JPH0785297B2 - Magnetic recording medium - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to a magnetic recording medium, and more particularly to a coating type high density magnetic recording medium using hexagonal ferrite powder.

[Technical background of the invention and its problems]

The magnetic recording medium is composed of a non-magnetic support such as polyethylene terephthalate, magnetic fine particles provided thereon, and a magnetic layer containing a binder as a main component. Conventionally, needle-shaped magnetic particles such as γFe ₂ O ₃ , CrO ₂ , and metallic Fe have been widely used as the magnetic fine particles, and the magnetic recording is based on the method using magnetization in the in-plane longitudinal direction. In manufacturing a recording medium by magnetization in the in-plane longitudinal direction, the magnetic particles can be oriented in the in-plane longitudinal direction without causing roughness on the surface of the magnetic layer. The filling rate can be increased. But,
In the recording method using only the magnetization in the in-plane longitudinal direction, when an attempt is made to improve recording / reproduction in a high frequency region, that is, to increase the recording density, the demagnetizing field in the recording medium increases, so that the recording density is increased. It cannot be improved so much.

Therefore, in order to solve such a problem, a perpendicular magnetic recording system has recently been proposed which uses magnetization in a direction perpendicular to the surface of the recording medium. In the perpendicular magnetic recording method, the higher the recording density, the smaller the demagnetizing field in the recording medium, so it can be said that the recording method is essentially suitable for high-density recording.

As a recording medium suitable for such a perpendicular recording system, a coating type recording medium using hexagonal ferrite powder in which an easy axis of magnetization is easily oriented in a direction perpendicular to the surface of the recording medium has been studied.

By the way, in the case where the recording medium is formed of a coating layer of magnetic particles, the following manufacturing method can be considered. That is, for example, BaFe ₁₂ O ₁₉ hexagonal ferrite powder is used as the magnetic particles. The reason for using hexagonal ferrite powder is that this ferrite has a flat plate shape, and since the axis of easy magnetization is perpendicular to the plate surface, vertical orientation can be easily performed by magnetic field orientation processing or mechanical orientation processing. Is. After mixing magnetic particles of such hexagonal ferrite powder with a binder and coating the mixture on the surface of a non-magnetic support, the coating layer is placed in a magnetic field so that the surface is orthogonal to the direction of the magnetic field. By doing so, the easy axis of magnetization of each magnetic particle is aligned with the direction of the magnetic field, and the coating material is dried, whereby a recording medium suitable for perpendicular magnetic recording can be obtained.

However, when trying to obtain a perpendicular magnetic recording medium by magnetic field orientation, it is difficult to set the timing of drying in a magnetic field,
It is difficult to obtain a vertically aligned coating film with good reproducibility. Further, if the magnetic particles are allowed to freely rotate and move under an orienting magnetic field for a long time, magnetic agglomeration of the particles occurs, and the coating film surface is roughened, which causes a problem of increasing noise during recording and reproduction.

[Object of the Invention]

The present invention has been made to solve the above problems, and in a coating type magnetic recording medium using hexagonal ferrite powder, effective magnetic recording is achieved by using both perpendicular and double-sided magnetization components simultaneously. It is an object of the present invention to provide a magnetic recording medium that can be formed and has excellent surface smoothness.

[Outline of Invention]

The magnetic recording medium according to the present invention is a magnetic recording medium comprising a magnetic layer containing hexagonal ferrite powder on a substrate, wherein the magnetic powder is oriented in the vertical direction on the surface and in the in-plane direction inside, The squareness ratio in the in-plane longitudinal direction and the direction perpendicular to the magnetic layer is larger than 0.5, and the squareness ratio in the in-plane direction perpendicular to both in-plane longitudinal directions is smaller than the squareness ratio in the in-plane longitudinal direction. It is a thing.

Hereinafter, the present invention will be described in more detail.

The magnetic powder in the present invention is a hexagonal ferrite powder, in which a part of Fe is substituted as a particularly preferable powder, which has a general formula of M _A O · n (Fe _1-X M _BX ) ₂ O _3. It is represented. Where M _A is Ba, Sr, Pb, M _B is In, Co, Ti, Ni, Mn, Cu, Zn, G
Represents at least one element such as e, Nb, Zr, V, Ta, Al, Cr, and Sb, which controls the coercive force. Also,
It is preferable that n is in the range of 5.0 to 6.0, x is in the range of 0 to 0.2, and the hexagonal ferrite powder has an average particle size of 0.01 to 0.3 μm and a particle size to thickness ratio of 2: 1 or more. It is the length of the diagonal line of the hexagonal plate surface.

The method for producing the hexagonal ferrite powder used in the present invention is not particularly limited, but the following method can be used, for example. That is, iron chloride, barium chloride or strontium chloride and, if necessary, an aqueous solution of a metal ion such as chloride of a substituting element is contacted with an alkaline solution such as NaOH to form a precipitate of the metal ion, which is washed with water and dried, and then heated to a high temperature. Co-precipitation method in which the metal ions are crystallized by a hydrothermal synthesis method in which the metal ions are crystallized from an aqueous solution containing the above metal ions in a high temperature and high pressure vessel and heated at a high temperature if necessary (JP-A-56-160328). ), A fluxing method such as NaCl, BaCl ₂ or the like, in which a compound containing iron, barium, strontium, or lead is crystallized at high temperature and then the fluxing agent is removed to obtain magnetic particles, BaO or B ₂ O ₃ , A glass-forming material that obtains magnetic particles by forming a glass from a glass-forming substance such as SiO ₂ and a compound containing iron, barium or strontium, lead and optionally a substituting element, and crystallizing at high temperature to remove the glass-forming substance Crystallization method (JP-A-56-67904).

Thus, the magnetic recording medium of the present invention can be manufactured as follows. That is, the magnetic particles obtained by the above manufacturing method, or a mixture of magnetic particles obtained by a different manufacturing method is made into a paint together with a binder or the like, coated on a substrate, and passed through a solenoid or a repulsive magnetic field to obtain magnetic properties. First, a coating film in which the particles are oriented in the in-plane longitudinal direction is obtained. Next, before this coating film is dried, it is mechanically sheared by a smoother or the like to orient the magnetic particles near the surface perpendicularly to the surface. The strength of the magnetic field for orienting in the in-plane longitudinal direction is 500 Oersted or more, preferably 800 to 3000 Oersted. Also, the shear strength for orienting in the vertical direction is 500 to 10 Kdyne / cm.
² , preferably 1000 to 5000 dyne / cm ² .

The method of mechanically shearing in the present invention is not particularly limited, and after orienting the magnetic particles in the in-plane longitudinal direction, a roll is applied to the coating surface before the coating is dried, or between twin rolls. It may be a direction such as passing it. Also, even after the coating film has dried, after melting the coating film surface with the solvent used for coating, mechanically shearing it and aligning the magnetic particles near the surface vertically to the surface. Good. Further, after further orienting in the in-plane longitudinal direction, the coating film is dried. Then
The same paint or a paint having the same composition except for the physical properties of the magnetic particles is applied over the coating film. Then, similarly, mechanical shear is applied to orient the magnetic particles near the surface perpendicularly to the surface. Alternatively, when performing overcoating, the magnetic particles may be oriented perpendicular to the surface by magnetic field orientation. In addition, the thickness of the coating film for vertical orientation in the multi-coating is preferably about the same as the recording / reproducing head gap width or about half thereof.

The binder used in the present invention is not particularly limited and various conventionally used thermoplastic resins, thermosetting resins, reactive resins and mixtures thereof can be used. Further, the magnetic layer of the present invention may contain abrasives such as aluminum oxide and chromium oxide, various fatty acids and fatty acid esters, lubricants such as silicone oil and fluorocarbon, etc., if necessary.

Further, as the substrate used in the present invention, various substrates such as a flexible substrate such as polyethylene terephthalate and a non-magnetic metal substrate can be used.

〔The invention's effect〕

In the magnetic recording medium obtained as described above, the in-plane orientation portion is provided in the lower portion of the magnetic layer, so that the packing ratio of magnetic particles in the longitudinal direction, that is, the magnetic recording direction is increased, and the perpendicular orientation is applied to the magnetic layer. Since it is not performed from the body surface to the entire surface, the smoothness of the surface is also improved, and the vertical alignment portion on the surface also improves the recording / reproducing output in the high range,
In addition, the magnetic circuit in the recording medium is well formed in the in-plane orientation portion and the vertical orientation portion, so that the electromagnetic conversion characteristics can be improved.

Example of Invention

 Hereinafter, the present invention will be described in more detail with reference to Examples.

Example 1 Co-Ti produced by a glass crystallization method as magnetic particles
Barium ferrite magnetic powder (average particle size 580
Å, particle size to thickness ratio 4.5: 1, coercive force 780Qe, saturation magnetization 54em
u / g) was used.

The barium ferrite magnetic powder was used to make a paint by thoroughly kneading the composition ratio below.

Barium ferrite magnetic particles 100 parts by weight Vinyl chloride monovinyl acetate copolymer 5 parts by weight Polyurethane 10 parts by weight Chromium oxide fine powder 3 parts by weight Lubricant 1.5 parts by weight Dispersant 4 parts by weight Methyl ethyl ketone 40 parts by weight Toluene 40 parts by weight Cyclohexane 40 parts by weight 6 parts by weight of a curing agent was added to this magnetic paint, and the composition was applied onto a polyethylene terephthalate film having a thickness of 15 μm and passed through a repulsive magnetic field of 1300 Qe to orient the magnetic particles in the plane. Next, before the coating film was dried, a shear of 2000 dyne / cm ² was applied by a smoother to vertically orient the magnetic particles on the surface of the coating film. Then, it was dried and subjected to surface smoothing treatment. After the coating film was hardened well, this film was made into a 1/2 inch width slit slit magnetic tape.

The squareness ratio was measured using a vibrating sample magnetometer in the in-plane longitudinal direction of the obtained magnetic tape, the direction perpendicular to the in-plane direction, and the direction perpendicular to the magnetic layer film. Further, the surface smoothness of the obtained tape was examined by a stylus type surface roughness meter, and the recording and reproducing characteristics in the video band were further examined. The results will be described later together with other examples and comparative examples.

Example 2 Co-Nb-substituted barium ferrite magnetic powder produced by hydrothermal synthesis method (average particle size 800Å, particle size to thickness ratio 3.
7: 1, coercive force 1100 Qe, saturation magnetization 57 emu / g) were used.

Using this barium ferrite magnetic powder, the same composition ratio as in Example 1 was thoroughly kneaded to form a paint. A curing agent (6 parts by weight) was added to this magnetic paint, which was applied onto polyethylene terephthalate having a thickness of 15 μm, and passed through a 2100 Qe solenoid to orient the magnetic particles in-plane.

Next, before the coating film was dried, a shear of 3500 dyne / cm ² was applied between the twin rolls to vertically align the magnetic particles. Then, it was dried and subjected to surface smoothing treatment. After the coating film was cured well, this film was slit into a 1/2 inch width to prepare a magnetic tape.

The same evaluation as in Example 1 was performed on the obtained tape.

Comparative Example 1 The same magnetic paint as used in Example 1 was applied onto polyethylene terephthalate having a thickness of 15 μm, passed through a magnetic field arranged so as to be orthogonal to the magnetic layer surface, and the magnetic particles were vertically aligned, A vertically oriented coating film was prepared by drying in a magnetic field. After the coating film was sufficiently cured, this film was slit into a 1/2 inch width to prepare a magnetic tape.

The same evaluation as in Example 1 was performed on the obtained tape.

Comparative Example 2 The same magnetic coating material as used in Example 1 was coated on 15 μm thick polyethylene terephthalate and dried in the non-oriented state. After the coating film was cured well, this film was slit into a 1/2 inch width to prepare a magnetic tape.

The same evaluation as in Example 1 was performed on the obtained tape. The results of each example are shown below.

As can be seen from the results, a high output can be obtained because the surface and the magnetic particles are well packed, and the improvement is observed especially in the low frequency region. The magnetic layer has in-plane orientation in the longitudinal direction, and the upper portion has vertical orientation, and Mrx / Ms> 0.5, Mrz / Ms> 0.5, Mrx / Ms> Mr.
It was found that an effect of y / Ms was observed.

Even in the strontium ferrite magnetic powder and the lead ferrite magnetic powder, the squareness ratio in the in-plane longitudinal direction and the vertical direction is 0.5 or more, and the squareness ratio in the in-plane direction perpendicular to the in-plane longitudinal direction is more than the squareness ratio in the in-plane longitudinal direction. Similar effects were obtained by reducing the size.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroki Yokoyama 1 Komukai Toshiba-cho, Kouki-ku, Kawasaki-shi, Kanagawa Toshiba Research Institute Ltd. (56) Reference JP-A-59-77628 (JP, A)

Claims (5)

[Claims] 1. A magnetic recording medium provided with a magnetic layer containing hexagonal ferrite powder on a substrate, wherein the hexagonal ferrite powder in the magnetic layer is perpendicular to the surface side of the magnetic layer rather than the inner side thereof. The degree of orientation in the direction is large, the in-plane longitudinal direction of the magnetic layer and the squareness ratio in the vertical direction are both 0.5 or more, and the squareness ratio in the in-plane direction perpendicular to the in-plane longitudinal direction is the in-plane longitudinal direction. A magnetic recording medium having a smaller squareness ratio. 2. The average particle size of hexagonal ferrite powder is 0.01.
The magnetic recording medium according to claim 1, wherein the magnetic recording medium has a thickness of 0.3 μm. 3. The ratio of grain size to thickness of hexagonal ferrite powder is
The magnetic recording medium according to claim 1, which is 2: 1 or more. 4. The coercive force of the hexagonal ferrite powder is 200 to 200.
The magnetic recording medium according to claim 1, which is 0 oersted. 5. The magnetic recording medium according to claim 1, wherein the hexagonal ferrite powder having a substitution element for controlling coercive force is represented by the following general formula. MA O ・ n (Fe _1-X MB _X ) ₂ O _{3 In the} formula, MA is one of Ba, Sr, and Pb, and MB is In, Co, Ti, Ni, Mn, C.
At least one of u, Zn, Nb, V, Ta, Sb, Al, Cr, x is 0
.About.0.2, n represents 5.0 to 6.0.