JP3046579B2 - Magnetic recording media - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a magnetic recording medium suitable for high-density recording.

[0002]

2. Description of the Related Art Magnetic recording / reproducing apparatuses have been required to have higher recording densities as their capacity, speed, and size have been reduced. To cope with this, magnetic recording media require high coercive force, thin film,
Attempts have been made to improve the recording density by high smoothing. However, the densification by such an approach has almost reached the limit.

On the other hand, on the magnetic recording / reproducing apparatus side, attempts have been made to improve reproduction sensitivity by performing reproduction with a magnetoresistive head instead of a conventional inductive magnetic head. The magnetoresistive head has the characteristic that a sufficient output can be obtained with a small amount of magnetic flux because the output is determined by the amount of magnetic flux entering the head, and is practically used in a hard disk drive. However, in the case of metal thin-film magnetic recording media used in hard disk drives, during high-frequency recording, the direction of magnetization of magnetic grains fluctuates due to the interaction between grains in the magnetization reversal part, and the boundary of magnetization transition becomes saw-toothed. Because of this, modulation noise is generated, and there is a limit in improving the linear recording density. On the other hand, in a coating type magnetic recording medium in which magnetic powder is dispersed in a binder, each magnetic powder forms a single magnetic domain, and the magnetic powder is separated by the binder. It is small and the above-mentioned modulation noise is hardly generated. Further, since the magnetoresistive head has high reproduction sensitivity, there is a possibility that high-density recording and reproduction can be performed even on a magnetic recording medium having a small amount of residual magnetization, that is, a coating type magnetic recording medium.

As a technique for reproducing a signal recorded on a coating type magnetic recording medium by using a magnetoresistive head, for example, a technique described in Japanese Patent Application Laid-Open No. Hei 6-342515 is known. However, the magnetic recording medium described in this publication is of a single-layer type and the thickness of the magnetic layer is large, so that it has not yet achieved a sufficiently high recording density that can meet current demands.

[0005]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a coating type magnetic recording medium capable of achieving high density recording.

The inventors of the present invention set the thickness of the upper magnetic layer, the product of the residual magnetic flux density and the thickness, and the coercive force in a multilayer coating type magnetic recording medium within specific ranges, and furthermore, the ferromagnetic layer contained in the upper magnetic layer. It has been found that the above object is achieved by setting the average particle size of the powder and the non-magnetic powder in a specific range.

The present invention has been made on the basis of the above findings, and has a magnetic recording medium in which a lower layer is provided on a support, and an upper magnetic layer in which a ferromagnetic powder and a nonmagnetic powder are dispersed in a binder is provided thereon. In the medium, the thickness of the upper magnetic layer is set to 0.
2 μm or less, the product of the residual magnetic flux density of the upper magnetic layer and the thickness of the upper magnetic layer is 0.005 to 0.045 Tμm, and the coercive force of the upper magnetic layer is 170 to 280 kA / m. Magnetic recording using a non-magnetic powder having an average particle diameter of 0.01 to 0.12 μm and an average particle diameter of 1/10 or more and 0.1 μm or less of the thickness of the upper magnetic layer as the nonmagnetic powder The above object has been achieved by providing a medium.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the magnetic recording medium of the present invention will be described below. In the magnetic recording medium of the present embodiment, a lower layer and an upper magnetic layer as an uppermost layer are provided on one surface of the support adjacent to the lower layer, and a back coat layer is provided on the other surface of the support. ing.

The upper magnetic layer is formed by dispersing a ferromagnetic powder and a non-magnetic powder in a binder. The upper magnetic layer has a product of the residual magnetic flux density Br and its thickness δ (hereinafter, this product is referred to as Brδ) of 0.005 to 0.045.
Tμm, preferably 0.005 to 0.038 Tμm, more preferably 0.005 to 0.03 Tμm. By combining this condition with other conditions described later, high-density recording becomes possible. Br δ is a measure of the amount of magnetic flux of the upper magnetic layer.
If it exceeds T μm, the head magnetic field is saturated and high-density recording cannot be performed. This is particularly noticeable when magnetically recorded information is reproduced by a head using a magnetoresistive element (hereinafter referred to as an MR head). Further, the amount of magnetic flux is too large, and the influence of the demagnetizing field is so large that high-density recording cannot be performed.
On the other hand, if the value of Brδ is less than 0.005 T μm, the amount of magnetic flux is too small to obtain a sufficient reproduction output. Since the value of Brδ depends on the value of the thickness δ of the upper magnetic layer, the value of Brδ can be made to fall within the above range by adjusting the value of δ, but the value of δ is 0.2 μm or less. There is a need to. If the value of δ is 0.2 μm or more, the influence of the demagnetizing field during high-frequency recording increases, and sufficient input / output characteristics cannot be obtained. The value of δ is 0.0
It is preferably from 1 to 0.2 μm, particularly preferably from 0.01 to 0.15 μm. The value of Brδ also depends on the value of the residual magnetic flux density Br of the upper magnetic layer.
It is 1-0.5T, especially 0.12-0.45T. still,
The value of Br is affected by the packing property of the ferromagnetic powder in the upper magnetic layer. When the value of Br is in the above range, the packing state of the ferromagnetic powder becomes good, and as a result, the high S /
N and high durability are compatible.

As described above, in the present invention, the value of .delta.
It is necessary to set the value of Brδ to not more than 2 μm and the value of Brδ to be not less than 0.005 Tμm and not more than 0.045 Tμm. However, this may cause a problem that the minimum recording bit volume is reduced and the S / N is reduced. In order to prevent this reduction in S / N, it is necessary to increase the number of ferromagnetic powders present in the minimum recording bit volume. For this purpose, in the present invention, a ferromagnetic powder having a small particle diameter, specifically one having an average particle diameter of 0.01 to 0.12 μm is used. If the average particle size of the ferromagnetic powder is more than 0.12 μm, it becomes difficult to increase the number of ferromagnetic powders present in the minimum recording bit volume, and the S / N is reduced. On the other hand, when the average particle size of the ferromagnetic powder is less than 0.01 μm, the dispersibility of the ferromagnetic powder is reduced, and a high output cannot be obtained. Further, by setting the average particle size of the ferromagnetic powder within the above range, it is possible to prevent the coercive force distribution from being varied due to the variation in the particle shape of the ferromagnetic powder, and hence the generation of modulation noise. The preferred average particle size of the ferromagnetic powder is 0.02 to 0.10. In the present specification, the particle size of the powder, regardless of whether the powder is magnetic or non-magnetic, means the major axis length when the powder shape is needle-like or spindle-like, and is plate-like The case means the plate diameter. In the case of a spherical shape, it means a diameter, and in the case of an amorphous shape, it means the length of the longest part.

Further, in the present invention, the coercive force Hc of the upper magnetic layer is set to 170 to 280 kA / in order to improve the recording density by preventing the generation of a demagnetizing field in the magnetization reversal region which hinders the improvement of the linear recording density. m, preferably 170-2
40 kA / m. The improvement in linear recording density is particularly advantageous when reproducing magnetically recorded information with an MR head that can reduce the track width. In order to keep the coercive force of the upper magnetic layer within the above range, a method such as using the above-mentioned ferromagnetic powder or controlling the magnetic field orientation conditions when forming the upper magnetic layer is used.

As the ferromagnetic powder contained in the upper magnetic layer, for example, acicular or spindle-shaped ferromagnetic powder and plate-shaped ferromagnetic powder can be used. Examples of the acicular or spindle-shaped ferromagnetic powder include a ferromagnetic metal powder mainly composed of iron and a ferromagnetic iron oxide-based powder. Examples of the plate-like ferromagnetic powder include a ferromagnetic hexagonal ferrite powder.

Examples of the ferromagnetic metal powder include an iron-based ferromagnetic metal powder in which the metal content is 40% by weight or more, and the metal content is 40% or more of iron. Specific examples of the ferromagnetic metal powder include columns 3 to 42 of JP-A-9-35246.
Those described in line 44 are mentioned. The coercive force Hc of this ferromagnetic metal powder is 150 to 200 kA / m, and the saturation magnetization σ
s is 100 to 170 Am ² / kg, BET specific surface area is 4
It is preferably from 0 to 70 m ² / g. As the ferromagnetic hexagonal ferrite powder, for example, JP-A-9-3524
No. 6, column 4, lines 1 to 5 described above. The coercive force Hc of the ferromagnetic hexagonal ferrite powder 135~260kA / m, saturation magnetization σs is 27~72Am ² / kg, BET specific surface area is preferably 30 to 70 m ² / g. In the present invention, since the thickness of the upper magnetic layer is as small as 0.2 μm or less, the iron-based ferromagnetic metal powder preferably contains cobalt from the viewpoint of preventing deterioration due to oxidation. In particular, it is preferable to contain cobalt so that cobalt / iron becomes 20 to 40 atomic%.

The upper magnetic layer contains a non-magnetic powder as a powder component in addition to the ferromagnetic powder. As the nonmagnetic powder, an abrasive made of a powder of a substance having a Mohs hardness of 7 or more such as α-alumina and chromium oxide, and a nonmagnetic powder such as carbon black as an antistatic agent are used. These non-magnetic powders have an average particle size of one thickness δ of the upper magnetic layer.
/ 10 or more and 0.1 μm or less. When two or more types of non-magnetic powder are used, the average particle size of each powder is set within the above range. When the average particle size is 0.1 μm or more, especially when reproduction is performed using an MR head, the temperature of the MR head increases due to sliding heat generated by contact between the projections on the medium surface and the head, and the resistance value decreases. This causes a problem of so-called thermal asperity in which noise occurs due to fluctuation. On the other hand, if the average particle size is less than 1/10 of the thickness δ of the magnetic layer, problems such as clogging of the head and deterioration of running performance occur. The preferred range of the average particle size is 1/6 or more of the thickness δ of the upper magnetic layer and 0.08 μm or less.

The binder contained in the upper magnetic layer together with the ferromagnetic powder and the non-magnetic powder is described in, for example,
No. 35246, column 4, lines 25 to 32. Among these, a polyurethane resin having a polar group such as a sulfate group, a sulfonate group, an epoxy group, a hydroxyl group or a carboxyl group in a molecule, a vinyl chloride copolymer, and a nitrocellulose resin are preferably used. The amount of the binder is preferably 5 to 30 parts by weight based on 100 parts by weight of the ferromagnetic powder. In particular,
A polyurethane resin and a vinyl chloride copolymer are used in combination, and the ratio of the two (former / latter) is 20/80 to 70/30.
It is preferable that

The performance of the magnetic recording medium can be further improved by including a lubricant such as a fatty acid or a fatty acid ester, a curing agent such as an isocyanate compound, etc. in the upper magnetic layer in addition to the above components. Preferred amounts of these components and the above-mentioned abrasive and carbon black as the nonmagnetic powder are as follows with respect to 100 parts by weight of the ferromagnetic metal powder. Abrasive: 1 to 20 parts by weight, especially 3 to 15 parts by weight Carbon black: 0.5 to 10 parts by weight Lubricant: 1 to 10 parts by weight Hardener: 5 parts by weight or less, especially 2 parts by weight or less

Next, the lower layer will be described. The lower layer may be a magnetic layer containing magnetic powder or a non-magnetic layer containing no magnetic powder. The lower layer contains a binder, an abrasive, a nonmagnetic powder such as carbon black as an antistatic agent, a lubricant, a hardener, and the like, regardless of whether it is magnetic or not. The details of these components are the same as those in the upper magnetic layer. Further, the lower layer contains a nonmagnetic filler such as nonmagnetic iron oxide (α-iron oxide), titanium oxide, and calcium carbonate. The preferable amount of these components contained in the lower layer is 100 parts by weight of the total amount of the ferromagnetic powder and the nonmagnetic filler (when the lower layer is a magnetic layer).
Or with respect to 100 parts by weight of the non-magnetic filler (when the lower layer is a non-magnetic layer), each is as follows. Binder: 5 to 50 parts by weight, especially 10 to 30 parts by weight Abrasive: 1 to 30 parts by weight, especially 2 to 18 parts by weight Carbon black: 0.3 to 30 parts by weight, especially 1-2
0 parts by weight Lubricant: 1 to 20 parts by weight, especially 3 to 10 parts by weight Hardener: 12 parts by weight or less, especially 8 parts by weight or less

The thickness of the lower layer is 0.2 to 3.0 μm from the viewpoint of improving the durability of the magnetic recording medium and preventing cupping.
m, particularly preferably 0.5 to 2.5 μm.

When the lower layer is a magnetic layer, the lower layer has a coercive force of 13 to control the magnetic properties of the upper magnetic layer.
It is preferably 5 to 260 kA / m, particularly preferably 160 to 260 kA / m, and the saturation magnetic flux density is 0.05 to 0.1.
T, particularly preferably 0.05 to 0.08T.

The lower and upper magnetic layers are formed by applying a lower paint for forming the lower layer and an upper paint for forming the upper magnetic layer. Lower layer paint and upper layer paint
It is obtained by dispersing the above various components in a predetermined amount of a solvent. As the solvent, ketone solvents, aromatic solvents, hydrocarbon solvents and the like are preferably used.

As the back coat layer formed on the other surface side of the support, a known one used for a magnetic recording medium can be used. Specifically, for example, Japanese Patent Application Laid-Open No. 9-3524
No. 6, JP-A-6, line 41 to column 9, line 4 can be used. Its thickness is 0.05 to 0.8 μm, especially 0.1 μm.
The thickness is preferably 1 to 0.7 μm from the viewpoint of improving durability and preventing occurrence of cupping.

As the support, a known support can be used as long as it is for a magnetic recording medium. Specifically, those described in column 2, lines 30 to 42 of JP-A-9-35246 can be used. Among these, polyethylene terephthalate (PE
Non-magnetic materials such as T), polyethylene naphthalate and polyamide are preferred. The thickness of the support is preferably 8 μm or less, particularly preferably 6 μm or less in order to increase the capacity of the magnetic recording medium. Further, an easy-adhesion layer may be provided on the surface of the support to enhance the adhesion to the lower layer and the back coat layer.

The total thickness of the magnetic recording medium is 4 to 10 μm, particularly 4 μm, from the viewpoint of increasing the capacity of the magnetic recording medium and ensuring durability.
It is preferably about 7 μm.

Next, an outline of a preferred method of manufacturing the magnetic recording medium of the above embodiment will be described. First, an upper layer coating and a lower layer coating are applied simultaneously on a support by a wet-on-wet method so that each layer has a predetermined thickness, thereby forming an upper magnetic layer and a lower coating film. Next, after performing a magnetic field orientation treatment on these coating films, a drying treatment is performed and the film is wound. By controlling the conditions of the magnetic field orientation treatment and the drying treatment, the magnetic properties of the upper magnetic layer can be made desired. Thereafter, a calendar process is performed. Further, a back coat paint is applied on the surface on the opposite side of the support and dried at a predetermined temperature to form a back coat layer. Subsequently, aging treatment is performed at 40 to 80 ° C. for 6 to 100 hours to obtain a wide magnetic recording medium. Then, for example, in the case of manufacturing a magnetic tape, this material is cut into a predetermined width along the longitudinal direction.

Although the present invention has been described based on the preferred embodiments, the present invention is not limited to the above embodiments.
Various changes can be made without departing from the spirit of the present invention. For example, the magnetic recording medium of the present invention may be a video / audio recording tape such as a DVC tape, an 8 mm video tape or a DAT tape, a DLT tape, a DDS tape, a 1/4 inch data cartridge tape, a data recording tape such as a data 8 mm tape, or the like. Suitable as a magnetic tape,
The present invention can also be applied to other magnetic recording media such as a magnetic disk such as a flexible disk.

[0026]

EXAMPLES In the following examples, parts and% mean parts by weight and% by weight, respectively, unless otherwise specified.

Example 1 The following ingredients (excluding the curing agent) were kneaded in a kneader, dispersed by a stirrer, and finely dispersed by a sand mill.
After filtering through a filter, a curing agent was finally added to prepare an upper layer paint, a lower layer paint and a back coat paint having the following compositions.

<Blending of upper layer paint> 100 parts of ferromagnetic powder (see Table 2) 5 parts of α-alumina (abrasive, average particle diameter: 70 nm) 5 parts of carbon black (average particle diameter: 20 nm) 2 parts Acid-base-containing vinyl chloride copolymer [Binder, MR104 (trade name) manufactured by Zeon Corporation] 12 parts ・ Sulfonate group-containing polyurethane resin [Binder, UR-8300 (trade name) manufactured by Toyobo] 8 parts -2 parts of butyl stearate (lubricant)-4 parts of polyisocyanate [curing agent, Cotonate L (trade name) manufactured by Nippon Polyurethane Industry, solid content 75%] 4 parts-100 parts of methyl ethyl ketone-60 parts of toluene-100 parts of cyclohexanone

<Blending of lower layer paint> 100 parts of needle-like α-Fe ₂ O ₃ (nonmagnetic filler, average particle diameter: 100 nm) 5 parts of α-alumina (abrasive, average particle diameter: 70 nm) 5 carbon Black (average particle diameter: 20 nm) 2 parts ・ Sulfonate group-containing vinyl chloride copolymer [Binder, MR104 (trade name) manufactured by Zeon Corporation] 12 parts ・ Sulfonate group-containing polyurethane resin [Binder, Toyobo UR-8300 (trade name)] 8 parts ・ Butyl stearate (lubricant) 2 parts ・ Polyisocyanate [curing agent, Nippon Polyurethane Industry Cotonate L (trade name), solid content 75%] 4 parts ・ Methyl ethyl ketone 100 Parts ・ Toluene 60 parts ・ Cyclohexanone 100 parts

<Blending of backcoat paint> 38 parts of carbon black (average particle size: 28 nm) 2 parts of carbon black (average particle size: 52 nm) 50 parts of "Nipporan 2301" [trade name, Nippon Polyurethane Industry Co., Ltd. ) Polyurethane, solid content 40%] Nitrocellulose 20 parts (Viscosity display 1/2 second manufactured by Hercules Powder Co.) Polyisocyanate (solid content 75%) 4 parts Copper phthalocyanine 5 parts Stearic acid 2 parts ・ Methyl ethyl ketone 120 parts ・ Toluene 120 parts ・ Cyclohexanone 120 parts

A lower layer paint and an upper layer paint were applied on a support made of a PET film having a thickness of 4 μm so that the lower layer had a dry thickness of 1.5 μm and the upper layer had a dry thickness shown in Table 1. Then, simultaneous multi-layer coating was performed with a die coater to form respective coating films. Next, while these coating films were in a wet state, they were passed through a solenoid of 400 kA / m to perform a magnetic field orientation treatment, further dried with hot air at 30 to 100 ° C., and wound up. The air volume after passing through the solenoid was adjusted so that the magnetic field orientation did not change during the drying process after passing through the solenoid. Next, 80 ° C., 2940 N / cm
, And a backcoat paint is applied on the back surface of the support and dried at 90 ° C to obtain a thickness of 0.1 mm.
A backcoat layer of 5 μm was formed. Thereafter, the magnetic tape was aged at 50 ° C. for 16 hours, and cut into 3.81 mm width to obtain a magnetic tape.

Examples 2 to 4 and Comparative Examples 1 to 3 In the same manner as in Example 1 except that the type of ferromagnetic powder to be mixed in the upper magnetic layer and the thickness of the upper magnetic layer were as shown in Table 1, A magnetic tape was obtained.

Example 5 Instead of 100 parts of needle-like α-Fe ₂ O ₃ compounded in the lower layer paint in Example 1, 50 parts of this α-Fe ₂ O ₃ and hexagonal plate barium shown in Table 2 were used. A magnetic tape was obtained in the same manner as in Example 1 except that 50 parts of ferrite E was blended. The magnetic properties of the lower layer of this magnetic tape are as follows: coercive force 234 kA / m, saturation magnetic flux density 0.06
T, SFD 0.3.

[Evaluation of Performance] In order to evaluate the performance of the magnetic tapes obtained in Examples and Comparative Examples, the magnetic properties and surface roughness Ra of the upper magnetic layer, and the S / N and storage properties of the magnetic tapes were evaluated by the following methods. Was measured. Table 1 shows the results.

<Magnetic Characteristics> A magnetic tape was punched out to a predetermined size, and an external magnetic field of 1194 kA /
m. However, with respect to the magnetic tape of Example 5, a material obtained by applying only the upper layer paint separately to a PET film by the same operation as in Example 5 was measured.

<Surface Roughness Ra> Optical surface roughness meter (Zy
According to the model Maxim (3D5700, manufactured by Go, Inc.), a Fizeau lens with a magnification of 40 was used, Cyinder correction was performed, and measurement was performed at five points. The average value was defined as the surface roughness Ra.

<S / N> A recording head having a track width of about 10 μm and a gap length of about 0.2 μm, and an MR reproducing head having a track width of about 5 μm and a gap length of about 0.2 μm are integrated. Recording density of 75, 100, 125, 150 kfc using a drum tester equipped with a composite type head
The reproduction output of the signal recorded in i was measured. The measurement result was a relative value based on Comparative Example 2.

<Storage characteristics> A magnetic tape was heated at 60 ° C. and 90% R.
H after storage for 7 days in an environment of H
r) was measured, and the Br reduction rate was calculated from the following equation. Br
The lower the rate of decrease, the better the storage characteristics. Br reduction rate (%) = (Br before storage−Br after storage) / Br × 100 before storage

[0039]

[Table 1]

[0040]

[Table 2]

As is clear from the results shown in Table 1, the magnetic tape of the embodiment (product of the present invention) has a higher S / N value during reproduction of a signal recorded at high density than the magnetic tape of the comparative example. It is high and is suitable for high-density recording. Also,
The magnetic tapes of Examples 1, 2, and 5 using the ferromagnetic metal powder containing Co in a specific amount or more as the ferromagnetic powder may have better storage characteristics than the magnetic tapes of Comparative Examples 1 and 2. I understand.

[0042]

As described above, according to the present invention,
A coating type magnetic recording medium capable of achieving high density recording is provided. The magnetic recording medium of the present invention has a particularly high effect of high-density recording when magnetically recorded information is reproduced by a reproducing head using a magnetoresistive element.

Continuation of the front page (56) References JP-A-7-182646 (JP, A) JP-A-10-162345 (JP, A) JP-A-11-238225 (JP, A) (58) Fields studied (Int .Cl. ⁷ , DB name) G11B 5/70 G11B 5/708

Claims (3)

(57) [Claims] 1. A magnetic recording medium comprising: a lower layer provided on a support; and an upper magnetic layer formed by dispersing a ferromagnetic powder and a non-magnetic powder in a binder. 0.2 μm or less, the product of the residual magnetic flux density of the upper magnetic layer and the thickness of the upper magnetic layer is 0.00
5 to 0.045 Tm, and the coercive force of the upper magnetic layer is 170
To 280 kA / m, and the ferromagnetic powder has an average particle size of 0.01 to 0.12 μm.
m, and the average particle size of the nonmagnetic powder is 1/10 or more of the thickness of the upper magnetic layer and 0.1 μm
A magnetic recording medium using: 2. The magnetic recording medium according to claim 1, wherein the magnetic recording information is reproduced by a reproducing head using a magnetoresistive element. 3. The ferromagnetic powder is an iron-based ferromagnetic metal powder containing cobalt, wherein cobalt / iron is 20 to 40.
3. The magnetic recording medium according to claim 1, wherein the content is at.