for evaluation.

  This apparatus is a traveling system to which two magnetic heads can be attached, and two magnetic heads are attached. The magnetic head is mounted on a precision piezo stage (moving resolution: 10 nm) that can be moved in the track width direction. Signal recording is performed with the upstream magnetic head, and AC erasing is performed with the downstream magnetic head in one run. A signal with a magnetization width (magnetization length in the width direction of the magnetic layer) of 0.8 μm is recorded on the magnetic tape for evaluation by offsetting the magnetic head on the side and the magnetic head on the downstream side by 0.8 μm in the track width direction. did.

  Next, the evaluation magnetic tape was run again to reproduce the signal, and the reproduced signal was amplified with a commercially available MR head read amplifier, and then a spectrum analyzer “N9020A” (product name) manufactured by Keysight Technology was used. Then, the fundamental wave component output (S) of the signal and the integrated noise (N) up to twice that frequency are measured, and the S / N ratio of Example 7 is used as a reference (0 dB), and other S / N ratios are measured. Is shown as a relative value (dB) to the S / N ratio of Example 7.

<Deviation of head-channel output characteristics (S / N ratio)>
In order to evaluate fluctuations in the output characteristics (S / N ratio) in the tape width direction, a head-channel between the output characteristics (S / N ratio) of the output characteristics (S / N ratio) (hereinafter referred to as “S / N ratio”) using a 16-channel (CH) head for LTO6 manufactured by Hewlett-Packard. The deviation between the head and CH) was measured. The deviation between the head and CH is about 835 μm, and the output characteristics (S / N ratio) of four CH heads (CH 1, CH 6, CH 11, CH 16), which are arranged at equal intervals apart from each other in the width direction, It measured using the said output characteristic evaluation method, and made the difference between the maximum value and the minimum value in it the deviation between head-CH.

  The above evaluation results are shown in Table 8.

  From Table 8, it can be seen that in Examples 1 to 9, the output characteristics (SN ratio) are high and the deviation between the head and the CH is small. Therefore, even in a multi-channel head having a narrow track width of 1 μm or less, the output characteristics between the head and CH are high, and the deviation can be reduced. On the other hand, Comparative Example 1 in which the average particle diameter of the magnetic particles exceeds 17 nm, Comparative Example 2 and Comparative Example 3 in which the reflectance variation rate A exceeds 10%, all have low output characteristics (S / N ratio). It is a value. Further, it can be seen from Comparative Examples 2 and 3 that the deviation between the head and the CH is large when the value of the reflectance fluctuation rate A is large.

  The high recording density magnetic recording medium of the present invention can be used as a magnetic recording medium excellent in electromagnetic conversion characteristics.

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

Claims (9)

A high recording density magnetic recording medium comprising a nonmagnetic support and a magnetic layer containing magnetic particles,
The average particle diameter of the magnetic particles is 17 nm or less,
The surface of the magnetic layer is irradiated with linearly polarized light from the longitudinal direction of the magnetic layer at an irradiation angle of 70 ° to obtain the refractive index nL and the extinction coefficient kL of the magnetic layer. The vertical reflectance RL at the time of vertical incidence of linearly polarized light is obtained,
The surface of the magnetic layer is irradiated with linearly polarized light at an irradiation angle of 70 ° from the width direction of the magnetic layer to obtain the refractive index nT and extinction coefficient kT of the magnetic layer. Obtain the vertical reflectivity RT at normal incidence of linearly polarized light,
When the fluctuation rate A (%) between RL and RT is A = | RL / RT-1 | × 100,
A magnetic recording medium for high recording density, wherein the relationship of A ≦ 10% is established.   2. The high recording density according to claim 1, wherein when the magnetization length of the signal recorded in the magnetic layer is 1 μm or less in the width direction of the magnetic layer, reproduction is performed by a TMR head. Magnetic recording media.   The residual magnetic flux density in the thickness direction of the magnetic layer is Mr, and the average thickness of the magnetic layer is t, and 0.0013 μT · m <Mr · t <0.0032 μT · m. Magnetic recording medium for high recording density.   The magnetic recording medium for high recording density according to any one of claims 1 to 3, wherein a squareness ratio in a thickness direction of the magnetic layer is 0.65 or more.   The magnetic recording medium for high recording density according to claim 1, wherein the magnetic particles are made of ε iron oxide.   The magnetic recording medium for high recording density according to claim 1, wherein the magnetic layer has a coercive force in the thickness direction of 3000 oersted [Oe] or more.   2. The spacing value is 5 nm or more and 12 nm or less when the spacing of the surface of the magnetic layer after the surface of the magnetic layer is washed with n-hexane is measured with TSA (Tape Spacing Analyzer). The magnetic recording medium for high recording density of any one of -6.   The magnetic recording medium for high recording density according to claim 1, wherein the magnetic layer has a thickness of 30 nm to 200 nm.   A recording / reproducing mechanism for a high recording density magnetic recording medium, comprising: the high recording density magnetic recording medium according to claim 1; and a TMR head.

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