JP4843080B2 - Magnetic recording medium - Google Patents

Description

  The present invention relates to a coating type magnetic recording medium excellent in high density recording characteristics, and more particularly to a magnetic recording medium capable of reducing medium noise in a high density recording system.

  A coating-type magnetic recording medium having a magnetic layer in which magnetic powder is dispersed in a binder is required to further improve the recording density as the recording / reproducing method shifts from an analog method to a digital method. In particular, this demand is increasing year by year for magnetic recording media used for high-density digital video tapes, computer backup tapes, and the like.

In order to cope with such high density, the magnetic powder has been made finer year by year, and at present, acicular metal magnetic powder having a particle size of about 100 nm is put into practical use. In addition, the coercive force and saturation magnetization have been improved year by year, and a metal iron-based magnetic powder having a coercive force of 199.0 kA / m or more and a saturation magnetization of 120 Am ² / kg or more has been realized by iron-cobalt alloying. (For example, Patent Document 1). Furthermore, in order to improve the output during short wavelength recording, it has been proposed to improve the packing density of the magnetic powder in the magnetic layer by highly dispersing the magnetic powder to a state close to the primary particles in the preparation of the magnetic coating. (For example, Patent Document 2).

  By utilizing such a magnetic powder of fine particles and a dispersion technique, for example, LTO Ultrium (registered trademark) of a computer backup tape currently using the shortest recording wavelength has a coercive force of about 210 kA / m and a residual magnetic flux. A magnetic characteristic in which the product (Br · δ) of the density Br and the magnetic layer thickness δ is about 28 nTm and the squareness ratio is about 0.9 is realized.

Japanese Patent Laid-Open No. 3-49026

JP 2002-352412 A

  By the way, in a data recording system for a computer, as a magnetic head used for reproducing recorded information, a magnetoresistive head (MR head), an anisotropic magnetoresistive magnet, instead of a conventional induction head, is used. Of magnetoresistive heads (hereinafter collectively referred to as MR heads) such as heads (AMR heads), giant magnetoresistive heads (GMR heads), or tunnel magnetoresistive heads (TMR heads) Application has been studied. In a system using such an MR head, it is known that noise caused by the system can be greatly reduced, so that medium noise derived from the magnetic recording medium dominates the S / N ratio of the system. ing. Therefore, in order to improve the S / N ratio of the system in future high-density recording, it is necessary to reduce the medium noise.

  On the other hand, in order to increase the density of magnetic recording media used for computer backup tapes and the like, it is required to increase the track density by narrowing the track at the same time as increasing the linear recording density by shortening the recording wavelength. However, when the linear recording density and the track density are increased, there is a problem that adjacent magnetic recording information interferes with each other and noise tends to increase.

  The present invention intends to provide an embodiment of a magnetic recording medium that can serve as a guideline for reducing the medium noise when further increasing the linear recording density and the track density.

The present inventor has focused on agglomeration of magnetic powder in a magnetic layer in a high-density recording system in which a high linear recording density and a high track density are used. It has been found that a magnetic recording medium in which medium noise is extremely reduced can be obtained if the agglomeration is isotropic in the longitudinal direction and the width direction.
That is, the present invention includes a non-magnetic support, a said magnetic recording medium on a nonmagnetic support you have a and undercoat layer and the magnetic layer,
The magnetic layer contains a magnetic powder having a particle size of 40 nm or less and a binder, has a thickness of 10 to 200 nm, has an autocorrelation length Ma of 10 to 70 nm in the longitudinal direction of the magnetic layer , and is magnetic. The autocorrelation length Mb in the width direction of the layer is 10 to 80 nm, and the ratio (Ma / Mb) of the autocorrelation length Ma in the longitudinal direction of the magnetic layer to the autocorrelation length Mb in the width direction of the magnetic layer is 0. The magnetic recording medium is 80 to 1.20.
However, the autocorrelation length is magnetic with a measurement 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) using a magnetic force microscope. By scanning the layer, a two-dimensional leakage magnetic field image of the magnetic layer is measured (scanning range: 5 μm square, scanning speed: 5 μm / sec), and magnetization intensity data of the obtained two-dimensional leakage magnetic field image is obtained. Based on the obtained magnetization intensity data, the magnetization intensity data when the magnetization intensity data is shifted by a certain distance within the scanning range in the longitudinal direction (X direction) and the width direction (Y direction) are the magnetization intensity data before movement. Then, the sum of the summation results over the entire area where the magnetization intensity data overlaps is obtained as the autocorrelation coefficient at the shifted position. An XY direction moving distance when a 1% before the autocorrelation coefficients.

According to the magnetic recording medium, by using fine magnetic powder having a particle size of 40 nm or less, particulate noise can be reduced , and the thickness of the magnetic layer containing the magnetic powder and the binder can be reduced. Since it is 10-200 nm, thickness loss can be reduced. In addition, since the autocorrelation length in the longitudinal direction of the magnetic layer is 10 to 70 nm, the aggregation of the magnetic powder in the direction can be suppressed, thereby reducing the magnetization transition region in the direction. Furthermore, since the autocorrelation length in the width direction of the magnetic layer is 10 to 80 nm, aggregation of the magnetic powder in that direction is suppressed, thereby increasing the off-track allowance and narrowing the track. Since the ratio of the autocorrelation length in the longitudinal direction to the width direction (Ma / Mb) is isotropic between 0.80 and 1.20, the bit aspect ratio can be reduced.

  The magnetic layer preferably contains an iron nitride magnetic powder as the magnetic powder. Since the iron nitride magnetic powder is a granular magnetic powder, it is possible to form a magnetic layer in which aggregation is further reduced in both the longitudinal direction and the width direction, and the ratio of autocorrelation lengths in both directions is more isotropic.

  According to the present invention, it is possible to provide a magnetic recording medium capable of reducing medium noise suitable for a system using a high linear recording density and a high track density.

It is a leakage magnetic field image of the magnetic tape which concerns on Example 1 of this invention. It is a figure which shows the magnetic correlation of the leakage magnetic field image of FIG. It is a leakage magnetic field image of the conventional magnetic tape for high-density recording. It is a figure which shows the magnetic correlation of the leakage magnetic field image of FIG.

  The coating-type high-density magnetic recording medium uses fine magnetic powder and highly disperses the magnetic powder to suppress medium noise and achieve high-density recording. For example, the above-mentioned LTO Ultrium (registered trademark) The computer backup system has achieved a linear recording density of 314 kbpi (shortest recording wavelength: 0.15 μm) and 5,080 tpi (reading track width: 5 μm). There is a demand for shorter wavelengths and narrower wavelengths. However, when the bit interval is narrowed to increase the linear recording density, the magnetizations face each other around the magnetization reversal in the magnetic layer, so that the demagnetizing field is in the direction of decreasing the magnetization around this magnetization reversal. Produces a large internal magnetic field called. Due to this demagnetizing field, a transition region having a finite width, that is, a magnetization transition region in which the magnetization does not reach a sufficient value is formed in the magnetization switching portion. Therefore, in the short wavelength recording, adjacent magnetization transition regions easily interfere with each other, and the magnetization transition region in the boundary region becomes a noise source. As the track density is increased, magnetic recording information between adjacent tracks interfere with each other, and the magnetization transition region between tracks tends to be a noise source. Furthermore, in the high-density recording as described above, the shape of the recording bit can be considered as a factor that greatly affects noise. That is, even if the magnetization transition regions in the longitudinal direction and the width direction are small, if the degree of aggregation of the magnetic powder in the aggregation is biased, the shape of the recording bit has anisotropy and the bit aspect ratio is lost. . For this reason, the balance between high linear recording density and high track density is lost, and noise is likely to occur.

  FIG. 3 is a leakage magnetic field image obtained by observing the magnetic layer of MAXELL LTO4 (trade name) for LTO Ultrium (registered trademark) with a magnetic force microscope (MFM). The measurement of the leakage magnetic field image is performed under the conditions used when measuring the leakage magnetic field image of a conventional coating type magnetic recording medium with a magnetic force microscope (scanning range: 0.5 to 40 μm square, scanning speed: 0.25). ˜20 μm / s, resolution: 12-1024 points / μm). In the figure, the portion that appears black is the aggregation (magnetic cluster) of the magnetic powder. It is known that magnetic clusters affect medium noise, and it has been studied to reduce the size of magnetic clusters by highly dispersing magnetic powder. However, since the magnetic cluster size is a scalar quantity and is observed by measuring the difference in magnetic force received from the magnetic layer, only the size of the aggregation of the magnetic powder is observed, It is not possible to evaluate to what degree the magnetic powder is present. That is, in order to reduce the medium noise, it is effective to suppress the aggregation of the magnetic powder and reduce the magnetic cluster size, but it has anisotropy that the aggregation state of the magnetic powder differs in the longitudinal direction or the width direction. In the case of agglomeration, it is conceivable that the bit aspect ratio collapses, thereby increasing the medium noise.

  As a method for observing the degree of aggregation of the magnetic material, an autocorrelation coefficient is used in a magnetic thin film such as a hard disk. By measuring this autocorrelation coefficient, it is possible to observe how similar a collection of magnetic bodies at a certain measurement point is with a collection of adjacent magnetic bodies. Therefore, it is considered that the coating type magnetic recording medium also has an isotropic autocorrelation coefficient when the magnetic powder is uniformly dispersed up to the primary particles.

FIG. 4 is a magnetic correlation diagram of the leakage magnetic field image of FIG. The magnetic correlation diagram the above magnetic forces longitudinal magnetization intensity data at any distance of the two-dimensional magnetic image obtained by a microscope (X direction), is moved in the width direction (Y-direction), magnetization intensity before moving integrating the data with magnetization intensity data after the movement, the value obtained by the summing over the entire region that overlaps the integration result is obtained by the autocorrelation coefficients after the movement. Therefore, the autocorrelation coefficient becomes the largest when the magnetization intensity data is not moved (X = 0, Y = 0). The autocorrelation length is a value obtained by measuring each movement distance in the XY directions that is 1% of the autocorrelation coefficient before the movement.

  In the figure, the center ellipse portion indicates a portion having an autocorrelation coefficient of 1%, that is, a portion where magnetic powder in a similar aggregation state exists from the center of the ellipse. In this elliptical portion, the autocorrelation length in the longitudinal direction of the magnetic layer is 75 nm, the autocorrelation length in the width direction is 120 nm, and the ratio is 0.63. As shown in the figure, the correlation spread in the width direction rather than the longitudinal direction. It has sex. For this reason, in such a magnetic recording medium, even if the autocorrelation length in the longitudinal direction or the width direction is small, the autocorrelation coefficient in the longitudinal direction and the width direction is destroyed, and the bit aspect ratio is expected to increase. Is done. Therefore, if the recording density is further increased in the future, it is considered that the medium noise increases.

  Further, from the result of the magnetic correlation diagram, it is recognized that the shape of the magnetic powder and the autocorrelation coefficient are in the following relationship. That is, since a coating type magnetic recording medium such as a magnetic tape uses needle-shaped magnetic powder and is magnetically oriented in the longitudinal direction, the correlation length in the longitudinal direction may be larger than that in the width direction. Though possible (that is, the degree of aggregation is more approximate in the longitudinal direction than in the width direction), as shown in the figure, the autocorrelation length in the width direction is rather larger than the autocorrelation length in the longitudinal direction. . On the other hand, in a magnetic recording medium using hexagonal barium ferrite magnetic powder which is a plate-like magnetic powder, the autocorrelation length in the longitudinal direction may be larger than that in the width direction, contrary to the needle-like magnetic powder. It has been confirmed. Therefore, it is understood that the autocorrelation coefficient indicating the degree of aggregation of the magnetic powder has an opposite relationship to the autocorrelation coefficient expected from the shape of the magnetic powder. The reason for this is not necessarily clear, but it is assumed that re-aggregation of the magnetic powder during the dispersion, coating, orientation, and drying process is likely to occur in a certain direction depending on the type of the magnetic powder.

From the above viewpoint, as a result of examining the autocorrelation length in the longitudinal direction and the width direction of the magnetic layer capable of reducing the medium noise and the relationship thereof, the autocorrelation length in the longitudinal direction is 10 to 70 nm, and the autocorrelation length in the width direction is 10 in ~ 80n m, if the magnetic layer ratio of the autocorrelation length (Ma / Mb) has a range of 0.80 to 1.20, the linear recording density and even when it began to further increase track density, medium It was confirmed that a magnetic recording medium with low noise could be obtained. That is, 10 to the autocorrelation length of the longitudinal 70n m, by the autocorrelation length in the width direction and 10 to 80n m, it is possible to narrow the magnetization transition region in each direction. Then, the ratio of the autocorrelation length in both directions (Ma / Mb) is 0.80 or more and 1.20 or less, preferably 0.80 or more and 1.05 or less, so that the recording bit is formed isotropically. Therefore, high linear recording density and high track density can be achieved in a well-balanced manner. For example, with the above magnetic recording medium, it is possible to obtain a magnetic recording medium having medium noise much lower than that of a conventional magnetic recording medium even in high-density recording with a linear recording density of 330 kbpi or more and a track density of 10 ktpi or more. . It has been studied so far that media noise can be reduced by specifying the autocorrelation length in the longitudinal direction and the width direction and the ratio thereof by using the size and degree of aggregation of the magnetic powder in the magnetic layer as a guideline. There are no examples.

  In this embodiment, in order to obtain a magnetic layer in which the autocorrelation length in the longitudinal direction and the width direction of the magnetic layer is small and the ratio of the autocorrelation length is isotropic, the magnetic powder is not only highly dispersed. It is preferable to suppress re-aggregation of the magnetic powder as much as possible in each process of coating, orientation and drying. According to the study by the present inventors, in order to form a magnetic layer having the autocorrelation lengths in the longitudinal direction and the width direction as described above and having an isotropic relationship, a high shear force is applied. It has been found that a method of dispersing fine magnetic powder, a method of drying a magnetic coating at a low speed during coating, a method of gradually orienting, and the like are effective. In particular, (1) a first composition containing a magnetic powder, a binder, and an organic solvent and having a non-solvent component content of 40% by mass or less is prepared and mixed while applying a shearing force to the first composition. A pre-dispersion treatment method in which a second composition is prepared by stirring and concentrated until the content of non-solvent components in the second composition reaches 80% by mass or more; (2) coated on a non-magnetic support; When the magnetic powder in the magnetic coating is magnetically oriented in a predetermined direction while performing a drying process to remove the solvent from the magnetic coating, a constant-rate drying period during which the surface temperature of the magnetic coating on the nonmagnetic support is substantially constant And (3) in the alignment treatment, at least two methods may be combined among the three methods of continuously providing the high magnetic field first alignment step and the low magnetic field second alignment step. preferable.

Next, a magnetic powder, a binder, a configuration of a magnetic layer, a nonmagnetic support, and a method for manufacturing the magnetic recording medium that can be suitably used for the magnetic recording medium of the present embodiment will be described.
In the present embodiment, fine magnetic powder having a particle size of 40 nm or less, preferably 5 to 30 nm is used as the magnetic powder. By using such fine magnetic powder, particulate noise can be reduced. Specific examples of the magnetic powder include acicular metal iron-based magnetic powders; plate-shaped hexagonal ferrite magnetic powders; and granular iron nitride-based magnetic powders. Among these, metal iron-based magnetic powder or iron nitride-based magnetic powder, which can easily obtain high coercive force and high saturation magnetization, is preferable. In particular, the iron nitride magnetic powder has a magnetocrystalline anisotropy and is a substantially spherical or substantially ellipsoidal magnetic powder, so the cohesive force of the magnetic powder is smaller than that of a needle-like or plate-like magnetic powder. Therefore, it is preferable. Such an iron nitride magnetic powder is described in, for example, Japanese Patent Application Laid-Open No. 2000-277311. The particle diameter means a major axis diameter in the case of acicular magnetic powder, a plate diameter in the case of plate-like magnetic powder, and a radius or major axis diameter in the case of granular magnetic powder. The particle diameter can be determined from the average value of the particle diameters of 100 magnetic powders taken with a transmission electron microscope (TEM) at a magnification of 200,000 times.

As the metallic iron-based magnetic powder, acicular α-Fe magnetic powder and Fe-Co-based magnetic powder are preferable, and among these, Fe-Co-based magnetic powder is more preferable. Fe-Co based magnetic powder (a) A method of calcining goethite powder to magnetite powder, ion exchange of divalent Fe ions and Co ions in an aqueous solution containing cobalt ions, and heat reduction. (B) A method of heating and reducing Co-containing acicular goethite powder obtained from an alkaline aqueous suspension containing an iron salt and a cobalt salt, (c) obtained from an iron salt and a cobalt salt added to an aqueous oxalic acid solution A method of reducing the coprecipitate, (d) a method of heating and reducing iron oxide powder having cobalt deposited on the surface, (e) a method of adding a reducing agent to a solution containing an iron salt and a cobalt salt, ( f) A method of evaporating a metal in an inert gas and colliding the metal with gas molecules to obtain an alloy magnetic powder, (g) a vapor of iron or cobalt chloride in a mixed gas of hydrogen, nitrogen or argon While flowing, reduce to metal By law, etc., it can be produced. Among these, it is preferable to use the methods (a) and (b) in combination with which a high amount of Co can be dissolved and the corrosion resistance is excellent. The amount of Co in the Fe-Co based magnetic powder can achieve the maximum saturation magnetization and coercive force in the vicinity of 30% with respect to (Fe + Co), but if it is too much, it cannot be alloyed with magnetic iron metal, Since the surplus becomes an oxide, the magnetic properties are easily deteriorated. Therefore, the amount of Co is preferably in the range of 0.3 to 0.5 in terms of the Co / Fe mass ratio. In addition, transition metals such as Zn, Sn, Ni, Mn, Ti, Cr, Cu, Nd, La, Sm, and Y, rare earth elements, and the like are added to the Fe—Co based magnetic powder. You can also In addition, it is desirable that the Fe—Co-based magnetic powder is coated with an inorganic oxide for the purpose of preventing sintering during heat reduction and improving the dispersibility in the magnetic paint. Examples of inorganic oxides include aluminum oxide and silicon oxide. Aluminum oxide is particularly preferable because it is excellent in hardness and can improve the wear resistance of the magnetic powder. In order to perform the coating, a method is used in which water is allowed to act on an alcohol solution containing a compound having aluminum, silicon, etc. in advance on the raw iron oxide powder, and these compounds are deposited on the surface of the iron oxide powder by hydrolysis. It is done. The coercive force of the Fe—Co based magnetic powder is preferably 160 to 320 kA / m, and more preferably 200 to 300 kA / m. The saturation magnetization is preferably ^{60~200Am 2 / kg, 80~180Am 2 /} kg is more preferable.

The iron nitride magnetic powder is preferably an iron nitride magnetic powder containing 1 to 20 atomic% of nitrogen with respect to iron. In the iron nitride magnetic powder, a part of iron may be substituted with another transition metal element. Specific examples of such other transition metal elements include Mn, Zn, Ni, Cu, and Co. Among these, Co and Ni are preferable, and Co is particularly preferable because it can improve saturation magnetization most. However, the Co content is preferably 10 atomic percent or less of Co with respect to Fe. If the Co content is too high, nitriding tends to take a long time. Further, the iron nitride magnetic powder may contain a rare earth element. In particular, an iron nitride magnetic powder having a two-layer structure including an inner layer portion mainly containing iron nitride mainly containing an Fe ₁₆ N ₂ phase and an outer layer portion mainly containing rare earth elements has a high coercive force and is high. This is preferable because it exhibits dispersibility and excellent shape maintenance. Specific examples of such rare earth elements include Y, Yb, Ce, Sm, Pr, La, Eu, and Nd. Among these, Y, Sm, and Nd are preferable because the effect of maintaining the particle shape during reduction is great. The rare earth element content is preferably 0.05 to 20 atomic%, more preferably 0.1 to 15 atomic%, and most preferably 0.5 to 10 atomic% with respect to iron. When there are too few rare earth elements, the improvement effect of a dispersibility will decrease and the particle shape maintenance effect at the time of a reduction | restoration will become small. When the amount of rare earth elements is too large, the amount of unreacted rare earth elements increases, which tends to hinder dispersion and coating processes, and excessively lower coercive force and saturation magnetization. Further, the iron nitride magnetic powder may contain B, Si, Al, and P. By containing such an element, a highly dispersible iron nitride magnetic powder can be obtained. Moreover, since these elements are cheaper than rare earth elements, they are advantageous in terms of cost. The content of these elements is preferably 0.1 to 20 atomic% in terms of the total content of B, Si, Al and P with respect to Fe. If these elements are too small, the shape maintaining effect is small. Moreover, when there are too many of these elements, saturation magnetization will fall easily. Note that the iron nitride-based magnetic powder may contain C, Ca, Mg, Zr, Ba, Sr, or the like, if necessary. By using these elements and rare earth elements in combination, higher shape maintenance and dispersion performance can be obtained.

  The method for producing the iron nitride magnetic powder is not particularly limited, but can be produced by the method described in, for example, JP-A-2004-273094. Specifically, iron-based oxides or iron-based hydroxides are used as starting materials. Examples of the iron-based oxide and iron-based hydroxide include hematite, magnetite, and goethite. The particle size of the starting material is not particularly limited, but is preferably 5 to 80 nm, more preferably 5 to 50 nm, and further preferably 5 to 30 nm. If the particle size is too small, interparticle sintering tends to occur during the reduction treatment. If the particle size is too large, the reduction treatment tends to be inhomogeneous, and it becomes difficult to control the particle size and magnetic properties of the obtained iron nitride magnetic powder.

  The above starting materials may be coated with the above rare earth elements. As a method for the deposition treatment, for example, a starting material is dispersed in an alkali or acid aqueous solution, a salt of a rare earth element is dissolved therein, and then a hydroxide containing the rare earth element in the starting material by a neutralization reaction or the like. And hydrates may be precipitated. Moreover, you may deposit elements, such as B, Si, Al, and P, to said starting material. As a method for depositing these elements, for example, a solution in which a compound containing the above elements is dissolved is prepared, a starting material is immersed in this solution, and B, Si, Al, P or the like is used as the starting material. The method of depositing is mentioned. In order to perform these deposition processes efficiently, additives such as a reducing agent, a pH buffering agent, and a particle size controlling agent may be further added to the solution. Furthermore, in the deposition process, a rare earth element and an element such as B, Si, Al, or P may be deposited on the starting material simultaneously or alternately.

  Next, the above starting materials are heated and reduced in a hydrogen stream. The reducing gas is not particularly limited, and a reducing gas such as carbon monoxide gas may be used in addition to hydrogen gas. The reduction temperature is preferably 300 to 600 ° C. When the reduction temperature is lower than 300 ° C., the reduction reaction does not proceed sufficiently. If the reduction temperature is higher than 600 ° C., sintering is likely to occur.

By performing nitriding after the heat reduction as described above, an iron nitride-based magnetic powder containing iron and nitrogen as constituent elements can be obtained. The nitriding treatment is desirably performed using a gas containing ammonia. In addition to ammonia gas alone, a mixed gas using hydrogen gas, helium gas, nitrogen gas, argon gas or the like as a carrier gas may be used. Nitrogen gas is particularly preferred because it is inexpensive. The nitriding temperature is preferably 100 to 300 ° C. If the nitriding temperature is too low, nitriding does not proceed sufficiently and the effect of increasing the coercive force is small. If the nitriding temperature is too high, nitriding is excessively promoted, the proportion of Fe ₄ N phase, Fe ₃ N phase, etc. is increased, the coercive force is rather lowered, and the saturation magnetization is likely to be excessively lowered. In the nitriding treatment, it is desirable to select the nitriding treatment conditions so that the nitrogen content with respect to iron is 1 to 20 atomic%. If the amount of nitrogen is too small, the amount of Fe ₁₆ N ₂ phase generated is reduced, and the effect of improving the coercive force is reduced. On the other hand, if the amount of nitrogen is too large, an Fe ₄ N phase, an Fe ₃ N phase or the like is likely to be formed, the coercive force is rather lowered, and the saturation magnetization is likely to be excessively lowered. The coercive force of the iron nitride magnetic powder is preferably 160 to 320 kA / m, more preferably 200 to 300 kA / m. Saturation magnetization is preferably ^{60~200Am 2 / kg, 80~180Am 2 /} kg is more preferable.

As the hexagonal ferrite magnetic powder, hexagonal barium ferrite magnetic powder is preferable. The hexagonal ferrite magnetic powder includes Al, Si, S, Sc, Ti, V, Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, W in addition to the predetermined elements. , Re, Au, Hg, Pb, Bi, La, Ce, Pr, Nd, P, Co, Mn, Zn, Ni, B, Ge, and Nb may be included. As a method for producing the hexagonal ferrite magnetic powder, a conventionally known production method can be used. For example, barium oxide, iron oxide, a metal oxide replacing iron, and boron oxide as a glass-forming substance are mixed and mixed so as to have a desired ferrite composition, rapidly cooled to an amorphous body, Examples thereof include a glass crystallization method in which barium ferrite crystal powder is obtained by washing and pulverizing after reheating treatment. The coercive force of the hexagonal ferrite magnetic powder is preferably 120 to 320 kA / m, and the saturation magnetization is preferably 40 to 60 Am ² / kg.

Examples of the binder include at least one selected from the group consisting of a vinyl chloride resin, a nitrocellulose resin, an epoxy resin, and a polyurethane resin. Specific examples of the vinyl chloride 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, and the like. Among these, the combined use of a vinyl chloride resin and a polyurethane resin is preferable, and the combined use of a vinyl chloride-hydroxyl group-containing alkyl acrylate copolymer resin and a polyurethane resin is more preferable. In addition, these binders preferably have a functional group in order to improve the dispersibility of the magnetic powder and increase the filling property. Specific examples of such functional groups include COOM, SO ₃ M, OSO ₃ M, P═O (OM) ₃ , and O—P═O (OM) ₂ (M is a hydrogen atom, alkali metal) Salt or amine salt), OH, NR ¹ R ² , NR ³ R ⁴ R ⁵ (R ¹ , R ² , R ³ , R ⁴ and R ⁵ are hydrogen or a hydrocarbon group, usually having 1 carbon atom) 10), and an epoxy group. When two or more kinds of resins are used in combination, it is preferable to use resins having the same functional group polarity, and among them, a combination of resins having a —SO ₃ M group is preferable. These binders are used in the range of 7 to 50 parts by mass, preferably 10 to 35 parts by mass with respect to 100 parts by mass of the magnetic powder. In particular, the combined use of 5 to 30 parts by mass of vinyl chloride resin and 2 to 20 parts by mass of polyurethane resin is preferable.

  Moreover, it is preferable to use together with 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; Examples include various polyisocyanates such as condensation products. A crosslinking agent is normally used in 10-50 mass parts with respect to 100 mass parts of binders.

  In the present embodiment, the thickness of the magnetic layer is preferably 200 nm or less, more preferably 10 to 200 nm, in order to solve the problem of output reduction due to demagnetization, which is an essential problem in longitudinal recording. More preferably, it is 10-100 nm. When the thickness of the magnetic layer exceeds 200 nm, the reproduction output becomes small due to the thickness loss, or the product of the residual magnetic flux density and the thickness becomes too large, and the distortion of the reproduction output due to magnetic saturation of the MR head is caused. It tends to happen. If the thickness of the magnetic layer is less than 10 nm, a uniform magnetic layer tends to be difficult to obtain.

  In the case of a magnetic tape, the coercive force in the longitudinal direction of the magnetic layer is preferably 79.6 to 318.4 kA / m, more preferably 119.4 to 318.4 kA / m. When the coercive force is less than 79.6 kA / m, there is a tendency for the output to decrease due to demagnetization in short wavelength recording. When the coercive force exceeds 318.4 kA / m, recording with a magnetic head tends to be difficult. Moreover, the squareness ratio (Br / Bm) in the longitudinal direction is usually 0.6 to 0.9, preferably 0.8 to 0.9. Furthermore, the product of the saturation magnetic flux density in the longitudinal direction and the thickness is preferably 0.001 to 0.1 μTm, more preferably 0.0015 to 0.05 μTm. When the product is less than 0.001 μTm, the reproduction output tends to be small even when the MR head is used. When the product exceeds 0.1 μTm, it tends to be difficult to obtain a high output in a short wavelength region.

  In the present embodiment, the magnetic layer may contain additives such as carbon black, a lubricant, and nonmagnetic powder for the purpose of improving characteristics such as conductivity, surface lubricity, and durability. Specifically, carbon black such as acetylene black, furnace black, and thermal black can be used as the carbon black. The content of carbon black is preferably 0.2 to 5 parts by mass with respect to 100 parts by mass of the magnetic powder. Specifically, for example, fatty acids, fatty acid esters, fatty acid amides and the like having 10 to 30 carbon atoms can be used as the lubricant. The content of the lubricant is preferably 0.2 to 3 parts by mass with respect to 100 parts by mass of the magnetic powder. Specifically, for example, nonmagnetic powders such as alumina and silica can be used as the nonmagnetic powder. The content of the nonmagnetic powder is preferably 1 to 20 parts by mass with respect to 100 parts by mass of the magnetic powder.

  In the preparation of the magnetic paint, the content of the above-mentioned magnetic powder, binder, organic solvent, and non-solvent component containing other additives as required is 40% by mass or less, preferably 1% by mass or more and 30% by mass or less. The first composition is prepared, mixed and stirred while applying shearing force to the first composition to prepare a second composition, and the content ratio of the non-solvent component of the second composition is 80% by mass or more. In addition, it is preferable to provide a pre-dispersion treatment step for concentrating to 90% by mass or more. By preparing a high-concentration composition in such a preliminary dispersion treatment step, a magnetic coating material in which fine magnetic powder is highly dispersed can be prepared. Specific examples of the organic solvent include ketone solvents such as methyl ethyl ketone, cyclohexanone and methyl isobutyl ketone; ether solvents such as tetrahydrofuran and dioxane; acetate solvents such as ethyl acetate and butyl acetate; ethylene glycol and propylene Examples include glycol solvents such as glycol, ethylene glycol monoethyl ether, and propylene glycol monomethyl ether. These organic solvents may be used alone or in combination. In addition to the above organic solvent, an aromatic organic solvent such as toluene may be used.

  As a mixing and stirring device used for preparing the second composition, a rotating shear type stirrer that rotates a rotating shaft with a rotating blade at high speed in a dispersion vessel; a rotating blade in a dispersion vessel containing a dispersion medium And an attritor and a sand mill that rotate a rotating shaft at a high speed; an ultrasonic disperser; a high-pressure spray impingement disperser, and the like.

In preparing the second composition, it is preferable that the shearing force be as high as possible, and it is preferable to apply the shearing force so that the shear rate is 10 ⁴ / sec or more, more preferably 10 ⁵ / sec or more. Such a high shearing force can be applied by using a stirrer that includes a rotating blade and a fixed portion, a gap between the rotating blade and the fixed portion is set small, and is capable of high-speed rotation. Examples of such a stirrer include Ultra Tarrax (manufactured by IKA), T.A. K. Homomixer (Primix), T. K. Examples thereof include batch stirrers such as Fillmix (manufactured by Primix) and Claremix (manufactured by M Technique). A continuous stirrer such as Ebara Milder (manufactured by Ebara Seisakusho) or Cavitron (manufactured by Eurotech) may also be used. The continuous stirrer may be used in a single treatment, or may be used in a plurality of treatments by forming a circulation line.

  As a method of concentrating the second composition prepared as described above to obtain a composition having a non-solvent component of 80% by mass or more, a method of evaporating the organic solvent by heating, reduced pressure or the like can be used.

  A magnetic paint can be prepared by kneading and dispersing the composition preliminarily dispersed as described above. In such a kneading and dispersing treatment, a kneading and dispersing treatment method conventionally used in a coating type magnetic recording medium can be applied. In the kneading process, for example, a batch kneader or a continuous biaxial kneader can be used. In the distribution process, for example, a media type disperser can be used. Examples of such media type dispersers include a disperser provided with a disc (including holes, notches, grooves, etc.), pins, and rings on a stirring shaft, and a rotor rotating disperser (for example, nano mill, 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.

  The magnetic recording medium of the present embodiment can be manufactured by applying the magnetic coating material prepared as described above onto a nonmagnetic support and subjecting it to an orientation treatment. As the nonmagnetic support, conventionally used nonmagnetic supports for magnetic recording media can be used. For example, a plastic film having a thickness of usually 2 to 20 μm made of polyesters such as polyethylene terephthalate and polyethylene naphthalate, polyolefins, cellulose triacetate, polycarbonate, polyamide, polyimide, polyamideimide, polysulfone, aramid, aromatic polyamide, etc. It is done. As the coating apparatus, a coating apparatus conventionally used in the production of magnetic recording media such as a gravure roll, a blade coater, and an extrusion type coater can be used.

  In performing magnetic field orientation of the magnetic powder in the magnetic coating in a predetermined direction while performing a drying process for removing the solvent from the magnetic coating applied on the nonmagnetic support, the surface temperature of the magnetic coating on the nonmagnetic support is It is preferable to provide a constant rate drying period that is substantially constant. By providing a coating process with a constant rate drying period in which the temperature of the magnetic coating is constant, the flow of intense magnetic powder accompanying the boiling, etc., and the generation of bubbles in a fluid magnetic coating containing an organic solvent Is suppressed. In addition, by actively providing a constant rate drying period during the drying process, the period during which the solvent content of the magnetic coating material decreases at a substantially constant rate becomes longer. Thereby, the space | gap which can generate | occur | produce in a magnetic coating material with the removal of an organic solvent also decreases. For this reason, the movement of the magnetic powder during drying and orientation treatment is suppressed. The constant rate drying period is preferably 0.2 seconds or more, and more preferably 1 second or more and 10 seconds or less. If the constant rate drying period is too short (that is, the solvent removal speed is too fast), convection is likely to occur in the magnetic paint during the drying process, and bubbles are likely to be generated, thereby increasing the autocorrelation length. The aggregation tends to be biased in the longitudinal direction or the width direction.

  The surface temperature of the magnetic coating during constant rate drying is the temperature of the hot air so that the latent heat of evaporation taken away from the magnetic paint by evaporation of the solvent from the magnetic paint and the amount of heat received by the magnetic paint from the surroundings can be balanced. It can be controlled by appropriately adjusting the wind speed, the distance between the magnetic paint and the heating means, and the like.

  Moreover, in the alignment process performed simultaneously with the application | coating process or continuously, it is preferable to provide continuously the 1st alignment process and 2nd alignment process from which magnetic field strength differs. By performing such a continuous magnetic field orientation process, re-aggregation of the magnetic powder due to the return orientation during the orientation process can be suppressed. For this purpose, the first alignment means used in the first alignment step and the second alignment means used in the second alignment step are disposed adjacent to each other, and both alignment means are as close as possible. Are preferably arranged. The magnetic field strength in the first alignment step is preferably 399 to 1197 kA / m, and the magnetic field strength in the second alignment step is preferably 120 to 798 kA / m. If the magnetic field strength is too weak, a sufficient orientation effect cannot be obtained. If the magnetic field strength is too strong, not only the orientation effect reaches saturation, but also the smoothness of the magnetic layer surface due to the magnetic field roughness may be impaired. As long as the magnetic field strength is within the above range, each orientation means may be a plurality of permanent magnets, solenoid magnets, or orientation means using these in combination.

  The first orientation means used in the first orientation step is preferably a repulsive magnet that faces the same pole of a permanent magnet because a high-intensity magnetic field can be obtained at a relatively low cost, but a solenoid magnet may be used. The second orientation means used in the second orientation step is preferably a solenoid magnet because a magnetic field with a relatively constant magnetic field strength is obtained with a constant length, but a combination of a solenoid magnet and a permanent magnet may be used. . In the second orientation step, the fluctuation rate of the magnetic field strength to which the magnetic layer is exposed is preferably 30% or less. If the variation rate of the magnetic field strength in the second orientation step is 30% or less, the return orientation of the magnetic powder can be further suppressed until the magnetic layer in the second orientation step is dried and fixed. The variation rate (%) of the magnetic field strength is a value obtained by [(Wmax−Wmin) / Wmax] × 100, where Wmax is the maximum magnetic field strength and Wmin is the minimum magnetic field strength in the second alignment step. is there.

  In the magnetic recording medium of the present embodiment, an undercoat layer may be provided between the nonmagnetic support and the magnetic layer. The thickness of the undercoat layer is preferably 0.1 to 3.0 μm, more preferably 0.15 to 2.5 μm. If the thickness of the undercoat layer is less than 0.1 μm, the durability of the magnetic tape tends to deteriorate. In addition, if the thickness of the undercoat layer exceeds 3.0 μm, the effect of improving the durability of the magnetic tape tends to be saturated, and the total tape thickness becomes thick, so the tape length per roll becomes short, There is a tendency for the storage capacity to decrease. For the undercoat layer, non-magnetic powders such as titanium oxide, iron oxide, aluminum oxide, etc. for the purpose of controlling paint viscosity and tape rigidity; γ-iron oxide, Co-γ-iron oxide, magnetite, chromium oxide, Fe-Ni Alloy, Fe-Co alloy, Fe-Ni-Co alloy, barium ferrite, strontium ferrite, Mn-Zn ferrite, Ni-Zn ferrite, Ni-Cu ferrite, Cu-Zn ferrite, Mg-Zn ferrite, etc. The magnetic powder can be included. These may be used alone or in combination. The undercoat layer may contain carbon black such as acetylene black, furnace black, or thermal black in order to impart conductivity. As the binder used in the undercoat layer, the same resin as the binder used in the magnetic layer can be used.

  In the magnetic recording medium of the present embodiment, a backcoat layer may be provided on the surface opposite to the surface on which the magnetic layer of the nonmagnetic support is provided. The thickness of the back coat layer is preferably 0.2 to 0.8 μm, more preferably 0.3 to 0.8 μm, and still more preferably 0.3 to 0.6 μm. The back coat layer preferably contains carbon black such as acetylene black, furnace black, or thermal black. As the binder for the backcoat layer, a resin similar to the resin used for the magnetic layer and the undercoat layer can be used. Among these, in order to reduce the coefficient of friction and improve the runnability, the combined use of a cellulose resin and a polyurethane resin is preferable.

  Hereinafter, the present invention will be described more specifically by way of examples. However, the present invention is not limited to these examples. In the following, “part” means “part by mass”, and “%” means “% by mass”.

<Preparation of magnetic paint>
[Preparation of magnetic paint (C-1)]
A first composition (solid content concentration: 30%) having the composition shown in Table 1 below is a rotating shear stirrer (Clearmix, manufactured by M Technique Co., Ltd., rotor blade diameter: 50 mm, gap: 2 mm, rotation speed: 2000 rpm, (Shear rate: 2.6 × 10 ⁴ / sec) and stirring was performed for 60 minutes.

The obtained composition was put into a vertical vibration dryer (VFD-01 manufactured by Chuo Kako Co., Ltd.), and the inside of the tank was vibrated (frequency: 1800 cpm, amplitude: 2.2 mm), under a reduced pressure of 20 kPa, 60 The mixture was heated to 0 ° C. and concentrated to obtain a second composition having a solid concentration of 90%.
Next, the kneading components shown in Table 2 below were added to the obtained second composition and kneaded with a continuous biaxial kneader.

  Next, in a diluting part of the continuous twin-screw kneader, a part of the diluting components shown in Table 3 below is added for dilution, and the remainder of the diluting components is further added to the taken-out composition, and the mixture is stirred at high speed to make it uniform A slurry before dispersion was obtained.

  The slurry before dispersion was dispersed with a sand mill (media: 0.5φ zirconia beads, filling rate: 80 vol%, blade peripheral speed: 10 m / s) with a residence time of 90 minutes. After stirring and filtering, the mixture was dispersed four times at a treatment pressure of 100 MPa using a high-pressure spray impingement dispersion disperser (manufactured by Sugino Machine Co., Ltd.) to obtain a magnetic paint (C-1).

[Preparation of magnetic paint (C-2)]
The magnetic paint (C-2) was prepared in the same manner as the magnetic paint (C-1) except that the amount of tetrahydrofuran in the first composition was changed to 174 parts (solid content concentration of the first composition: 40%). ) Was prepared.

[Preparation of magnetic paint (C-3)]
Magnetic paint (C) except that the amount of tetrahydrofuran in the first composition was changed to 464 parts (solid content concentration of the first composition: 20%) and the solid content concentration of the second composition was changed to 95%. A magnetic paint (C-3) was prepared in the same manner as in the preparation of -1).

[Preparation of magnetic paint (C-4)]
As magnetic powder, needle-like Fe-Co magnetic powder (additional elements: Al, Y) [σs: 120 Am ² / kg, Hc: 207 kA / m, particle size (major axis diameter): 35 nm, axial ratio: 5] A magnetic paint (C-4) was prepared in the same manner as in the preparation of the magnetic paint (C-1) except that was used.

[Preparation of magnetic paint (C-5)]
As magnetic powder, needle-like Fe-Co magnetic powder (additional elements: Al, Y) [σs: 120 Am ² / kg, Hc: 207 kA / m, particle size (major axis diameter): 35 nm, axial ratio: 5] Except that the amount of tetrahydrofuran in the first composition was changed to 174 parts (solid content concentration of the first composition: 40%) and the solid content concentration of the second composition was changed to 80%. A magnetic paint (C-5) was prepared in the same manner as in the preparation of the paint (C-1).

[Preparation of magnetic paint (C-6)]
The magnetic paint (C-6) was prepared in the same manner as in the preparation of the magnetic paint (C-1) except that the amount of tetrahydrofuran in the first composition was changed to 142 parts (solid content concentration of the first composition: 45%). ) Was prepared.

[Preparation of magnetic paint (C-7)]
A magnetic paint (C-7) was prepared in the same manner as the magnetic paint (C-1) except that the solid content concentration of the second composition was changed to 70%.

[Preparation of magnetic paint (C-8)]
As magnetic powder, needle-like Fe-Co magnetic powder (additional elements: Al, Y) [σs: 120 Am ² / kg, Hc: 207 kA / m, particle size (major axis diameter): 35 nm, axial ratio: 5] The magnetic paint (C-1) was prepared in the same manner as in the preparation of the magnetic paint (C-1) except that the amount of tetrahydrofuran in the first composition was changed to 142 parts (solid content concentration of the first composition: 45%). C-8) was prepared.

<Preparation of paint for undercoat layer>
After kneading the coating component for the undercoat layer in Table 5 below with a kneader, dispersion treatment (residence time: 60 minutes) is performed with a sand mill, to which the ingredients for the undercoat layer in Table 6 are added, stirred, filtered, and undercoat A layer coating was prepared.

<Preparation of paint for back coat layer>
The coating component for the backcoat layer shown in Table 7 below is subjected to a dispersion treatment (residence time: 45 minutes) with a sand mill, and 8.5 parts of polyisocyanate is added thereto, stirred and filtered to obtain a coating material for the backcoat layer. Prepared.

<Preparation of magnetic tape>
[Preparation of magnetic tape (T-1)]
The undercoat layer coating material is applied on a polyethylene naphthalate film having a thickness of 8 μm so that the thickness after drying and calendering becomes 0.9 μm, and the above-described magnetic coating material ( C-1) was applied with an extrusion coater at a speed of 150 m / min so that the thickness after drying and calendering was 0.08 μm. The conditions shown in Table 8 were used as the drying and orientation treatment conditions. The coating material for the back coat layer was applied and dried on the opposite surface of the obtained magnetic sheet on which the magnetic layer was formed. The magnetic sheet thus obtained was mirror-finished (calendar treatment) under the conditions of a temperature of 100 ° C. and a linear pressure of 196 kN / m with a seven-stage calendar made of a metal roll. A magnetic sheet for evaluation was prepared by aging at 48 ° C. for 48 hours. Then, it cut | judged to the 1/2 inch width | variety and produced the magnetic tape (T-1).

[Production of Magnetic Tape (T-2) to (T-1 5 )]
Magnetic tapes (T-2) to (T-1 5 ) were produced in the same manner as the magnetic tape (T-1) except that the conditions shown in Table 8 were changed.

  About each magnetic tape produced as mentioned above, the following autocorrelation length and noise were measured.

<Autocorrelation length>
As a magnetic force microscope, Digital Scope's Nano Scope III was used, and the leakage magnetic field image of the magnetic layer was measured by the 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, the scanning range is 5 μm square, and the scanning speed is 5 μm. / Sec.
Based on the obtained magnetization intensity data of the two-dimensional leakage magnetic field image, the magnetization intensity data when the magnetization intensity data is shifted by a certain distance within the scanning range in the longitudinal direction (X direction) and the width direction (Y direction) is not moved. The value obtained by summing the magnetization intensity data of the leakage magnetic field image and adding up the summation result over the entire range where the data overlap is used as the autocorrelation coefficient at the shifted position. Then, the movement distance in the XY directions, which is 1% of the autocorrelation coefficient before the movement, was evaluated as the autocorrelation length.

<Noise>
For the evaluation of noise, a drum tester equipped with an electromagnetic induction head (track width 25 μm, gap 0.1 μm) and MR head (gap length: 8 μm) was used. Both heads are installed at different locations with respect to the rotating drum, and tracking can be adjusted by operating both heads in the vertical direction. A magnetic tape having a length of about 60 cm is wound around the rotating drum of this drum tester, a rectangular wave signal having a wavelength of 0.1 μm is recorded with an induction type head, and the signal is reproduced with an MR head. The output was amplified by a preamplifier and then read into a spectrum analyzer (linear recording density: 350 kbpi, track density: 11 ktpi). The integrated value of the value obtained by subtracting the output and system noise from the read spectral component corresponding to the wavelength component longer than 0.1 μm was obtained as the noise value (dB). This noise value was evaluated as a relative value when the noise value of MAXELL LTO4 measured in the same manner was set to 0 dB.

  Table 8 shows the results.

  FIG. 1 is a leakage magnetic field image of the magnetic layer of the magnetic tape (T-1), and FIG. 2 is a diagram showing a magnetic correlation of the leakage magnetic field image of FIG. The magnetic tape (T-1) has an autocorrelation length Ma in the longitudinal direction of 42 nm, an autocorrelation length Mb in the width direction of 44 nm, and a ratio (Ma / Mb) of 0.95. Therefore, it can be seen that the magnetic layer of this magnetic tape has little aggregation of the magnetic powder in both the longitudinal direction and the width direction, and the aggregation has an isotropic aggregation degree. When comparing magnetic tapes using the same kind of magnetic powder, it can be seen that the magnetic tape having a smaller autocorrelation length in the longitudinal direction and the width direction and having an isotropic degree of aggregation can further reduce noise. As shown in Table 8, the magnetic tape having a small autocorrelation length in the longitudinal direction and the width direction and a small ratio as in the magnetic tape (T-1) is a conventional magnetic tape for high-density recording in high-density recording. It can be seen that noise can be further reduced. On the other hand, a magnetic tape having a large autocorrelation length in either the longitudinal direction or the width direction, or a magnetic tape in which aggregation is biased in the longitudinal direction or the width direction even if the autocorrelation length is small has a small noise reduction effect I understand.

Claims (3)

A magnetic recording medium having a nonmagnetic support and an undercoat layer and a magnetic layer on the nonmagnetic support,
The magnetic layer contains a magnetic powder having a particle size of 40 nm or less and a binder, has a thickness of 10 to 200 nm, and has an autocorrelation length Ma in the longitudinal direction of the magnetic layer of 10 to 70 nm. The autocorrelation length Mb in the width direction of the magnetic layer is 10 to 80 nm, and the ratio of the autocorrelation length Ma in the longitudinal direction of the magnetic layer to the autocorrelation length Mb in the width direction of the magnetic layer (Ma / Mb) is 0.80 to 1. 20 is a magnetic recording medium.
However, the autocorrelation length is magnetic with a measurement 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) using a magnetic force microscope. By scanning the layer, a two-dimensional leakage magnetic field image of the magnetic layer is measured (scanning range: 5 μm square, scanning speed: 5 μm / sec), and magnetization intensity data of the obtained two-dimensional leakage magnetic field image is obtained. Based on the obtained magnetization intensity data, the magnetization intensity data when the magnetization intensity data is shifted by a certain distance within the scanning range in the longitudinal direction (X direction) and the width direction (Y direction) are the magnetization intensity data before movement. Then, the sum of the summation results over the entire area where the magnetization intensity data overlaps is obtained as the autocorrelation coefficient at the shifted position. An XY direction moving distance when a 1% before the autocorrelation coefficients. The magnetic recording medium according to claim 1, wherein the magnetic layer contains an iron nitride magnetic powder as the magnetic powder.   The magnetic recording medium according to claim 1, wherein the magnetic powder has a particle size of 5 to 40 nm.