JP4459248B2 - Magnetic recording medium, magnetic signal reproduction system, and magnetic signal reproduction method - Google Patents

Description

  The present invention relates to a magnetic recording medium, and more particularly to a magnetic recording medium having excellent electromagnetic conversion characteristics during high-density recording, and a magnetic signal reproducing system and reproducing method using the magnetic recording medium.

  In recent years, means for transmitting terabyte-class information at a high speed have been remarkably developed, and it has become possible to transfer images and data having enormous information. Along with the improvement of this data transfer technique, recording / reproducing apparatuses and recording media for recording, reproducing and storing information are required to have a higher recording capacity.

  Regarding magnetic tape, there are various uses such as audio tape, video tape, computer tape, etc. Especially in the field of data backup tape, with the increase in the capacity of the hard disk to be backed up, several 10 to 800 GB per volume. Those with recording capacity have been commercialized. Also, a large capacity backup tape exceeding 1 TB has been proposed, and it is essential to increase the recording capacity.

  As a means of increasing the recording capacity, the approach from the magnetic tape manufacturing side, high recording such as making fine particles of magnetic powder and packing them in high density, smoothing the coating, thinning the magnetic layer, etc. Densification techniques have been proposed. For example, Patent Document 1 proposes that the dispersibility of the inorganic powder in the lower layer is improved and the surface property of the magnetic layer is ensured by containing a phosphorus-containing organic compound in the lower layer. Patent Document 2 proposes that the spatial frequency intensity is taken as a ratio of long wavelength to short wavelength as an index of the surface smoothness of the magnetic layer.

However, when the surface smoothness of the magnetic layer is increased, there is a possibility that winding disturbance or deterioration in running performance may occur. Therefore, in order to prevent this turbulence disturbance and deterioration in running performance, a back coat layer containing a particulate material such as carbon black is provided on the surface of the support opposite to the magnetic layer (for example, (See Patent Document 3).
JP-A-8-306032 Japanese Patent Laid-Open No. 11-25442 JP 2004-30870 A

As an index for evaluating the surface properties of the magnetic layer, the magnetic layer surface roughness Ra is widely used. On the other hand, Patent Document 2 proposes to take the spatial frequency intensity at a ratio of long wavelength to short wavelength. This is because Ra is an averaged numerical value, and attention is paid to the fact that there is a large difference in characteristics depending on the difference in the waviness component even with the same Ra.
However, in a magnetic recording medium having a backcoat layer, the magnetic recording medium can be stored in a roll state during the manufacturing process, or the magnetic tape can be stored in a state wound on a reel hub after production, simply by controlling the surface properties of the magnetic layer. Then, the projection of the backcoat layer is transferred to the surface of the magnetic layer, and so-called “show-through” occurs in which a minute dent is formed. This show-through causes a problem that electromagnetic conversion characteristics, in particular, BB-SNR and K-SNR (neighboring noise) deteriorate. Since show-through occurs remarkably after long-term storage or after storage at high temperature, even if the surface properties of the magnetic layer in the initial state are controlled, the magnetic properties of the magnetic layer with the backcoat layer are roughened by storage after storage. There is a problem of end.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a magnetic recording medium having excellent electromagnetic conversion characteristics before and after long-term storage or high-temperature storage.

  As a result of intensive studies to achieve the above object, the present inventors have controlled the undulation component on the surface of the magnetic layer and also controlled the undulation component on the surface of the backcoat layer, particularly the undulation of 10 μm pitch, It was found that the show-through of the magnetic layer during long-term storage or high-temperature storage can be suppressed. Although the reason for this is not clear, when the show-through occurs, the shape of the back surface itself is not completely reflected on the magnetic layer, but the way of reflection changes depending on the pitch and intensity (frequency, amplitude), and particularly 10 μm on the back surface. It is presumed that by suppressing the reflection of the pitch, it is possible to suppress the reflection of the roughness related to the noise in the vicinity of the output. The present invention has been completed based on the above findings.

That is, the means for achieving the above object is as follows.
[1] A magnetic recording medium having a magnetic layer containing hexagonal ferrite powder and a binder on one side of a nonmagnetic support and a backcoat layer on the other side,
The spectral density at a 10 μm pitch on the surface of the magnetic layer is in the range of 800 to 10,000 nm ³ ,
The spectral density at a 10 μm pitch on the backcoat layer surface is in the range of 20000 to 80000 nm ³ ,
The center plane average surface roughness Ra of the magnetic layer measured by an atomic force microscope is in the range of 0.5 to 2.5 nm, and the average plate diameter of the hexagonal ferrite powder is in the range of 10 to 40 nm. Characteristic magnetic recording tape .
[2] The magnetic recording tape according to [1], which is used in a magnetic signal reproducing system using a giant magnetoresistive head as a reproducing head.
[3] A magnetic signal reproducing system including the magnetic recording tape according to [1] and a giant magnetoresistive magnetic head as a reproducing head.
[4] A magnetic signal reproducing method for reproducing the magnetic signal recorded on the magnetic recording tape according to [1] using a giant magnetoresistive head.

  According to the present invention, there is provided a high-capacity magnetic recording medium that can satisfactorily maintain electromagnetic conversion characteristics (BB-SNR, K-SNR (near noise)) in a high-density region even after long-term storage / high-temperature storage. be able to.

The magnetic recording medium of the present invention has a magnetic layer containing hexagonal ferrite powder and a binder on one side of a nonmagnetic support, a backcoat layer on the other side, and a 10 μm pitch on the surface of the magnetic layer. The spectral density in the range of 800 to 10000 nm ^3, the spectral density in the 10 μm pitch on the back coat layer surface is in the range of 20000 to 80000 nm ³ , and the average surface roughness of the center plane of the magnetic layer measured by an atomic force microscope Ra is in the range of 0.5 to 2.5 nm, and the average plate diameter of the hexagonal ferrite powder is in the range of 10 to 40 nm.

Furthermore, the present invention provides
A magnetic signal reproducing system including the magnetic recording medium of the present invention and a giant magnetoresistive head as a reproducing head;
A magnetic signal reproducing method for reproducing a magnetic signal recorded on the magnetic recording medium of the present invention using a giant magnetoresistive magnetic head;
About.
The magnetic recording medium of the present invention will be described in detail below.

The spectral density (Power Spectrum Density) at a 10 μm pitch is a value measured by the following method, and is used as an index of waviness at a 10 μm pitch. Hereinafter, the spectral density at a pitch of 10 μm is also referred to as PSD (10 μm).
Using a non-contact optical roughness measuring device (apparatus: HD2000 manufactured by wyko), averages the profile data of the surface roughness in the longitudinal direction of the medium with a measurement area of 240 μm × 180 μm and obtains a frequency analysis result . From this analysis result, the intensity at each wavelength is calculated to obtain the intensity corresponding to a 10 μm pitch. This is PSD (10 μm).

In the present invention, the measurement conditions of an atomic force microscope (AFM) for measuring the center surface average surface roughness Ra of the magnetic layer are as follows.
Equipment: Nanoscope III manufactured by Veeco Japan
Mode: AFM mode (contact mode)
Measurement range: 40μm square Scan line: 512 * 512
Scan speed: 2Hz
Scan direction: Medium longitudinal direction

  The center plane average roughness Ra on the surface of the magnetic layer is related to the output and the SNR. When the average roughness Ra exceeds 2.5 nm, the output decreases, and as a result, the SNR deteriorates. In particular, in a system used at a high linear recording density (for example, a linear recording density of 100 to 400 KFCI), the SNR deterioration due to a decrease in output becomes significant. On the other hand, if it is smaller than 0.5 nm, the friction coefficient on the surface of the magnetic layer becomes high, and the running durability deteriorates. Therefore, in the magnetic recording medium of the present invention, Ra on the surface of the magnetic layer is set in the range of 0.5 to 2.5 nm. Preferably it is the range of 1-2 nm.

Furthermore, in the present invention, in order to improve electromagnetic conversion characteristics, the center surface average roughness Ra of the magnetic layer surface is controlled, and the swell component of the magnetic layer surface is also controlled. In the magnetic recording medium of the present invention, the PSD (10 μm) on the surface of the magnetic layer is in the range of 800 to 10,000 nm ³ . The surface of the magnetic layer PSD (10 [mu] m) is less than 800 n m ^3, by the magnetic layer surface is too smooth, the friction coefficient increases, the running durability is deteriorated. In addition, when the PSD (10 μm) on the surface of the magnetic layer exceeds 10,000 nm ³ , the nearby noise is significantly degraded. The surface of the magnetic layer PSD (10 [mu] m) is preferably in the range of 800～6000Nm ^3, and still more preferably in the range of 800~3000nm ^3.

Furthermore, in the present invention, the undulation component on the surface of the backcoat layer is also controlled in order to prevent deterioration of electromagnetic conversion characteristics due to show-through on the magnetic layer. As a result of the study by the present inventors, it has been newly found that when the PSD (10 μm) on the surface of the backcoat layer exceeds 80000 nm ³ , the influence of the reflection on the surface of the magnetic layer is large, which causes a decrease in electromagnetic conversion characteristics. It was. Therefore, in the magnetic recording medium of the present invention, the PSD (10 μm) on the surface of the backcoat layer is set to 80000 nm ³ or less. However, if the PSD (10 μm) of the backcoat layer is less than 20000 nm ³ , the winding shape of the cartridge after driving is deteriorated, and running durability is deteriorated due to edge breakage or the like. Therefore, in the magnetic recording medium of the present invention, the PSD (10 μm) on the surface of the backcoat layer is in the range of 20000 to 80000 nm ³ . This makes it possible to achieve both running durability and electromagnetic conversion characteristics. Backcoat layer surface of the PSD (10 [mu] m) is preferably in the range of 20000～60000Nm ^3, and still more preferably in the range of 20000~40000nm ^3.

  Furthermore, in the magnetic recording medium of the present invention, the ferromagnetic powder contained in the magnetic layer is hexagonal ferrite powder having an average plate diameter of 10 to 40 nm. In order to smooth the surface, it is desirable that the ferromagnetic powder has fine particles and good dispersibility. Hexagonal ferrite has a high dispersibility due to its low σs compared to conventional acicular metal ferromagnetic milling, and because the shape is plate-like, the longest particle size can be reduced, that is, it is advantageous for micronization. It is. On the other hand, if the average plate diameter is less than 10 nm, the influence of thermal fluctuation is large, and a desired coercive force (Hc) cannot be ensured, and electromagnetic conversion characteristics deteriorate. When the average plate diameter exceeds 40 nm, it becomes difficult to ensure high output and low noise at a high linear recording density. The average plate diameter of the hexagonal ferrite powder is preferably in the range of 10 to 30 nm, more preferably 15 to 25 nm.

Examples of methods for controlling Ra and PSD (10 μm) on the magnetic layer surface and PSD (10 μm) on the backcoat layer surface include the following methods.
(A) A support with Ra and PSD (10 μm) suppressed is used.
(B) A smoothing layer such as a radiation curable resin is provided on one side or both sides of the support to suppress PSD (10 μm).
In the above (a), (b), the magnetic layer side, PSD backcoat layer side of the support (10 [mu] m) is preferably 6000 nm ³ or less, more preferably 4000 nm ³ or less, particularly preferably 2000 nm ³ or less. The lower limit is, for example, 100 nm ³ .
(C) Fine particles and finely dispersed powder are used for the back layer forming coating liquid (also referred to as back liquid). For example, the backcoat layer coating solution can have the same composition as the nonmagnetic layer coating solution.
(D) Set conditions in the smoothing process (smoothing process, calendar process).
(E) The non-magnetic layer coating solution and the back coating layer coating solution are subjected to classification treatment (centrifugal sedimentation of coarse particles, filter filtration).

The smoothing process is a process of applying shear in the coating direction while the coating layer is still wet immediately after coating the nonmagnetic layer, and is effective for destroying aggregates in the coating layer. Usually, it is carried out by bringing a hard plate-like smooth smoother (for example, center surface average surface roughness Ra ≦ 2.5 nm) into contact with the wet surface and applying shear.
In the calendar process, the temperature, pressure, speed, material, surface property, roll configuration, etc. of the calendar roll are appropriately set. Details thereof will be described later.

The above (e) will be described below.
It is presumed that the occurrence of waviness is greatly affected by (i) insufficient dispersion of the coating solution and (ii) dry aggregation after coating. Therefore, in order to prevent swell, it is preferable to strengthen the dispersion condition to prevent the above (i). On the other hand, regarding the above (ii), it is considered that what becomes a nucleus in the aggregation during drying is a coarse aggregate contained in the coating solution. This is presumed to be due to the fact that coarse aggregates attract large light particles around them because of their large mass. For this reason, it is effective to suppress the dry aggregation by subjecting the coating liquid after dispersion to a classification treatment and removing coarse aggregates. The classification treatment can be performed with a centrifuge. Specifically, it is preferable that the coating solution is kneaded with an open kneader, and then dispersed with a sand mill using zirconia beads, followed by classification treatment.
By the way, in order to improve the surface smoothness, the mixing ratio of the nonmagnetic powder or the inorganic powder and the carbon black in the nonmagnetic layer coating liquid and the backcoat layer coating liquid is expressed by volume ratio, the former: the latter = 8: 2. ˜5: 5 is preferable. In terms of mass ratio, the former: the latter = 9: 1 to 7: 3 is preferable. However, since the specific gravity of nonmagnetic powder or inorganic powder and carbon black differ greatly, the coating solution may not be able to sufficiently remove coarse agglomerates by one-stage classification treatment. In this case, the coating solution after the dispersion treatment is subjected to a classification process by filtering (filter filtration) to remove coarse aggregates of carbon black having a relatively low specific gravity and then non-magnetic powder having a relatively high specific gravity by centrifugal sedimentation. It is preferable to perform a two-stage classification process of removing coarse aggregates of inorganic powder. Centrifugal sedimentation is a treatment for allowing coarse particles to settle by leaving the coating solution after centrifugation for a predetermined time.

  Hereinafter, the magnetic recording medium of the present invention will be described in more detail.

Nonmagnetic supports Nonmagnetic supports include polyesters such as polyethylene terephthalate and polyethylene naphthalate, polyolefins, cellulose triacetate, polycarbonate, polyamide, polyimide, polyamideimide, polysulfone, polyaramid, aromatic polyamide, polybenzoxazole, etc. Any known film can be used. It is preferable to use a high strength support such as polyethylene naphthalate or polyamide. If necessary, a laminated type support as disclosed in JP-A-3-224127 can be used to change the surface roughness of the magnetic surface and the nonmagnetic support surface. These supports may be subjected in advance to corona discharge treatment, plasma treatment, easy adhesion treatment, heat treatment, dust removal treatment, and the like. It is also possible to apply an aluminum or glass substrate as the support.

Of these, a polyester support (hereinafter simply referred to as polyester) is preferred. The polyester is preferably a polyester composed of a dicarboxylic acid and a diol, such as polyethylene terephthalate and polyethylene naphthalate.
The main constituent dicarboxylic acid components include terephthalic acid, isophthalic acid, phthalic acid, 2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid, diphenylsulfone dicarboxylic acid, diphenyl ether dicarboxylic acid, diphenylethanedicarboxylic acid, Examples thereof include cyclohexane dicarboxylic acid, diphenyl dicarboxylic acid, diphenyl thioether dicarboxylic acid, diphenyl ketone dicarboxylic acid, and phenylindane dicarboxylic acid.
Examples of the diol component include ethylene glycol, propylene glycol, tetramethylene glycol, cyclohexanedimethanol, 2,2-bis (4-hydroxyphenyl) propane, 2,2-bis (4-hydroxyethoxyphenyl) propane, bis ( 4-Hydroxyphenyl) sulfone, bisphenol fluorene hydroxyethyl ether, diethylene glycol, neopentyl glycol, hydroquinone, cyclohexanediol and the like.

Among the polyesters comprising these as main constituents, terephthalic acid and / or 2,6-naphthalenedicarboxylic acid as a dicarboxylic acid component and ethylene glycol as a diol component from the viewpoint of transparency, mechanical strength, dimensional stability, etc. Polyesters having and / or 1,4-cyclohexanedimethanol as the main constituent are preferred.
Among them, a polyester mainly composed of polyethylene terephthalate or polyethylene-2,6-naphthalate, a copolymer polyester composed of terephthalic acid, 2,6-naphthalenedicarboxylic acid and ethylene glycol, and two or more kinds of these polyesters A polyester having a mixture as a main constituent is preferred. Particularly preferred is a polyester having polyethylene-2,6-naphthalate as a main constituent.

  The polyester may be biaxially stretched or a laminate of two or more layers.

  Further, the polyester may be further copolymerized with other copolymerization components, or may be mixed with other polyesters. Examples of these include the dicarboxylic acid components and diol components mentioned above, or polyesters composed thereof.

Polyester has an aromatic dicarboxylic acid having a sulfonate group or an ester-forming derivative thereof, a dicarboxylic acid having a polyoxyalkylene group or an ester-forming derivative thereof, and a polyoxyalkylene group in order to prevent delamination during film formation. A diol or the like may be copolymerized.
Of these, 5-sodium sulfoisophthalic acid, 2-sodium sulfoterephthalic acid, 4-sodium sulfophthalic acid, 4-sodium sulfo-2,6-naphthalenedicarboxylic acid, and the like in terms of polyester polymerization reactivity and film transparency. Compounds in which sodium is substituted with other metals (for example, potassium, lithium, etc.), ammonium salts, phosphonium salts, etc., or ester-forming derivatives thereof, polyethylene glycol, polytetramethylene glycol, polyethylene glycol-polypropylene glycol copolymers and their A compound obtained by oxidizing the hydroxy groups at both ends to form a carboxyl group is preferred. The proportion of copolymerization for this purpose is preferably 0.1 to 10 mol% based on the dicarboxylic acid constituting the polyester.
For the purpose of improving heat resistance, a bisphenol compound, a compound having a naphthalene ring or a cyclohexane ring can be copolymerized. The copolymerization ratio is preferably 1 to 20 mol% based on the dicarboxylic acid constituting the polyester.

The polyester can be produced according to a conventionally known polyester production method. For example, a direct esterification method in which a dicarboxylic acid component is directly esterified with a diol component. First, a dialkyl ester is used as a dicarboxylic acid component, this is transesterified with the diol component, and this is heated under reduced pressure. A transesterification method of polymerizing by removing excess diol component can be used. At this time, if necessary, a transesterification catalyst or a polymerization reaction catalyst can be used, or a heat-resistant stabilizer can be added.
In addition, anti-coloring agents, antioxidants, crystal nucleating agents, slipping agents, stabilizers, anti-blocking agents, UV absorbers, viscosity modifiers, antifoaming and clearing agents, antistatic agents, pH adjustments in each process during synthesis One or more of various additives such as an agent, a dye, a pigment, and a reaction terminator may be added.

Further, a filler may be added to the polyester. Examples of the filler include inorganic powders such as spherical silica, colloidal silica, titanium oxide, and alumina, and organic fillers such as crosslinked polystyrene and silicone resin.
Further, in order to increase the rigidity of the support, these materials can be highly stretched, or a metal, semimetal, or oxide layer thereof can be provided on the surface.

  The thickness of the nonmagnetic support is preferably 3 to 80 μm, more preferably 3 to 50 μm, and particularly preferably 3 to 10 μm.

When the surface forming the magnetic layer of the nonmagnetic support is not provided with a smooth layer (intermediate layer aiming at smoothing), the center plane average roughness (Ra) is preferably 4 nm or less, more preferably 2 nm or less, More preferably, it is 1 nm or less. The lower limit is, for example, 0.3 nm.
Further, the surface on which the back coat layer is formed, when a smooth layer (intermediate layer aiming at smoothing) is not provided, the center plane average roughness (Ra) is preferably 6 nm or less, more preferably 3 nm or less, and even more preferably It is 1.5 nm or less. Ra regarding said nonmagnetic support body is the value measured by HD2000 by WYKO.

The PSD (10 [mu] m) of the non-magnetic support surface, the surface forming the magnetic layer, if not provided a smooth layer (intermediate layer aimed at smoothing), preferably 15000 nm ³ or less, more preferably 10000 nm ³ or less, More preferably, it is 5000 nm ³ or less. The lower limit is, for example, 100 nm ³ . When a smooth layer (intermediate layer aiming at smoothing) is not provided on the surface on which the backcoat layer is formed, PSD (10 μm) is preferably 50000 nm ³ or less, more preferably 20000 nm ³ or less, and even more preferably 10000 nm ^3. It is as follows. The lower limit is, for example, 500 nm ³ . Moreover, when providing a smooth layer, after providing a smooth layer, what satisfy | fills the said surface roughness and PSD (10 micrometers) is preferable.
Further, the Young's modulus in the longitudinal direction and the width direction of the nonmagnetic support is preferably 6.0 GPa or more, and more preferably 7.0 GPa or more.

  The magnetic recording medium of the present invention has a magnetic layer containing ferromagnetic powder and a binder on at least one surface of the nonmagnetic support, and is substantially between the nonmagnetic support and the magnetic layer. It is preferable to have a nonmagnetic layer (also referred to as a lower layer or a nonmagnetic lower layer) that is nonmagnetic.

Magnetic Layer In the magnetic recording medium of the present invention, the ferromagnetic powder contained in the magnetic layer contains hexagonal ferrite powder having an average plate diameter of 10 to 40 nm. Its volume is preferably 1000～20000Nm ^3, further preferably 2000~8000nm ^3. By setting it within this range, it is possible to effectively suppress a decrease in magnetic characteristics due to thermal fluctuations, and to obtain good C / N (S / N) while maintaining low noise. As the ferromagnetic powder, hexagonal ferrite powder is essential, but it may be used in the same layer as the ferromagnetic metal powder or iron nitride-based powder or in another layer.

The volume of the acicular powder can be determined from the major axis length and minor axis length assuming that the shape is a cylinder.
In the case of a plate-like powder, the volume can be determined from the plate diameter and the axial length (plate thickness) assuming that the shape is a prism (hexagonal prism in the case of hexagonal ferrite powder).
In the case of an iron nitride-based powder, the volume can be obtained assuming that the shape is a sphere.
The size of the magnetic material can be determined by the following method.
First, an appropriate amount of the magnetic layer is peeled off. N-Butylamine is added to 30 to 70 mg of the peeled magnetic layer, sealed in a glass tube, set in a thermal decomposition apparatus, and heated at 140 ° C. for about 1 day. After cooling, the contents are taken out from the glass tube and centrifuged to separate the liquid and solids. The separated solid is washed with acetone to obtain a powder sample for TEM. This sample is photographed with a Hitachi transmission electron microscope H-9000, and the particles are photographed at a photographing magnification of 100000 times and printed on a photographic paper to obtain a total magnification of 500,000 times to obtain a particle photograph. The target magnetic material is selected from the particle photograph, the outline of the powder is traced with a digitizer, and the particle size is measured with image analysis software KS-400 made by Carl Zeiss. Measure the size of 500 particles.

In the present invention, the size of the powder such as a magnetic material (hereinafter referred to as “powder size”) is (1) the shape of the powder is needle-like, spindle-like, or column-like (however, the height is the bottom surface). In the case of larger than the maximum major axis, etc., it is represented by the length of the major axis constituting the powder, that is, the major axis length, and (2) the shape of the powder is plate-shaped or columnar (however, the thickness or height is (Smaller than the maximum major axis of the plate surface or the bottom surface), and (3) the shape of the powder is spherical, polyhedral, unspecified, etc. When the major axis constituting the body cannot be specified, it is represented by the equivalent circle diameter. The equivalent circle diameter is a value obtained by a circle projection method.
The average powder size of the powder is an arithmetic average of the above powder sizes, and is obtained by carrying out the measurement as described above for 500 primary particles. Primary particles refer to an independent powder without aggregation.

The average acicular ratio of the powder is determined by measuring the short axis length of the powder in the above measurement, that is, the short axis length, and calculating the arithmetic average of the values of (long axis length / short axis length) of each powder. Point to. Here, the minor axis length is defined by the above-mentioned powder size in the case of (1), the length of the minor axis constituting the powder, and in the case of (2), the thickness or height, respectively. In the case of (3), since there is no distinction between the major axis and the minor axis, (major axis length / minor axis length) is regarded as 1 for convenience.
When the shape of the powder is specific, for example, in the case of the definition (1) of the powder size, the average powder size is referred to as an average major axis length, and in the case of the definition (2), the average powder size Is called the average plate diameter, and the arithmetic average of (maximum major axis / thickness to height) is called the average plate ratio. In the case of definition (3), the average powder size is referred to as an average diameter (also referred to as an average particle diameter or an average particle diameter). In powder size measurement, the standard deviation / average value expressed as a percentage is defined as the coefficient of variation.

Hexagonal ferrite powders include, for example, barium ferrite, strontium ferrite, lead ferrite, calcium ferrite, and their substitutes such as Co. More specifically, magnetoplumbite type barium ferrite and strontium ferrite, magnetoplumbite type ferrite whose particle surface is coated with spinel, and magnetoplumbite type barium ferrite and strontium ferrite partially containing a spinel phase, etc. Is mentioned. In addition to predetermined atoms, Al, Si, S, Sc, Ti, V, Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, W, Re, Au, Hg , Pb, Bi, La, Ce, Pr, Nd, P, Co, Mn, Zn, Ni, Sr, B, Ge, and Nb may be included. In general, elements such as Co—Zn, Co—Ti, Co—Ti—Zr, Co—Ti—Zn, Ni—Ti—Zn, Nb—Zn—Co, Sb—Zn—Co, and Nb—Zn are added. You can use things. Some raw materials and production methods contain specific impurities.

As described above, the average plate diameter of the hexagonal ferrite powder is 10 to 40 nm, preferably 10 to 30 nm, and more preferably 15 to 25 nm.
The average plate ratio [arithmetic average of (plate diameter / plate thickness)] is preferably 1 to 15, and more preferably 1 to 7. When the average plate ratio is 1 to 15, sufficient orientation can be obtained while maintaining high filling properties in the magnetic layer, and noise increase due to stacking between particles can be suppressed. Further, the specific surface area (S _BET ) by the BET method within the above particle size range is preferably 40 m ² / g or more, more preferably 40 to 200 m ² / g, and 60 to 100 m ² / g. Is most preferred.

  The distribution of the particle plate diameter and plate thickness of the hexagonal ferrite powder is generally preferably as narrow as possible. The particle plate diameter and plate thickness can be compared by randomly measuring 500 particles from a particle TEM photograph. In many cases, the distribution of the particle plate diameter and plate thickness is not a normal distribution, but when calculated and expressed as a standard deviation with respect to the average size, generally σ / average size = 0.1 to 1.0. In order to sharpen the particle size distribution, the particle generation reaction system is made as uniform as possible, and the generated particles are subjected to a distribution improvement process. For example, a method of selectively dissolving ultrafine particles in an acid solution is also known.

  In general, a hexagonal ferrite powder having a coercive force (Hc) of about 143.3 to 318.5 kA / m (1800 to 4000 Oe) can be produced. The coercive force (Hc) of the hexagonal ferrite powder is preferably 159.2 to 238.9 kA / m (2000 to 3000 Oe). More preferably, it is 191.0-214.9 kA / m (2200-2800 Oe). The coercive force (Hc) can be controlled by the particle size (plate diameter / plate thickness), the type and amount of the contained element, the substitution site of the element, the particle generation reaction conditions, and the like.

The saturation magnetization (σs) of the hexagonal ferrite powder is preferably 30 to 80 A · m ² / kg (emu / g). Higher saturation magnetization (σs) is preferable, but tends to be smaller as the particles become finer. In order to improve the saturation magnetization (σs), it is well known to combine spinel ferrite with magnetoplumbite ferrite, and to select the type and amount of contained elements. It is also possible to use W-type hexagonal ferrite. When dispersing the magnetic material, the surface of the magnetic material particles may be treated with a material suitable for the dispersion medium and polymer. As the surface treatment agent, inorganic compounds and organic compounds can be used. Typical examples of the main compound include oxides or hydroxides such as Si, Al, and P, various silane coupling agents, and various titanium coupling agents. The addition amount is, for example, 0.1 to 10% by mass with respect to the mass of the magnetic substance. The pH of the magnetic material is also important for dispersion. Usually, about 4 to 12 has optimum values depending on the dispersion medium and polymer, but about 6 to 11 can be selected from the chemical stability and storage stability of the medium. Water contained in the magnetic material also affects the dispersion. Although there is an optimum value depending on the dispersion medium and the polymer, 0.01 to 2.0% is usually selected.

The hexagonal ferrite powder is manufactured by mixing (1) barium oxide / iron oxide / metal oxide replacing iron and boron oxide as a glass forming substance so as to have a desired ferrite composition, and then melting and quenching. A glass crystallization method for obtaining a barium ferrite crystal powder by re-heating treatment after being reheated, and (2) neutralizing the barium ferrite composition metal salt solution with an alkali to produce a by-product Hydrothermal reaction method to obtain barium ferrite crystal powder by liquid phase heating at 100 ° C. or higher after removal of water, (3) neutralization of barium ferrite composition metal salt solution with alkali, by-product There is a coprecipitation method in which the product is removed and then dried, treated at 1100 ° C. or less, and pulverized to obtain a barium ferrite crystal powder. The hexagonal ferrite powder may be subjected to surface treatment with Al, Si, P, or an oxide thereof as required. The amount thereof is, for example, 0.1 to 10% by mass with respect to the ferromagnetic powder, and surface treatment is preferable because adsorption of a lubricant such as a fatty acid is 100 mg / m ² or less. The ferromagnetic powder may contain inorganic ions such as soluble Na, Ca, Fe, Ni, and Sr. Although it is preferable that these are essentially not present, if they are 200 ppm or less, they do not particularly affect the characteristics.

Binders , lubricants, dispersants, additives, solvents, dispersion methods and other known techniques for the binder magnetic layer, nonmagnetic layer, and backcoat layer can be applied to each other as appropriate. In particular, known techniques relating to the amount and type of binder, the additive, and the amount and type of dispersant can be applied.

  As the binder, conventionally known thermoplastic resins, thermosetting resins, reactive resins, and mixtures thereof can be used. A thermoplastic resin having a glass transition temperature of −100 to 150 ° C., a number average molecular weight of 1,000 to 200,000, preferably 10,000 to 100,000, and a polymerization degree of about 50 to 1000 is used. can do.

  Examples include vinyl chloride, vinyl acetate, vinyl alcohol, maleic acid, acrylic acid, acrylic ester, vinylidene chloride, acrylonitrile, methacrylic acid, methacrylic ester, styrene, butadiene, ethylene, vinyl butyral, vinyl acetal, There are polymers or copolymers containing vinyl ether as a structural unit, polyurethane resins, and various rubber resins. Thermosetting resins or reactive resins include phenolic resins, epoxy resins, polyurethane curable resins, urea resins, melamine resins, alkyd resins, acrylic reactive resins, formaldehyde resins, silicone resins, epoxy-polyamide resins, polyester resins. And a mixture of an isocyanate prepolymer, a mixture of polyester polyol and polyisocyanate, a mixture of polyurethane and polyisocyanate, and the like. These resins are described in detail in “Plastic Handbook” published by Asakura Shoten. It is also possible to use a known electron beam curable resin for each layer. These examples and their production methods are described in detail in JP-A No. 62-256219. The above resins can be used alone or in combination, but are preferably selected from vinyl chloride resin, vinyl chloride vinyl acetate copolymer, vinyl chloride vinyl acetate vinyl alcohol copolymer, vinyl chloride vinyl acetate vinyl maleic anhydride copolymer. A combination of at least one selected from the above and a polyurethane resin, or a combination of these with a polyisocyanate.

As the polyurethane resin, those having a known structure such as polyester polyurethane, polyether polyurethane, polyether polyester polyurethane, polycarbonate polyurethane, polyester polycarbonate polyurethane, and polycaprolactone polyurethane can be used. For all binding agents indicated herein, if necessary in order to obtain more excellent dispersibility and _{durability, -COOM, -SO 3 M, -OSO} 3 M, -P = O (OM) 2, - O—P═O (OM) ₂ (wherein M is a hydrogen atom or an alkali metal base), —OH, —NR ₂ , —N ⁺ R ₃ (R is a hydrocarbon group), epoxy group, —SH, — It is preferable to use one in which at least one polar group selected from CN and the like is introduced by copolymerization or addition reaction. The amount of such a polar group is, for example, 10 ⁻¹ to 10 ⁻⁸ mol / g, preferably 10 ⁻² to 10 ⁻⁶ mol / g.

  Specific examples of these binders used in the present invention include VAGH, VYHH, VMCH, VAGF, VAGD, VROH, VYES, VYNC, VMCC, XYHL, XYSG, PKHH, PKHJ, PKHC, PKFE, manufactured by Dow Chemical Co., Ltd. MPR-TA, MPR-TA5, MPR-TAL, MPR-TSN, MPR-TMF, MPR-TS, MPR-TM, MPR-TAO, Denki Kagaku 1000W, DX80, DX81, DX82, manufactured by Nissin Chemical Industry Co., Ltd. DX83, 100FD, Nippon Zeon MR-104, MR-105, MR110, MR100, MR555, 400X-110A, Nippon Polyurethane Nipporan N2301, N2302, N2304, Dainippon Ink, Pandex T-5105, T- R3080, T-5201 Bernock D-400, D-210-80, Crisbon 6109, 7209, Byron UR8200, UR8300, UR-8700, Toyobo Co., Ltd., RV530, RV280, Daiferamin 4020, 5020, 5100, 5300, 9020, 9022 7020, MX5004 manufactured by Mitsubishi Chemical Corporation, Samprene SP-150 manufactured by Sanyo Chemical Co., Ltd., Saran F310, F210 manufactured by Asahi Kasei Corporation, and the like.

For the nonmagnetic layer and the magnetic layer, a binder can be used in the range of, for example, 5 to 50% by mass, and preferably 10 to 30% by mass with respect to the nonmagnetic powder or the magnetic powder. It is preferable to use a combination of 5 to 30% by mass when using a vinyl chloride resin, 2 to 20% by mass when using a polyurethane resin, and 2 to 20% by mass of polyisocyanate. However, for example, when head corrosion occurs due to a small amount of dechlorination, it is possible to use only polyurethane or only polyurethane and isocyanate. When polyurethane is used, the glass transition temperature is −50 to 150 ° C., preferably 0 ° C. to 100 ° C., the breaking elongation is 100 to 2000%, the breaking stress is 0.05 to 10 kg / mm ² (0.49 to 98 MPa), The yield point is preferably 0.05 to 10 kg / mm ² (0.49 to 98 MPa).

  Examples of polyisocyanates include tolylene diisocyanate, 4,4′-diphenylmethane diisocyanate, hexamethylene diisocyanate, xylylene diisocyanate, naphthylene-1,5-diisocyanate, o-toluidine diisocyanate, isophorone diisocyanate, triphenylmethane triisocyanate and the like. Moreover, the product of these isocyanates and polyalcohol, the polyisocyanate produced | generated by condensation of isocyanate, etc. can be used. Commercially available product names of these isocyanates include Coronate L, Coronate HL, Coronate 2030, Coronate 2031, Millionate MR, Millionate MTL, Takeda Pharmaceutical Taketake D-102, Takenate D-110N, There are Takenate D-200, Takenate D-202, Death Module L, Death Module IL, Death Module N, Death Module HL, etc. manufactured by Sumitomo Bayer. These are used alone or two or more by utilizing the difference in curing reactivity. Each layer can be used in the above combination.

Additives can be added to the magnetic layer as necessary. Examples of the additive include an abrasive, a lubricant, a dispersant / dispersion aid, an antifungal agent, an antistatic agent, an antioxidant, a solvent, and carbon black. Examples of these additives include molybdenum disulfide, tungsten disulfide, graphite, boron nitride, graphite fluoride, silicone oil, silicone having a polar group, fatty acid-modified silicone, fluorine-containing silicone, fluorine-containing alcohol, fluorine-containing ester, Polyolefin, polyglycol, polyphenyl ether, phenylphosphonic acid, benzylphosphonic acid, phenethylphosphonic acid, α-methylbenzylphosphonic acid, 1-methyl-1-phenethylphosphonic acid, diphenylmethylphosphonic acid, biphenylphosphonic acid, benzylphenylphosphonic acid , Α-cumylphosphonic acid, toluylphosphonic acid, xylylphosphonic acid, ethylphenylphosphonic acid, cumenylphosphonic acid, propylphenylphosphonic acid, butylphenylphosphonic acid, heptylph Aromatic ring-containing organic phosphonic acids such as enylphosphonic acid, octylphenylphosphonic acid, nonylphenylphosphonic acid and alkali metal salts thereof, octylphosphonic acid, 2-ethylhexylphosphonic acid, isooctylphosphonic acid, isononylphosphonic acid, isodecylphosphonic acid Acids, isoundecyl phosphonic acid, isododecyl phosphonic acid, isohexadecyl phosphonic acid, isooctadecyl phosphonic acid, isoeicosyl phosphonic acid, alkylphosphonic acids and alkali metal salts thereof, phenyl phosphate, benzyl phosphate, phosphoric acid Phenethyl, α-methylbenzyl phosphate, 1-methyl-1-phenethyl phosphate, diphenylmethyl phosphate, biphenyl phosphate, benzylphenyl phosphate, α-cumyl phosphate, toluyl phosphate, xylyl phosphate, ethyl phosphate Phenyl Aromatic phosphates such as cumenyl phosphate, propylphenyl phosphate, butylphenyl phosphate, heptylphenyl phosphate, octylphenyl phosphate, nonylphenyl phosphate, and alkali metal salts thereof, octyl phosphate, 2-phosphate Alkyl phosphates such as ethylhexyl, isooctyl phosphate, isononyl phosphate, isodecyl phosphate, isoundecyl phosphate, isododecyl phosphate, isohexadecyl phosphate, isooctadecyl phosphate, isoeicosyl phosphate, and alkali metal salts thereof, alkyl sulfones Acid esters and alkali metal salts thereof, fluorine-containing alkyl sulfates and alkali metal salts thereof, lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, butyl stearate, oleic acid, linoleic acid, linolenic acid , Monobasic fatty acids which may contain or be branched, such as elaidic acid and erucic acid, and their metal salts, or butyl stearate, octyl stearate, amyl stearate, stearin One or more unsaturated bonds having 10 to 24 carbon atoms such as isooctyl acid, octyl myristate, butyl laurate, butoxyethyl stearate, anhydrosorbitan monostearate, anhydrosorbitan tristearate, etc. A basic fatty acid and a 1 to 6-valent alcohol which may contain or be branched with an unsaturated bond having 2 to 22 carbon atoms, an alkoxy alcohol which may contain or be branched with an unsaturated bond with 12 to 22 carbon atoms, or A module comprising any one of monoalkyl ethers of alkylene oxide polymer Fatty acid esters, difatty acid esters or polyvalent fatty acid esters, fatty acid amides having 2 to 22 carbon atoms, and aliphatic amines having 8 to 22 carbon atoms can be used. In addition to the above hydrocarbon groups, alkyl groups, aryl groups, and aralkyl groups substituted with nitro groups and groups other than hydrocarbon groups such as halogen-containing hydrocarbons such as F, Cl, Br, CF ₃ , CCl ₃ , and CBr ₃ You may have something.

  In addition, nonionic surfactants such as alkylene oxide, glycerin, glycidol, and alkylphenol ethylene oxide adducts, cyclic amines, ester amides, quaternary ammonium salts, hydantoin derivatives, heterocycles, phosphonium or sulfoniums, etc. Amphoteric interfaces such as cationic surfactants, anionic surfactants containing acidic groups such as carboxylic acid, sulfonic acid, and sulfate ester groups, amino acids, aminosulfonic acids, sulfuric or phosphate esters of aminoalcohols, and alkylbetaines An activator or the like can also be used. These surfactants are described in detail in “Surfactant Handbook” (published by Sangyo Tosho Co., Ltd.).

  The above-mentioned lubricant, antistatic agent and the like are not necessarily pure and may contain impurities such as isomers, unreacted materials, side reaction products, decomposition products, oxides, etc. in addition to the main components. These impurities are preferably 30% by mass or less, more preferably 10% by mass or less.

  Specific examples of these additives include, for example, manufactured by NOF Corporation: NAA-102, castor oil hardened fatty acid, NAA-42, cation SA, Naimine L-201, Nonion E-208, Anon BF, Anon LG, Takemoto Oils and fats: FAL-205, FAL-123, Shin Nippon Chemical Co., Ltd .: NJ Lube OL, Shin-Etsu Chemical Co., Ltd .: TA-3, Lion Corporation: Armide P, Lion Corporation: Duomin TDO, Nisshin Oilio Co., Ltd .: BA-41G, Sanyo Kasei Co., Ltd .: Profan 2012E, New Pole PE61, Ionette MS-400, etc. are mentioned.

Carbon black can be added to the magnetic layer as necessary. Examples of carbon black that can be used in the magnetic layer include rubber furnace, rubber thermal, color black, and acetylene black. Specific surface area is 5 to 500 m ² / g, DBP oil absorption is 10 to 400 ml / 100 g, particle size is 5 to 300 nm, pH is 2 to 10, moisture content is 0.1 to 10%, tap density is 0.1 to 1 g / ml is preferred.

  Specific examples of carbon black include BLACKPEARLS 2000, 1300, 1000, 900, 905, 800, 700, VULCAN XC-72, manufactured by Cabot Corporation, # 80, # 60, # 55, # 50, # 50 manufactured by Asahi Carbon Co., Ltd. 35, Mitsubishi Chemical Corporation # 2400B, # 2300, # 900, # 1000, # 30, # 40, # 10B, Colombian Carbon Corporation CONDUCTEX SC, RAVEN150, 50, 40, 15, RAVEN-MT-P, Ketjen・ Ketjen Black EC manufactured by Black International, etc. Carbon black may be surface-treated with a dispersant, or may be used after being grafted with a resin, or may be obtained by graphitizing a part of the surface. Carbon black may be dispersed with a binder in advance before being added to the magnetic coating. These carbon blacks can be used alone or in combination. When using carbon black, it is preferable to use 0.1-30 mass% with respect to the mass of a magnetic body. Carbon black functions to prevent the magnetic layer from being charged, reduce the coefficient of friction, impart light-shielding properties, and improve the film strength. These differ depending on the carbon black used. Therefore, these carbon blacks used in the present invention have different types, amounts, and combinations in the magnetic layer and the non-magnetic layer, and are based on the above-mentioned characteristics such as particle size, oil absorption, conductivity, pH, etc. Of course, it is possible to use them properly according to the purpose, but rather they should be optimized in each layer. For the carbon black that can be used in the magnetic layer of the present invention, for example, “Carbon Black Handbook” edited by Carbon Black Association can be referred to.

As abrasives , α-alumina, β-alumina, silicon carbide, chromium oxide, cerium oxide, α-iron oxide, corundum, artificial diamond, silicon nitride, silicon carbide titanium carbide, oxide with an α conversion rate of 90% or more Known materials having a Mohs hardness of 6 or more, such as titanium, silicon dioxide, and boron nitride, can be used alone or in combination. Moreover, you may use the composite_body | complex (what surface-treated the abrasive | polishing agent with the other abrasive | polishing agent) of these abrasive | polishing agents. These abrasives may contain compounds or elements other than the main component, but the effect is not changed if the main component is 90% or more. The particle size of these abrasives is preferably from 0.01 to 2 μm, and in particular, in order to improve electromagnetic conversion characteristics, it is preferable that the particle size distribution is narrow. Further, in order to improve the durability, it is possible to combine abrasives having different particle sizes as necessary, or to obtain the same effect by widening the particle size distribution even with a single abrasive. The tap density is preferably 0.3 to ² g / cc, the water content is preferably 0.1 to 5%, the pH is 2 to 11, and the specific surface area is preferably 1 to 30 m ² / g. The shape of the abrasive may be any of acicular, spherical, dice, and plate shapes, but those having a corner in a part of the shape are preferable because of high abrasiveness. Specifically, AKP-12, AKP-15, AKP-20, AKP-30, AKP-50, HIT-20, HIT-30, HIT-55, HIT-60, HIT-70, HIT- manufactured by Sumitomo Chemical Co., Ltd. 80, HIT-100, Reynolds ERC-DBM, HP-DBM, HPS-DBM, Fujimi Abrasives WA10000, Uemura Kogyo UB20, Nippon Kagaku Kogyo G-5, Chromex U2, Chromex U1, Toda Kogyo TF100, TF140, Ibiden Beta Random Ultra Fine, Showa Mining B-3, and the like. These abrasives can be added to the nonmagnetic layer as required. By adding to the nonmagnetic layer, the surface shape can be controlled, and the protruding state of the abrasive can be controlled. Of course, the particle size and amount of the abrasive added to the magnetic layer and the nonmagnetic layer should be set to optimum values.

  Known organic solvents can be used. As the organic solvent, specifically, ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, cyclohexanone, isophorone, tetrahydrofuran, etc., methanol, ethanol, propanol, butanol, isobutyl alcohol, isopropyl alcohol, in any ratio Alcohols such as methylcyclohexanol, esters such as methyl acetate, butyl acetate, isobutyl acetate, isopropyl acetate, ethyl lactate, glycol acetate, glycol ethers such as glycol dimethyl ether, glycol monoethyl ether, dioxane, benzene, toluene, xylene , Aromatic hydrocarbons such as cresol, chlorobenzene, methylene chloride, ethylene chloride, carbon tetrachloride, chloroform, ethyl Nkuroruhidorin, chlorinated hydrocarbons such as dichlorobenzene, N, N- dimethylformamide, may be used hexane.

  These organic solvents are not necessarily 100% pure, and may contain impurities such as isomers, unreacted materials, side reaction products, decomposition products, oxides, and moisture in addition to the main components. These impurities are preferably 30% by mass or less, more preferably 10% by mass or less. The organic solvent used in the present invention is preferably the same in the magnetic layer and the nonmagnetic layer. The amount added may be changed. Use non-magnetic layers with high surface tension solvents (cyclohexanone, dioxane, etc.) to increase coating stability. Specifically, the arithmetic average value of the upper layer solvent composition may not fall below the arithmetic average value of the nonmagnetic layer solvent composition. It is essential. In order to improve the dispersibility, it is preferable that the polarity is somewhat strong, and it is preferable that 50% by mass or more of the solvent having a dielectric constant of 15 or more is included in the solvent composition. Moreover, it is preferable that a solubility parameter is 8-11.

  These dispersants, lubricants, and surfactants used in the present invention can be properly used in the magnetic layer and further in the nonmagnetic layer described later as needed. For example, of course, the dispersant is not limited to the example shown here, but the dispersant has a property of adsorbing or binding with a polar group, and in the magnetic layer, mainly on the surface of the ferromagnetic metal powder and nonmagnetic. In the layer, it is presumed that the organic phosphorus compound adsorbed or bonded mainly to the surface of the non-magnetic powder with the polar group, for example, is difficult to desorb from the surface of the metal or the metal compound once adsorbed. Therefore, since the surface of the ferromagnetic metal powder or the nonmagnetic powder of the present invention is in a state of being coated with an alkyl group, an aromatic group, etc., the affinity for the binder component of the ferromagnetic metal powder or the nonmagnetic powder. And the dispersion stability of the ferromagnetic metal powder or nonmagnetic powder can be improved. In addition, since the lubricant exists in a free state, fatty acids with different melting points are used in the non-magnetic layer and the magnetic layer, and bleeding is controlled on the surface using esters with different boiling points and polarities. It is conceivable to improve the coating stability by controlling the amount of the surfactant, to improve the lubrication effect by increasing the additive amount of the lubricant in the nonmagnetic layer. All or a part of the additives used in the present invention may be added in any step during the production of the coating liquid for the magnetic layer or nonmagnetic layer. For example, when mixing with a ferromagnetic powder before the kneading step, when adding at a kneading step with a ferromagnetic powder, a binder and a solvent, when adding at a dispersing step, when adding after dispersing, when adding just before coating, etc. There is.

Non-magnetic layer Next, detailed contents regarding the non-magnetic layer will be described. The magnetic recording medium of the present invention can have a nonmagnetic layer containing a nonmagnetic powder and a binder on a nonmagnetic support. The nonmagnetic powder that can be used in the nonmagnetic layer may be an inorganic substance or an organic substance. Carbon black or the like can also be used. Examples of the inorganic substance include metals, metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, and metal sulfides.

Specifically, titanium oxide such as titanium dioxide, cerium oxide, tin oxide, tungsten oxide, ZnO, ZrO ₂ , SiO ₂ , Cr ₂ O ₃ , α-alumina, β-alumina having an α conversion of 90 to 100%, γ-alumina, α-iron oxide, goethite, corundum, silicon nitride, titanium carbide, magnesium oxide, boron nitride, molybdenum disulfide, copper oxide, MgCO ₃ , CaCO ₃ , BaCO ₃ , SrCO ₃ , BaSO ₄ , silicon carbide Titanium carbide or the like can be used alone or in combination of two or more. Preferred are α-iron oxide and titanium oxide.

  The shape of the nonmagnetic powder may be any of acicular, spherical, polyhedral and plate shapes. The crystallite size of the nonmagnetic powder is preferably 4 nm to 500 nm, more preferably 40 to 100 nm. If the crystallite size is in the range of 4 nm to 500 nm, it is preferable because dispersion does not become difficult and it has a suitable surface roughness. The average particle size of these non-magnetic powders is preferably 5 nm to 500 nm. However, if necessary, non-magnetic powders having different average particle sizes may be combined, or a single non-magnetic powder may have a wide particle size distribution. It can also have an effect. The average particle size of the particularly preferred nonmagnetic powder is 10 to 200 nm. The range of 5 nm to 500 nm is preferable because the dispersion is good and the surface roughness is suitable.

The specific surface area of the nonmagnetic powder is, for example, 1 to 150 m ² / g, preferably 20 to 120 m ² / g, and more preferably 50 to 100 m ² / g. A specific surface area in the range of 1 to 150 m ² / g is preferred because it has a suitable surface roughness and can be dispersed with a desired amount of binder. The oil absorption using dibutyl phthalate (DBP) is, for example, 5 to 100 ml / 100 g, preferably 10 to 80 ml / 100 g, and more preferably 20 to 60 ml / 100 g. The specific gravity is, for example, 1 to 12, preferably 3 to 6. The tap density is, for example, 0.05 to 2 g / ml, preferably 0.2 to 1.5 g / ml. When the tap density is in the range of 0.05 to 2 g / ml, there are few particles to be scattered, the operation is easy, and there is a tendency that it is difficult to adhere to the apparatus. The pH of the non-magnetic powder is preferably 2 to 11, and particularly preferably 6 to 9. When the pH is in the range of 2 to 11, it is possible to prevent the friction coefficient from increasing due to high temperature, high humidity, or liberation of fatty acids. The water content of the nonmagnetic powder is, for example, 0.1 to 5% by mass, preferably 0.2 to 3% by mass, and more preferably 0.3 to 1.5% by mass. A water content in the range of 0.1 to 5% by mass is preferable because the dispersion is good and the viscosity of the paint after dispersion is stable. The ignition loss is preferably 20% by mass or less, and the ignition loss is preferably small.

When the nonmagnetic powder is an inorganic powder, the Mohs hardness is preferably 4-10. If the Mohs hardness is in the range of 4 to 10, durability can be ensured. The nonmagnetic powder has a stearic acid adsorption amount of, for example, 1 to 20 μmol / m ² , and more preferably ² to 15 μmol / m ² . The heat of wetting of the nonmagnetic powder into water at 25 ° C. is preferably in the range of 200 to 600 erg / cm ² (200 to 600 mJ / m ² ). Moreover, the solvent which exists in the range of this heat of wetting can be used. The amount of water molecules on the surface at 100 to 400 ° C. is suitably 1 to 10 / 100Å. The pH of the isoelectric point in water is preferably between 3 and 9. It is preferable that Al ₂ O ₃ , SiO ₂ , TiO ₂ , ZrO ₂ , SnO ₂ , Sb ₂ O ₃ and ZnO are present on the surface of these nonmagnetic powders by surface treatment. Particularly preferred for dispersibility are Al ₂ O ₃ , SiO ₂ , TiO ₂ and ZrO ₂ , and more preferred are Al ₂ O ₃ , SiO ₂ and ZrO ₂ . These may be used in combination or may be used alone. Further, a surface-treated layer co-precipitated according to the purpose may be used, or a method of treating the surface layer with silica after first treating with alumina, or vice versa may be employed. The surface treatment layer may be a porous layer depending on the purpose, but it is generally preferable that the surface treatment layer is homogeneous and dense.

  Specific examples of the nonmagnetic powder used for the nonmagnetic layer include, for example, Showa Denko Nanotite, Sumitomo Chemical HIT-100, ZA-G1, Toda Kogyo DPN-250, DPN-250BX, and DPN-245. , DPN-270BX, DPB-550BX, DPN-550RX, manufactured by Ishihara Sangyo TTO-51B, TTO-55A, TTO-55B, TTO-55C, TTO-55S, TTO-55D, SN-100, MJ-7, α-iron oxide E270, E271, E300, STT-4D, STT-30D, STT-30, STT-65C, manufactured by Titanium Industry, MT-100S, MT-100T, MT-150W, MT-500B, T-600B manufactured by Teika , T-100F, T-500HD, Sakai Chemical FINEX-25, BF-1, BF-10, BF- 0, ST-M, Dowa Mining Ltd. DEFIC-Y, DEFIC-R, manufactured by Japan Aerosil AS2BM, TiO2P25, manufactured by Ube Industries, Ltd. 100A, 500A, include those fired Titan Kogyo Ltd. Y-LOP and it. Particularly preferred nonmagnetic powders are titanium dioxide and α-iron oxide.

Carbon black can be mixed in the nonmagnetic layer together with nonmagnetic powder to lower the surface electrical resistance, reduce the light transmittance, and obtain a desired micro Vickers hardness. The micro-Vickers hardness of the nonmagnetic layer is generally 25~60kg / mm ² (245~588MPa), preferably in order to adjust the head contact, a 30~50kg / mm ² (294~490MPa), thin film hardness meter ( Using a HMA-400 manufactured by NEC, measurement can be performed using a diamond triangular pyramid needle having a ridge angle of 80 degrees and a tip radius of 0.1 μm at the tip of the indenter. For details, refer to Realize Co., Ltd. It is standardized that the light transmittance is generally 3% or less for absorption of infrared rays having a wavelength of about 900 nm, for example, 0.8% or less for a VHS magnetic tape. For this purpose, rubber furnace, rubber thermal, color black, acetylene black and the like can be used.

The specific surface area of the carbon black employed in the nonmagnetic layer is, for example, 100 to 500 m ² / g, preferably from 150 to 400 m ² / g, DBP oil absorption, for example, 20 to 400 ml / 100 g, preferably 30 to 200 ml / 100 g is there. The particle size of carbon black is, for example, 5 to 80 nm, preferably 10 to 50 nm, and more preferably 10 to 40 nm. Carbon black preferably has a pH of 2 to 10, a water content of 0.1 to 10%, and a tap density of 0.1 to 1 g / ml.

  Specific examples of carbon black that can be used for the nonmagnetic layer include Cabot's BLACKPEARLS 2000, 1300, 1000, 900, 800, 880, 700, VULCAN XC-72, Mitsubishi Chemical Corporation # 3050B, # 3150B. , # 3250B, # 3750B, # 3950B, # 950, # 650B, # 970B, # 850B, MA-600, Columbia Carbon Corporation CONDUCTEX SC, RAVEN8800, 8000, 7000, 5750, 5250, 3500, 2100, 2000, 1800 1500, 1255, 1250, and Ketjen Black EC manufactured by Ketjen Black International.

  Carbon black may be surface-treated with a dispersant, or may be grafted with a resin, or may be obtained by graphitizing a part of the surface. Moreover, before adding carbon black to a coating liquid, you may disperse | distribute with a binder previously. These carbon blacks can be used in a range not exceeding 50% by mass with respect to the inorganic powder and in a range not exceeding 40% of the total mass of the nonmagnetic layer. These carbon blacks can be used alone or in combination. The carbon black that can be used in the nonmagnetic layer of the present invention can be referred to, for example, “Carbon Black Handbook” edited by Carbon Black Association.

  Further, an organic powder can be added to the nonmagnetic layer according to the purpose. Examples of such organic powder include acrylic styrene resin powder, benzoguanamine resin powder, melamine resin powder, and phthalocyanine pigment, but polyolefin resin powder, polyester resin powder, polyamide resin powder, polyimide resin, and the like. Resin powder and polyfluorinated ethylene resin can also be used. As the production method, those described in JP-A Nos. 62-18564 and 60-255827 can be used.

  As the binder, lubricant, dispersant, additive, solvent, dispersion method and the like of the nonmagnetic layer, those of the magnetic layer can be applied. In particular, known techniques relating to the magnetic layer can be applied to the amount of binder, type, additive, and amount of dispersant added, and type.

  The magnetic recording medium of the present invention may be provided with an undercoat layer. By providing the undercoat layer, the adhesive force between the support and the magnetic layer or the nonmagnetic layer can be improved. As an undercoat layer for improving adhesion, a polyester resin soluble in a solvent can be used. As will be described later, a smoothing layer can be provided as an undercoat layer.

Layer Structure As described above, the thickness structure of the magnetic recording medium of the present invention is such that the thickness of the nonmagnetic support is preferably 3 to 80 μm, more preferably 3 to 50 μm, and particularly preferably 3 to 10 μm. When an undercoat layer is provided between the nonmagnetic support and the nonmagnetic layer or the magnetic layer, the thickness of the undercoat layer is, for example, 0.01 to 0.8 μm, preferably 0.02 to 0.6 μm.

In addition, an intermediate layer for smoothing can be provided between the support and the nonmagnetic layer or magnetic layer, and between the support and the backcoat layer. For example, the surface of the nonmagnetic support contains a polymer. Apply and dry the applied coating solution, or apply a coating solution containing a compound having a radiation curable functional group in the molecule (radiation curable compound), and then irradiate with radiation to cure the coating solution Can be formed.
The number average molecular weight of the radiation curable compound is preferably in the range of 200 to 2,000. When the molecular weight is within this range, the molecular weight is relatively low, so that the coating film is easy to flow in the calendar process, has high moldability, and a smooth coating film can be formed.
Preferred as the radiation curable compound is a bifunctional acrylate compound having a molecular weight of 200 to 2000, and more preferred are bisphenol A, bisphenol F, hydrogenated bisphenol A, hydrogenated bisphenol F, and these alkylene oxide adducts. Acid and methacrylic acid are added.

The radiation curable compound may be used in combination with a polymer-type binder. Examples of the binder used in combination include conventionally known thermoplastic resins, thermosetting resins, reactive resins, and mixtures thereof. When ultraviolet rays are used as radiation, it is preferable to use a polymerization initiator in combination. As the polymerization initiator, known radical photopolymerization initiators, cationic photopolymerization initiators, photoamine generators, and the like can be used.
The radiation curable compound can also be used for the nonmagnetic layer.

  The thickness of the magnetic layer is optimized according to the saturation magnetization amount, head gap length, and recording signal band of the magnetic head to be used, but is generally 10 to 150 nm, preferably 20 to 120 nm, and more preferably. Is 30 to 100 nm, particularly preferably 30 to 80 nm. Further, the thickness variation rate (σ / δ) of the magnetic layer is preferably within ± 50%, and more preferably within ± 30%. There may be at least one magnetic layer, and the magnetic layer may be separated into two or more layers having different magnetic characteristics, and a configuration related to a known multilayer magnetic layer can be applied.

  The thickness of the nonmagnetic layer is, for example, 0.1 to 3.0 μm, preferably 0.3 to 2.0 μm, and more preferably 0.5 to 1.5 μm. The non-magnetic layer of the magnetic recording medium of the present invention exhibits its effect if it is substantially non-magnetic. For example, even if it contains a small amount of magnetic material as an impurity or intentionally, This shows the effect of the invention and can be regarded as substantially the same configuration as the magnetic recording medium of the invention. Note that “substantially the same” means that the residual magnetic flux density of the nonmagnetic layer is 10 mT or less or the coercive force is 7.96 kA / m (100 Oe) or less, and preferably has no residual magnetic flux density and coercive force. Means.

Backcoat Layer The magnetic recording medium of the present invention has a backcoat layer on the surface opposite to the surface having the magnetic layer of the nonmagnetic support. The back coat layer preferably contains carbon black and inorganic powder. For the binder and various additives, the formulation of the magnetic layer and the nonmagnetic layer can be applied. It is particularly preferable to apply the prescription for the nonmagnetic layer. The thickness of the back coat layer is preferably 0.9 μm or less, and more preferably 0.1 to 0.7 μm.

[Production method]
The process for producing the coating liquid for forming the magnetic layer coating material, the nonmagnetic layer or the backcoat layer comprises at least a kneading process, a dispersing process, and a mixing process provided before and after these processes. Each process may be divided into two or more stages. All raw materials such as ferromagnetic powder, non-magnetic powder, binder, carbon black, abrasive, antistatic agent, lubricant, and solvent used in the present invention may be added at the beginning or during any step. In addition, individual raw materials may be added in two or more steps. For example, polyurethane may be divided and added in a kneading step, a dispersing step, and a mixing step for adjusting the viscosity after dispersion. In order to achieve the object of the present invention, a conventional known manufacturing technique can be used as a partial process. In the kneading step, it is preferable to use a kneading force such as an open kneader, a continuous kneader, a pressure kneader, or an extruder. Details of these kneading treatments are described in JP-A-1-106338 and JP-A-1-79274. Further, glass beads can be used to disperse the magnetic layer paint, nonmagnetic layer paint or back layer paint. Such glass beads are preferably zirconia beads, titania beads, and steel beads, which are high specific gravity dispersion media. The particle diameter and filling rate of these dispersion media are optimized. A well-known thing can be used for a disperser.

  In the production process of the coating liquid, it is preferable to enhance the dispersion according to the dispersion conditions (bead type used for dispersion, bead amount, peripheral speed, dispersion time). Furthermore, as described above, in order to effectively suppress re-aggregation during drying, it is preferable to classify the coating solution before coating. Classification processing includes natural sedimentation in which the particle size distribution is controlled by the liquid concentration and time, liquid concentration, rotation speed of the centrifuge, centrifugal sedimentation method in which the particle size distribution is controlled by the processing time, and any method is used in the present invention. It may be used.

  In the method of manufacturing a magnetic recording medium, for example, a magnetic layer coating solution is formed on the surface of a nonmagnetic support under running so as to have a predetermined film thickness. Here, a plurality of magnetic layer coating solutions may be applied sequentially or simultaneously, or a nonmagnetic layer coating solution and a magnetic layer coating solution may be applied sequentially or simultaneously. In general, the sequential multi-layer coating method tends to reduce interfacial fluctuations in the upper and lower layers compared to the simultaneous multi-layer coating method, and also tends to apply high shear to the coating solution when applying a thin magnetic layer. For this purpose, it is preferable to adopt a sequential multilayer coating method. The coating machine for applying the magnetic layer coating liquid or the nonmagnetic layer coating liquid includes air doctor coat, blade coat, rod coat, extrusion coat, air knife coat, squeeze coat, impregnation coat, reverse roll coat, transfer roll coat, gravure A coat, a kiss coat, a cast coat, a spray coat, a spin coat, etc. can be used. As for these, for example, “Latest Coating Technology” (May 31, 1983) issued by General Technology Center Co., Ltd. can be referred to.

  In the case of a magnetic tape, the magnetic layer coating liquid coating layer may be subjected to magnetic field orientation treatment using a cobalt magnet or solenoid on the ferromagnetic powder contained in the magnetic layer coating liquid coating layer. In the case of a disk, a sufficiently isotropic orientation may be obtained even without non-orientation without using an orientation device, but known random methods such as alternately arranging cobalt magnets obliquely and applying an alternating magnetic field with a solenoid. It is preferable to use an alignment device. In the case of a ferromagnetic metal powder, the isotropic orientation is generally preferably in-plane two-dimensional random, but can also be three-dimensional random with a vertical component. Further, isotropic magnetic characteristics can be imparted in the circumferential direction by using a well-known method such as a counter-polarized magnet and making it vertically oriented. In particular, when performing high density recording, vertical alignment is preferable. Moreover, circumferential orientation can also be achieved using spin coating.

  It is preferable that the drying position of the coating film can be controlled by controlling the temperature, air volume, and coating speed of the drying air, the coating speed is preferably 20 m / min to 1000 m / min, and the temperature of the drying air is preferably 60 ° C. or higher. Also, moderate pre-drying can be performed before entering the magnet zone.

The coating raw material thus obtained is once taken up by a take-up roll and then unwound from the take-up roll and subjected to a calendar process.
For the calendar process, for example, a super calendar roll or the like is used. The calendering improves the surface smoothness and eliminates the voids generated by the removal of the solvent during drying and improves the filling rate of the ferromagnetic powder in the magnetic layer. Obtainable. The step of calendering is preferably performed while changing the calendering conditions according to the smoothness of the surface of the coating raw material.

In general, the gloss value of the coating raw material decreases from the core side of the winding roll toward the outside, and the quality may vary in the longitudinal direction. It is known that the gloss value has a correlation (proportional relationship) with the surface roughness Ra. Therefore, in the calendar processing step, if the calendar processing conditions, for example, the calendar roll pressure is kept constant without changing, no measures are taken for the difference in smoothness in the longitudinal direction caused by the winding of the coating raw material. As a result, the final product also varies in quality in the longitudinal direction.
Accordingly, it is preferable to cancel the difference in smoothness in the longitudinal direction caused by winding the coating raw material by changing the calendar processing conditions, for example, the calendar roll pressure, in the calendar processing step. Specifically, it is preferable to reduce the pressure of the calendar roll from the core side of the coating raw material unwound from the winding roll toward the outside. According to the study by the present inventors, it has been found that when the pressure of the calender roll is lowered, the gloss value is lowered (smoothness is lowered). As a result, the difference in smoothness in the longitudinal direction caused by the winding of the coating raw material is offset, and a final product having no variation in quality in the longitudinal direction can be obtained.

  In addition, although the example which changes the pressure of a calendar roll was demonstrated above, it can carry out by controlling a calendar roll temperature, a calendar roll speed, and a calendar roll tension besides this. In consideration of the characteristics of the coating type medium, it is preferable to control the calender roll pressure and the calender roll temperature. By decreasing the calender roll pressure or lowering the calender roll temperature, the surface smoothness of the final product is lowered. Conversely, increasing the calender roll pressure or the calender roll temperature increases the surface smoothness of the final product.

  Apart from this, the magnetic recording medium obtained after the calendering process can be thermo-processed to allow thermosetting to proceed. Such thermo treatment may be appropriately determined depending on the formulation of the magnetic layer coating solution, and is, for example, 35 to 100 ° C, and preferably 50 to 80 ° C. The thermo treatment time is, for example, 12 to 72 hours, preferably 24 to 48 hours.

  As the calender roll, a heat-resistant plastic roll such as epoxy, polyimide, polyamide, polyamideimide or the like can be used. Moreover, it can also process with a metal roll.

  As calendering conditions, the temperature of the calender roll is, for example, in the range of 60 to 100 ° C., preferably in the range of 70 to 100 ° C., particularly preferably in the range of 80 to 100 ° C., and the pressure is, for example, 100 to 500 kg / cm. (98 to 490 kN / m), preferably 200 to 450 kg / cm (196 to 441 kN / m), particularly preferably 300 to 400 kg / cm (294 to 392 kN / m). .

As described above, the magnetic layer in the magnetic recording medium of the present invention has an average surface roughness Ra of 0.5 to 2.5 nm measured using an atomic force microscope (AFM), preferably 1 to 2.5 nm. 2 nm.
The ten-point average roughness Rz of the magnetic layer is preferably 30 nm or less. These can be controlled by controlling the surface properties by the filler of the support or the surface shape of the calendered roll. The curl is preferably within ± 3 mm.

  The obtained magnetic recording medium can be cut into a desired size using a cutting machine or the like. The cutting machine is not particularly limited, but is preferably provided with a plurality of pairs of rotating upper blades (male blades) and lower blades (female blades). The slitting speed, the engagement depth, and the upper blade (male blade) are appropriately selected. The peripheral speed ratio (upper blade peripheral speed / lower blade peripheral speed) between the blade and the lower blade (female blade), the continuous use time of the slit blade, and the like are selected.

Physical characteristics The saturation magnetic flux density of the magnetic layer of the magnetic recording medium of the present invention is preferably 100 to 400 mT. The coercive force (Hc) of the magnetic layer is preferably 143.2 to 318.3 kA / m (1800 to 4000 Oe), more preferably 159.2 to 278.5 kA / m (2000 to 3500 Oe). The coercive force distribution is preferably narrow, and SFD and SFDr are preferably 0.6 or less, and more preferably 0.3 or less.

The friction coefficient with respect to the head of the magnetic recording medium of the present invention is 0.50 or less, preferably 0.3 or less in the range of temperature -10 to 40 ° C. and humidity 0 to 95%. The surface resistivity is preferably 10 ⁴ to 10 ⁸ Ω / sq of the magnetic surface, and the charging position is preferably within −500 V to +500 V. The elastic modulus at 0.5% elongation of the magnetic layer is preferably 0.98 to 19.6 GPa (100 to 2000 kg / mm ² ) in each in-plane direction, and the breaking strength is preferably 98 to 686 MPa (10 to 70 kg). / Mm ² ), the elastic modulus of the magnetic recording medium is preferably 0.98 to 14.7 GPa (100 to 1500 kg / mm ² ) in each in-plane direction, and the residual spread is preferably 0.5% or less, 100 ° C. The thermal shrinkage at any of the following temperatures is preferably 1% or less, more preferably 0.5% or less, and most preferably 0.1% or less.

The glass transition temperature of the magnetic layer (the maximum point of the loss elastic modulus of the dynamic viscoelasticity measurement measured at 110 Hz by a dynamic viscoelasticity measuring device (for example, Leo Vibron)) is preferably 50 to 180 ° C., and that of the nonmagnetic layer is 0-180 degreeC is preferable. The loss elastic modulus is preferably in the range of 1 × 10 ⁷ to 8 × 10 ⁸ Pa (1 × 10 ⁸ to 8 × 10 ⁹ dyne / cm ² ), and the loss tangent is preferably 0.2 or less. If the loss tangent is too large, adhesion failure is likely to occur. These thermal characteristics and mechanical characteristics are preferably almost equal within 10% in each in-plane direction of the medium.

The residual solvent contained in the magnetic layer is preferably 100 mg / m ² or less, more preferably 10 mg / m ² or less. The porosity of the coating layer is preferably 40% by volume or less, more preferably 30% by volume or less for both the nonmagnetic layer and the magnetic layer. The porosity is preferably small in order to achieve high output, but it may be better to ensure a certain value depending on the purpose. For example, in the case of a disk medium in which repeated use is important, a larger void ratio is often preferable for running durability.

  In the magnetic recording medium of the present invention, these physical characteristics can be changed between the nonmagnetic layer and the magnetic layer according to the purpose. For example, the elastic modulus of the magnetic layer can be increased to improve running durability, and at the same time, the elastic modulus of the nonmagnetic layer can be made lower than that of the magnetic layer to improve the contact of the magnetic recording medium with the head.

As a magnetic recording / reproducing system using the magnetic recording medium of the present invention, a magnetism for reproducing a high-density recorded signal by an anisotropic magnetoresistive head (AMR head) or a giant magnetoresistive head (GMR head). A recording / reproducing system is preferred.
Normally, two types of fci and bpi are generally used as units representing linear recording density. fci represents the density physically recorded on the medium with the number of bit inversions per inch. On the other hand, bpi depends on the system in terms of the number of bits per inch including signal processing. For this reason, fci is usually used for pure performance evaluation of the medium. A preferable range of linear recording density when recording a signal on the magnetic recording medium of the present invention is 100 to 400 kfci. Furthermore, it is 175 kfci-400 kfci. In a system actually used, since it depends on signal processing, it is not uniquely determined, but as a guideline, performance at fci 0.5 to 1 times bpi is reflected. For this reason, the range of 200 kbpi to 800 kbpi, more preferably 350 kbpi to 800 kbpi is particularly preferable.

  The distance between the shields (sh-sh) of the reproducing head is, for example, 0.08 μm to 0.18 μm, and the reproducing track width is, for example, 0.5 μm to 3.5 μm. The GMR head uses a magnetoresistive effect that responds to the magnitude of the magnetic flux applied to the thin film magnetic head, and has an advantage that a high reproduction output that cannot be obtained by the induction head can be obtained. This is mainly due to the fact that the reproduction output of the GMR head is based on the change in the magnetic resistance and thus does not depend on the relative speed between the medium and the head. In particular, the read sensitivity of the GMR head is almost three times higher than that of the AMR head. By using such a GMR head as a reproducing head, it is possible to reproduce a high-density recorded signal with high sensitivity.

  Further, a high-sensitivity AMR head can be used as the reproducing head. A magnetoresistive coefficient is generally used as an index of head sensitivity. Normally used magnetoresistive elements have a thickness of 200 to 300 nm and a magnetoresistive coefficient of about 2%, while high-sensitivity AMR heads have about 2 to 5%. Even when a high-sensitivity AMR head is used, a signal recorded on the magnetic recording medium of the present invention can be reproduced with high sensitivity, and a high SNR can be obtained.

  When the magnetic recording medium of the present invention is a tape-like magnetic recording medium, a GMR head can be used as a reproducing head, so that even a signal recorded in a high frequency region can be reproduced with a higher S / N ratio. Therefore, the magnetic recording medium of the present invention is most suitable as a magnetic tape for computer data recording for higher density recording and a disk-shaped magnetic recording medium.

  Hereinafter, the present invention will be described more specifically with reference to examples. It should be noted that the components, ratios, operations, order, and the like shown here can be changed without departing from the spirit of the present invention, and should not be limited to the following examples. Further, “parts” in the examples means parts by mass unless otherwise indicated.

Preparation of magnetic layer coating solution (ferromagnetic powder: BaFe)
Ferromagnetic tabular hexagonal ferrite powder 100 parts Composition excluding oxygen (molar ratio): Ba / Fe / Co / Zn = 1/9 / 0.2 / 1
Hc: 176 kA / m (2200 Oe), average plate diameter: 20 nm, average plate ratio: 3
BET specific surface area: 65 m ² / g
σs: 49 A · m ² / kg (49 emu / g)
Polyurethane resin 17 parts Branched side chain-containing polyester polyol / diphenylmethane diisocyanate system, —SO ₃ Na = 400 eq / ton
α-Al ₂ O ₃ (particle size 0.15 μm) 5 parts diamond powder (average particle size: 60 nm) 1 part carbon black (average particle size: 20 nm) 1 part cyclohexanone 110 parts methyl ethyl ketone 100 parts toluene 100 parts butyl stearate 2 1 part stearic acid

Preparation of coating solution for nonmagnetic layer 85 parts of nonmagnetic inorganic powder α-iron oxide Surface treatment layer: Al ₂ O ₃ , SiO ₂
Average long axis length: 0.15 μm
Tap density: 0.8
Average needle ratio: 7
BET specific surface area: 52 m ² / g
pH 8
DBP oil absorption: 33g / 100g
Carbon black 15 parts
DBP oil absorption: 120ml / 100g
pH: 8
BET specific surface area: 250 m ² / g
Volatile content: 1.5%
10 parts of vinyl chloride resin MR110 manufactured by Nippon Zeon Co., Ltd.
Polyretane resin: 10 parts Branched side chain-containing polyester polyol / diphenylmethane diisocyanate system, —SO ₃ Na = 150 eq / ton
Phenylphosphonic acid 3 parts Cyclohexanone 140 parts Methyl ethyl ketone 170 parts Butyl stearate 2 parts Stearic acid 1 part

Preparation of Backcoat Layer Coating Liquid A Non-magnetic inorganic powder 85 parts α-iron oxide Surface treatment layer: Al ₂ O ₃ , SiO ₂
Average long axis length: 0.15 μm
Tap density: 0.8
Average needle ratio: 7
BET specific surface area: 52 m ² / g
pH 8
DBP oil absorption: 33g / 100g
20 parts of carbon black
DBP oil absorption: 120ml / 100g
pH: 8
BET specific surface area: 250 m ² / g
Volatile content: 1.5%
Vinyl chloride copolymer (Nippon Zeon MR104) 13 parts Polyurethane resin (Toyobo Co., Ltd. Byron UR820 6 parts Phenylphosphonic acid 3 parts Alumina powder (average particle size: 0.25 μm) 5 parts Cyclohexanone 140 parts Methyl ethyl ketone 170 parts Butyl stearate 2 parts Stearic acid 1 part

Preparation of Backcoat Layer Coating Solution B Carbon black (average particle size: 25 nm) 40.5 parts Carbon black (average particle size: 370 nm) 0.5 parts Barium sulfate 4.05 parts Nitrocellulose 28 parts Polyurethane resin (SO ₃ Na Group) 20 parts cyclohexanone 100 parts toluene 100 parts methyl ethyl ketone 100 parts

  About each of the said magnetic layer coating liquid, nonmagnetic layer coating liquid, and backcoat layer coating liquid, after kneading each component for 240 minutes with an open kneader, it disperse | distributed with the sand mill (dispersion time is described in Table 1). Four parts each of a trifunctional low molecular weight polyisocyanate compound (Coronate 3041 manufactured by Nippon Polyurethane) was added to the obtained dispersion, and the mixture was further stirred and mixed for 20 minutes, and then filtered using a filter having an average pore size of 0.5 μm. As for the non-magnetic layer coating solution and the back coat layer coating solution, the supernatant liquid treated at 3000 rpm (described in Table 1 centrifugal sedimentation time) with a Hitachi High-Tech centrifuge is used. A coating solution was prepared.

Next, on the non-magnetic support (PEN, thickness 6 μm, surface properties are described in “Support A surface” (magnetic layer forming side) and “Support B surface” (back layer forming side) in Table 1)) The non-magnetic coating solution is applied so that the thickness after drying is 1.5 μm, and then dried at 100 ° C., and then the magnetic layer coating solution 1 is applied so that the thickness after drying is 60 nm. Dried. The backcoat layer coating solution A was applied to the surface of the support opposite to the surface on which the magnetic layer was provided, and dried to form a backcoat layer having a thickness of 0.5 μm.
Thereafter, treatment was carried out at a speed of 100 m / min, a linear pressure of 350 kg / cm (343 kN / m), and a temperature of 100 ° C. with a seven-stage calendar made of a metal roll. Thereafter, heat treatment was performed at 70 ° C. for 24 hours, and slitting to a width of ½ inch was performed to produce the magnetic tape of Example 1.

[Examples 2-9, Comparative Examples 1-9]
The average plate diameter of hexagonal ferrite contained in the magnetic layer coating solution, the surface roughness of the support used, the dispersion time of each layer forming coating solution, the centrifugation time, or the back coating layer forming layer coating solution used A magnetic tape was produced in the same manner as in Example 1, except that the change was as described in 1.

  Each tape sample was evaluated by the following method using the sample in the initial state and the sample after storage at 60 ° C. and 10% for 1 week. The results are shown in Table 1.

Measuring method 1. Measurement of electromagnetic conversion characteristics
(i) BB-SNR
The electromagnetic conversion characteristics were measured using a drum tester (relative speed 2 m / sec). Using a write head with Bs = 1.7T and a gap length of 0.2 μm, linear recording density 100 kFCI (recording wavelength λ = 0.504 μm, carrier-signal 3.937 MHz), 400 kFCI (recording wavelength λ = 0.127 μm carrier-signal) 15.748 MHz) signal was recorded and reproduced with a GMR head (reproduction track width (Tw): 3.0 μm, sh-sh = 0.12 μm).
BB-SNR was determined by measuring the ratio of the output of 100 k and 400 FCI to the integrated noise of 0 to 200 kFCI and 0 to 400 Kfci.
(ii) K-SNR
The K-SNR was recorded and reproduced in the same manner as the BB-SNR measurement. The K-SNR was obtained by measuring the ratio of output to noise by using the value obtained by adding integral noise from −1 MHz to −0.3 MHz and +0.3 MHz to 1 MHz in the vicinity of the carrier signal as a noise value.

2. Measurement of Friction Coefficient The friction coefficient of the magnetic layer surface was measured with an ILC μ value measuring machine under the following conditions.
Target member AlTiC bar (φ = 2mm, surface roughness Ra = 4nm)
Load 50gf
Speed 20mm / sec

3. Back side winding figure (number of jumps out)
Each tape sample was incorporated in an LTO reel, and the drive run was performed once, and then the number of popping balls was counted.

Evaluation results As shown in Table 1, each of the magnetic layer coating solution, the non-magnetic layer coating solution, and the backcoat layer coating solution is subjected to advanced dispersion treatment and classification treatment, whereby the magnetic layer Ra is 0.5-2. the range of .5Nm, the surface of the magnetic layer PSD to (10 [mu] m) in range of 800~10000nm ^3, it was possible to control the back coat layer PSD to (10 [mu] m) in the range of 20000~80000nm ³ (examples 1-9 ). As described above, in Examples 1 to 9 in which the roughness and waviness of the magnetic layer and the backcoat layer were controlled, the change in electromagnetic conversion characteristics before and after storage was small. Also, the evaluation results of the winding shape and the friction coefficient were good.
On the other hand, in Comparative Example 1, since the Ra on the surface of the magnetic layer was too low, the friction coefficient was high, and sticking occurred during running. For this reason, the electromagnetic conversion characteristics could not be measured.
In Comparative Example 2, since the Ra on the surface of the magnetic layer was too high, the SNR was lowered due to the decrease in output.
In Comparative Example 3, since the PSD (10 μm) on the surface of the magnetic layer was too low, the friction coefficient was high, and sticking occurred during running. For this reason, the electromagnetic conversion characteristics could not be measured.
In Comparative Example 4, since the PSD (10 μm) on the surface of the magnetic layer was too high, the K-SNR was lowered due to increased noise.
In Comparative Example 5, since the PSD (10 μm) on the surface of the backcoat layer was low and the smoothness was too high, the winding shape was deteriorated and the protrusion was large.
In Comparative Examples 6 and 9, since the PSD (10 μm) on the surface of the backcoat layer was too high, the K-SNR was lowered due to an increase in noise due to reflection after storage. In particular, in Comparative Example 9, the K-SNR was lowered due to the image after storage.
In Comparative Example 7, since the particle size of the hexagonal ferrite powder was too small, the output could not be secured and the SNR was lowered.
In Comparative Example 8, since the particle size of the hexagonal ferrite powder was too large, noise increased and SNR decreased.

  The magnetic recording medium of the present invention is suitable as a magnetic recording medium for high density recording.

Claims (4)

A magnetic recording medium having a magnetic layer containing a hexagonal ferrite powder and a binder on one side of a nonmagnetic support and a backcoat layer on the other side,
The spectral density at a 10 μm pitch on the surface of the magnetic layer is in the range of 800 to 10,000 nm ³ ,
The spectral density at a 10 μm pitch on the backcoat layer surface is in the range of 20000 to 80000 nm ³ ,
The center plane average surface roughness Ra of the magnetic layer measured by an atomic force microscope is in the range of 0.5 to 2.5 nm, and the average plate diameter of the hexagonal ferrite powder is in the range of 10 to 40 nm. Characteristic magnetic recording tape . 2. The magnetic recording tape according to claim 1, which is used in a magnetic signal reproducing system using a giant magnetoresistive head as a reproducing head. A magnetic signal reproducing system comprising the magnetic recording tape according to claim 1 and a giant magnetoresistive head as a reproducing head. A magnetic signal reproducing method for reproducing a magnetic signal recorded on the magnetic recording tape according to claim 1 using a giant magnetoresistive magnetic head.