JPH1092620A - Magnetic powder for magnetic recording - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide magnetic power which has an average particle size of 20-100nm, small SFD(switching field distribution), and highly symmetric coercive force distribution. SOLUTION: Magnetic powder having excellent characteristics is obtained by controlling a crystallizing process so that the following two relations can be met when the half-value width ΔH of a differential curve (dσ/dH) in the second quadrant is divided into two parts by drawing a perpendicular from the vertex of the curve: (|ΔH1 -ΔH2 |)/Hm<=0.12 and ΔH/Hm=(ΔH1 +ΔH2 )/ Hm<=1.2. where, ΔH1 , ΔH2 , and Hm respectively represent the low magnetic field side of the halved ΔH, the high magnetic field side of the ΔH, and the magnetic field which gives the maximum differential coefficient.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic powder suitable for a coated magnetic recording medium capable of high-density recording and reproduction.

[0002]

2. Description of the Related Art With the rapid progress of the information society, the demand for higher recording density of magnetic recording is increasing, and the demand for short-wavelength recording / reproducing is also increasing in the field of coating type magnetic recording media. ing. For the purpose of increasing the linear recording density of a magnetic recording medium to meet these demands, for example, a magnetic material used as a magnetic powder, for example, has a smaller particle size, a larger coercive force, and a width of magnetization reversal distribution. (Swic
Development is being made in such a direction as to reduce recording field demagnetization as much as possible (hing field distribution: hereinafter abbreviated as SFD). At present, for example, in the case of hexagonal ferrite, magnetic powder having a particle size of 300 nm to about several tens nm is widely used as magnetic powder. However, although a short wavelength output and an improvement in S / N ratio have been observed in a magnetic recording medium manufactured using a magnetic material whose particle size has been advanced, it cannot be said that it is still sufficient. Therefore, there is a demand for a fine particle magnetic powder having not only a small particle size but also excellent magnetic properties.

[0003] The SFD means that a magnetic field is applied to a magnetic powder to measure a remanent magnetization (σr) of the magnetic powder to prepare a magnetization curve, and a half-value width ΔH of a differential curve (dσ / dH) in the magnetization curve of the second quadrant. Is divided by the coercive force Hc of the magnetic powder, and represents the gradient of the magnetization curve near the coercive force Hc. This SFD is known as a parameter indicating the spread of the coercive force distribution of the magnetic powder, and it is generally said that the shorter the value is, the longer the short wavelength characteristic is.

[0004]

By the way, as the particle size of a magnetic material is reduced, for example, when the particle size of a hexagonal ferrite or the like is reduced to a size of 40 nm or less, a remarkable increase in the value of SFD occurs. It turned out to be. In addition, when a magnetic coating is prepared using this type of magnetic powder, which has been made finer, and a magnetic film is formed by coating, the orientation property is higher than when a magnetic powder having a conventional particle size is used. -It became clear that the filling property was inferior. As a result, the saturation magnetization and the residual magnetization decrease, and S
The inconvenience of deterioration of FD characteristics has occurred. Therefore, when a magnetic recording medium is manufactured, although a reduction in noise is recognized, a reduction in the reproduction output is remarkable, and the gain of improving the S / N has not yet been achieved.

SUMMARY OF THE INVENTION The present invention has been made in order to solve such inconveniences caused by fine magnetic powder and to cope with high-density recording. The present invention provides a magnetic powder having a small SFD and suitable for high-density recording and reproduction. That is its purpose.

[0006]

As a result of intensive study for the above purpose, the cause of the above-mentioned inconvenience is that the magnetic powder obtained by micronization has a wide particle size distribution and is symmetrical away from the normal distribution type. It was speculated that the lack was caused, and that the coercive force distribution was widened and offset.

As a method of expressing whether or not the coercive force distribution of the magnetic powder is balanced, SFD, that is, a differential curve (dσ) in the magnetization curve of the second quadrant of the magnetic powder is used.
/ DH) is defined as a numerical value defined as a value obtained by dividing the half width of the magnetic powder by the coercive force Hc of the magnetic powder, and a numerical value representing the symmetry of the shape near the apex of the differential curve is introduced as the index. did. By defining the characteristics of the magnetic powder with these two indices, the spread of SFD is suppressed,
In addition, a magnetic powder suitable for high-density recording / reproduction is obtained without impairing the filling property and orientation in the coating film.

That is, the magnetic powder for magnetic recording of the present invention comprises:
When the half width ΔH of the differential curve (dσ / dH) in the magnetization curve of the second quadrant is divided into two by a perpendicular from the apex,
Equation (| ΔH ₁ −ΔH ₂ |) /Hm≦0.12 ΔH / Hm = (ΔH ₁ + ΔH ₂ ) /Hm≦1.2 (where ΔH ₁ is the low magnetic field side of ΔH divided into two, ΔH
₂ represents the high magnetic field side, and Hm represents the magnetic field giving the maximum derivative. ).

In the present invention, (| ΔH ₁ −ΔH ₂ |)
/ Hm is preferably 0.12 or less, more preferably 0.1 or less. (| ΔH ₁
An increase in the value of −ΔH ₂ |) / Hm indicates that the asymmetry of the differential curve (dσ / dH) increases.

In the case of ΔH ₁ > ΔH ₂ , when the peak area of the differential curve is divided into a low magnetic field side including ΔH ₁ and a high magnetic field side including ΔH ₂ , the low magnetic field side becomes large. At this time, if the value of (| ΔH ₁ −ΔH ₂ |) / Hm exceeds 0.12, the ratio of the particles having a low coercive force, that is, the ratio of the soft magnetic particles is large as compared with the average coercive force of the magnetic powder. This is not preferable because it causes problems such as excessive recording and difficulty in holding records.

On the other hand, when ΔH ₁ <ΔH ₂ , (| ΔH ₁
When −ΔH ₂ |) / Hm exceeds 0.12, the high magnetic field side of the differential curve becomes large. In this case, the ratio of the particles having a large coercive force becomes too large as compared with the coercive force of the magnetic powder, which causes a problem such as difficulty in writing.

ΔH / Hm, that is, (ΔH ₁ + ΔH ₂ )
The SFD of the magnetic powder represented by / Hm is preferably as small as possible, and in the present invention, it is desirably 1.2 or less, preferably 1.0 or less. If the value of SFD exceeds 1.2, the distribution of Hc is too wide and the short-wavelength characteristic required for high recording density is impaired, which is not preferable.

Note that ΔH, ΔH ₁ , ΔH
The values of H ₂ and Hm can be easily obtained using a VSM (vibration sample type magnetometer) or the like.

In the present invention, the average particle size of the magnetic powder is 20
It is preferably in the range of -100 nm, more preferably in the range of 20-60 nm. It has been confirmed that when the average particle diameter exceeds 100 nm, the particle size distribution is too wide and the noise component increases when a medium is produced.
It is not suitable for high density recording intended by the present invention. Average particle size 2
Below 0 nm, the so-called superparamagnetism (superparamagnetism) in which the magnetic moment deviates from the magnetization stable axis due to thermal vibration due to the small individual particle volume and the coercive force disappears constantly.
The ratio of particles exhibiting amagnetism increases, which is also unsuitable for magnetic recording.

Hereinafter, the magnetic powder of the present invention will be described in more detail by taking hexagonal ferrite as an example. In preparing the hexagonal ferrite magnetic powder, various methods known in the art, for example, a glass crystallization method, a coprecipitation method,
Although it can be produced by a flux method or a hydrothermal synthesis method, a glass crystallization method is particularly effective as a method for producing the magnetic powder of the present invention.

Further, in producing the magnetic powder of the present invention, it is very important to strictly control the heat treatment process for depositing and growing crystals.

That is, in producing the magnetic powder of the present invention, nucleation is selectively performed by taking a sufficiently long holding time at the primary crystallization temperature (crystal nucleus precipitation temperature), Crystal growth is performed selectively at a secondary crystallization temperature (crystal growth temperature) higher than the crystallization temperature so as to grow to a desired grain size. Also, if the rate of temperature rise from the primary crystallization temperature to the secondary crystallization temperature is slow, new nucleation is likely to occur during this time, so the primary crystallization temperature is reduced to the secondary crystallization temperature. It is important to make the transition to as soon as possible. When it takes time to shift to the secondary crystallization temperature, for example, as in a conventional manufacturing method using a one-step heat treatment, crystal growth is performed simultaneously without sufficient nucleation. In other words, a difference in temperature and a difference in growth rate during nucleation results in an increase in the particle size distribution or an offset in the differential curve, which is not preferable.

The hexagonal ferrite magnetic powder of the present invention, which can be produced by the above method, has, for example, the following composition formula: MO.Fe _12-x M ' _x O ₁₈ (where M is Ca, Sr, Ba, and Pb Represents at least one or more elements selected, and M ′ represents an atomic group adjusted so that the average number of valences becomes trivalent.)
It can be expressed by

In the present invention, the coercive force of the magnetic powder is 300
It is desirable to be in the range of ~ 3000 Oe. 300
If it is less than Oe, recording demagnetization is remarkably not suitable for high-density recording,
In the case of exceeding 3000 Oe, since the head which sufficiently magnetizes this is not present, saturation occurs, and both cases are not preferable.

In order to control the coercive force in such a range, it is desirable to replace a part of Fe in the above composition formula with an appropriate metal element. At this time, it is desirable that the valence of the substituted ion be trivalent on average. M '
Represents an atomic group adjusted so that the average valence of atoms becomes trivalent. For example, when a divalent metal element is used as one type of M ′, tetravalent, pentavalent, and hexavalent elements are used in combination so that the valence of the substitution element is adjusted so that the average number of atoms is trivalent. Here, as divalent elements, Mn, Fe, Co, Ni, Cu, Z
n, Mg, Cd, etc. are exemplified.
Al, Y, Ga, In, Tl, Rh and the like; tetravalent elements such as Ti, Zr, Hf, Sn, Ge, Δe and Ru; and pentavalent elements as V, Nb, Ta, Bi, Sb and the like; Examples of the valence element include Mo, W, and the like.

The saturation magnetization of the magnetic powder of the present invention at room temperature is 4
It is desirable to be in the range of 0 to 75 emu / g. 40
If the amount of magnetization is less than emu / g, the output of the manufactured magnetic recording medium at a long wavelength is insufficient, which is not preferable. The larger the saturation magnetization is, the more preferable. However, it is difficult to achieve a magnetization exceeding 75 emu / g with the magnetic powder having the M-type structure and the particle size range of the present invention.

In the present invention, the magnetic powder is in the above-mentioned range of particle size and shape, and has a specific surface area of 2 as determined by the BEΤ method.
It is desirably in the range of 5 to 70 m ² / g. The reason for limiting the specific surface area to such a numerical range is that the specific surface area is
This is because the amount is related to the degree to which the magnetic particles interact with the resin binder during the preparation of the magnetic paint in the production of the medium. In other words, if it is less than 25 m ² / g, although the resistance received from the binder is small and the orientation of the magnetic powder is improved, it is difficult to secure the dispersion stability of the magnetic powder in the magnetic paint, which is not preferable. On the other hand, if it exceeds 70 m ² / g, the orientation and filling properties of the magnetic powder are reduced, and the magnetic powder is not suitable for high-density recording.

In the magnetic powder of the present invention, the plate ratio is preferably from 2 to 9 in consideration of the filling property and orientation in the medium, and more preferably from 3 to 6. As the plate ratio increases, the orientation improves,
Although the FD is narrowed, the filling property is reduced. In order to increase the reproduction output of the manufactured magnetic recording medium,
In magnetic powder, improvement of filling property, improvement of orientation, and S
It is important that the three characteristics of narrowing the FD be well balanced. In the present invention, from such a viewpoint,
The preferred range of the plate ratio is 2 to 9, and the more preferred range is 3 to 6.

Although the present invention has been described above by taking hexagonal ferrite as an example, the present invention is not limited to hexagonal ferrite magnetic powders, but includes Fe, Fe--Co, F
It can be suitably applied to acicular magnetic powder such as e-Co-Ni.

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to embodiments. As the magnetic powder, the following composition formula: BaO · Fe _{12-3 (x + y) / 2} Co _x Zn _y Nb _{(x + y) / 2} O
_The hexagonal ferrite magnetic powder represented by ₁₈ was picked up.

Example 1 A magnetic powder was produced by a glass crystallization method using BaO-B ₂ O ₃ as a glass matrix. According to a known method, a raw material component of ferrite having a composition where x = 0.2 and y = 0.6 in the above composition formula and a glass matrix component are mixed well, and the melt obtained by heating and melting is subjected to a twin-cast process. It was quenched with a roll to produce an amorphous body. In addition, in mixing the raw materials, the mass ratio of the matrix component / ferrite component was set to 60/40.

Next, a heat treatment for 25 hours is performed at a primary crystallization temperature of 480 ° C., mainly for nucleation, and then at 15 ° C.
The temperature was raised to the secondary crystallization temperature of 760 ° C. at a heating rate of / min, and heat treatment was performed at 760 ° C. for 4 hours. afterwards,
A glass matrix component was dissolved and removed by a washing treatment according to a known method to obtain a hexagonal ferrite powder which is a magnetic powder of the present invention.

When the magnetic properties of the obtained magnetic powder were examined using a VSM, the coercive force Hc was 1590 Oe, the magnetization was 55 emu / g, and the SFD was 0.7. The SFD of the magnetic powder is a value obtained from a major hysteresis loop obtained by filling a nonmagnetic cell having a diameter of 4 mm and a thickness of 1 mm with the powder and not performing demagnetization correction.
When (| ΔH ₁ −ΔH ₂ |) / Hm was calculated from the same loop, it was 0.06.

The particle size and shape of the magnetic powder were determined by randomly selecting 200 particles from a transmission electron microscope image at a magnification of 200,000, measuring the particle size and plate thickness, and calculating the arithmetic mean of each. . As a result, the average particle size was 30 nm and the plate ratio was 4
Met. The value of the specific surface area by the BET method (gas adsorption method) was measured using a high-sensitivity area meter.
m ² / g.

Next, in order to examine the properties of the obtained magnetic powder, a magnetic paint having the following composition was prepared and applied to a polyethylene terephthalate film using an applicator to form a coating film. In preparing the magnetic paint, kneading was performed for 5 hours using a sand grinder.

<Magnetic coating composition> Magnetic powder 100 parts by mass Polar group-containing polyurethane resin 5 parts by mass Polar group-containing vinyl chloride resin 5 parts by mass Solvent (methyl ethyl ketone / cyclohexanone / toluene) 300 parts by mass 6 coats before drying
The coating film was placed in a kOe magnetic field so as to be parallel to the magnetic field, and was naturally dried in the magnetic field, and the coating film was subjected to an orientation treatment.

Then, the ratio of residual magnetization amount / saturation magnetization amount of the obtained alignment film was determined by using VSM, and the ratio was 75% as an orientation ratio. The SFD of this oriented coating film was 0.2 as a result.

Example 2 A hexagonal ferrite powder was produced in the same manner as in Example 1 except that the secondary crystallization temperature was changed to 800 ° C. The magnetic properties of the obtained magnetic powder were determined in the same manner as in Example 1.
(| ΔH ₁ −ΔH ₂ |) / Hm, and the particle size and shape were measured. Further, an alignment film was prepared, and the alignment ratio and SFD were measured. The above measurement results are shown in Table 1 below together with the measurement results of Example 1.

Example 3 The ferrite composition was determined as follows: x = 0.1, y
= 0.7, heat treatment at a primary crystallization temperature of 520 ° C. for 20 hours, then heating at a rate of 10 ° C./min to a secondary crystallization temperature of 725 ° C., and a heat treatment for 5 hours A hexagonal ferrite powder was produced in the same manner as in Example 1 except that the application was performed. The magnetic properties of the obtained magnetic powder were calculated in the same manner as in Example 1; (| ΔH ₁ −ΔH ₂ |) /
The Hm, the particle size and the shape were measured, and an orientation film was further prepared to measure the orientation ratio and SFD. The above measurement results are shown in Table 1 below.

Example 4 A hexagonal ferrite powder was produced in the same manner as in Example 3 except that the secondary crystallization temperature was changed to 830 ° C. The magnetic properties of the obtained magnetic powder were determined in the same manner as in Example 1.
(| ΔH ₁ −ΔH ₂ |) / Hm, and the particle size and shape were measured. Further, an alignment film was prepared, and the alignment ratio and SFD were measured. The measurement results are shown in Table 1 below.

Example 5 In preparing a hexagonal ferrite magnetic powder having the same composition as in Example 1 by a coprecipitation method, a metal salt solution containing an element constituting ferrite was mixed at 50 ° C. with twice the equivalent of caustic soda. And coprecipitated. After the obtained coprecipitate was filtered off, washed with water and dried, the primary crystallization temperature was 500 ° C.
After the heat treatment for 30 hours, the temperature was raised to the secondary crystallization temperature of 780 ° C. at a rate of 20 ° C./min, and the heat treatment was performed for 2 hours. Then, a hexagonal ferrite powder as the magnetic powder of the present invention was obtained. Example 1 about the obtained magnetic powder
Magnetic properties, (| ΔH ₁ −ΔH ₂ |) / H
m, and the particle size and shape were measured, and an orientation film was further prepared to measure the orientation ratio and SFD. The above measurement results are shown in Table 1 below.

[0037]

[Table 1] Next, as a comparative example, a hexagonal ferrite magnetic powder not according to the present invention was manufactured. The properties of the obtained magnetic powder were evaluated in the same manner as in Example 1. The measurement results are shown in Table 2 below.

Comparative Example 1 A hexagonal ferrite powder was obtained in the same manner as in Example 1 except that the primary crystallization step at 480 ° C. for 25 hours was omitted.

Comparative Example 2 A hexagonal ferrite powder was obtained in the same manner as in Example 3 except that the primary crystallization step at 520 ° C. for 20 hours was omitted.

Comparative Example 3 A hexagonal ferrite powder was obtained in the same manner as in Example 5, except that the first heat treatment step at 500 ° C. for 30 hours was omitted.

[0041]

[Table 2] As is clear from the comparison between Tables 1 and 2, the magnetic powder of the present invention, which has excellent symmetry of the differential curve of the magnetization curve and low SFD, has a large saturation magnetization, a high orientation ratio when forming a magnetic film, and a high density. Suitable for magnetic recording.

[0042]

As described above, the magnetic powder of the present invention has excellent magnetic characteristics such as a small SFD value and good symmetry of the coercive force distribution despite being fine particles. Therefore, it is suitable for a magnetic powder for producing a coating medium for high recording density.

[0043]

Claims (2)

[Claims] When a half width ΔH of a differential curve (dσ / dH) in a magnetization curve of a second quadrant is divided into two by a perpendicular line from a vertex, the following two formulas (| ΔH ₁ −ΔH ₂ |) / Hm ≦ 0.12 ΔH / Hm = (ΔH ₁ + ΔH ₂ ) /Hm≦1.2 (where ΔH ₁ is the low magnetic field side of ΔH divided into two, ΔH
₂ represents the high magnetic field side, and Hm represents the magnetic field giving the maximum derivative. A) a magnetic powder for magnetic recording, which satisfies both the above conditions. 2. The magnetic powder for magnetic recording according to claim 1, wherein the average particle diameter is in the range of 20 to 100 nm.