JP2006073125A - Magnetic recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic recording medium of a multilayer structure wherein an uppermost magnetic layer securing practical durability and a satisfactory C/N ratio in high density recording contains extremely particulate magnetic powder and having a smooth surface and extremely thin thickness. <P>SOLUTION: In the magnetic recording medium wherein the magnetic powder contained in the uppermost magnetic layer has ≤35 nm particle diameter, a magnetic layer has 10 to 100 nm average thickness, a subbing layer has 0.50 to 1.5 μm average thickness, non-magnetic powder contained in the subbing layer has 30 to 200 nm average particle diameter (defined as Rf) and a value of Rf/Du is specified to be 0.06 to 0.2 when the average thickness of the subbing layer is defined as Du. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a magnetic recording medium in which a substantially non-magnetic undercoat layer and a magnetic layer containing fine particle magnetic powder are formed thereon on a non-magnetic support. In particular, the present invention relates to a coating type magnetic recording medium that is most suitable for ultra-high density recording, such as a digital video tape and a backup tape for computers.

  In recent years, in the field where information magnetic recording media such as video video and computers are used, the recording / playback method has become the mainstream in the video field, and the amount of information has increased more in the computer backup field. Remarkably, in a recording / reproducing system, since the reproducing head on the drive side does not use an induction coil, equipment noise such as impedance noise is greatly reduced, and a large C / N is obtained by reducing noise of the magnetic recording medium. Obtained MR (magnetoresistance effect type) heads have become mainstream. In order to cope with such a tendency, a magnetic recording medium is required to further improve recording density and capacity.

  In response to these requirements, so-called coating-type magnetic recording media in which a magnetic layer is formed by coating and drying a magnetic coating containing magnetic powder on a non-magnetic support increases the linear recording density and supports short wavelength recording. In order to do this, various measures have been devised.

  That is, for the magnetic powder used in the magnetic layer, (1) use a magnetic powder having a small particle volume and a large coercive force, or (2) make the magnetic layer as thin as possible in order to reduce the thickness loss. In addition, for the purpose of improving the surface smoothness of the magnetic layer, a so-called multi-layer structure in which an undercoat layer and a magnetic layer are provided on the support is employed.

  To increase the capacity, the capacity per volume is increased by reducing the total thickness of the magnetic recording medium. It is important to sufficiently improve the surface smoothness of the magnetic layer in order to improve the recording density and increase the reproduction output in a short wavelength region by applying these measures and to improve the C / N.

Regarding (1), the elemental composition and production conditions of the magnetic powder have been studied in order to reduce the coercive force and saturation magnetization, and to reduce the coercive force distribution and particle volume. At present, iron and Co are used as the magnetic powder. The metal fine particle powder as the main component has become the mainstream. The shape is needle-like, rice-grained, and has a long axis length of 0.1 to 0.2 μm and a coercive force of about 130 to 250 kA / m (for example, Patent Document 1). More recently, finer particles and higher coercive force have been developed. Further, an iron nitride magnetic powder mainly composed of substantially spherical Fe ₁₆ N ₂ that can secure a coercive force by crystal anisotropy regardless of shape anisotropy and reduce the particle volume has been developed (Patent Literature). 2).

  (2) is widely used in computer backup tapes of large capacity exceeding 10 GB / volume and some digital video recording tapes used in recent DDS, DLT, LTO systems and the like.

In the future high-density, large-capacity coating-type magnetic recording media will mainly have a multilayer structure, and a technique for ensuring good C / N becomes important. That is, how to control the thickness of the uppermost magnetic layer, smooth the surface property, and obtain high C / N. In Patent Document 3, it is best not to disturb the region between the uppermost magnetic layer and the generally employed substantially nonmagnetic intermediate layer (hereinafter also referred to as a nonmagnetic lower layer, lower layer or undercoat layer). It is disclosed that it is important for improving the reproduction output and C / N by improving the surface smoothness of the upper magnetic layer. The thickness of the uppermost magnetic layer is also greatly involved in C / N in high density recording. The thickness of the uppermost magnetic layer can be determined by the relationship with the recording wavelength of the system to be used, but is preferably 0.1 μm or less in order to support high density recording exceeding 0.5 GB / in ² . When the thickness exceeds 0.1 μm, the reproduction output is reduced due to thickness loss at the shortest wavelength of 0.3 μm or less corresponding to this class of recording density, or the product of residual magnetic flux density and thickness is smaller than the MR head used. It becomes too large and distortion of playback output due to head saturation is likely to occur. If the thickness is less than 0.01 μm, the amount of magnetization of the uppermost magnetic layer is too small and the output signal becomes small, so that the noise of the entire system cannot be ignored and the output noise ratio (C / N) becomes small. Moreover, it is difficult to obtain a uniform top magnetic layer at the current technical level.

  When the thickness of the uppermost magnetic layer is 0.1 μm or less, even if there is a slight variation in thickness, the ratio will increase as compared with the case where it is thicker and other than the magnetic powder in the uppermost magnetic layer. The influence of minute irregularities caused by the nonmagnetic powder and the nonmagnetic powder contained in the nonmagnetic lower layer is also increased, and the smoothness of the uppermost magnetic layer that greatly affects C / N is impaired.

  Since the nonmagnetic powder in the uppermost magnetic layer greatly affects the surface smoothness of the uppermost magnetic layer, this is not included in the uppermost magnetic layer when the thickness of the uppermost magnetic layer is 0.5 μm or less. For example, the content, shape and particle diameter of magnetic powder are defined (Patent Documents 1 and 2), and the relationship between the thickness of the magnetic layer and the average particle diameter of nonmagnetic powder is defined (Patent Documents 4 and 5). Many techniques for improving the surface smoothness of the upper magnetic layer are known.

  On the other hand, the influence on the surface smoothness of the uppermost magnetic layer of the nonmagnetic lower layer is also significant. This is because the disturbance state of the boundary region between the lower layer and the uppermost magnetic layer corresponds to the dispersion state of the nonmagnetic powder contained in the lower layer, and the influence is directly reflected on the surface smoothness of the uppermost magnetic layer.

As means for keeping the boundary area between the lower layer and the uppermost magnetic layer smooth, that is, for improving the surface smoothness of the uppermost magnetic layer, Patent Documents 6 and 7 describe the shape and particles of nonmagnetic powder to be contained in the lower layer. The diameter and the thickness range of the lower layer are disclosed. Furthermore, in Patent Document 8, in addition to these, the relationship between the thickness of the lower layer and the particle diameter of the nonmagnetic powder contained in the lower layer is also disclosed. However, any of the methods disclosed in these documents is insufficient to obtain a high C / N in a short wavelength region corresponding to high density recording of 0.5 GB / in ² or more.

  That is, in References 6 and 7, the thickness of the uppermost magnetic layer is described as 0.5 μm or less, but in the examples, they are 0.3 μm, 0.2 μm, and 0.4 μm, respectively, and less than 0.1 μm. The case of an ultrathin magnetic layer is not disclosed. In the example of Document 8, an example in which the thickness of the uppermost magnetic layer is 0.1 μm is shown, but the major axis length of the acicular magnetic powder is as large as 0.1 μm (100 nm). With this, high C / N cannot be secured.

As described above, various techniques are combined to improve output and C / N in the ultra-short wavelength region corresponding to high-density recording. However, it is expected to be higher than 0.5 GB / in ² expected in the future. In the coating type magnetic recording medium corresponding to the density recording, in the multi-layer structure, the magnetic powder contained in the magnetic layer is finer and the thickness of the magnetic layer is considered to be thinner than 0.1 μm. However, sufficient C / N cannot be ensured by sufficiently improving the surface smoothness in consideration of practical running durability.

JP 2003-115107 A WO03 / 079332 pamphlet Japanese Patent No. 2666610 JP 2003-071114 A JP 2003-115105 A JP 2000-293835 A JP 11-316937 A JP 2001-0667650 A

  Therefore, an object of the present invention is to ensure practical durability and good C / N in high-density recording, and the uppermost magnetic layer contains extremely fine magnetic powder, has a smooth surface and has a thickness. An object of the present invention is to provide a magnetic recording medium having an extremely thin multilayer structure.

As a result of diligent research on improving the surface properties of a very thin magnetic layer in order to provide a magnetic recording medium exhibiting high C / N in high-density magnetic recording, the inventors have realized this by the conventional technology. The lower non-magnetic layer and the lower non-magnetic layer in a coating type medium having a multi-layer structure in which the thickness of the uppermost magnetic layer corresponding to high density recording of 0.5 GB / in ² or more that could not be achieved is less than 100 nm (0.1 μm) The present invention has been clarified with respect to the relationship with the particle size of the nonmagnetic powder to be contained in the present invention.

  That is, the present invention includes a nonmagnetic support, and at least one undercoat layer (hereinafter also referred to as a lower layer or a nonmagnetic lower layer) containing a nonmagnetic powder and a binder resin formed on one surface of the nonmagnetic support. An uppermost magnetic layer having at least one magnetic layer containing magnetic powder and a binder resin formed on the undercoat layer, and a back layer formed on the other surface of the nonmagnetic support. In the magnetic recording medium in which the particle diameter of the magnetic powder is 35 nm or less, the average thickness of the uppermost magnetic layer is 10 to 100 nm, and the average particle diameter (Rf) of the nonmagnetic powder contained in the undercoat layer is 30 to 200 nm. Further, the present invention relates to a magnetic recording medium characterized in that the value of Rf / Du is 0.06 to 0.2, where Du is the average thickness of the undercoat layer.

In the above magnetic recording medium of the present invention, the following embodiments are preferable.
1. The magnetic recording medium, wherein the magnetic powder contained in the uppermost magnetic layer is a rice grain having a major axis length of 20 to 35 nm and a ratio of a major axis diameter to a minor axis diameter of 2 or more and 4 or less.
2. The magnetic powder is a magnetic recording medium having a substantially spherical shape with a particle diameter of 5 nm to 20 nm and an axial ratio of less than 2 (particularly an axial ratio of less than 1.5).
3. A magnetic recording medium wherein the undercoat layer has an average thickness of 0.5 to 1.5 μm.
4). A magnetic recording medium comprising an uppermost magnetic layer made of a magnetic layer coating material comprising magnetic powder, a binder resin, an organic component, and a nonmagnetic powder having a Mohs hardness of less than 5.
5. The magnetic recording medium wherein the thickness of the nonmagnetic support is 2.5 μm or more and less than 6 μm.

  The nonmagnetic lower layer having a coating composition that satisfies the relationship between the thickness of the nonmagnetic lower layer and the particle size of the nonmagnetic powder contained in the nonmagnetic lower layer according to the present invention does not deteriorate the surface smoothness of the uppermost magnetic layer, and is 100 nm. The uppermost magnetic layer, which is a thin film of less than (0.1 μm), has a good surface smoothness, realizes a high C / N suitable for high recording density, and can provide a magnetic recording medium with good durability.

  The magnetic recording medium of the present invention comprises an undercoat layer obtained by coating a nonmagnetic support on which a nonmagnetic powder is dispersed in a binder resin, and a magnetic layer in which magnetic powder is dispersed in a binder resin. The present invention is applied to a so-called multilayer coating type magnetic recording medium formed by applying a paint. That is, the magnetic recording medium of the present invention comprises a nonmagnetic support, at least one subbing layer comprising a nonmagnetic powder and a binder resin formed on one surface of the nonmagnetic support, and an upper layer of the subbing layer. And at least one magnetic layer containing magnetic powder and a binder resin, and a back layer formed on the other surface of the nonmagnetic support, and the particle diameter of the magnetic powder of the uppermost magnetic layer is In a magnetic recording medium of 35 nm or less, the average thickness of the uppermost magnetic layer (hereinafter also referred to as a magnetic layer or an upper layer) is 10 to 100 nm, and the average particle diameter (Rf) of the nonmagnetic powder contained in the undercoat layer is 30. The magnetic recording medium is characterized in that the value of Rf / Du is 0.06 to 0.2, when the average thickness of the undercoat layer is Du.

Embodiments of the present invention will be described below in detail according to the components of the present invention.
<Magnetic layer>
The magnetic layer is composed of an uppermost magnetic layer as a recording layer provided via at least the above-mentioned nonmagnetic layer (undercoat layer). In addition to the uppermost magnetic layer, for example, for the purpose other than the recording layer, Another magnetic layer (for example, the lower magnetic layer for servo signal recording described in the WO03 / 079333 A1 pamphlet) can be formed on the lower side (inside) of the nonmagnetic layer. Hereinafter, the uppermost magnetic layer as the recording layer will be described in detail.

In order to cope with a magnetic recording medium of high density recording of 0.5 GB / in ² or more, the thickness of the uppermost magnetic layer should be 10 to 100 nm in order to minimize thickness loss and self-demagnetization. Is preferred. More preferably, it is 30-80 nm. When the thickness exceeds 100 nm, the reproduction output becomes smaller due to the thickness loss at the shortest wavelength of 0.3 μm or less corresponding to this class of recording density, or the product of the residual magnetic flux density and the thickness is larger than the reproduction MR head used. This is because it becomes too large and distortion of the reproduction output due to head saturation is likely to occur. If the thickness is less than 10 nm, the amount of magnetization of the magnetic layer is too small and the output signal becomes small, so that the noise of the entire system cannot be ignored and the output noise ratio (C / N) becomes small. If the thickness is less than 10 nm, it is difficult to obtain a uniform magnetic layer with the current coating technique.

  The surface roughness of the magnetic layer is preferably 2 to 7 nm, more preferably 2 to 5 nm in terms of centerline average roughness (Ra). More preferably, it is 2-4 nm. This range is preferable because if it is less than 2 nm, the friction coefficient becomes large and running becomes unstable, and if it exceeds 7 nm, the spacing loss during recording / reproduction increases and the short wavelength output decreases.

  The coercive force in the recording direction (longitudinal direction in the case of a magnetic tape) of the uppermost magnetic layer is preferably 127 kA / m to 400 kA / m, more preferably 143 to 380 kA / m, still more preferably 143 to 350 kA / m, and 220 to 350 kA / m is even more preferred. When the coercive force in the recording direction is less than 127 kA / m, the reproduction output tends to decrease due to demagnetization when the shortest recording wavelength is 0.3 μm or less, and when it exceeds 400 kA / m, recording with a magnetic head may be difficult. It is.

  In addition to magnetic powder and binder resin, the magnetic layer contains a lubricant for imparting lubricity, carbon black for maintaining conductivity, non-magnetic powder for ensuring abrasiveness and running durability, etc. It is common to add.

  That is, when the head is contaminated due to contamination on the tape surface or so-called powder removal, the head is polished, or a nonmagnetic powder having a polishing function of Mohs hardness of 5 or more is included in the magnetic layer as a reinforcing material for the coating film. However, it is also used in a magnetic recording medium having a multilayer structure for improving output in a short wavelength region (References 4, 5, and 6). However, in the present invention in which the thickness of the uppermost magnetic layer is 100 nm or less, in order to ensure high C / N in high-density recording, the uppermost magnetic layer coating material does not contain nonmagnetic powder having a Mohs hardness of 5 or more. Method is also an option. This is because the non-magnetic powder is hard when the Mohs hardness is 5 or more, so even if the particle size is small, the smoothing effect by the so-called calendar smoothing treatment is difficult to occur, and the non-magnetic powder is oriented by a magnetic field. This is because there is no improvement in smoothness due to alignment in a certain direction. In addition, the inclusion of nonmagnetic powder in the magnetic layer leads to a reduction in the amount of magnetization per unit volume of the magnetic layer, which is disadvantageous for high output during reproduction. However, even in this case, since the nonmagnetic powder bites into the uppermost magnetic layer from the nonmagnetic lower layer in the drying process or the calendar process, the durability of the uppermost magnetic layer can be maintained.

  Conventionally known carbon black for maintaining the conductivity of the uppermost magnetic layer and a lubricant having a lubricating function can be appropriately used.

  As this carbon black (CB), acetylene black, furnace black, thermal black, etc. can be used. Usually, carbon black having a particle size of 5 to 200 nm is used, but a particle size of 10 to 100 nm is preferable, and a particle size of 10 to 50 nm is more preferable. This range is preferable because when the particle size is less than 10 nm, it is difficult to disperse the carbon black, and when it exceeds 100 nm, it is necessary to add a large amount of carbon black. In either case, the surface becomes rough and the output decreases. This is because it causes. The amount of carbon black added is preferably 0.2 to 5 parts by weight, more preferably 0.5 to 2 parts by weight with respect to 100 parts by weight of the magnetic powder.

  Moreover, you may add the dispersing agent which improves the dispersibility of magnetic powder to a magnetic layer. The surface of the magnetic powder may be treated in advance with these dispersants. Conventionally known dispersants can be used as appropriate.

(Magnetic powder)
1. Acicular magnetic powder The shape of the magnetic powder used for the uppermost magnetic layer can be any of acicular, granular, and dice, but acicular magnetic powder has a particle diameter (major axis length) of 20 to 35 nm and a shaft. Rice grains having a ratio (major axis length / minor axis length) of 2 or more and 4 or less are preferred. If the particle size is less than 20, the coercive force and saturation magnetization may be reduced and the reproduction output (C) may be low, and if it exceeds 35 nm, the reproduction noise (N) will be high. In either case, the output noise ratio (C / This is because N) becomes low. When the axial ratio exceeds 4, when the thickness of the uppermost magnetic layer is as thin as 100 nm, a part of the magnetic powder rising in the film surface direction bites into the nonmagnetic lower layer, and the interface between the uppermost magnetic layer and the nonmagnetic lower layer is disturbed. If the axial ratio is less than 2, the coercive force is low and the reproduction output (C) may be low. In either case, the reproduction output noise ratio (C / N) is low. It is.
In addition, although this invention does not exclude mixing of the particle | grains whose particle diameter is less than 20 nm, and particle | grains exceeding 35 nm, in that case, a number average particle diameter of 20-35 nm is preferable. Although it does not exclude mixing of particles having an axial ratio of more than 4 and particles having an axial ratio of less than 2, the number average axial ratio is preferably 2 or more and 4 or less.

2. Substantially Granular Magnetic Powder For a substantially granular magnetic powder such as a substantially spherical shape, a substantially ellipsoidal shape or a substantially regular polyhedral shape, a substantially granular magnetic powder having a particle diameter of 5 to 20 nm is preferred. Among these magnetic powders, a substantially granular iron nitride-based magnetic powder having a particle diameter of 5 to 20 nm is more preferable because a magnetic powder having a high coercive force and high saturation magnetization can be easily obtained. Fe ₁₆ N ₂ phase is mainly contained in the core part, and elements such as rare earth elements, Al, Si, Zr, Ti, etc. whose oxides are not reduced by hydrogen reduction at 600 ° C. (especially rare earth elements, Al Further, a substantially granular iron nitride-based magnetic powder containing Si, mainly in the outer layer portion is more preferable. The axial ratio is preferably less than 2, preferably 1.5 or less, and more preferably less than 1.5. The particle diameter is the maximum particle diameter of the particle, and the axial ratio of approximately spherical and approximately ellipsoidal particles is the maximum particle diameter and the maximum particle diameter in the direction perpendicular to the maximum particle diameter. Say. The axial ratio of substantially regular polyhedral particles corresponds to a deviation from the regular polyhedron.

When the particle diameter exceeds 35 nm, the particle volume of the magnetic powder becomes large and the magnetization reversal volume becomes large, which is disadvantageous for high-density recording. If it is less than 5 nm, the dispersion becomes poor, or thermal fluctuations occur, and the coercive force tends to decrease. In addition, it is difficult to ensure the saturation magnetization amount, and it is difficult to uniformly coat the surface oxide layer imparting corrosion resistance with the metal magnetic powder whose main component is magnetic powder. In addition, production efficiency deteriorates and costs increase. At the current technical level, it is difficult to manufacture industrially.
In addition, although this invention does not exclude mixing of the particle | grains exceeding 20 nm or the particle | grains less than 5 nm, the number average particle diameter of 5-20 nm is preferable in that case. Moreover, although mixing of particles with an axial ratio of 2 or more is not excluded, in that case, a number average axial ratio of less than 2 is preferable.

  According to the simulation, when the degree of dispersion and packing properties are the same, if a substantially granular fine particle magnetic material of 30 nm is used, a needle (ultrafine particle magnetic material having a major axis length of 75 nm and an axial ratio of 5) and noise ( N), magnetic recording media having substantially the same particle diameter are obtained, and if a substantially granular fine particle magnetic material having a diameter of 20 nm is used, the noise (N) is similar to that using a needle-shaped ultrafine particle magnetic material having a major axis length of 45 nm and an axial ratio of 4. A substantially equal magnetic recording medium is obtained. Further, if a substantially granular fine particle magnetic material of 14 nm is used, a magnetic recording medium having a N of 4.6 dB lower than that using a needle-shaped fine particle magnetic material having a major axis length of 45 nm and an axial ratio of 4 can be obtained. Further, if a substantially granular fine particle magnetic material of 12 nm is used, N is further lowered by 2 dB compared to the case of using a substantially granular fine particle magnetic material of 14 nm.

The saturation magnetization of the magnetic powder used for the uppermost magnetic layer is preferably 50 to 130 A · m ² / kg. More preferred is 70 to 110 A · m ² / kg. The coercive force of the magnetic powder used for the uppermost magnetic layer is appropriately selected so that the coercive force in the recording direction (longitudinal direction in the case of a magnetic tape) of the uppermost magnetic layer is 127 kA / m to 400 kA / m. it can. In the case of rice granular magnetic powder, it is usually 100 to 180 kA / m, and in the case of substantially granular iron nitride magnetic powder, it is usually 180 to 320 kA / m. Conventionally known magnetic powder can be used for the uppermost magnetic layer regardless of the shape as long as the magnetic characteristics, particle diameter, and axial ratio are in the above-mentioned ranges.

<Undercoat layer>
In order to stably apply a smooth uppermost magnetic layer having a thickness of 10 nm or less in the present invention and to ensure the durability of the uppermost magnetic layer, a lower layer is provided. The thickness of the lower layer is preferably set in the range of 0.5 to 1.5 μm, and more preferably in the range of 0.5 to 1.0 μm. This range is preferable if the thickness is less than 0.5 μm, the effect of reducing the thickness unevenness of the uppermost magnetic layer and the effect of improving the durability are reduced, and if it exceeds 1.5 μm, the total thickness of the magnetic recording medium becomes too thick. This is because the recording capacity per roll is reduced.

  Examples of the nonmagnetic powder used for the nonmagnetic lower layer include titanium oxide, iron oxide, and aluminum oxide. In many cases, titanium oxide alone, iron oxide alone, or a mixed system of any of these and aluminum oxide is used. Usually, nonmagnetic iron oxide having a major axis length of 50 to 200 nm and a minor axis length of 5 to 100 nm is mainly contained, if necessary, carbon black having a particle size of 10 to 100 nm and aluminum oxide having a particle size of 50 to 200 nm. There are many. In order to apply the lower layer smoothly and with less thickness unevenness, it is preferable to use nonmagnetic powder and carbon black having a sharp particle size distribution.

Here, the present invention is used for the lower layer thickness and the lower layer in the case of a thin multi-layer recording medium in which the average particle diameter of the magnetic powder contained in the uppermost magnetic layer is 35 nm or less and the uppermost magnetic layer thickness is 100 nm or less. Rf / Du when the average particle diameter (Rf) of the nonmagnetic powder contained in the lower layer is 30 to 200 nm and the average thickness of the lower layer is Du. The value of 0.06 to 0.2 is limited so that the surface smoothness of the uppermost magnetic layer coated on the lower layer is improved and high density recording of 0.5 GB / in ² or more is achieved. It was found that C / N corresponding to the above also was obtained and durability was good.

  In addition, the average particle diameter (Rf) of the nonmagnetic powder contained in the lower layer of the present invention means that the shape of the nonmagnetic powder contained in the lower layer is granular or dice, needle-like, rice-grained or plate-like. If there is a mixture of particles, the average particle diameter of the granular or dice-like nonmagnetic powder, the average minor axis length of the acicular or rice-shaped nonmagnetic powder, and the average plate thickness of the plate-like nonmagnetic powder The largest average particle diameter. This is because when a needle-like, rice-grained or plate-like non-magnetic powder is used, the major axis of the non-magnetic powder is oriented in the lower layer and mainly in the direction of the minor axis or the plate thickness. This is because the size greatly affects the boundary with the uppermost magnetic layer, that is, the surface smoothness and durability of the uppermost magnetic layer.

In order to keep the boundary area between the lower layer and the uppermost magnetic layer smooth, that is, to improve the surface smoothness of the uppermost magnetic layer (Reference 6, Reference 7), the shape and particle diameter of the nonmagnetic powder contained in the lower layer are set. Reference 6 discloses the thickness range of the lower layer. In addition to these, Document 8 also discloses the relationship between the thickness of the lower layer and the particle size of the nonmagnetic powder contained in the lower layer. However, all of the methods disclosed in these documents provide the surface smoothness of the magnetic layer of the magnetic recording medium for obtaining a high C / N in a short wavelength region corresponding to high density recording of 0.5 GB / in ² or more. As described in the “Background Art” section above, it is insufficient to secure. That is, in Reference 6 and Reference 7, there is a description that the uppermost magnetic layer is 0.5 μm (500 nm) or less, but the thickness of the uppermost magnetic layer in the examples is 0.3 μm, 0.2 μm, and 0.4 μm, respectively. In the case of an ultrathin uppermost magnetic layer of less than 0.1 μm, it is not disclosed.

  On the other hand, although the example of Reference 8 shows an example in which the thickness of the uppermost magnetic layer is 0.1 μm, the major axis length of the acicular magnetic powder is as large as 0.1 μm (100 nm). With this, high C / N cannot be secured.

  In any case, the uppermost magnetic layer coating material contains a nonmagnetic powder having a Mohs hardness of 5 or more, which is disadvantageous for obtaining a high C / N ratio. There is no suggestion or implementation that the above nonmagnetic powder is not included in the uppermost magnetic layer coating.

  The range of the value of Rf described above is preferably 30 to 200 nm. If it is smaller than 30 nm, the particle diameter is too small, and the reinforcing effect on the lower layer coating and the uppermost layer magnetic coating is not expressed. This is because even if the relationship Rf / Du described below is satisfied, the smoothness of the magnetic layer may be lowered.

  When the value of Rf / Du in the present invention is smaller than 0.06, the reinforcing effect of the non-magnetic powder lower layer coating and magnetic coating is lost. In addition, when the thickness of the lower layer becomes thinner than 1.0 μm, the corresponding non-magnetic powder has a smaller average particle size and is difficult to disperse. When the value of Rf / Du exceeds 0.2, the height of a part of the nonmagnetic powder that protrudes from the lower surface cannot be ignored to impair the surface smoothness of the uppermost magnetic layer. This phenomenon is clarified from the results of extensive studies by the present inventors. The reason is considered as follows. That is, generally non-magnetic powders that are commercially available have a particle size distribution, and the inventors have conducted experiments using non-magnetic powders that are commercially available on the market. It has been empirically found that powders having a large particle size of about 5 times the average particle size at the maximum are included. As a result, when the value of Rf / Du exceeds 0.2, the uppermost layer When the magnetic layer is as thin as 100 nm or less, depending on the form of the non-magnetic powder in the lower layer, some of them protrude onto the surface of the coating film and reduce the smoothness of the uppermost magnetic layer. it is conceivable that. Therefore, by setting the value of Rf / Du to 0.2 or less, the smoothness of the uppermost magnetic layer is not affected and the uppermost magnetic coating film can have a reinforcing effect. is there.

<Binder resin>
In the present invention, the following conventionally known binder resins can be used for the nonmagnetic layer and the magnetic layer. Vinyl chloride resin, vinyl chloride-vinyl acetate copolymer resin, vinyl chloride-vinyl alcohol copolymer resin, vinyl chloride-vinyl acetate-vinyl alcohol copolymer resin, vinyl chloride-vinyl acetate-maleic anhydride copolymer resin, vinyl chloride- A combination of at least one selected from a hydroxyl group-containing alkyl acrylate copolymer resin, nitrocellulose, and the like and a polyurethane resin can be used. 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.

In addition, —COOH, —SO ₃ M, —OSO ₂ 2M, —P═O (OM) ₃ , —O—P═O (OM) ₂ , [M is a hydrogen atom, an alkali metal base, or an amine salt as a functional group. ], -OH, -NR'R ", -N + + R '''R''''R''''' [R ', R'',R''', R '''', R It is preferable to use a binder resin such as a urethane resin composed of a polymer having a hydrogen group or a hydrocarbon group] or an epoxy group. Such a binder resin is preferable because the dispersibility of the magnetic powder and the like is improved as described above. When two or more 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.

  These binder resins are used in the range of 7 to 50 parts by weight, preferably 10 to 35 parts by weight with respect to 100 parts by weight of the ferromagnetic iron-based metal powder in the magnetic 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 parts by weight of a polyurethane resin as a binder resin.

  The nonmagnetic lower layer is used in an amount of 7 to 50 parts by weight, preferably 10 to 35 parts by weight, based on 100 parts by weight of the total amount of nonmagnetic powder and carbon black. 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 parts by weight of a polyurethane resin as a binder resin.

  Along with these binder resins, it is desirable to use a thermosetting crosslinking agent that is bonded to a functional group contained in the binder resin and crosslinked. Examples of such crosslinking agents 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. And various polyisocyanates are preferred. These crosslinking agents are usually used in a proportion of 10 to 50 parts by weight with respect to 100 parts by weight of the binder resin. More preferably, it is 15 to 35 parts by weight.

<Lubricant>
In the present invention, as the uppermost magnetic layer and the nonmagnetic lower layer, conventionally known lubricants can be used as lubricants that reduce the friction coefficient of the magnetic recording medium and improve running durability. The addition amount is 0.3 to 5.0% by weight of higher fatty acid based on the total powder contained in the uppermost magnetic layer and the nonmagnetic lower layer, combining the uppermost magnetic layer and the nonmagnetic lower layer, It is preferable to contain 0.2 to 3.0% by weight of an ester of higher fatty acid because the coefficient of friction with the head becomes small.

  The addition amount is preferable because if the amount is less than 0.3% by weight, the effect of reducing the friction coefficient is small, and if it exceeds 5.0% by weight, the coating film may be plasticized and the toughness may be lost. . In addition, ester addition of higher fatty acids within this range is preferable, if less than 0.2% by weight, the effect of reducing the friction coefficient is small. This is because side effects such as sticking of the tape and the head may occur. As the fatty acid, it is preferable to use a fatty acid having 10 or more carbon atoms. The fatty acid having 10 or more carbon atoms may be any of isomers such as linear, branched and cis / trans, but is preferably a linear type having excellent lubricating performance. Examples of such fatty acids include lauric acid, myristic acid, stearic acid, palmitic acid, behenic acid, oleic acid, linoleic acid, and the like. Among these, myristic acid, stearic acid, palmitic acid and the like are preferable. The amount of fatty acid (same for fatty acid ester) added to the magnetic layer is not particularly limited because the fatty acid is transferred between the lower layer and the uppermost magnetic layer. The addition amount of the above may be the above amount. If fatty acid is added to the undercoat layer, it is not always necessary to add fatty acid to the magnetic layer.

  When the uppermost magnetic layer contains 0.5 to 3.0% by weight of fatty acid amide with respect to the magnetic powder and 0.2 to 3.0% by weight of higher fatty acid ester, This is preferable because the friction coefficient is small. Fatty acid amides in this range are preferred if less than 0.5% by weight, the direct contact at the head / magnetic layer interface tends to occur, and the effect of preventing seizure is small. This is because defects such as out may occur. As fatty acid amides, amides of fatty acids having 10 or more carbon atoms such as palmitic acid and stearic acid can be used. Further, the higher fatty acid ester addition in the above range is preferable because the effect of reducing the friction coefficient is small if it is less than 0.2% by weight, and there is a side effect such as sticking to the head if it exceeds 3.0% by weight. The addition of the fatty acid amide does not exclude the mutual movement of the lubricant in the uppermost magnetic layer and the lubricant in the lower layer.

<Back coat layer>
The magnetic recording medium of the present invention needs to have a configuration in which a backcoat layer is coated on the surface opposite to the surface on which the magnetic layer on the nonmagnetic support is coated. As described above, the smoothness of the surface of the magnetic layer corresponding to high-density recording is very smooth with Ra of 2 to 5 nm as a more preferable range. For this reason, it is necessary to reduce the load applied to the surface of the smooth magnetic layer by coating a backcoat layer having a lower coefficient of friction than that of the nonmagnetic support to ensure good running durability.

  As the material for the back coating used as the back coat layer, conventionally known non-magnetic powders containing a binder resin and carbon black can be used. The thickness of the back coat layer is preferably 0.3 to 1.0 μm, and more preferably 0.3 to 0.7 μm. If it is thinner than 0.3 μm, it is too thin to obtain a uniform surface, and the function of a general back coat is not exhibited. When the thickness exceeds 1.0 μm, the thickness of the entire tape as a recording medium is increased, and the capacity per tape roll is reduced. Among the nonmagnetic powders contained, hard nonmagnetic powders with a Mohs hardness of 5 or more adhere to the uppermost magnetic layer when the portion protruding from the surface of the backcoat layer is wound on the tape, so-called show-through phenomenon occurs. This causes an emboss (dent) on the surface of the uppermost magnetic layer and impairs smoothness. Therefore, in the present invention, the average particle diameter of the nonmagnetic powder having a Mohs hardness of 5 or more contained in the backcoat layer is preferably smaller than 1/3 of the thickness of the backcoat layer. The ratio of the thickness of the contained layer is larger than that of the nonmagnetic powder contained in the lower layer because the lower nonmagnetic powder has a smaller effect on the uppermost magnetic layer, and the Mohs hardness of 5 or more contained in the backcoat layer. This is because the nonmagnetic powder has a much smaller proportion of the total nonmagnetic powder in the layer compared to the amount contained in the lower layer, and is usually only 1 to 5 parts by weight. Examples of the nonmagnetic powder having a Mohs hardness of 5 or more include iron oxide, alumina, titanium oxide, silicon oxide, and chromium oxide.

<Non-magnetic support>
The thickness of the non-magnetic support used in the present invention varies depending on the application, but usually a thickness of 2 to 8 μm is used. More preferably, it is 2.5-6 micrometers. Nonmagnetic supports with a thickness in this range are used because film formation is difficult if the thickness is less than 2 μm, and the tape strength decreases. If the thickness exceeds 8 μm, the total thickness of the tape increases, and the recording capacity per tape roll This is because becomes smaller.

The longitudinal Young's modulus of the nonmagnetic support is preferably 5.8 GPa (590 kg / mm ² ) or more, and more preferably 7.8 GPa (800 kg / mm ² ) or more. The Young's modulus in the longitudinal direction of the nonmagnetic support is preferably 5.8 GPa (590 kg / mm ² ) or more. If the Young's modulus in the longitudinal direction is less than 5.8 GPa (590 kg / mm ² ), the tape running becomes unstable. Because. In the helical scan type, the Young's modulus (MD) in the longitudinal direction / Young's modulus (TD) in the width direction is preferably in a specific range of 0.60 to 0.80. The Young's modulus in the longitudinal direction / Young's modulus in the width direction is more preferably in the range of 0.65 to 0.75. The specific range of Young's modulus in the longitudinal direction / Young's modulus in the width direction is preferably in the range of 0.60 to 0.80. When the ratio is less than 0.60 or exceeds 0.80, the mechanism is currently unknown. This is because the output variation (flatness) between the entrance side and the exit side of the track of the magnetic head increases. This variation is minimized when the Young's modulus in the longitudinal direction / Young's modulus in the width direction is around 0.70. Furthermore, in the linear recording type, the Young's modulus in the longitudinal direction / Young's modulus in the width direction is preferably 0.7 to 1.30, although the reason is not clear. Nonmagnetic supports satisfying such properties include biaxially stretched aromatic polyamide base films, aromatic polyimide films, polyethylene terephthalate, polyethylene naphthalate, and the like.

<Organic solvent for paint>
Conventionally used organic solvents can be used as organic solvents used in the production of magnetic paints, undercoat paints, and back coat paints. For example, ketone solvents such as methyl ethyl ketone, cyclohexanone, methyl isobutyl ketone, and acetone, aromatic hydrocarbon solvents such as toluene, xylene, and benzene, ether solvents such as tetrahydrofuran and dioxane, ethyl acetate, butyl acetate, and isobutyl acetate Examples include acetate solvents, alcohol solvents such as ethanol, butanol and isopropyl alcohol, hexane, N, N-dimethylformamide and the like.

  These solvents are used alone or in combination. These organic solvents are not necessarily 100% pure and may contain impurities other than the main components, such as decomposition products, oxides, and moisture. The proportion of these impurities is preferably 20% or less. The organic solvent used in the present invention is preferably the same type in the magnetic layer, the nonmagnetic layer, and the backcoat layer. Needless to say, the mixing ratio and addition amount are appropriately changed in consideration of the function of each layer.

<Method of manufacturing magnetic recording medium>
The magnetic recording medium of the present invention can be produced by applying and drying paints for forming the uppermost magnetic layer, the nonmagnetic lower layer, and the backcoat layer.

  In the production of magnetic paints, undercoat paints, and back coat paints, it is preferable to use a kneading process and a dispersion process in which a conventionally known paint production process can be used, and a mixing process provided before and after these processes. In the dispersion step, it is desirable to use a sand mill because the surface properties can be controlled as well as improving the dispersibility of the magnetic powder and the like.

  Conventionally known coating methods such as gravure coating, roll coating, blade coating, and extrusion coating are used as a method of applying a magnetic paint, undercoat paint, and backcoat paint on the nonmagnetic support. The undercoat paint and magnetic paint can be applied by applying the undercoat paint on the non-magnetic support and drying, then applying the magnetic paint, or by applying the successive overlay coating method or by applying the undercoat paint and the magnetic paint simultaneously. Any of the application methods (wet-on-wet) may be employed.

  As in the present invention, considering that the uppermost magnetic layer is an extremely thin layer and the non-magnetic lower layer is also thin, and considering the leveling of the upper and lower layer paints, the undercoat paint applies the magnetic paint while it is wet. It is preferable to employ a simultaneous multilayer coating method. When the simultaneous multi-layer coating method is adopted, it may be applied by a coating head containing two slits through which the coating liquid passes, as disclosed in JP-A-63-088080 and JP-A-2-179712, The coating material may be applied by arranging two extrusion-type coating heads having one slit along the traveling method of the non-magnetic support. The back coat paint may be applied before or after the upper and lower layers are applied, or after the smoothing treatment.

  For the surface smoothing treatment (hereinafter referred to as calendering treatment), conventionally known calendering rolls made of a material can be used alone or in combination.

  The so-called surface post-treatment process, which is performed to remove protrusions and impurities on the surface of the coating film after the magnetic layer and back coat layer are provided, can be performed either after the smoothing process or after cutting into a tape shape. Good.

  Hereinafter, the present invention will be described in detail using examples, but the present invention is not limited thereto. In the examples, “part” means “part by mass”. Moreover, an average particle diameter shows a number average particle diameter.

Example 1.
{{Non-magnetic underlayer paint component}}
(1)
・ Nonmagnetic acicular iron oxide powder (major axis average particle size: 90 nm, axial ratio 5) 68 parts ・ Particulate alumina powder (average particle size: 90 nm) 12 parts ・ Carbon black (average particle size: 25 nm) 20 parts Acid 2.0 parts / vinyl chloride-hydroxypropyl acrylate copolymer 8.8 parts (containing-SO ₃ Na group: 0.7 × 10 ⁻⁴ equivalent / g)
Polyester polyurethane resin 4.4 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)
・ Stearic acid 1 part ・ Butyl stearate 1 part ・ Cyclohexanone 70 parts ・ Methyl ethyl ketone 50 parts ・ Toluene 20 parts (3)
・ Polyisocyanate 1.4 parts ・ Cyclohexanone 10 parts ・ Methyl ethyl ketone 15 parts ・ Toluene 10 parts

{{Magnetic paint component}}
(1) Kneading process / magnetic powder (YN-Fe) 100 parts (Y / Fe: 5.5 at%,
N / Fe: 11.9 at%
σs: 103 A · m ² / kg (103 emu / g),
Hc: 211.0 kA / m (2650 Oe),
Average particle diameter: 17 nm, axial ratio: 1.1)
Vinyl chloride - hydroxypropyl acrylate copolymer 13 parts (containing _-SO 3 Na group: 0.7 × 10 ^-4 eq / g)
Polyester polyurethane resin (PU) 4.5 parts (containing _-SO 3 Na group: 1.0 × 10 ^-4 eq / g)
Granular alumina powder (average particle size: 80 nm) 10 parts Methyl acid phosphate (MAP) 2 parts Tetrahydrofuran (THF) 20 parts Methyl ethyl ketone / cyclohexanone (MEK / A) 9 parts (2) Dilution step Palmitic acid amide ( PA) 1.5 parts, n-butyl stearate (SB) 1 part, methyl ethyl ketone / cyclohexanone (MEK / A) 350 parts (3) blending step, polyisocyanate 1.5 parts, methyl ethyl ketone / cyclohexanone (MEK / A) 29 Part

  After kneading (1) with a batch kneader in the above-mentioned undercoat paint component, add (2) and stir, and then disperse with a sand mill with a residence time of 60 minutes. Add (3) to this and stir and filter After that, an undercoat paint (nonmagnetic underlayer paint) was obtained.

Apart from this, the kneading process component (1) is previously mixed at high speed in the above-mentioned components of the magnetic paint, the mixed powder is kneaded with a continuous biaxial kneader, and the dilution process component (2) is further mixed. In addition, dilute in at least two stages with a continuous twin-screw kneader, disperse with a sand mill with a residence time of 45 minutes, add the blending process component (3) to this, and stir and filter to obtain a magnetic paint. .
Further, the above nonmagnetic paint components were mixed with stirring to obtain a nonmagnetic paint.

  On the nonmagnetic support (base film) made of an aromatic polyamide film (thickness: 3.9 μm, MD = 11 GPa, MD / TD = 0.7, trade name: Miktron, Toray Industries, Inc.) , Dried and coated so that the thickness after calendering is 1.5 μm, and the magnetic coating is further coated on the undercoat layer with a magnetic layer having a thickness of 0. 0 after magnetic field orientation treatment, drying and calendering treatment. The coating was applied wet-on-wet with an extrusion coater to a thickness of 09 μm, and after magnetic field orientation treatment, it was dried using a dryer and far infrared rays to obtain a magnetic sheet.

  In this magnetic sheet, the coercive force in the longitudinal direction of the uppermost magnetic layer was 240 kA / m, the square was 0.81, and Br · δ was 0.018 μTm.

{{Paint component for back coat layer}}
Carbon black (average particle diameter: 25 nm) 87 parts Carbon black (average particle diameter: 350 nm) 10 parts Granular alumina powder (average particle diameter: 80 nm) 3 parts Nitrocellulose 45 parts Polyurethane resin (-SO ₃ Na 30 parts ・ Cyclohexanone 260 parts ・ Toluene 260 parts ・ Methyl ethyl ketone 525 parts Disperse the above coating component for the backcoat layer in a sand mill with a residence time of 45 minutes, and then add 15 parts of polyisocyanate to adjust the coating material for the backcoat layer. After filtration, it was applied to the opposite surface of the magnetic layer of the magnetic sheet prepared above so that the thickness after drying and calendering was 0.5 μm and dried.

  The magnetic sheet thus obtained was mirror-finished under the conditions of a temperature of 100 ° C. and a linear pressure of 196 kN / m with a seven-stage calendar made of a metal roll, and the magnetic sheet was wound around the core at 70 ° C. at 72 ° C. Time-aging was performed to obtain a magnetic sheet with a back layer.

The magnetic sheet was cut into ½ inch width by a slit machine.
A slit machine (an apparatus for cutting a magnetic tape original into a magnetic tape having a predetermined width) was used in which various constituent elements were improved as follows. A tension cut roller was provided in the web path from the unwinding raw fabric to the slit blade group, this tension cut roller was a suction type, and the suction part was a mesh suction in which a porous metal was embedded. A direct drive directly connected to a motor without a mechanism for transmitting power to the blade drive unit was used.

  The tape pancake obtained by cutting was subjected to surface treatment of the magnetic layer with a super steel blade and tressy.

  The magnetic tape obtained as described above was assembled in a cartridge to produce a computer tape.

Example 2:
A computer tape of Example 2 was produced in the same manner as in Example 1 except that the thickness of the nonmagnetic lower layer was changed from 1.5 μm to 0.75 μm.

Example 3:
A computer tape of Example 3 was produced in the same manner as in Example 1 except that the thickness of the nonmagnetic lower layer was changed from 1.5 μm to 0.6 μm.

Example 4:
A computer tape of Example 4 was produced in the same manner as in Example 1 except that the thickness of the nonmagnetic lower layer was changed from 1.5 μm to 0.45 μm.

Example 5:
The computer tape of Example 5 was obtained in the same manner as in Example 1 except that granular titanium oxide having an average particle diameter of 30 nm was used instead of the granular alumina powder (average particle diameter: 90 nm) in the nonmagnetic lower layer coating material. Produced.

Example 6:
A computer tape of Example 6 was produced in the same manner as in Example 1 except that granular alumina having an average particle diameter of 200 nm was used instead of the granular alumina powder (average particle diameter: 90 nm) in the nonmagnetic lower layer coating material. did.

Example 7:
A computer tape of Example 7 was produced in the same manner as in Example 1 except that the thickness of the magnetic layer was changed from 90 nm to 13 nm and the thickness of the nonmagnetic lower layer was changed from 1.5 μm to 0.9 μm.

Example 8:
A computer tape of Example 8 was produced in the same manner as in Example 1 except that the thickness of the magnetic layer was changed from 90 nm to 50 nm and the thickness of the nonmagnetic lower layer was changed from 1.5 μm to 0.9 μm.

Example 9:
The computer tape of Example 9 in the same manner as in Example 1 except that the magnetic powder was changed to the following Fe—Co based metal magnetic powder and the thickness of the nonmagnetic lower layer was changed from 1.5 μm to 1.0 μm. Was made.
Y / (Fe + Co): 7.9 at%, Al / (Fe + Co) = 4.7 wt%, Co / Fe: 25.0 at%, σs: 105 A · m ² / kg (105 emu / g), Hc: 163.1 kA / M (2050 Oe), long axis average particle diameter: 35 nm, axial ratio: 4.0
In this magnetic sheet, the coercive force in the longitudinal direction of the uppermost magnetic layer was 209.9 kA / m, the square was 0.81, and Br · δ was 0.017 μTm.

Example 10:
A computer tape of Example 10 was produced in the same manner as in Example 1 except that the granular alumina powder in the magnetic paint was not added.

Example 11:
A computer tape of Example 11 was produced in the same manner as Example 10 except that the average particle size of the magnetic powder was changed from 17 nm to 20 nm and the thickness of the nonmagnetic lower layer was changed from 1.5 μm to 0.75 μm. did.

Comparative Example 1:
In Example 1, except that the granular alumina powder (average particle diameter: 90 nm) was changed to a granular alumina powder having an average particle diameter of 70 nm, and the thickness of the nonmagnetic lower layer was changed from 1.5 μm to 1.3 μm, A computer tape of Comparative Example 1 was produced in the same manner as Example 1.

Comparative Example 2:
A computer tape of Comparative Example 2 was produced in the same manner as in Example 1 except that the thickness of the nonmagnetic lower layer was changed from 1.5 μm to 0.04 μm.

Comparative Example 3:
A computer tape of Comparative Example 3 was produced in the same manner as in Example 1 except that the granular alumina powder (average particle size: 90 nm) in the nonmagnetic lower layer coating material was changed to 250 nm granular alumina powder.

Comparative Example 4:
In Comparative Example 1, the granular alumina powder (average particle size: 90 nm) in the nonmagnetic lower layer coating material was changed to granular silicon oxide having an average particle size of 20 nm, and the thickness of the nonmagnetic lower layer was changed from 1.3 μm to 0.3 μm. A computer tape of Comparative Example 4 was produced in the same manner as in Example 1 except that the above was changed.

Comparative Example 5:
A computer tape of Comparative Example 5 was produced in the same manner as in Example 1 except that the thickness of the magnetic layer was changed from 90 nm to 120 nm and the thickness of the nonmagnetic lower layer was changed from 1.5 μm to 1.3 μm.

Comparative Example 6:
A computer tape of Comparative Example 6 was produced in the same manner as in Example 1 except that the average particle size of the magnetic powder was changed from 17 nm to 40 nm.

Comparative Example 7:
A computer tape of Comparative Example 7 was produced in the same manner as in Example 1 except that the magnetic powder was changed to the following Fe—Co metal magnetic powder.
Y / (Fe + Co): 7.9 at%, Co / Fe: 24.0 at%, σs: 116 A · m ² / kg (116 emu / g), Al / (Fe + Co): 4.7 wt%, Hc: 164.7 kA / M (2070 Oe), long axis average particle diameter: 45 nm, axial ratio: 3.0

  The obtained sample was measured for C / N as an electromagnetic conversion characteristic and a still life as a measure of durability.

The evaluation method was performed as follows.
<C / N measurement>
A drum tester was used for measuring the electromagnetic conversion characteristics of the tape. The drum tester was equipped with an electromagnetic induction head (track width 25 μm, gap length 0.2 μm) and an MR head (track width 8 μm), and recording was performed with the induction head and reproduction was performed with the MR head. 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 pulled out from the state of being wound around the cartridge, cut out of 60 cm, further processed into a width of 4 mm, and wound around the outer periphery of the rotating drum.

  For the output and noise, a rectangular wave was input to the recording current generator by a function generator, a signal having a wavelength of 0.2 μm was written, the output of the MR head was amplified by a preamplifier, and then read into a spectrum analyzer. The carrier value of 0.2 μm was set as the medium output C. Further, when a rectangular wave of 0.2 μm was written, an integrated value obtained by subtracting the output and system noise from the spectral component corresponding to the recording wavelength of 0.2 μm or more was used as the noise value N. Furthermore, the ratio of both was taken as C / N, and C / N was determined as a relative value to the value of the tape of Comparative Example 5.

<Still durability>
Still durability was similarly evaluated using a drum tester. The tape is mounted as described above, a carrier signal of 0.9 μm is written by the same writing method, and the output is continuously measured with both heads being applied. After that, the time (minute) when the initial output value dropped to 80% was defined as the still life.

<Measurement of Ra>
Measurements were made under the following conditions using a WYKO HD-2000 model.
Objective lens: × 50 Intermediate lens × 0.5 Measurement range: 242μm × 184μm
After subjecting the data to tilt correction and cylindrical correction, the center plane average roughness (Ra) was measured.
(Table 1) and (Table 2) show the evaluation results of the prototyped computer tapes. As is apparent from the table, each of the computer tapes of Examples 1 to 12 according to the present invention includes the magnetic layer thickness, the particle diameter of the nonmagnetic powder contained in the nonmagnetic lower layer, and the particle diameter of the nonmagnetic powder. Since the thickness of the non-magnetic lower layer is a magnetic recording medium within the scope of the present invention and the value of Rf / Du is in a preferred range, it does not satisfy claim 1 or the particle diameter of the non-magnetic powder, Rf / Du However, compared with the computer tapes of Comparative Examples 1 to 7 which are not the subject of the present invention, the C / N is good and the still durability is good.

  Next, the critical significance of various numerical values of the present invention will be clarified with reference to FIGS. First, FIG. 1 shows the relationship between the thickness of the uppermost magnetic layer and the C / N when a magnetic powder having a substantially spherical shape is used. The experiment was conducted with Example 1 as the basic composition and the thickness of the magnetic layer varied in the range of 13 to 150 nm. The C / N value (dB) of the obtained computer tape is shown with each point. As is apparent from the figure, when the thickness of the magnetic layer is larger than 100 nm, the output C does not decrease, but noise increases and C / N decreases. In addition, it was difficult to technically apply a uniform coating with a thickness of less than 10 nm, and it was impossible to produce a tape to be used for C / N evaluation.

  Next, FIG. 2 shows the relationship between the values of Rf and Du and C / N. The experiment was performed with Example 1 as the basic composition, with the average particle size of the nonmagnetic powder being changed in the range of 20 to 250 nm and the thickness of the nonmagnetic lower layer in the range of 0.3 μm to 1.8 μm. The C / N value (dB) of the obtained computer tape is shown with “◯” points (C / N ≧ 0.2 dB) and “×” points (C / N <0.2 dB). As is apparent from the figure, the C / N ratio decreases when the average particle diameter of the region above the line or the nonmagnetic powder exceeds 200 nm from the line (shown by a broken line) where Rf / Du is 0.2. I understand that.

  FIG. 3 shows the relationship between the Rf and Du values and the still durability. The experiment was conducted with Example 1 as the basic composition, with the average particle size of the nonmagnetic powder being changed in the range of 20 to 250 nm and the thickness of the nonmagnetic lower layer in the range of 0.3 to 1.8 μm. The value (minute) of the still time of the obtained computer tape is shown with “◯” point (still life ≧ 10 minutes) and “×” point (still life <10 minutes). As is apparent from the figure, the region below the line where Rf / Du is 0.06 (shown by a broken line), the region where the average particle diameter of the nonmagnetic powder is less than 30 nm, and the nonmagnetic lower layer thickness is 500 nm. It turns out that still durability falls when it becomes the area below.

  FIG. 4 shows the results of FIGS. 2 and 3 collectively. The range enclosed by the broken line, that is, the relationship of 0.06 ≦ Rf / Du ≦ 0.2, the range of Rf and Du satisfying 30 nm ≦ Rf ≦ 200 nm and satisfying 500 nm ≦ Du is C / N, still durability It can be seen that a good range of properties is shown.

The figure which shows the relationship between the thickness of a magnetic layer, and C / N The figure which shows the relationship between the value of Rf, Du and C / N The figure which shows the relationship between the value of Rf, Du and still durability The figure which shows the relationship between the value of Rf, Du and C / N and still durability

Claims (1)

A nonmagnetic support, at least one subbing layer comprising a nonmagnetic powder and a binder resin formed on one surface of the nonmagnetic support, and a magnetic powder formed on the subbing layer, In a magnetic recording medium having at least one magnetic layer containing a binder resin and a backcoat layer formed on the other surface of the nonmagnetic support, and the particle size of the magnetic powder of the uppermost magnetic layer is 35 nm or less ,
The average thickness of the uppermost magnetic layer is 10 to 100 nm, the average thickness of the undercoat layer is 0.50 to 1.5 μm, and the average particle diameter (Rf) of the nonmagnetic powder contained in the undercoat layer ) Is 30 to 200 nm, and the value of Rf / Du is 0.06 to 0.2, where Du is the average thickness of the undercoat layer.

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