JP2007242161A - Magnetic recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a coating type magnetic recording medium attaining high recording density. <P>SOLUTION: The magnetic recording medium is characterized in that an SFD value of a part in the range from the surface of a magnetic layer to t/5 of the magnetic layer is included in the range of 0.5 to 0.8 of the SFD value of the whole magnetic layer when the film thickness of the magnetic layer is defined as t. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a coating-type magnetic recording medium obtained by forming a magnetic layer coating film on a flexible support, and to a magnetic recording medium having a high recording capacity, access speed, and transfer speed.

  Magnetic tape, which is one of the magnetic recording media, has various uses such as audio tape, video tape, and computer tape. In particular, in the field of data backup tape, as the capacity of a hard disk to be backed up increases, A storage capacity of 300 GB or more is commercialized, and it is essential to increase the capacity of the backup tape in order to cope with the further increase in the capacity of the hard disk in the future.

  In order to realize a high recording density, it is necessary to shorten the recording wavelength and the track width. Together, both can realize a high recording density. However, due to these, the SNR in recording / reproduction is lowered, and sufficient recording / reproduction cannot be performed. For this reason, magnetic recording media and recording / reproducing heads that can ensure SNR even with short wavelengths and narrow track widths have been developed. In particular, when an MR head using the magnetoresistive effect is put to practical use, a high reproduction output can be obtained and the SNR can be increased.

  When the MR head is used, the reproduction output can be increased without increasing the noise dependent on the electric circuit such as an amplifier, so that the SNR can be increased. However, since the magnetization transition width caused by the magnetic recording medium cannot be reduced, the recording resolution cannot be improved. Both high SNR and good resolution are necessary for high recording density.

  As described above, when an MR head is used to increase the recording density, it is important to achieve a high output in the magnetic recording medium, but it is also necessary to reduce the magnetization transition width as described above. come. The magnetization transition width of the magnetic recording medium depends on the head-medium spacing, the magnetic recording layer thickness, the residual magnetization of the magnetic recording medium, the coercive force, and the variation in coercive force represented by SFD. In particular, it is effective to reduce the SFD in order to reduce the magnetization transition width.

  In a metal thin film type magnetic recording medium in which a magnetic layer is formed by vapor deposition or sputtering, the SFD value has been reduced by optimizing materials and manufacturing conditions. In a so-called coating-type magnetic recording medium mainly containing ferromagnetic magnetic powder and a binder, SFD can be reduced by narrowing the variation in the particle diameter of the ferromagnetic magnetic powder.

  Ferromagnetic magnetic powder used for normal coating type magnetic recording media is fine metal powder mainly composed of iron-cobalt alloy. To ensure coercive force, the shape is anisotropic such as needle or spindle. It was necessary to make the shape that can be applied. Coating type magnetic recording media using such ferromagnetic magnetic powder are described in JP-A-2004-319838, JP-A-2005-025936, JP-A-2005-026603, JP-A-2005-032367, etc. We are trying to achieve high resolution with a magnetic recording medium using small particles, but it is still not enough.

JP 2004-319838 A JP 2005-025936 A JP-A-2005-026603 JP 2005-032367 A

  As described above, even when fine particles having a small particle size distribution are used, one reason why high resolution cannot be realized is a problem in the configuration of the magnetic recording medium. Higher coercive force and lower SFD value can improve the resolution of the portion near the recording head in the magnetic recording medium, but higher permeability is more advantageous in increasing the resolution in the portion away from the head. become. With a single magnetic layer, it has been difficult to achieve such a configuration.

  The present invention mainly addresses the above-mentioned problems in magnetic recording media, and its purpose is to change the magnetic characteristics in the thickness direction of the magnetic layer, despite the use of a single magnetic layer, and to increase the capacity. Provided is a magnetic recording medium that realizes high resolution corresponding to the increase in resolution.

  As a result of intensive studies to achieve the above-mentioned object, the present inventors pay attention to the SFD value of the magnetic layer, and by making the SFD near the surface relatively smaller than the SFD of other parts. And found that high resolution can be realized.

  The present invention has been completed based on the above findings. That is, the present invention is a magnetic recording medium having at least a magnetic layer on a flexible support, and the thickness of the magnetic layer is t / 5 within the range of t / 5 from the surface of the magnetic layer. The SFD value of the partial magnetic layer is included in the range of 0.5 to 0.8 of the SFD value of the entire magnetic layer. The major axis length and the minor axis length of 95% or more of the magnetic particles in the field image are obtained in the ratio of the major axis length to the minor axis length of the magnetic particles including the secondary particles contained in the magnetic layer obtained from the observed field image. The axial length ratio of the axial length is 1.5 or less, and the present invention relates to the magnetic recording medium according to claim 1 (claim 2).

  By using the magnetic recording medium described in the present invention, a sufficiently high resolution can be obtained in recording and reproduction, and thus a sufficiently low error rate can be realized at a high recording density.

  Hereinafter, preferred embodiments of the magnetic recording medium of the present invention will be described. However, the present invention is not limited to the following examples. The magnetic recording medium is for digital recording, and an upper magnetic layer is provided on at least one surface of the flexible magnetic support. In particular, in a recording medium that requires a high recording density, a lower nonmagnetic layer can be provided adjacently between the flexible support and the upper magnetic layer. In the case of a tape medium that particularly requires high running reliability, a backcoat layer can be provided on the other surface of the surface coating layer comprising the lower nonmagnetic layer and the upper magnetic layer of the support. Hereinafter, the nonmagnetic support, the lower nonmagnetic layer, the upper magnetic layer, and the backcoat layer for carrying out the present invention will be described in detail.

The Young's modulus in the longitudinal direction of the nonmagnetic support is preferably 5.9 GPa (600 kg / mm ² ) or more, and the Young's modulus in the width direction is preferably 3.9 GPa (400 kg / mm ² ) or more. More preferably, the Young's modulus is 9.9 GPa (1000 kg / mm ² ) or more, and the Young's modulus in the width direction is 7.9 GPa (800 kg / mm ² ) or more. The Young's modulus in the longitudinal direction of the non-magnetic support is preferably 5.9 GPa (600 kg / mm ² ) or more. If the Young's modulus in the longitudinal direction is less than 5.9 GPa (600 kg / mm ² ), the tape running becomes unstable. Because it becomes. The non-magnetic support preferably has a Young's modulus in the width direction of 3.9 GPa (400 kg / mm ² ) or more. If the Young's modulus in the width direction is less than 3.9 GPa (400 kg / mm ² ), tape edge damage occurs. It is because it becomes easy to do.

  Nonmagnetic supports satisfying such properties include polyethylene terephthalate film, polyethylene naphthalate film, biaxially stretched aromatic polyamide base film, aromatic polyimide film and the like. In addition, although the thickness of a nonmagnetic support body changes with uses, a 1-7 micrometers thing is used normally. More preferably, it is 2.5-4.5 micrometers. Nonmagnetic supports with a thickness in this range are used because film formation is difficult if the thickness is less than 1 μm, and the tape strength decreases. If the thickness exceeds 7 μm, the total thickness of the tape increases, and the storage capacity per tape roll. This is because becomes smaller. The surface center line average roughness (Ra) of the nonmagnetic support on which the magnetic layer is formed is more preferably 2.5 nm or more and 20 nm or less. The reason why the thickness is 20 nm or less is that if the thickness is 20 nm or less, the unevenness on the surface of the lower nonmagnetic layer and the surface of the magnetic layer is reduced even if the lower nonmagnetic layer is made thinner.

  The lower nonmagnetic layer contains a nonmagnetic inorganic powder for the purpose of increasing strength, and the inorganic powder is preferably a metal oxide, an alkaline earth metal salt, or the like. Further, as the inorganic powder added to the lower nonmagnetic layer, iron oxide is preferable, the particle diameter is more preferably 50 to 400 nm, and the addition amount is 35 to 83% by weight based on the weight of the total inorganic powder. preferable. The particle size within this range is preferred because uniform dispersion is difficult when the particle size is less than 50 nm, and unevenness at the interface between the lower non-magnetic layer and the layer immediately above it increases when the particle size exceeds 400 nm. The addition amount in this range is preferable because the effect of improving the coating film strength is small if it is less than 35% by weight, and the coating film strength is lowered if it exceeds 83% by weight.

  It is preferable to add alumina to the lower nonmagnetic layer. The addition amount of alumina is more preferably 2 to 30% by weight, further preferably 8 to 20% by weight, and still more preferably 11 to 20% by weight based on the weight of the total nonmagnetic powder. The particle size of the alumina to be added is preferably 100 nm or less, more preferably 10 to 100 nm, more preferably 30 to 90 nm, and even more preferably 50 to 90 nm. Moreover, the alumina of the lower nonmagnetic layer is particularly preferably alumina mainly composed of a corundum phase. The amount of alumina added in the above range is preferable because if the amount is less than 2% by weight, the fluidity of the coating becomes insufficient, and if it exceeds 30% by weight, the unevenness between the lower nonmagnetic layer and the layer immediately above it becomes large. Alumina of 100 nm or less is preferable when a nonmagnetic support having a low surface roughness of 2.5 nm or more is used and the lower nonmagnetic layer is as thin as 1.5 μm or less. This is because if the particle size of alumina exceeds 100 nm, the smoothing effect on the surface of the lower nonmagnetic layer becomes insufficient. Alumina mainly composed of corundum phase (alpha conversion rate: 30% or more) is particularly good because the Young's modulus of the lower non-magnetic layer is increased in a small amount compared to the case of using σ, θ, γ alumina, etc. This is because the strength increases. Further, since the tape strength is also increased, the output variation due to the tape edge undulation (edge weave) is also improved.

  It is not excluded to add 100 to 800 nm α-alumina of less than 3% by weight based on the weight of the total inorganic powder together with the alumina having the above particle diameter.

  Carbon black (CB) is added to the lower nonmagnetic layer for the purpose of improving conductivity. As CB to be added, acetylene black, furnace black, thermal black or the like can be used. A particle size of 5 nm to 200 nm is used, but a particle size of 10 to 100 nm is preferable. This range is preferable because CB has a structure, so that dispersion of CB is difficult when the particle size is 10 nm or less, and smoothness is deteriorated when the particle size is 100 nm or more. The amount of CB added varies depending on the particle size of CB, but is preferably 15 to 40% by weight with respect to the total nonmagnetic powder. This range is preferable because if the amount is less than 15% by weight, the effect of improving conductivity is poor, and if it exceeds 40% by weight, the effect is saturated. It is more preferable to use 15 to 35% by weight of CB having a particle size of 15 nm to 80 nm, and it is even more preferable to use 20 to 30% by weight of CB having a particle size of 20 to 50 nm. By adding CB having such a particle size and amount, electric resistance is reduced, and generation of electrostatic noise and tape running unevenness are reduced.

As the magnetic powder added to the upper magnetic layer, acicular or substantially spherical ferromagnetic magnetic powder is used. In any case, the coercive force is preferably 135 kA / m to 360 kA / m (1700 to 4500 Oe), and preferably 175 kA / m to 290 kA / m (2200 to 3600 Oe). Saturation magnetization is ^preferably 70~200A · m 2 / kg (70~200emu / g) ^{is, 90~180A · m 2 / kg (} 90~180emu / g) is more preferable. The magnetic characteristics of the magnetic layer and the magnetic characteristics of the ferromagnetic magnetic powder are both measured by a sample vibration type magnetometer with an external magnetic field of 1.28 MA / m (16 kOe).

Moreover, as a maximum particle diameter of the acicular and substantially spherical ferromagnetic magnetic powder of this invention, 2-50 nm is preferable, 2-25 nm is more preferable, 3-20 nm is still more preferable. This range is preferable when the particle diameter is larger than 50 nm, particle noise based on the particle size increases, and it becomes difficult to improve the C / N characteristics. Further, when the maximum particle size is less than 2 nm, the coercive force is lowered, and at the same time, the cohesive force of the magnetic powder is increased, so that dispersion in the coating becomes difficult. The average major axis length is obtained by measuring the particle size of a photograph taken with a scanning electron microscope (SEM) and calculating the average value per 100 particles. Further, BET specific surface area of the acicular or substantially spherical ferromagnetic magnetic powder is preferably 35~100m ² / g, more preferably ^{40~80m 2 / g, 50~70m 2 /} g being most preferred.

  As the abrasive to be added to the upper magnetic layer, α-alumina and β-alumina having a number average particle size of 5 to 150 nm and a particle size distribution of 10 nm or less with a standard deviation and Mohs hardness of 6 or more are mainly used alone or in combination. The Among these, corundum type alumina (α conversion rate: 30% or more) is particularly good compared to the case of using σ, θ, γ alumina, etc., and has a head cleaning effect with a small addition amount. It is particularly preferable because it is excellent. Furthermore, single crystal alumina prepared by the CVD method is particularly preferable since it has a narrow particle size distribution and no sintering. As for the particle size of the alumina abrasive, although it depends on the thickness of the magnetic layer, it is usually more preferable that the average particle size is 20 to 100 nm, more preferably 30 to 90 nm. The addition amount is preferably 5 to 20% by weight with respect to the ferromagnetic iron-based metal powder. More preferably, it is 8 to 18% by weight.

  The reason why alumina having a particle size of 150 nm or less is preferable is that the wear resistance of the head increases when alumina having a particle size of 150 nm or more is present in the magnetic layer. The reason why alumina having a particle size of 50 nm or more is preferable is that durability and cleaning properties deteriorate when the particle size of alumina present in the magnetic layer is 50 nm or less. Furthermore, the standard deviation of the particle size distribution is preferably 10 nm or less. When alumina having a distribution wider than 10 nm is used, the wear of the head is increased in part where the large particle size alumina exists in the magnetic layer, and small particle size alumina exists. Durability and cleanability will deteriorate at the part where it does. This is because when such partial deterioration occurs, the characteristics vary, and finally sufficient performance cannot be achieved.

  The reason why the amount of alumina added is preferably 5% by weight or more is that when it is less than 5% by weight, the coating strength of the magnetic layer is lowered and the durability is deteriorated. In addition, the cleaning performance of the head by the coating film is extremely deteriorated, so that the dirt attached to the head cannot be scraped off. Further, the reason why 15% by weight or less is preferable is that the C / N characteristic is lowered when it exceeds 15% by weight.

  Further, conventionally known carbon black (CB) can be added to the upper magnetic layer of the present invention for the purpose of improving conductivity and improving surface lubricity. Examples of these CB include acetylene black, furnace black, thermal black, and thermal black. Black, etc. can be used. A particle size of 5 nm to 200 nm is used, but a particle size of 10 nm to 100 nm is preferable. This range is preferable because when the particle size is 10 nm or less, it is difficult to disperse CB. When the particle size is 100 nm or more, it is necessary to add a large amount of CB. In any case, the surface becomes rough and the output decreases. Because. The addition amount is preferably 0.2 to 5% by weight with respect to the ferromagnetic iron-based metal powder. More preferably, it is 0.5 to 4% by weight.

  Lubricants with different roles are used for the lower nonmagnetic layer and the upper magnetic layer. When the lower non-magnetic layer contains 0.5 to 4.0% by weight of higher fatty acid and 0.2 to 3.0% by weight of higher fatty acid ester relative to the total inorganic powder, And the coefficient of friction between the rotating cylinder and the head island is small. The addition of higher fatty acids within this range is preferable when the content is less than 0.5% by weight, and the effect of reducing the friction coefficient is small, and when the content exceeds 4.0% by weight, the undercoat layer is plasticized and the toughness is lost. In addition, the addition of higher fatty acid esters within this range is preferable when the content is less than 0.5% by weight, since the effect of reducing the friction coefficient is small, and when the content exceeds 3.0% by weight, the amount transferred to the magnetic layer is too large. This is because there are side effects such as sticking of the rotating cylinder or the head island.

  When the upper magnetic layer contains 0.5 to 3.0% by weight of fatty acid amide with respect to the ferromagnetic magnetic powder and 0.2 to 3.0% by weight of higher fatty acid ester, rotation with the tape This is preferable because the coefficient of friction with the cylinder becomes small. Fatty acid amides in this range are preferred when less than 0.2% by weight, the direct contact at the head / magnetic layer interface is likely to occur, and the effect of preventing seizure is small. Defects such as out occur. As the fatty acid amide, amides such as palmitic acid and stearic acid can be used. Also, the addition of higher fatty acid esters in the above range is preferred because the effect of reducing the friction coefficient is small if it is less than 0.2% by weight, and if it exceeds 3.0% by weight, there are side effects such as sticking of the tape and the rotating cylinder. It is. The mutual movement of the lubricant in the magnetic layer and the lubricant in the lower nonmagnetic layer is not excluded.

  The binder used for the lower non-magnetic layer and the upper magnetic layer is vinyl chloride resin, vinyl chloride-vinyl acetate copolymer resin, vinyl chloride-vinyl alcohol copolymer resin, vinyl chloride-vinyl acetate-vinyl alcohol copolymer resin, There is a combination of at least one selected from vinyl chloride-vinyl acetate-maleic anhydride copolymer resin, vinyl chloride-hydroxyl group-containing alkyl acrylate copolymer resin, nitrocellulose and the like, and a polyurethane resin. Among these, it is preferable to use a vinyl chloride-hydroxyl group-containing alkyl acrylate copolymer resin and a polyurethane resin in combination. Examples of the polyurethane resin include polyester polyurethane, polyether polyurethane, polyether polyester polyurethane, polycarbonate polyurethane, and polyester polycarbonate polyurethane. These binders are used in the range of 7 to 50% by weight, preferably 10 to 35% by weight, based on the ferromagnetic iron-based metal powder in the magnetic layer and the total nonmagnetic powder in the lower nonmagnetic layer. In particular, it is most preferable to use a composite of 5 to 30 parts by weight of a vinyl chloride resin and 2 to 20% by weight of a polyurethane resin as a binder.

COOH, SO ₃ M, OSO ₂ M, P═O (OM) ₃ , O—P═O (OM) ₂ , [M is a hydrogen atom, alkali metal base or amine salt], OH, NR ₁ R as functional groups ₂ , N ⁺ R ₃ R ₄ R ₅ [R ₁ , R ₂ , R ₃ , R ₄ , R ₅ are hydrogen or hydrocarbon groups], and a binder made of a polymer having an epoxy group is used. The reason why such a binder is used is that the dispersibility of magnetic powder and the like is improved as described above. When two or more kinds of resins are used in combination, the polarities of the functional groups are preferably matched, and among them, a combination of —SO ₃ M groups is preferable.

  In addition to these binders, it is desirable to use a thermosetting crosslinking agent that is bonded to a functional group contained in the binder and crosslinked. Examples of the crosslinking agent include tolylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, reaction products of these isocyanates with a plurality of hydroxyl groups such as trimethylolpropane, and condensation products of the above isocyanates. Various polyisocyanates are preferred. These crosslinking agents are usually used in a proportion of 10 to 50% by weight based on the binder. More preferably, it is 15 to 35% by weight.

  Organic solvents used in both layers are ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, cyclohexanone, isophorone and tetrahydrofuran, alcohols such as methanol, ethanol, propanol, butanol, isobutyl alcohol, isopropyl alcohol and 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, cresol, chlorobenzene, etc. Aromatic hydrocarbons, methylene chloride, ethylene chloride, carbon tetrachloride, chloroform, ethylene chlorohydrin Chlorinated hydrocarbons such as dichlorobenzene, N, N- dimethylformamide, may be used as a mixture in a ratio such as hexane, alone or in any.

  The back coat layer provided on the back surface of the surface coating layer is a conventionally known one and is intended to improve running performance. The thickness of the back coat layer is preferably 0.2 to 0.8 μm. This range is good because if it is less than 0.2 μm, the effect of improving the running performance is insufficient, and if it exceeds 0.8 μm, the total thickness of the tape becomes thick and the storage capacity per roll becomes small. The backcoat layer can be applied by a conventionally known gravure coating, roll coating, blade coating, die coating apparatus or the like.

  As carbon black (CB) used for the back coat layer, acetylene black, furnace black, thermal black, and the like can be used. Usually, small particle size carbon and large particle size carbon are used. As the small particle size carbon, those having a particle size of 5 nm to 200 nm are used, and those having a particle size of 10 nm to 100 nm are more preferable. This range is more preferable because when the particle size is 10 nm or less, it is difficult to disperse CB, and when the particle size is 100 nm or more, it is necessary to add a large amount of CB. This is because it causes a setback (embossing). When the large particle size carbon is 5 to 15% by weight of the small particle size carbon and the large particle size carbon having a particle size of 300 to 400 nm is used, the surface is not roughened and the effect of improving running performance is increased. The total addition amount of the small particle size carbon and the large particle size carbon is preferably 60 to 98% by weight, more preferably 70 to 95% by weight based on the weight of the inorganic powder. The surface roughness Ra is preferably 3 to 8 nm, and more preferably 4 to 7 nm.

  Further, it is preferable to add iron oxide having a particle size of 100 nm to 600 nm, more preferably 200 nm to 500 nm, for the purpose of improving strength. The addition amount is preferably 2 to 40% by weight, more preferably 5 to 30% by weight based on the weight of the inorganic powder.

  The main method for producing a magnetic paint includes the following methods. First, using a powerful kneader such as a kneader or a biaxial continuous kneader (extruder), magnetic powder and a small amount of binder resin are kneaded, and a solvent is added to obtain a solid content concentration of 35 to 45% ( A paste-like mill base is obtained by stirring on the basis of weight (the same applies hereinafter). The biaxial continuous kneader used in the kneading step is equipped with a device that can be heated and cooled in the kneading part (barrel), and the temperature of the kneading part is 20 to 50 ° C., preferably 25 to 35 ° C. It is adjusted by controlling. Here, if the temperature of the kneading part is less than 20 ° C., the wettability to the kneaded product cannot be improved and the dispersibility cannot be improved. If the temperature exceeds 50 ° C., the viscosity of the kneaded product decreases. Thus, a desired shear force cannot be applied. The kneading conditions for kneading in the kneading step are preferably a kneading time of 2 to 5 minutes and a feed rate of the kneaded product of 5 to 15 kg / h. Next, a dispersion operation is performed by a sand mill or the like to improve the dispersion state of the solid content.

  In order to finally obtain a good dispersion state of the paint, it is preferable to carry out finish dispersion. For finishing dispersion, dispersion by a sand mill using fine bead media of 3 mm or less, a method using a jet collision type disperser, and a method using ultrasonic dispersion are particularly preferable. These dispersions are preferably at a solid content concentration equivalent to that of the final coating material, and preferably 10 to 30%. In particular, sand mill dispersion using bead media and jet collision type dispersers are alternately installed, and by combining them in multiple stages, a more excellent divided state can be realized.

  Applying the average dry thickness of the upper magnetic layer with an arbitrary thickness of 1 nm to 100 nm with high accuracy and high productivity means that the lower nonmagnetic layer is placed immediately below the upper magnetic layer and the lower nonmagnetic layer is in a wet state. This can be realized by using a wet-on-wet simultaneous multi-layer coating method in which the upper magnetic layer is applied in an overlapping manner. The lower nonmagnetic layer and the upper magnetic layer are applied almost simultaneously by a single die application head incorporating two coating liquid passage slits. In order to improve the coating stability, the surface tension of the solvent used for the lower nonmagnetic layer is preferably higher than the surface tension of the solvent used for the upper magnetic layer. Examples of the solvent having a high surface tension include cyclohexanone and dioxane.

  Further, in order to manufacture a magnetic recording medium that realizes surface smoothness, a magnetic field orientation and a solvent drying step are provided after the surface layer coating step. In particular, in the solvent drying step, more appropriate magnetic properties can be obtained by combining a temporary cooling step by spraying with liquid nitrogen in the drying step in addition to the drying step that is generally performed.

  After applying the surface coating layer, the effect of the present invention can be increased by calendaring between metal rolls. Further, a heat-resistant plastic roll such as epoxy, polyimide, polyamide, polyimide amide or the like can be used as a calender roll. The treatment temperature is preferably 70 ° C. or higher, more preferably 80 ° C. or higher. The linear pressure is preferably 200 kg / cm, more preferably 300 kg / cm or more, and the speed is in the range of 20 m / min to 700 m / min. The effect of the present invention can be further enhanced with a linear pressure of 300 kg / cm or higher at a temperature of 80 ° C. or higher.

  The back coat layer is applied in any of the steps before and after the application of the surface coating layer and the calendar process. Further, after the coating of the surface coating layer and the back coating layer and the calendar treatment, an aging treatment at 40 ° C. to 80 ° C. may be performed in order to accelerate the curing of the surface coating layer and the back coating layer.

  The thickness of the upper magnetic layer of the magnetic recording medium prepared as described above is preferably 1 nm to 200 nm, more preferably 10 nm to 90 nm. This range is preferable because if the magnetic layer is less than 1 nm, the head output is small because the leakage magnetic field is small, and if it exceeds 200 nm, the head output is small due to thickness loss.

  The coercive force of the upper magnetic layer as a magnetic recording medium is preferably 135 kA / m to 360 kA / m (1700 to 4500 Oe) in the head running direction, and the residual magnetic flux density is preferably 0.25 T (2500 G) or more in the head running direction. This range is preferable because when the coercive force is less than 135 kA / m, the output decreases due to the demagnetizing field, and when it exceeds 360 kA / m, writing by the head becomes difficult. The reason why the residual magnetic flux density is preferably 0.25T (2500G) or more is that the output is reduced below 0.35T. Those having a coercive force of 175 kA / m to 290 kA / m (2200 to 3600 Oe) and a residual magnetic flux density of 0.3 T to 0.5 T (3,000 to 5000 G) are more preferable.

When the MR head is used as a reproducing head, the Mrt value, which is the product of the residual magnetic flux density in the longitudinal direction of the upper magnetic layer and the thickness of the magnetic layer, is preferably 72 Tnm (6.0 memu / cm ² ) or less. . The reason why Mrt is 72 Tnm or less is that most MR heads are saturated at 72 Tnm or more. Mrt is more preferably in the range of ^{2 to} 24 Tnm (0.2 to 2.00 memu / cm ² ).

  The SFD value of the upper magnetic layer is preferably in the range of 0.05 to 0.6, more preferably in the range of 0.05 to 0.5, and still more preferably in the range of 0.05 to 0.4. This range is preferable because it is extremely difficult to manufacture a magnetic recording medium when the SFD is smaller than 0.05. In addition, if the SFD exceeds 0.6, sufficient reproduction resolution cannot be obtained. Furthermore, it is preferable that the SFD value of the part of the magnetic layer in the range of t / 5 from the surface of the magnetic layer is in the range of 0.5 to 0.8 of the SFD value of the entire magnetic layer. This is because the resolution can be remarkably improved when the SFD in the portion near the recording head in the range of t / 5 from the surface is smaller than 0.5. However, in a portion away from the head, a higher magnetic permeability is advantageous for increasing the resolution, and therefore the degree of orientation in the longitudinal direction needs to be lowered. At this time, since the SFD in the portion away from the recording head is increased, an SFD distribution is generated in the depth direction of the head. Note that the SFD of the portion away from the recording head substantially matches the SFD value of the entire magnetic recording layer.

  Such SFD distribution is realized by optimizing the above-described upper magnetic layer finish dispersing step and drying step by appropriately combining them. In addition, it may be produced under normal drying conditions by combining three or more stages of bead mill dispersion, jet collision dispersion, and ultrasonic dispersion used for finishing dispersion. If the finishing dispersion is one stage, it may be 40 ° C. or higher during drying. You may manufacture on the conditions to cool.

  As described above, the SFD of the entire upper magnetic layer is measured by the vibration type magnetometer. However, since the SFD value of the magnetic layer in the range of t / 5 from the surface of the magnetic layer is too small, this magnetometer Insufficient sensitivity to measure with. Therefore, the magnetic measurement of this portion was performed using a SQUID magnetometer with a maximum external applied magnetic field of 0.78 MA / m (10 kOe).

  The particle size of the magnetic particles containing secondary particles contained in the magnetic layer obtained as follows is the maximum particle size (major axis length) of 95% of the magnetic particles in the field image obtained by SEM observation of the magnetic layer cross section. The standard deviation is preferably in the range of 0.05 nm to 3 nm. Furthermore, regarding the axial length ratio between the major axis length and the minor axis length of this magnetic particle, the major axis length to minor axis length ratio of 95% of the magnetic particles in the field image is 1.1-7. Is preferable, and 1.5 or less is more preferable.

  First, the recording surface of the magnetic recording medium was observed with a scanning electron microscope (SEM) and photographed. Thereafter, image processing was performed to identify the major axis length and minor axis length of each spherical magnetic particle, and the average value of major axis length / minor axis length was defined as the axis length ratio. Also, the field of view of the photograph was enlarged, and the statistical distribution of the particle size and axial length ratio in this field of view was determined. And the particle size of the particle | grain located in + 47.5% and -47.5% from the average particle diameter in distribution was made into the maximum particle size and the minimum particle size for convenience, respectively.

  The average line center roughness Ra of the surface coating layer is preferably included in the range of 0.2 to 3.0 nm, and more preferably in the range of 0.3 to 2.0 nm. The reason why Ra is preferably 3.0 nm or less is that when the wavelength exceeds 3.0 nm, the short wavelength component of the output is drastically reduced and the reproduction resolution is deteriorated. On the other hand, if Ra is less than 0.3 nm, the friction with the head and the traveling guide is increased, the durability is deteriorated, and it is difficult to produce Ra within this range, and the process cost is excessively high. The surface smoothness is a value obtained by measuring the visual field of 5 μm × 5 μm with 512 × 512 pixels using an atomic force microscope (AFM), and arithmetically averaging the absolute values from the average line of each point.

  The thickness of the lower nonmagnetic layer is usually 0.5 to 3 μm. More preferably, it is 1-2 micrometers. The lower non-magnetic layer having a thickness in this range is used because it is difficult to apply if the thickness is less than 0.5 μm, and the productivity is poor. If the thickness exceeds 3 μm, the total thickness of the tape becomes thick, This is because the storage capacity is reduced. A known undercoat layer may be provided between the nonmagnetic support and the lower nonmagnetic layer in order to improve adhesion. This thickness is 0.01 to 2 μm, preferably 0.05 to 0.5 μm.

  The Young's modulus of the surface coating layer composed of the lower nonmagnetic layer and the upper magnetic layer is preferably 40 to 100% of the average Young's modulus in the longitudinal direction and the width direction of the nonmagnetic support. When the Young's modulus of the coating layer is in this range, the durability of the tape is great and the contact property (head touch) between the tape and the head is improved. The range of 50 to 100% is more preferable, and the range of 60 to 90% is more preferable. This range is preferable because if the amount is less than 40%, the durability of the coating film becomes small, and if it exceeds 100%, the tape-head touch becomes poor. In the present invention, as a method for controlling the Young's modulus of the coating layer composed of the lower nonmagnetic layer and the upper magnetic layer, a control method based on calendar conditions is used.

  Furthermore, the Young's modulus of the lower nonmagnetic layer is preferably 80 to 99% of the Young's modulus of the upper magnetic layer. The reason why the Young's modulus of the lower nonmagnetic layer is preferably lower than that of the magnetic layer is that the lower nonmagnetic layer acts as a kind of cushion during calendar processing.

The friction coefficient with respect to the stainless steel of the surface coating layer surface of the magnetic recording medium of the present invention and the opposite back coating layer surface is preferably 0.5 or less, more preferably 0.3 or less. The surface resistivity of the surface coating layer is preferably 10 ⁴ to 10 ¹¹ ohm / sq, and the surface electrical resistance of the backcoat layer is preferably 10 ³ to 10 ⁹ ohm / sq. A cassette tape in which the medium prepared as described above is incorporated into a tape has a large capacity per roll and high reliability, and is particularly excellent as a data backup tape.

  EXAMPLES The present invention will be described in detail below by examples, but the present invention is not limited to these. In addition, the part of an Example and a comparative example shows a weight part.

Example 1
<< Coating ingredients for magnetic layer >>
(1) 100 parts of ferromagnetic iron-based metal magnetic powder (Co / Fe: 30 at%, Y / Fe: 2 at%,
Al / Fe: 5 wt%, σs: 110 A · m ² / kg,
(Hc: 280 kA / m, pH: 9.5, long axis length: 45 nm)
10 parts of vinyl chloride-hydroxypropyl acrylate copolymer (containing -SO ₃ Na group: 0.7 × 10 ^-4 equivalent / g)
4 parts Polyester polyurethane resin (containing _-SO 3 Na group: 1.0 × ^{10 -4} eq / g)
α-alumina 15 parts (center particle size 100 nm)
Carbon black 2 parts (average particle size: 75 nm, DBP oil absorption: 72 cc / 100 g)
Methyl acid phosphate 2 parts Palmitic acid amide 1.5 parts Tetrahydrofuran 65 parts Methyl ethyl ketone 245 parts Toluene 85 parts (2) Polyisocyanate 4 parts Cyclohexanone 167 parts
<< Coating ingredients for lower non-magnetic layer >>
(1) Iron oxide powder (particle size: 0.11 × 0.02 μm) 68 parts Alumina (α conversion: 50%, particle size: 70 nm) 8 parts Carbon black (particle size: 25 nm) 24 parts Stearin Acid 2 parts Vinyl chloride copolymer 10 parts (containing -SO ₃ Na group: 0.7 × 10 ^-4 equivalent / g)
Polyester polyurethane resin 4.5 parts (Tg: 40 ° C., contained -SO ₃ Na group: 1 × 10 ⁻⁴ equivalent / g)
Cyclohexanone 25 parts Methyl ethyl ketone 40 parts Toluene 10 parts (2) Butyl stearate 1 part Cyclohexanone 70 parts Methyl ethyl ketone 50 parts Toluene 20 parts (3) Polyisocyanate 4.5 parts Cyclohexanone 10 parts Methyl ethyl ketone 15 parts Toluene 10 parts The above coating for magnetic layer After kneading component (1) with a kneader, dispersion is performed using a zirconia bead having a bead diameter of 0.5 mm with a sand mill with a residence time of 45 minutes. After adding the magnetic layer coating component (2) to this, stirring and filtering, The paint for the magnetic layer was used. Further, a sand mill having a bead diameter of 0.1 mm and jet collision type dispersers were alternately combined in three stages, and the final dispersion was performed with the dispersion time of each sand mill being 50 seconds and the collision pressure of each jet collision type disperser being 10 GPa. Separately, after kneading (1) with the kneader in the coating component for the lower non-magnetic layer described above, (2) is added, and after stirring, the dispersion time is set to 60 minutes with a sand mill. After adding 3), stirring and filtering, it was set as the coating material for lower layer nonmagnetic layers.

  The lower layer nonmagnetic layer coating is coated on a support made of a polyethylene terephthalate film (thickness: 6 μm, MD = 5.9 GPa, TD = 3.9 GPa, manufactured by Toray), and the upper layer after drying and calendaring. Simultaneously coat the magnetic layer so that the thickness of the magnetic layer is 100 nm, the upper magnetic layer and the lower non-magnetic layer are combined, and the thickness of the surface coating layer is 1.1 μm. As a result, a magnetic sheet having a lower magnetic layer and an upper magnetic layer laminated on one surface was obtained. In the magnetic field orientation treatment, an NN counter magnet (0.5 T) is installed in front of the dryer, and 2 NN counter magnets (0.5 T) are placed in the dryer from the front side 75 cm of the position where the coating is dry. The installation was performed at intervals of 50 cm. As drying conditions, four drying zones of 15 m were provided, and the drying temperatures were 40/100/40/100 ° C., respectively. The coating speed was 100 m / min.

<< Backcoat layer coating ingredients >>
Carbon black (particle size: 25 nm) 80 parts Carbon black (particle size: 370 nm) 10 parts Iron oxide (particle size: 400 nm) 10 parts Nitrocellulose 45 parts Polyurethane resin (containing SO ₃ Na group) 30 parts 260 parts of cyclohexanone 260 parts of toluene Methyl ethyl ketone 525 parts After the above coating component for the backcoat layer was dispersed in a sand mill with a residence time of 45 minutes, 15 parts of polyisocyanate was added to adjust the coating material for the backcoat layer and filtered. It applied to the opposite surface of the magnetic layer of the produced magnetic sheet so that the thickness after drying and calendering was 0.5 μm, and dried. The magnetic sheet thus obtained was calendered with a seven-stage calendar made of a metal roll under the conditions of a temperature of 100 ° C. and a linear pressure of 150 kg / cm, and the magnetic sheet was wound around the core at 70 ° C. and 72 ° C. After aging for a time, the magnetic layer surface was cut into ½ inch widths, and the magnetic layer surface was polished with a wrapping tape, polished with a blade, and wiped off the surface while running at a speed of 200 m / min to produce a magnetic tape. At this time, K10000 was used for the wrapping tape, a carbide blade was used for the blade, and Toraysee was used for wiping the surface. The magnetic tape obtained as described above was assembled in a cartridge to produce a computer tape.

(Example 2)
A computer tape was prepared in the same manner as in Example 1 except that the drying temperature in the drying step was 40/60/40/100 ° C.

(Example 3)
A computer tape was prepared in the same manner as in Example 1 except that the combination of the sand mill and the jet collision type disperser in the finishing dispersion step was changed to one stage.

Example 4
The magnetic powder used was N / Fe: 11 at%, Y / Fe: 2 at%, Al / Fe: 5 wt%, σs: 100 A · m ² / kg, Hc: 280 kA / m, pH: 9.5, particle size: A computer tape was prepared in the same manner as in Example 1 except that the magnetic powder was changed to a 17 nm spherical ferromagnetic iron-based metal magnetic powder.

(Example 5)
A computer tape was produced in the same manner as in Example 4 except that the combination of the sand mill and the jet collision type disperser in the finishing dispersion step was changed to 6 stages.

(Example 6)
A computer tape was prepared in the same manner as in Example 1 except that liquid nitrogen was sprayed between the second and third zones in the drying step.

(Comparative Example 1)
A computer tape was prepared in the same manner as in Example 1 except that the drying temperature in the drying step was 40/60/100/100 ° C.

(Comparative Example 2)
A computer tape was prepared in the same manner as in Example 1 except that the drying temperature in the drying step was 100/40/40/100 ° C.

  The evaluation method was performed as follows. The results are shown in Table 1.

  The thickness of the magnetic layer was cut out in the thickness direction with a focused ion beam processing apparatus, and the cross section was photographed with 10 fields of view at a magnification of 50,000 times with a scanning electron microscope (SEM). Border the boundary of the layer-undercoat layer interface. Next, select any five locations (total of 50 locations) where no non-magnetic powder is applied per field of view of the photograph, measure the distance between the bordered lines as the thickness of the magnetic layer, and average them all. The magnetic layer thickness was used.

  The magnetic properties of all the magnetic layers were measured using a sample vibration type magnetometer at a maximum external applied magnetic field of 1.28 MA / m (16 kOe). The part of the magnetic layer in the range of t / 5 from the surface of the magnetic layer was performed as follows. First, the surface coating layer is peeled from the support with a peeling tape, and this sample is further cut to a predetermined thickness by ion edging. Thereafter, a sample having a size of 1 mm × 1 mm was used with a maximum external applied magnetic field of 0.78 MA / m (10 kOe) using a SQUID magnetometer (MPMS-5 manufactured by Quantum Design).

  Regarding the particle size of the magnetic powder particles, the recording surface of the magnetic recording medium was observed with a scanning electron microscope (SEM) at a magnification of 50000 times and photographed. Thereafter, image processing was performed to identify the major axis length and minor axis length of each spherical powder magnetic particle containing secondary particles. The average value of the long axis length and the short axis length was defined as the average particle diameter of the spherical magnetic powder particles, and the long axis length / short axis length value was defined as the axial length ratio.

  A drum tester was used for measuring the electromagnetic conversion characteristics of the tape. The drum tester is equipped with an electromagnetic induction head (track width 25 μm, gap 0.2 μm) and an MR head (track width 5.5 μm, shield interval 0.17 μm). Recording is performed with the induction head, and reproduction is performed with the MR head. It was. Both heads are installed at different locations with respect to the rotating drum, and tracking can be adjusted by operating both heads in the vertical direction. An appropriate amount of the magnetic tape was drawn out from the state of being wound around the cartridge and discarded, and a 60 cm portion was cut out and wound around the outer periphery of the rotating drum.

As for output and noise, a rectangular wave was input to the recording current generating amplifier by a function generator, a signal having a wavelength of 20 μm was written, the output of the MR head was amplified by a preamplifier, and then read into a digital oscilloscope. Observed isolated waveforms were captured for 50 wavelengths, the half widths of each were calculated and averaged, and the one obtained by correcting the length dimension was taken as the PW50 value.

As is apparent from the results shown in Table 1, it can be seen that the magnetic tape of the example (product of the present invention) has a lower PW50 value than the magnetic tape of the comparative example and realizes good resolution.

Claims (2)

  A magnetic recording medium having at least a magnetic layer on a flexible support, and a portion of the magnetic layer in the range of t / 5 from the surface of the magnetic layer, where t is the thickness of the magnetic layer A magnetic recording medium characterized in that the SFD value falls within a range of 0.5 to 0.8 of the SFD value of the entire magnetic layer. In the ratio of the major axis length to the minor axis length of the magnetic particles including the secondary particles contained in the magnetic layer obtained from the field image obtained by SEM observation of the cross section of the magnetic layer, 95% or more of the magnetic particles in the field image are included. 2. The magnetic recording medium according to claim 1, wherein an axial length ratio between the major axis length and the minor axis length is 1.5 or less.