JP3576332B2 - Magnetic powder for magnetic recording - Google Patents

Description

次に、比較例として、本発明にしたがわない六方晶系フェライト磁性粉末を製造した。得られた磁性粉末についても、実施例１と同様にして特性を評価した。その測定結果を、後出の表２に示した。
【００３８】
比較例１
４８０℃で２５時間の第一次結晶化工程を省略した他は実施例１と同様にして、六方晶系フェライト粉末を得た。
【００３９】
比較例２
５２０℃で２０時間の第一次結晶化工程を省略した他は実施例３と同様にして、六方晶系フェライト粉末を得た。
【００４０】
比較例３
５００℃で３０時間の第一次熱処理工程を省略した他は実施例５と同様にして、六方晶系フェライト粉末を得た。
【００４１】
【表２】

表１および表２の比較からも明らかなように、磁化曲線の微分曲線の対称性にすぐれＳＦＤが小さい本発明の磁性粉は、飽和磁化も大きく磁性膜形成時の配向率も高く、高密度の磁気記録に適している。
【００４２】
【発明の効果】
以上説明したように、本発明の磁性粉は微粒子でありながらＳＦＤの値が小さく、しかも保磁力分布の対称性が良好というすぐれた磁気特性を有している。それゆえ、高記録密度用の塗布型媒体を作製するための磁性粉に、好適である。
【００４３】[0001]
TECHNICAL 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]
[Prior art]
With the rapid progress of the information-oriented society, the demand for higher recording density of magnetic recording is increasing more and more, and the demand for short-wavelength recording / reproduction is also increasing in the field of coating type magnetic recording media. 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 has a smaller particle size, a larger coercive force, and a width of magnetization reversal distribution. (Switching field distribution: hereafter abbreviated as SFD), and development has been made in such a direction as to minimize recording demagnetization. At present, for example, in hexagonal ferrite, magnetic powder that is finely divided to a particle size of about 300 nm to about several tens of nm is frequently used. However, although short-wavelength output and improvement in S / N ratio have been observed in a magnetic recording medium manufactured using a magnetic material whose particle size has been reduced, it is still not sufficient. Therefore, there is a demand for 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 residual 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 determined by this magnetic field. It is defined as the value divided by the coercive force Hc of the 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 smaller this value is, the longer the short wavelength characteristic is.
[0004]
[Problems to be solved by the invention]
By the way, it has been found that as the magnetic material becomes finer, if the particle size of a hexagonal ferrite or the like is reduced to a particle size of 40 nm or less, the value of SFD significantly increases. Furthermore, when a magnetic paint is prepared using this type of magnetic powder, which has been made finer, and a magnetic film is formed by coating, the orientation 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, inconveniences such as a decrease in the amount of saturation magnetization / remaining magnetization and deterioration of the SFD characteristics have 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.
[0005]
The present invention has been made in order to solve such inconveniences associated with the micronization of magnetic powder and to cope with high-density recording, and to provide a magnetic powder having a small SFD and suitable for high-density recording and reproduction. That is the purpose.
[0006]
[Means for Solving the Problems]
As a result of intensive studies 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 departs from the normal distribution form and lacks symmetry. It was speculated that the width of the coercive force distribution would be widened and offset.
[0007]
Then, as a method of expressing whether or not the coercive force distribution of the magnetic powder is balanced, the SFD, that is, the half value width of the differential curve (dσ / dH) in the magnetization curve of the second quadrant of the magnetic powder is determined. The numerical value defined as the value divided by the magnetic force Hc is used as an index, and a numerical value representing the symmetry of the shape near the apex of the differential curve is introduced as the index. By defining the characteristics of the magnetic powder with these two indices, the spread of the SFD can be suppressed, and the magnetic powder suitable for high-density recording and reproduction can be obtained without impairing the filling property and orientation in the coating film. I got it.
[0008]
That is, the hexagonal ferrite magnetic powder for magnetic recording of the present invention has a composition formula of MO.Fe _12-x M ′ _x O ₁₈ (where M represents at least one or more elements selected from Ca, Sr, Ba, and Pb, and M ′ represents an atomic group adjusted so that the average number of atoms is three. A) a hexagonal ferrite magnetic powder for magnetic recording, represented by the formula (1), having an average particle size of 20 to 100 nm, a plate-like ratio of 2 to 9 and a specific surface area of 25 to 70 m ² / g by BEＢ method. When the half width ΔH of the differential curve (dσ / dH) of the second quadrant in the magnetization curve of the magnetic powder is divided into two by a perpendicular from the apex, the following two equations (| ΔH1−ΔH2 |) / Hm ≦ 0.12
ΔH / Hm = (ΔH1 + ΔH2) /Hm≦1.2
(However, ΔH1 represents the low magnetic field side of ΔH divided into two, ΔH2 represents the high magnetic field side, and Hm represents the magnetic field that gives the maximum differential coefficient.)
[0009]
In the present invention, (| ΔH ₁ −ΔH ₂ |) / Hm is preferably 0.12 or less, more preferably 0.1 or less. An increase in the value of (| ΔH ₁ −ΔH ₂ |) / Hm indicates that the asymmetry of the differential curve (dσ / dH) increases.
[0010]
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, when 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.
[0011]
On the other hand, when (| ΔH ₁ −ΔH ₂ |) / Hm exceeds 0.12 in the case of ΔH ₁ <ΔH ₂ , the high magnetic field side of the differential curve increases. In this case, the ratio of particles having a large coercive force becomes too large as compared with the coercive force of the magnetic powder.
[0012]
The smaller the SFD of the magnetic powder represented by ΔH / Hm, that is, (ΔH ₁ + ΔH ₂ ) / Hm, the better, and it is desirable in the present invention to be 1.2 or less, preferably 1.0 or less. If the SFD value exceeds 1.2, the distribution of Hc is too wide, and the short-wavelength characteristics required for higher recording density are impaired.
[0013]
Note that the values of ΔH, ΔH ₁ , ΔH ₂ , and Hm of the magnetic powder described above are VSM
(Vibration sample type magnetization measurement device) or the like.
[0014]
In the present invention, the average particle size of the magnetic powder is preferably in the range of 20 to 100 nm, more preferably in the range of 20 to 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 manufactured, which is not suitable for the high-density recording intended by the present invention. When the average particle diameter is 20 nm or less, the proportion of particles exhibiting so-called superparamagnetism, in which the magnetic moment deviates from the magnetization stable axis due to thermal oscillation and continually loses the coercive force due to the small particle volume, is reduced. It becomes too large for magnetic recording.
[0015]
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, it can be produced by various methods known in the art, for example, a glass crystallization method, a coprecipitation method, a flux method, or a hydrothermal synthesis method. The glass crystallization method is effective as a method for producing the magnetic powder of the present invention.
[0016]
Further, in producing the magnetic powder of the present invention, it is very important to strictly control the heat treatment process for precipitating and growing crystals.
[0017]
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), and then the primary crystallization temperature is reduced. Crystal growth is performed selectively at a higher secondary crystallization temperature (crystal growth 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. If 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. Therefore, a difference in temperature and a difference in growth rate at the time of nucleation are not preferable because they result in broadening of the particle size distribution or deviation of the differential curve.
[0018]
Hexagonal ferrite magnetic powders capable of manufacturing the present invention by the above method, for example, the following composition formula _{MO · Fe 12-x M'x} O 18
(However, M represents at least one or more elements selected from Ca, Sr, Ba, and Pb, and M ′ represents an atomic group adjusted so that the average number of valences becomes trivalent.)
It can be expressed by
[0019]
In the present invention, the coercive force of the magnetic powder is desirably in the range of 300 to 3000 Oe. If it is less than 300 Oe, recording demagnetization is remarkably unsuitable for high-density recording, and if it exceeds 3000 Oe, since a head for sufficiently magnetizing this is not present, saturation occurs, and both cases are not preferable.
[0020]
In order to control the coercive force in such a range, it is desirable to replace a part of Fe with an appropriate metal element in the above composition formula. At this time, it is desirable that the valence of the substituted ion is trivalent on average. M ′ means an atomic group adjusted so that the average valence of atoms becomes trivalent. For example, when a divalent metal element is used as one kind 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, examples of the divalent element include Mn, Fe, Co, Ni, Cu, Zn, Mg, and Cd, and examples of the trivalent element include Sc, Al, Y, Ga, In, Tl, and Rh. Examples thereof include Ti, Zr, Hf, Sn, Ge, Τe, and Ru, and pentavalent elements such as V, Nb, Ta, Bi, and Sb, and hexavalent elements such as Mo and W.
[0021]
The saturation magnetization of the magnetic powder of the present invention at room temperature is preferably in the range of 40 to 75 emu / g. If the amount of magnetization is less than 40 emu / g, the long-wavelength output of the manufactured magnetic recording medium becomes insufficient, which is not preferable. The larger the saturation magnetization is, the more preferable. However, it is difficult to achieve the magnetization exceeding 75 emu / g with the magnetic powder having the M-type structure and the particle size range of the present invention.
[0022]
In the present invention, it is desirable that the magnetic powder is in the above-mentioned range of particle size and shape, and that the specific surface area is in the range of 25 to 70 m ² / g as determined by the BEΤ method. The specific surface area is limited to such a numerical range because the specific surface area is an amount 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. 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.
[0023]
In the magnetic powder of the present invention, the plate ratio is preferably 2 to 9 in consideration of the filling property and orientation in the medium, and more preferably 3 to 6. As the plate ratio increases, the orientation is improved and the SFD is narrowed, but the fillability is reduced. In order to increase the reproduction output of the manufactured magnetic recording medium, it is important that the magnetic powder has a good balance of the three characteristics of improving the filling property, improving the orientation, and narrowing the SFD. In the present invention, from such a viewpoint, a desirable range of the plate ratio is 2 to 9, and a more preferable range is 3 to 6.
[0024]
As described above, the present invention has been described using hexagonal ferrite as an example. However, the present invention is not limited to hexagonal ferrite magnetic powder, and needles such as Fe, Fe-Co, Fe-Co-Ni, etc. The present invention can also be suitably applied to magnetic powders in the form of particles.
[0025]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail with reference to examples. As the magnetic powder, the following composition formula: BaO · Fe _{12-3 (x + y) / 2} Co _x Zn _y Nb _{(x + y) / 2} O ₁₈
Hexagonal ferrite magnetic powder represented by
[0026]
Example 1
Manufacture of magnetic powder of BaO-B ₂ O ₃ was carried out by the glass crystallization method and the glass matrix. According to a known method, a raw material component of ferrite having a composition in which x = 0.2 and y = 0.6 in the above composition formula is well mixed with a glass matrix component, and the melt obtained by heating and melting is mixed. 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.
[0027]
Next, heat treatment is performed at a primary crystallization temperature of 480 ° C for 25 hours mainly for nucleation, and then the temperature is increased to 760 ° C of a second crystallization temperature at a heating rate of 15 ° C / min. Heat treatment was performed at 4 ° C. for 4 hours. Thereafter, a washing process was performed according to a known method to dissolve and remove the glass matrix component, thereby obtaining a hexagonal ferrite powder as the magnetic powder of the present invention.
[0028]
When the magnetic properties of the obtained magnetic powder were examined using 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 non-magnetic 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.
[0029]
As for the particle diameter and shape of the magnetic powder, 200 particles were randomly selected from a transmission electron microscope image at a magnification of 200,000, their particle diameter and plate thickness were measured, and the arithmetic average of each was obtained. As a result, the average particle size was 30 nm, and the plate ratio was 4. In addition, the value of the specific surface area by the BET method (gas adsorption method) was 44 m ² / g when measured using a high-sensitivity area meter.
[0030]
Next, in order to examine the characteristics 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.
[0031]
<Magnetic paint 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 After applying the magnetic paint, apply the coating before the coating liquid is completely dried. The coating film was arranged in a magnetic field of 6 kOe so as to be parallel to the magnetic field, air-dried as it was in the magnetic field, and subjected to an orientation treatment.
[0032]
Then, the ratio of residual magnetization amount / saturation magnetization amount was determined for the obtained alignment film using VSM, and the ratio was 75% when the ratio was determined as the orientation ratio. The SFD of the coated film in this oriented state was 0.2.
[0033]
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, (| ΔH ₁ −ΔH ₂ |) / Hm, and the particle size and shape of the obtained magnetic powder were measured in the same manner as in Example 1, and an orientation film was prepared to determine the orientation ratio and SFD. It was measured. The above measurement results are shown in Table 1 below together with the measurement results of Example 1.
[0034]
Example 3
The ferrite composition is set to x = 0.1 and y = 0.7 in the above composition formula, and heat treatment is performed at a primary crystallization temperature of 520 ° C. for 20 hours, and then at a rate of 10 ° C./min. A hexagonal ferrite powder was produced in the same manner as in Example 1 except that the crystallization temperature was raised to 725 ° C. and the heat treatment was performed for 5 hours. The magnetic properties, (| ΔH ₁ −ΔH ₂ |) / Hm, and the particle size and shape of the obtained magnetic powder were measured in the same manner as in Example 1, and an orientation film was prepared to determine the orientation ratio and SFD. It was measured. The above measurement results are shown in Table 1 below.
[0035]
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, (| ΔH ₁ −ΔH ₂ |) / Hm, and the particle size and shape of the obtained magnetic powder were measured in the same manner as in Example 1, and an orientation film was prepared to determine the orientation ratio and SFD. It was measured. The measurement results are shown in Table 1 below.
[0036]
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 coprecipitated at 50 ° C. by charging twice the equivalent of caustic soda. Was. The obtained coprecipitate was filtered off, washed with water and dried. After a heat treatment at a primary crystallization temperature of 500 ° C. for 30 hours, a secondary crystallization temperature of 780 was increased at a rate of 20 ° C./min. C., and heat-treated for 2 hours. Then, a hexagonal ferrite powder as the magnetic powder of the present invention was obtained. The magnetic properties, (| ΔH ₁ −ΔH ₂ |) / Hm, and the particle size and shape of the obtained magnetic powder were measured in the same manner as in Example 1, and an orientation film was prepared to determine the orientation ratio and SFD. It was measured. 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.
[0038]
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.
[0039]
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.
[0040]
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 having excellent symmetry of the differential curve of the magnetization curve and small SFD has a large saturation magnetization, a high orientation ratio at the time of forming a magnetic film, and a high density. Suitable for magnetic recording.
[0042]
【The invention's effect】
As described above, the magnetic powder of the present invention has excellent magnetic properties 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 (1)

Composition formula MO · Fe _12-x M ′ _x O ₁₈ (where M represents at least one or more elements selected from Ca, Sr, Ba, and Pb, and M ′ represents an atomic group adjusted so that the average number of atoms is three. A) hexagonal ferrite magnetic powder for magnetic recording represented by
Average particle size is 20 to 100 nm,
The plate ratio is 2 to 9,
The specific surface area by BEΤ method is 25 to 70 m ² / g,
When the half width ΔH of the differential curve (dσ / dH) of the second quadrant in the magnetization curve of the magnetic powder is divided into two by a perpendicular line from the apex, the following two equations (| ΔH1−ΔH2 |) /Hm≦0.12
ΔH / Hm = (ΔH1 + ΔH2) /Hm≦1.2
(However, .DELTA.H1 the two divided downfield [Delta] H, [Delta] H2 is also represents a high magnetic field side, Hm represents the magnetic field that gives the maximum differential coefficient.) The magnetic recording hexagonal and satisfies the both Ferrite magnetic powder.