JP2935385B2 - Composite oxide magnetic powder and magneto-optical recording medium using the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention
The present invention relates to a composite oxide magnetic powder suitable for applications such as an optical isolator, an optical circulator, an optical switch, an optical waveguide, and an optical memory, and a magneto-optical recording medium using the same.

[0002]

2. Description of the Related Art Heretofore, as a magnetic material having an excellent magneto-optical effect, a magnetic material comprising an amorphous alloy of a rare earth metal and a transition metal has been known. However, the magnetic material made of such an amorphous alloy has a drawback that it is susceptible to oxidative corrosion and its magneto-optical characteristics deteriorate. In addition, amorphous alloys have a low light transmittance, and therefore utilize the magneto-optical effect (Kerr effect) due to reflection on the surface. However, amorphous alloys generally have a small Kerr rotation angle and thus have a problem of low sensitivity. there were.

On the other hand, Japanese Patent Publication No. 56-15125 proposes a magneto-optical material using a garnet polycrystalline oxide thin film. A magnetic body using this oxide has excellent corrosion resistance, and has an advantage of high sensitivity because reproduction is performed using a magneto-optical effect (Faraday effect) due to light transmitted through the magnetic film. However, since it is polycrystalline, there is a disadvantage that noise is increased due to light scattering, birefringence, and domain wall motion at crystal grain boundaries.

Further, when the above-mentioned magnetic thin film is formed on a substrate, there is a problem that only a substrate having heat resistance can be used because the manufacturing temperature is as high as 500 ° C. or higher.

On the other hand, Japanese Patent Application Laid-Open No. 62-119758 discloses a coating type magneto-optical recording material using yttrium iron garnet particles. In such a coating medium, there is no adverse effect of crystal grain boundaries as in the polycrystalline oxide thin film, but the garnet particles described in the publication are:
The particle diameter is as large as 1.5 μm, and when such particles are used, light is scattered, so that they are not suitable for applications utilizing light of a submicron wavelength.

Therefore, the present applicant has proposed rare-earth iron garnet fine particles having an average particle diameter of 30 to 1000 ° as a magnetic powder which does not cause light scattering and is suitable for applications utilizing light of a submicron wavelength. (JP-A-3-159
915)

However, when the particle diameter is reduced to 1000 ° or less, there is a problem that the influence of the surface layer of the particles becomes large and the original magnetic properties are not sufficiently exhibited as compared with the bulk material. Further, when a thermal decomposition product such as a nitrate is used as a binder when forming a medium, there is a problem that the preparation process of the binder becomes complicated due to poor acid resistance.

[0008]

SUMMARY OF THE INVENTION The present invention solves the above problems, has excellent corrosion resistance, has a large magneto-optical effect, has excellent light transmittance, has little influence of a surface layer due to fine particles, has an optical isolator, an optical circulator, and an optical device. An object of the present invention is to provide a composite oxide magnetic powder suitable for applications such as a switch, an optical waveguide, and an optical memory, and a method for producing the same.

Further, the present invention enables high-density recording,
An object is to provide a magneto-optical recording medium having a high S / N ratio and excellent productivity.

[0010]

[Means for solving the problems]

The present invention have the general formula _{R a Bi b Fe c M d} O e ( wherein, R represents one or more rare earth elements selected from the group consisting of Y and lanthanides, M is Al, Ga, Cr, M
n, Sc, In, Ru, Rh, Co, Fe (II), Cu, Ni, Zn, Li, Si, Ge, Zr, Ti, H
at least one element selected from the group consisting of f, Sn, Pb, Mo, V, Nb and Ta, and a + b + c + d = 7.0-8.0;
a + b = 2.0-3.5, c + d = 4.0-6.0, a
= 0.5-3.5, b = 0-2.5, c = 3.0-6.
0, d = 0 to 2.0, and e is the number of oxygen atoms satisfying the valence of another element. ) And a composite oxide magnetic powder in which silica is coated on the surface of rare earth iron garnet fine particles having an average particle diameter of 30 to 1000 °.

[0011] The present invention also relates to a magneto-optical recording medium characterized in that a magnetic layer comprising the above-mentioned composite oxide magnetic powder and a binder is provided on a substrate.

The rare earth iron garnet fine particles in the present invention may be garnet represented by R ₃ Fe ₅ O ₁₂ or a garnet in which a part of the rare earth element of the garnet is substituted with Bi, and / or a part of Fe is M Is used.

R in the above general formula is Y, La, Ce, Pr, N
One or more rare earth elements selected from the group consisting of d, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. M is an element that can be substituted for iron, and Al, Ga, Cr, Mn, Sc, In, Ru, Rh, C
o, Fe (II), Cu, Ni, Zn, Li, Si, Ge, Zr, Ti, Hf, Sn, Pb, Mo, V,
One or more elements selected from the group consisting of Nb and Ta. Among the elements of M, trivalent elements Al, Ga, Cr, Mn, Sc, In, Ru, R
h and Co alone, the divalent element Co, Fe, Cu, Ni and Zn, or the monovalent element Li is the tetravalent element Si, Ge, Zr, Ti, Hf, Sn, Pb and Mo, or 3 in combination with pentavalent elements V, Nb and Ta
It is preferable to substitute as an element equivalent to a valence.

The ratio of each element in the above general formula is as follows: a + b + c + d = 7.0-8.0, a + b = 2.0
3.5, c + d = 4.0-6.0, a = 0.5-3.
5, b = 0-2.5, c = 3.0-6.0, d = 0
2.0, and e is the number of oxygen atoms that satisfies the valence of another element.

In the present invention, the Faraday rotation angle can be increased by substituting a part of the rare earth element of garnet with Bi of preferably 0.2 to 2.5.
Further, by partially replacing iron with M of preferably 0.3 to 2.0, the Curie temperature can be lowered and the saturation magnetization can be reduced. In the present invention, a part of R of the rare earth iron garnet fine particles further includes Pb, Ca, Mg.
And the like, and in this case, M 4
Charge compensation may be performed using a valence or pentavalent element.

The rare earth iron garnet fine particles in the present invention have an average particle size of 30 to 1000 °, preferably 100 to 1000 °.
600 °. If the average particle diameter is smaller than 30 °, it becomes superparamagnetic due to thermal disturbance. Also, 10
If the angle is larger than 00 °, light scattering occurs and noise is generated, which is not preferable. Further, the particle shape is preferably optically symmetric and preferably spherical, but may be polyhedral or irregular.

The composite oxide magnetic powder of the present invention is obtained by coating the surface of the rare earth iron garnet fine particles with silica. The coating amount of the silica may be an amount that covers the entire surface of the rare earth iron garnet fine particles, and usually, the thickness of the coating layer is in the range of 5 to 40 °, preferably 10 to 30 °. For example, for particles having an average particle diameter of 400 to 500 °, a range of 0.5 to 15 wt% in terms of SiO ₂ weight is preferable. When the coating amount of silica is less than the above range,
There is no effect of improving the magnetic properties, and if it exceeds the above range, the magnetic properties will no longer be improved, and conversely, the proportion of the rare earth iron garnet fine particles will decrease, and the magnetic properties will deteriorate.

In the present invention, the reason why the magnetic properties are improved by coating the surface of the rare earth iron garnet fine particles with silica is that, as shown in FIG. Are present, which reduces the magnetic properties of the whole particle. On the other hand, in the composite oxide magnetic particles of the present invention in which the surface of the fine particles is coated with silica, it is considered that the non-magnetic region of the particle surface layer is reduced and the magnetic properties are improved. Furthermore, in the composite oxide magnetic powder of the present invention, since the surface of the fine particles is coated with silica, the acid resistance is also improved.

The method for producing the rare earth iron garnet fine particles is not particularly limited as long as the particles having the above-mentioned characteristics can be obtained, and may be produced by a hydrothermal synthesis method, a coprecipitation method, a melting method, an alkoxide method, a gas phase method, or the like. A method is used.

A method for producing rare earth iron garnet fine particles by the coprecipitation method will be described below. A solution containing each of the element ions selected in a ratio constituting the rare earth iron garnet represented by the above general formula, and an alkali are mixed so that the alkali concentration after mixing becomes 0.1 to 8 M. Is generated, and the precipitate is calcined at 600 to 900 ° C. to obtain rare earth iron garnet fine particles.

The solution containing each element ion is obtained by dissolving a compound of each element, for example, a nitrate, a sulfate, a chloride or the like in a solvent. The solvent may be any as long as the compound of each of the above elements can be dissolved, and is usually water, alcohols,
Ethers and mixed solvents thereof are used.

The solution containing each of the above-mentioned element ions has a total ion concentration of 0.01 to 2.
It is desirable to adjust to be 0M. If the total ion concentration is less than 0.01 M, the amount of garnet generated is small, and if it is more than 2.0 M, the particles become large or a heterogeneous phase is generated, which is not preferable.

The proportion of each element ion can be the proportion of each element constituting the rare earth iron garnet represented by the above-mentioned general formula. R and Bi are 10 mol% more than their required amounts. It may be used in excess to the extent.

As the alkali, sodium hydroxide, potassium hydroxide, aqueous ammonia, diethylamine, ethanolamine and the like are used. The amount of the alkali used is such that the alkali concentration in the solution after mixing the alkali is 0.1 to 8;
M, preferably 0.5-2M. If the amount of the alkali is too small, the generated particles become large or unreacted substances remain. Too much alkali is not economical.

As a method of mixing the solution containing each element ion with an alkali, for example, there is a method of adding a solution containing each element ion to an aqueous alkali solution, or a method of continuously mixing both. The precipitation may be performed at once or in multiple stages.

Next, the resulting precipitate is washed with water to remove free alkali, filtered, dried and calcined to obtain rare earth iron garnet fine particles. The firing temperature varies depending on the composition of the rare-earth iron garnet, but can usually be in the range of 600 to 900 ° C. If the firing temperature is too low, crystallization is not sufficient, and if the temperature is too high, the particles become large or sintering occurs, which is not preferable. The firing time is suitably about 10 minutes to 30 hours.
The firing atmosphere is not particularly limited, but an air atmosphere is generally convenient.

In the present invention, a composite oxide magnetic powder is obtained by coating the surface of the rare earth iron garnet fine particles with silica. The silica is coated by applying a silicon compound to the rare earth iron garnet fine particles, adsorbing or dipping the silicon compound, and then heat-treating the processed product at a temperature of 300 to 600 ° C. As the silicon compound, water glass, sodium metasilicate, silane coupling agent, silicone oil and the like are used. The heat treatment temperature is 300 ~
The range is 600 ° C. When the temperature of the heat treatment is lower than 300 ° C., the bond between the SiO ₂ and the rare earth iron garnet fine particles is weak,
Magnetic properties do not improve. On the other hand, when the temperature is higher than 600 ° C., diffusion of silicon occurs, which causes a decrease in magnetic characteristics.

The composite oxide magnetic powder of the present invention is excellent in corrosion resistance, has a large magneto-optical effect, is excellent in light transmittance, is little affected by the surface layer due to fine particles, and has an optical isolator, an optical circulator, an optical switch, an optical switch, and the like. It is suitably used for applications such as wave paths and optical memories.

In the present invention, a magneto-optical recording medium can be obtained by providing a magnetic layer comprising the above-mentioned composite oxide magnetic powder and a binder on a substrate.

As the binder, an amorphous binder of an inorganic oxide or an organic binder is used. As the inorganic binder, a nitrate aqueous solution according to the garnet composition,
A deflocculated coprecipitated product according to the garnet composition, a metal alcoholate, a silanol compound and its derivatives, an organosilicon compound, an acetylacetone metal salt, etc., and an amorphous inorganic oxide or molten glass formed by heat treatment. Is mentioned.

Examples of the organic binder include vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinylidene chloride copolymer, cellulose derivatives such as nitrocellulose resin, acrylic resin, polyvinyl butyral resin, epoxy resin, polyurethane resin, and phenoxy resin. Resins, polyester resins, phenol resins, polyamide resins, melamine resins, and the like.

In particular, in order to reduce noise due to light scattering, it is desirable that the refractive index of the binder matches the refractive index of the rare earth iron garnet fine particles, and the deviation from the refractive index of the rare earth iron garnet fine particles is ± 20% or less. In particular, it is preferably ± 10% or less.

The thickness of the magnetic layer is 0.05 to 2.0 μm,
In particular, the range of 0.2 to 1.0 μm is preferable from the viewpoint of recording bit stability.

The magnetic layer is prepared by dissolving or dispersing a composite oxide magnetic powder and a binder in water or an organic solvent to prepare a magnetic paint, applying this on a substrate, and curing the binder by heat treatment or the like. It is formed.

At this time, the magnetization of the composite oxide magnetic fine particles can be aligned in the direction perpendicular to the substrate by applying stress due to the curing of the binder to the composite oxide magnetic fine particles. Further, an orientation treatment such as a magnetic field orientation may be performed.

The substrate is not particularly limited, and may be a single crystal substrate, a polycrystal substrate, an amorphous substrate such as glass, an inorganic material substrate such as a composite substrate, or an acrylic resin, a polycarbonate resin, a polyester resin, a polyamide resin, or the like. An organic material substrate such as a polyimide resin can be used.

A light reflecting layer may be provided between the substrate and the magnetic layer or on the magnetic layer. As the light reflecting layer, C
u, Cr, Al, Ag, Au, TiN, etc. are used. This light reflection layer,
It is formed on a substrate or a magnetic layer by a coating method, a plating method, a vapor deposition method, or the like.

[0038]

The present invention will be described in more detail with reference to the following examples. Examples 1 to 3 and Comparative Example 1 1.93 mol of ammonia was dissolved in 80 ml of water, and separately, 0.020 mol of bismuth nitrate, 0.040 mol of dysprosium nitrate, and 0.10 mol of iron nitrate.
096 mol and 0.004 mol of aluminum nitrate in 5N-nitric acid solution 2
Dissolved in 20 ml. Then, while stirring the ammonia solution, a nitric acid solution was gradually added dropwise to neutralize the solution, thereby producing a precipitate. After washing the obtained precipitate with water, filtering and drying,
A mixture having a molar ratio of NaCl: LiCl = 3: 7 as a flux was mixed at 100 wt% with respect to the precipitate, and the mixture was calcined at 690 ° C.
The obtained fired product was washed with water to remove a flux, filtered and dried to obtain garnet fine particles. The obtained garnet fine particles had an average particle size of 450 °, and as a result of X-ray powder diffraction spectrum and composition analysis, Bi ₁ Dy ₂ Fe _4.8 Al _0.2 O
₁₂ , which was garnet single phase.

After adding the No. 4 water glass in the amount shown in Table 1 to the slurry in which the obtained garnet fine particles were dispersed with acetic acid, the slurry was neutralized with ammonia to adsorb silica on the surface of the garnet fine particles. Then, after filtration and drying,
Heat treatment was performed at 400 ° C. to obtain a composite oxide magnetic powder. Table 1 shows the results of measuring the saturation magnetization of the obtained composite oxide magnetic powder. It can be seen that the magnetic properties of the garnet fine particles are improved by coating with silica. Also,
FIG. 2 shows the result of measuring the elution amount of the obtained composite oxide magnetic powder with respect to concentrated nitric acid with respect to the elution amount of Fe. It turns out that the acid resistance of the garnet fine particles is improved by coating with silica.

Example 4 3.61 mol of ammonia was dissolved in 750 ml of water. Separately, 0.04 mol of bismuth nitrate, 0.08 mol of dysprosium nitrate, and iron nitrate
0.18 mol and 0.02 mol of aluminum nitrate in 5N nitric acid solution
Dissolved in 400 ml. Then, while stirring the ammonia solution, a nitric acid solution was gradually added dropwise to neutralize the solution, thereby producing a precipitate. After the obtained precipitate was washed with water, filtered and dried, a mixture having a molar ratio of KBr: KI = 1: 1 as a flux was mixed with the precipitate by 25 wt%, and the mixture was calcined at 670 ° C. The obtained fired product was washed with water to remove a flux, filtered and dried to obtain garnet fine particles. The obtained garnet fine particles have an average particle size of 440 °, and as a result of X-ray powder diffraction spectrum and composition analysis, Bi ₁ Dy ₂ Fe _4.5 Al _0.5 O ₁₂
And garnet single phase.

The slurry obtained by dispersing the garnet microparticles in acetic acid acid was mixed with No. 4 water glass at a concentration of 6.82 wt% based on the weight of SiO _{2 based on} the weight of the microparticles. It was adsorbed on the surface of the fine particles. Next, after filtration and drying, heat treatment was performed at 400 ° C. to obtain a composite oxide magnetic powder. The saturation magnetization of the obtained composite oxide magnetic powder was 7% higher than that of the garnet fine particles before coating with silica.

Next, this composite oxide magnetic powder was converted to Bi: Y: Fe by using bismuth nitrate, yttrium nitrate and ferric nitrate.
= 28: 10: 62 in an aqueous solution containing the mixture at a molar ratio of 28:10:62, sufficiently dispersed, and then applied onto a glass substrate having a diameter of 3 inches and a thickness of 1 mm. Next, the resultant was thermally decomposed at 250 ° C. to form an amorphous BiYFe oxide film containing the composite oxide magnetic powder. The thickness of the obtained film was 0.5 μm. A reflective film of aluminum was formed on this coating film by a vapor deposition method to obtain a magneto-optical recording medium.

633 in the direction perpendicular to the film surface of the obtained medium
The Faraday rotation angle with respect to light of nm was measured by a polarization plane modulation method, and was found to be 1.0 deg. When the S / N of this medium was evaluated, it was 50 dB.

Example 5 3.61 mol of ammonia was dissolved in 750 ml of water, and separately 0.068 mol of bismuth nitrate, 0.052 mol of dysprosium nitrate and iron nitrate
0.160mol and 0.040mol of aluminum nitrate in 5N nitric acid solution
Dissolved in 400 ml. Then, while stirring the ammonia solution, a nitric acid solution was gradually added dropwise to neutralize the solution, thereby producing a precipitate. After the obtained precipitate was washed with water, filtered and dried, a mixture having a molar ratio of KBr: KI = 1: 1 as a flux was mixed with the precipitate by 50 wt%, and the mixture was calcined at 670 ° C. The obtained fired product was washed with water to remove a flux, filtered and dried to obtain garnet fine particles. The obtained garnet fine particles have an average particle size of 380 °, and as a result of X-ray powder diffraction spectrum and composition analysis, Bi _1.7 Dy _1.3 Fe ₄ Al ₁ O ₁₂
And garnet single phase.

The slurry obtained by dispersing the garnet fine particles in acetic acid acid was mixed with No. 4 water glass in an amount of 3.98 wt% of the fine particles in terms of the weight of SiO ₂ , and then the slurry was neutralized with ammonia to convert the silica into garnet. It was adsorbed on the surface of the fine particles. Next, after filtration and drying, heat treatment was performed at 400 ° C. to obtain a composite oxide magnetic powder. The saturation magnetization of the obtained composite oxide magnetic powder was 12% higher than that of the garnet fine particles before the silica was coated.

Next, this composite oxide magnetic powder was converted to bismuth nitrate, yttrium nitrate and ferric nitrate by Bi: Y: Fe
= 28: 10: 62 in an aqueous solution containing the mixture at a molar ratio of 28:10:62, sufficiently dispersed, and then applied onto a glass substrate having a diameter of 3 inches and a thickness of 1 mm. Next, the resultant was thermally decomposed at 250 ° C. to form an amorphous BiYFe oxide film containing the composite oxide magnetic powder. The thickness of the obtained film was 0.5 μm. A reflective film of aluminum was formed on this coating film by a vapor deposition method to obtain a magneto-optical recording medium.

633 in the direction perpendicular to the film surface of the obtained medium
The Faraday rotation angle with respect to light of nm was measured by a polarization plane modulation method, and was 1.7 deg. When the S / N of this medium was evaluated, it was 52 dB.

Example 6 3.34 mol of diethylamine was dissolved in 750 ml of water. Separately, 0.040 mol of bismuth nitrate, 0.080 mol of dysprosium nitrate, 0.188 mol of iron nitrate, 0.008 mol of cobalt nitrate and 0.004 mol of vanadium oxide were added to 400 ml of a 5N nitric acid solution. Dissolved. Then, while stirring the diethylamine solution, a nitric acid solution was gradually added dropwise to neutralize the solution, thereby producing a precipitate. After the obtained precipitate was washed with water, filtered and dried, KBr: KI was used as a flux.
= 1: 1 molar ratio of the mixture to the precipitate was mixed at 25 wt%,
The mixture was fired at 700 ° C. The obtained fired product was washed with water to remove a flux, filtered and dried to obtain garnet fine particles. The obtained garnet fine particles have an average particle size of 430.
Å, and as a result of X-ray powder diffraction spectrum and composition analysis, it was Bi ₁ Dy ₂ Fe _4.7 Co _0.2 V _0.1 O ₁₂ , which was a garnet single phase.

The slurry obtained by dispersing the obtained garnet fine particles in acetic acid acid was mixed with No. 4 water glass in an amount of 5.28 wt% of the fine particles in terms of the weight of SiO ₂ , and the slurry was neutralized with ammonia to convert the silica into garnet. It was adsorbed on the surface of the fine particles. Next, after filtration and drying, heat treatment was performed at 400 ° C. to obtain a composite oxide magnetic powder. The saturation magnetization of the obtained composite oxide magnetic powder was 10% higher than that of the garnet fine particles before coating with silica.

Next, this composite oxide magnetic powder was converted to Bi: Y: Fe by using bismuth nitrate, yttrium nitrate and ferric nitrate.
= 28: 10: 62 in an aqueous solution containing the mixture at a molar ratio of 28:10:62, sufficiently dispersed, and then applied onto a glass substrate having a diameter of 3 inches and a thickness of 1 mm. Next, the resultant was thermally decomposed at 250 ° C. to form an amorphous BiYFe oxide film containing the composite oxide magnetic powder. The thickness of the obtained film was 0.5 μm. A reflective film of aluminum was formed on this coating film by a vapor deposition method to obtain a magneto-optical recording medium.

633 in the direction perpendicular to the film surface of the obtained medium
The Faraday rotation angle with respect to light of nm was measured by the polarization plane modulation method, and was found to be 1.1 deg. When the S / N of this medium was evaluated, it was 55 dB.

Comparative Example 2 The silica-free garnet fine particle powder obtained in Comparative Example 1 contains bismuth nitrate, yttrium nitrate and ferric nitrate in a molar ratio of Bi: Y: Fe = 28: 10: 62. The solution was sufficiently dispersed in an aqueous solution, and then applied on a glass substrate having a diameter of 3 inches and a thickness of 1 mm. Then 2
By heat decomposition at 50 ° C., an amorphous BiYFe oxide film containing garnet fine particle powder was formed. The thickness of the obtained film was 0.5 μm. A reflective film of aluminum was formed on this coating film by a vapor deposition method to obtain a magneto-optical recording medium.

633 in the direction perpendicular to the film surface of the obtained medium
The Faraday rotation angle with respect to light of nm was measured by a polarization plane modulation method, and was 0.9 deg. When the S / N of this medium was evaluated, it was 45 dB.

[0054]

The composite oxide magnetic powder of the present invention is excellent in corrosion resistance, large in magneto-optical effect, excellent in light transmittance, and suitable for applications such as optical isolators, optical circulators, optical switches, optical waveguides and optical memories. Used for In particular, the influence of the surface layer due to the atomization is small, and the magnetic properties such as the saturation magnetization and the Faraday rotation angle are improved. Furthermore, it has excellent acid resistance. The magneto-optical recording medium using the composite oxide magnetic powder of the present invention has excellent corrosion resistance, a large magneto-optical effect, a large S / N ratio, and is suitable for high-density magneto-optical recording. Furthermore, it can be manufactured by a coating method,
The productivity is also good.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing the state of a surface layer of garnet fine particles depending on the presence or absence of a silica coating layer.

FIG. 2 is a graph showing the relationship between the thickness of a silica coating layer and the amount of Fe eluted with respect to concentrated nitric acid in the composite oxide magnetic powder of the present invention.

[Table 1]

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H01F 1/11 H01G 49/00 G02F 1/09 501

Claims (3)

(57) [Claims] 1. A general formula _{R a Bi b Fe c M d} O e ( wherein, R represents one or more rare earth elements selected from the group consisting of Y and lanthanides, M is Al, Ga, Cr, M
n, Sc, In, Ru, Rh, Co, Fe (II), Cu, Ni, Zn, Li, Si, Ge, Zr, Ti, H
at least one element selected from the group consisting of f, Sn, Pb, Mo, V, Nb and Ta, and a + b + c + d = 7.0-8.0;
a + b = 2.0-3.5, c + d = 4.0-6.0, a
= 0.5-3.5, b = 0-2.5, c = 3.0-6.
0, d = 0 to 2.0, and e is the number of oxygen atoms satisfying the valence of another element. ) And a composite oxide magnetic powder in which silica is coated on the surface of rare earth iron garnet fine particles having an average particle diameter of 30 to 1000 °. 2. The method according to claim 1, wherein the rare earth iron garnet fine particles are coated with a silicon compound, adsorbed or immersed, and then heat-treated at a temperature of 300 to 600 ° C. A method for producing a composite oxide magnetic powder. 3. A magneto-optical recording medium comprising a magnetic layer comprising the composite oxide magnetic powder of claim 1 and a binder provided on a substrate.