JPH0677030A - Magnetic powder for magnetic recording and magnetic recording medium using same - Google Patents

Abstract

(57) [Summary] [Structure] The magnetic powder for magnetic recording of the present invention is a magnetic powder for magnetic recording mainly composed of a plate-like ferrite having crystal anisotropy, and is 200 Oe ≤ Hc ≤ 3000 Oe, 0 <(Hr-Hc) / (H
c) ^1/2 ≤ 7 (where Hc is the coercive force of the magnetic powder for magnetic recording, H
r is the residual magnetization coercive force of the magnetic powder for magnetic recording. ) Is satisfied. [Effect] According to the present invention, without impairing durability,
Good O / W characteristics can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic powder for magnetic recording and a magnetic recording medium using the magnetic powder.

[0002]

2. Description of the Related Art In recent years, magnetic recording media represented by 3.5-inch size floppy disks and magnetic tapes for VTRs have been widely used in various fields.

In general, such a magnetic recording medium comprises a magnetic layer in which magnetic powder is dispersed in a binder resin on a non-magnetic support. Examples of such magnetic powder include γ-Fe ₂ O ₃ , CrO ₂ , Co-γ-F.
Needle-like magnetic powders such as e ₂ O ₃ and more recently magnetic recording magnetic powders such as hexagonal ferrite magnetic powders represented by Ba ferrite have been used.

In such a magnetic recording medium, improvement of reproduction output and electromagnetic conversion characteristics such as S / N have been achieved by improvement of magnetic powder for magnetic recording and progress of coating technology. One of the important characteristics of the above is an overwrite (overwrite, hereinafter abbreviated as O / W) characteristic.

As a method of improving the O / W characteristic, there is a method of reducing the coercive force of the magnetic layer. However, reducing the coercive force of the magnetic layer causes a decrease in output in the high recording frequency range, and is not a preferable method.

A method of reducing the thickness of the magnetic layer itself can be considered. This is because by making the thickness of the magnetic layer as thick as the gap length of the head, the magnetic field can be sufficiently penetrated into the deep portion of the magnetic layer, thereby improving the O / W characteristics. Is.

[0007]

By the way, in recent years, higher density recording of magnetic recording media has been promoted, and in response to this, the gap length of the magnetic head tends to be further reduced. Therefore, due to the thinning of the magnetic layer described above, O
In order to improve the / W characteristic, it is necessary to further reduce the thickness of the magnetic layer.

However, there is a limit to thinning the magnetic layer in consideration of durability of the magnetic recording medium due to sliding with the head.

Further, since it is very difficult to apply the thin film magnetic layer with high accuracy, there arises a problem that productivity is lowered.

The present invention has been made in order to solve the above problems, and it is possible to obtain O without impairing durability and electromagnetic conversion characteristics.
An object of the present invention is to provide a magnetic powder for magnetic recording capable of improving the / W characteristic and a magnetic recording medium using the magnetic powder.

[0011]

The magnetic powder for magnetic recording of the present invention is a magnetic powder for magnetic recording mainly composed of a plate-like ferrite having crystal anisotropy, and the coercive force of the magnetic powder for magnetic recording. (Hc) is 200 Oe ≦ Hc ≦ 3000 Oe, and 0
<(Hr-Hc) / (Hc) ^1/2 ≦ 7 (where Hc is the coercive force of the magnetic powder and Hr is the residual coercive force of the magnetic powder, respectively).

Here, the residual coercive force (Hr) is the absolute value of the magnetic field at which the remanent magnetization curve intersects the horizontal axis (magnetic field axis), that is, the saturation magnetic field is applied, and then the magnetic field (H) is applied in the opposite direction. It shows the absolute value of the above-mentioned magnetic field (H) such that the residual magnetization becomes 0 when the magnetic field is made 0 thereafter.

The magnetic recording medium of the present invention is a magnetic recording medium in which a magnetic layer comprising magnetic powder for magnetic recording dispersed in a binder resin is disposed on at least one main surface of a support, The magnetic powder is mainly plate-like ferrite having crystal anisotropy, coercive force Hc is 200 Oe ≦ Hc ≦ 3000 Oe, and 0 <(Hr-Hc) / (Hc) ^1/2 ≦ 7 ( However, Hc represents the coercive force of the magnetic powder, and Hr represents the residual coercive force of the magnetic powder.).

The measurement of Hr and Hc is based on a measuring method in which the influence of the demagnetizing field can be ignored.

[0015]

From various experiments, the inventors of the present invention have found that the O / W characteristics of the magnetic recording medium greatly depend on the characteristics of the magnetic powder for magnetic recording, and in particular, the coercive force Hc is 200 Oe ≤ Hc ≤ 3000 Oe 0
It was found that excellent O / W characteristics can be secured by using magnetic powder for magnetic recording satisfying <(Hr-Hc) / (Hc) ^1/2 ≦ 7. Hc is the coercive force of the magnetic powder, and Hr is the residual coercive force of the magnetic powder.

Here, the coercive force (Hc) is set to 200 to 3000 Oe, because if the coercive force (Hc) is less than 200 Oe, sufficient recording / reproducing characteristics cannot be obtained in the magnetic recording medium.
This is because if it exceeds 00 Oe, it tends to be difficult to write and erase signals with the magnetic head. By the way, the reason why the O / W characteristic of the magnetic recording medium largely depends on [(Hr-Hc) / (Hc) ^1/2 ] of the magnetic powder for magnetic recording in the present invention has not been sufficiently clarified. However, it is considered that the reason is as follows. That is, [(Hr-Hc) /
This is because when (Hc) ^1/2 ] is small, magnetization reversal easily occurs in the same magnetic field, and therefore [(Hr-Hc) / (H
c) It is considered that the smaller the value of ^1/2 ], the better the O / W characteristics. In addition, the magnetic powder larger than 7 is difficult to permeate the magnetization, so that the O / W characteristic is poor.

As the above-mentioned magnetic powder for high density magnetic recording, one selected from hexagonal ferrite is particularly preferable. This is because the hexagonal ferrite has an easy axis of magnetization in the direction perpendicular to the plate surface of the plate crystal, and therefore high density recording can be achieved by utilizing the perpendicular magnetization component. As the hexagonal ferrite, for example, M
Type, W type and other hexagonal Ba ferrites, Sr ferrites, Pb ferrites, and ferrites in which Ba, Sr, and Pb are partially replaced by Ca, or those mainly composed of solid solutions or ion-substituted products thereof Can be mentioned.

The hexagonal ferrite magnetic powder is represented by, for example, the general formula AO.n (Fe _{12 -P} M _P O _18-α ) (1) (where A is Ba, Sr, Pb and At least one metal element selected from the group consisting of Ca is shown, and M is 2
To at least one metal element selected from hexavalent metal elements, P is a substitution amount per chemical formula and is a number of 0 or more,
and n is 0.7 to 4, and α is a number of 0 or more. And, as the magnetic powder for magnetic recording of the present invention,
A substance obtained by substituting a part of Fe atoms with a specific metal element, for example, the general formula AO.n (Fe _12-XYZ Co _X Zn _Y M (I) _Z O _18-α ) of the following formula (2) ... ( 2) (wherein A represents at least one element selected from Ba, Sr, Pb and Ca, and M (I) represents at least one metal element selected from tetravalent to hexavalent metal elements, X ,
Y and Z are preferably those represented by the substitution amounts of Co, Zn and M (I) per one chemical formula, n is 0.7 to 4 and α is a number of 0 or more).

Further, in the above formula (2), the substitution amount X per chemical formula is 0 or more and 0.5 or less, and Y is 0.5 or more and 1.5 or less.
The following is preferable. Further, it is better to replace a part of the Fe atoms including some Co atoms with Fe.
Compared with the case where some of the atoms are not replaced with Co atoms, the saturation magnetization (σs) of the magnetic powder is less likely to decrease, which is particularly preferable.

Furthermore, among the hexagonal ferrites as described above, M type (n = 0.7 to 1.1) with W = 0, W
Type (n to 1.5) hexagonal ferrite is very good O
/ W characteristic is obtained, which is particularly preferable.

Here, if valence compensation is performed for the M type in which α = 0 in the above chemical formula, the substitution amount Z of M (I) per chemical formula is determined so that the ionic valence is compensated.

For example, if M (I) is selected from only tetravalent metal elements, the substitution amount Z is X + Y, and if it is selected from only pentavalent metal elements, the substitution amount Z is (X + Y) / 2, 6 valences. Substitution amount Z is (X + Y) / 3
Becomes Incidentally, as the tetravalent metal element, Ti, Zr, H
f, Sn, etc. are Nb, Sb, as pentavalent metal elements,
Examples of the hexavalent metal element such as Ta and V include W and Mo.

However, the fact that the magnetic powder for magnetic recording is constituted with such a composition does not mean that the conditions of the present invention are always satisfied, and the manufacturing conditions and magnetic properties for magnetic recording adapted to each composition are not always satisfied. It must be in the form of powder particles.

The magnetic powder for magnetic recording of the present invention can be produced by a known method such as a glass crystallization method, a hydrothermal synthesis method or a coprecipitation method. Among them, the glass crystallization method is preferable as the method for obtaining the magnetic powder for magnetic recording of the present invention.

The average particle size of the magnetic powder for magnetic recording is preferably 0.02 to 0.2 μm. Average particle size is
If it is less than 0.02 μm, the saturation magnetization (σs) and coercive force (Hc) will be small, and sufficient reproduction output will not be obtained. Is recognized.

The magnetic recording medium according to the present invention is obtained by forming a magnetic layer by applying a magnetic coating material, in which the above-mentioned magnetic powder for magnetic recording is dispersed together with a binder resin, onto the surface of a non-magnetic support. The thickness of the magnetic layer is preferably 0.3 μm or more. This is because if the film thickness of the magnetic layer is less than 0.3 μm, it becomes difficult to secure the durability of the magnetic layer by sliding the head. Also, considering the O / W characteristics,
The thickness of the magnetic layer is preferably thin, but 3.0 μm
The following is particularly preferable to be 2.5 μm or less.

Examples of the binder resin used in the magnetic recording medium of the present invention include polyurethane resins, polyester resins, vinyl chloride resins, fibrin resins such as nitrocellulose, or carboxyl groups or sulfones at the ends of these resins. Those containing a polar group such as acid metal base can be used.

Further, in addition to the above-mentioned magnetic powder and binder resin, the magnetic layer may appropriately contain additives such as a dispersant, a lubricant, an abrasive and an antistatic agent, if necessary. Alumina, chromium trioxide, etc. can be used as the polishing agent, but it is necessary to select such that the electromagnetic conversion characteristics are not deteriorated and the durability can be improved. Preferably, the Mohs hardness is 6 or more, and the average particle size is It is recommended to use the one having a thickness of 0.1 μm to 1 μm.

[0029]

EXAMPLES Examples of the present invention and comparative examples will be described below.

Example 1 First, starting materials BaCO ₃ and Fe were used so as to form a desired Ba ferrite composition.
₂ O ₃ , CoO, ZnO, Nb ₂ O ₅ , TiO ₂ or the like is appropriately mixed and further mixed with a predetermined amount of BaCO ₃ , H ₃ BO ₃ for forming BaO.B ₂ O ₃ ,
It was melted at 1350 ° C., and the melt was dropped on a rotating twin roll and rapidly cooled to obtain an amorphous body. Next, this amorphous material was heated to 780 ° C. and kept at this temperature for 5 hours to precipitate Ba ferrite in the matrix.
Next, this was washed with acetic acid to extract Ba ferrite magnetic powder, washed with water, and then dried. The resulting magnetic powder, M type Ba ferrite magnetic powder (BaFe _9.9 Zn _1.4 Nb _{0. 7}
O ₁₉ ), and its average particle size was also suitable for high density recording.

Using the magnetic powder for magnetic recording thus obtained, a magnetic recording medium was prepared with the following magnetic coating composition.

(Magnetic coating composition) 100 parts by weight of magnetic powder for magnetic recording Dispersant (phosphate ester) 2 〃 Vinyl chloride-vinyl acetate copolymer 9 〃 Sulfonate-containing polyurethane (molecular weight 4500) 6 〃 Aluminum oxide (average particle size) Diameter 0.3 μm) 5 〃 Carbon (average particle size 30 nm) 3 〃 Oleic acid 1.5 〃 BES 0.5 〃 Methyl ethyl ketone 70 〃 Toluene 70 〃 Cyclohexanone 40 〃 Disperse each of the above magnetic paint components thoroughly using a sand grinder After that, the mixed impurities were removed. Furthermore,
After adding 5 parts by weight of polyisocyanate to make a magnetic paint, apply this magnetic paint on the surface of the base film with a thickness of 75 μm using a reverse coater and perform calendar processing to form a magnetic layer with a thickness of 1.0 μm. The magnetic recording medium was formed and cured in a constant temperature bath.

(Examples 2 to 6) Magnetic powders for magnetic recording and magnetic recording media were obtained in the same manner as in Example 1 except that the magnetic powder and the magnetic layer thickness were changed as shown in Tables 1 and 2, respectively. .

Comparative Examples 1 to 4 Magnetic powders for magnetic recording and magnetic recording media were obtained in the same manner as in Example 1 except that the magnetic powder and the magnetic layer thickness were changed as shown in Tables 1 and 2, respectively. .

Table 1 shows the characteristics of the magnetic powders of Examples 1 to 6 and Comparative Examples 1 to 4, and Table 2 shows the characteristics of the magnetic recording medium.

[0036]

[Table 1]

[Table 2] The value of n in Table 1 and the substitution amount (p) per chemical formula
Is n, the substitution amount shown in the formula (1) in the present specification.
(p). In addition, the coercive force (Hc) of magnetic powder for magnetic recording,
The saturation magnetization (σs) and the residual coercive force (Hr) were measured by a vibrating sample type magnetometer (VSM) at a maximum applied magnetic field of 10 (kOe). The magnetic powder sample for measurement was pressed into a plate shape having a packing density of 1.7 g / cc and a shape ratio of 1:10, and a magnetic field was applied in a direction parallel to the longitudinal direction of the sample. For electromagnetic conversion characteristics and durability, each medium is punched out into a 3.5-inch size disk, a center core is installed in the center, and it is rotatably housed in a jacket to make a 3.5-inch size floppy disk. It was measured.

For reproduction output, a ferrite head having a gap length of 0.4 μm and a track width of 120 μm was used, and the recording density was 35 K.
Evaluated against FRPI and 5KFRPI. S / N is 35kFRP
It was evaluated by I. In addition, the O / W characteristics have a gap length of 0.6 μ.
m, using a ferrite head with a track width of 120 μm, 2.
The signal written with 6 kFRPI was overwritten with 10.4 kFRPI and evaluated by the residual rate of the first written 2.6 kFRPI signal.

The durability was evaluated in a cycle environment in which a low temperature / low humidity environment having a temperature of 5 ° C. and a humidity of 10% and a high temperature environment having a temperature of 52 ° C. and a humidity of 50% were changed in a 24-hour cycle. The measurement track was Tr.12, and the evaluation standard was when the output dropped to 70% of the initial output or when surface damage was observed by visual inspection.

As can be seen from the table, No. 1-No. Three
All of the magnetic powders for magnetic recording of [(Hr-Hc) / (Hc)
The value of [ ^1/2 ] is controlled to be larger than 0 and equal to or less than 7, so that the magnetic recording media of Examples 1 to 5 in which this is used have very excellent O / W characteristics. .
Therefore, the thickness of the magnetic layer is equivalent to that of the magnetic recording medium in which other magnetic recording magnetic powder is used, and even if the magnetic layer is thicker, the same overwrite characteristic can be secured, and thus the durability can be significantly improved. You can

[0040]

According to the magnetic powder for magnetic recording of the present invention, excellent O / W characteristics can be secured without significantly reducing the film thickness of the magnetic layer.

Therefore, the magnetic recording medium of the present invention can secure excellent O / W characteristics without impairing durability.

[0042]

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tatsumi Maeda 1 Komukai Toshiba-cho, Sachi-ku, Kawasaki-shi, Kanagawa Stock company Toshiba Research Institute

Claims (5)

[Claims] 1. A magnetic powder for magnetic recording, which is mainly composed of a plate-like ferrite having crystal anisotropy, and has 200 Oe ≦ Hc ≦ 30.
00 Oe, 0 <(Hr-Hc) / (Hc) ^1/2 ≤ 7 (where Hc is the coercive force (Oe) of the magnetic powder and Hr is the residual coercive force (Oe) of the magnetic powder). Magnetic powder for magnetic recording characterized by satisfying. 2. The magnetic powder for magnetic recording according to claim 1, wherein the magnetic powder for magnetic recording is hexagonal ferrite. 3. A part of Fe atoms constituting the magnetic powder for magnetic recording according to claim 2, wherein a substitution amount per chemical formula is 0.5 or more and 1.5 or less Zn atoms, and 0 or more and 0.5 or less Co atoms. Magnetic powder for magnetic recording, which is substituted with at least one kind of metal atom selected from tetravalent to hexavalent metal elements. 4. The magnetic powder for magnetic recording according to claim 3, wherein a part of Fe atoms constituting the magnetic powder for magnetic recording is replaced by Co atoms. 5. A magnetic recording medium in which a magnetic layer comprising magnetic powder for magnetic recording dispersed in a binder resin is arranged on at least one main surface of a support, wherein the magnetic powder for magnetic recording is plate-shaped. Mainly ferrite with crystal anisotropy,
200 Oe ≤ Hc ≤ 3000 Oe, 0 <(Hr-Hc) / (Hc) ^1/2 ≤ 7
The magnetic recording medium is characterized in that Hc represents the coercive force (Oe) of the magnetic powder and Hr represents the residual coercive force (Oe) of the magnetic powder.