JP2007302914A - Magnetic nanoparticle, method for producing the same and magnetic recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide magnetic nanoparticles exhibiting satisfactory output while suppressing the interaction between the particles; to provide a method for producing the same; and to provide a magnetic recording medium whose magnetic layer comprises the magnetic nanoparticles. <P>SOLUTION: In the magnetic nanoparticle of a CuAu type or Cu<SB>3</SB>Au type ferromagnetic ordered alloy phase composed of transition metal and noble metal, the magnetic nanoparticle comprises an oxide layer of the transition metal on the surface. The magnetic recording medium comprises the magnetic nanoparticle in a magnetic layer. Further, the method for producing a magnetic nanoparticle comprises: a process where an alloy particle capable of forming a CuAu type or Cu<SB>3</SB>Au type ferromagnetic ordered alloy or the magnetic nanoparticle of a CuAu type or Cu<SB>3</SB>Au type ferromagnetic ordered alloy phase is oxidized. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to a magnetic nanoparticle of CuAu type or Cu ₃ Au type ferromagnetic ordered alloy phase, a method for producing the same, and a magnetic recording medium containing the nanoparticle in a magnetic layer.

It is necessary to reduce the size of the magnetic particles contained in the magnetic layer in order to increase the magnetic recording density. For example, in a magnetic recording medium widely used as a video tape, a computer tape, a disk, etc., when the mass of the ferromagnetic material is the same, the SNR is higher when the particle size is reduced.
As a magnetic particle material promising for improving the magnetic recording density, there is a CuAu type or Cu ₃ Au type ferromagnetic ordered alloy (for example, see Patent Document 1). It is known that the ferromagnetic ordered alloy has large magnetocrystalline anisotropy due to strain generated during ordering, and exhibits ferromagnetism even when the size of the magnetic particles is reduced.

  However, even if the size of the magnetic particles is simply reduced, if there is interaction between the particles, the particles are aggregated or fused, and the magnetization reversal unit (magnetic cluster) becomes large. Therefore, the effect is reduced even if the particles are made fine. In addition, if the distance between the particles is too large to disperse the particles, the packing density of the magnetic material is lowered, and the output is lowered. From this, it is understood that control of the interparticle distance of the magnetic material is important.

Regarding such interparticle distance, the numerical value obtained and disclosed by Sun et al. By TEM observation or the like was about 4 nm (for example, see Non-Patent Document 1).
However, according to experiments by the present inventors, it has been found that it is difficult to set the magnetic cluster size in a desired low range only by controlling the interparticle distance.
JP 2003-73705 A Science 287 (5460), 1989-1992 (2000)

This invention is made | formed in view of the said conventional problem, and makes it a subject to achieve the following objectives. That is,
An object of the present invention is to provide a magnetic nanoparticle that exhibits good output while suppressing interaction between particles, a method for producing the magnetic nanoparticle, and a magnetic recording medium including the magnetic nanoparticle in a magnetic layer.

As a result of intensive studies to achieve the above object, the present inventors have found that the object can be achieved by the present invention described below. That is,
<1> A magnetic nanoparticle of a CuAu type or Cu ₃ Au type ferromagnetic ordered alloy phase composed of a transition metal and a noble metal, wherein the magnetic nanoparticle has an oxide layer of the transition metal on the surface.
Interaction between particles can be blocked by the transition metal oxide layer on the surface of the particle.

<2> A magnetic recording medium comprising the magnetic nanoparticle according to <1> in a magnetic layer.

<3> CuAu type or _Cu 3 Au-type ferromagnetic ordered alloy particles alloy capable of forming, and / or, characterized in that it comprises the step of oxidizing the magnetic nanoparticles CuAu type or _Cu 3 Au-type ferromagnetic ordered alloy phase This is a method for producing magnetic nanoparticles.

<4> The method for producing magnetic nanoparticles according to <3>, wherein the ratio of the transition metal in the composition of the alloy particles is larger than the ratio of the noble metal.
<5> The magnetic nanostructure according to <4>, wherein the ratio of transition metal in the composition of alloy particles capable of forming the CuAu type or Cu ₃ Au type ferromagnetic ordered alloy is 51 at% to 65 at% A method for producing particles.

  ADVANTAGE OF THE INVENTION According to this invention, the magnetic nanoparticle which exhibits favorable output, suppressing the interaction between particle | grains, its manufacturing method, and the magnetic recording medium which contains this magnetic nanoparticle in a magnetic layer can be provided.

<< Magnetic nanoparticles and production method thereof >>
Hereafter, each process of the manufacturing method of the magnetic nanoparticle of this invention is demonstrated, and the magnetic nanoparticle of this invention is demonstrated through this description.

<Alloy particle production process>
Alloy particles that become magnetic particles by annealing can be produced by a gas phase method or a liquid phase method. In view of excellent mass productivity, the liquid phase method is preferable. As the liquid phase method, various conventionally known methods can be applied. However, it is preferable to apply a reduction method obtained by improving these methods. Among the reduction methods, the particle size can be easily controlled. The micelle method is particularly preferred.

(Reverse micelle method)
The reverse micelle method includes at least (1) a reduction step in which two types of reverse micelle solutions are mixed to perform a reduction reaction, and (2) an aging step in which aging is performed at a predetermined temperature after the reduction reaction.
Hereinafter, each step will be described.

(1) Reduction process:
First, a reverse micelle solution (I) in which a water-insoluble organic solvent containing a surfactant and a reducing agent aqueous solution are mixed is prepared.

An oil-soluble surfactant is used as the surfactant. Specifically, sulfonate type (for example, aerosol OT (manufactured by Wako Pure Chemical Industries), quaternary ammonium salt type (for example, cetyltrimethylammonium bromide), ether type (for example, pentaethylene glycol dodecyl ether) and the like can be mentioned. It is done.
The amount of the surfactant in the water-insoluble organic solvent is preferably 20 to 200 g / liter.

Preferable examples of the water-insoluble organic solvent for dissolving the surfactant include alkanes, ethers and alcohols.
The alkane is preferably an alkane having 7 to 12 carbon atoms. Specifically, heptane, octane, isooctane, nonane, decane, undecane, dodecane and the like are preferable.
As the ether, diethyl ether, dipropyl ether, dibutyl ether and the like are preferable.
As the alcohol, ethoxyethanol, ethoxypropanol and the like are preferable.

The reducing agent in the reducing agent solution, alcohols, poly _{alcohols; H 2; HCHO, S 2} O 6 2-, H 2 PO 2 -, BH 4 -, N 2 H 5 +, H 2 PO 3 - Are preferably used alone or in combination of two or more.
The amount of the reducing agent in the aqueous solution is preferably 3 to 50 mol with respect to 1 mol of the metal salt.

  Here, the mass ratio (water / surfactant) between water and the surfactant in the reverse micelle solution (I) is preferably 20 or less. If the mass ratio exceeds 20, precipitation may easily occur and particles may be uneven. The mass ratio is preferably 15 or less, and more preferably 0.5 to 10.

Separately from the above, a reverse micelle solution (II) in which a water-insoluble organic solvent containing a surfactant and a metal salt aqueous solution are mixed is prepared.
The conditions (substance used, concentration, etc.) of the surfactant and the water-insoluble organic solvent are the same as in the case of the reverse micelle solution (I).
In addition, the same kind or different kind of reverse micelle solution (I) can be used. The mass ratio of water and surfactant in the reverse micelle solution (II) is the same as that of the reverse micelle solution (I), and may be the same as or different from the mass ratio of the reverse micelle solution (I). It may be.

The metal salt contained in the metal salt aqueous solution is preferably selected as appropriate so that the magnetic particles to be produced can form a CuAu type or Cu ₃ Au type ferromagnetic ordered alloy.
Here, examples of the CuAu type ferromagnetic ordered alloy include FeNi, FePd, FePt, CoPt, and CoAu, and among them, FePd, FePt, and CoPt are preferable.
Examples of the Cu ₃ Au type ferromagnetic ordered alloy include Ni ₃ Fe, FePd ₃ , Fe ₃ Pt, FePt ₃ , CoPt ₃ , Ni ₃ Pt, CrPt ₃ , and Ni ₃ Mn. Among them, FePd ₃ , FePt ₃ , CoPt ₃ , Fe ₃ Pd, Fe ₃ Pt, and Co ₃ Pt are preferable.

Specific examples of the metal salt include H ₂ PtCl ₆ , K ₂ PtCl ₄ , Pt (CH ₃ COCHCOCH ₃ ) ₂ , Na ₂ PdCl ₄ , Pd (OCOCH ₃ ) ₂ , PdCl ₂ , Pd (CH ₃ COCHCOCH ₃ ) _2. , HAuCl ₄ , Fe ₂ (SO ₄ ) ₃ , Fe (NO ₃ ) ₃ , (NH ₄ ) ₃ Fe (C ₂ O ₄ ) ₃ , Fe (CH ₃ COCHCOCH ₃ ) ₃ , NiSO ₄ , CoCl ₂ , Co ( OCOCH ₃ ) _{2 and the} like.

  The concentration (as the metal salt concentration) in the aqueous metal salt solution is preferably 0.1 to 1000 μmol / ml, and more preferably 1 to 100 μmol / ml.

By appropriately selecting the metal salt, alloy particles capable of forming a CuAu type or Cu ₃ Au type ferromagnetic ordered alloy in which a base metal and a noble metal form an alloy are produced.

  The alloy particles need to transform the alloy phase from an irregular phase to an ordered phase by an annealing process, which will be described later. In order to lower the transformation temperature, Sb, Pb, Bi, Cu, Ag, It is preferable to add a third element such as Zn or In. As for these third elements, it is preferable to add a precursor of each third element to the metal salt solution in advance. The addition amount is preferably 1 to 30 at% and more preferably 5 to 20 at% with respect to the binary alloy.

The reverse micelle solutions (I) and (II) prepared as described above are mixed. The mixing method is not particularly limited, but in consideration of the reduction uniformity, the reverse micelle solution (II) may be added and mixed while stirring the reverse micelle solution (I). preferable. The reduction reaction is allowed to proceed after completion of the mixing, and the temperature at that time is preferably in the range of −5 to 30 ° C. and constant.
When the reduction temperature is less than −5 ° C., there arises a problem that the aqueous phase condenses and the reduction reaction becomes non-uniform. A preferable reduction temperature is 0 to 25 ° C, more preferably 5 to 25 ° C.
Here, the “constant temperature” means that when the set temperature is T (° C.), the T is in the range of T ± 3 ° C. Even in this case, the upper and lower limits of T are in the range of the reduction temperature (−5 to 30 ° C.).

  The time for the reduction reaction needs to be appropriately set depending on the amount of the reverse micelle solution and the like, but is preferably 1 to 30 minutes, and more preferably 5 to 20 minutes.

Since the reduction reaction has a great influence on the monodispersity of the particle size distribution, it is preferable to carry out the reduction reaction while stirring as fast as possible.
A preferred stirring device is a stirring device having a high shearing force. Specifically, the stirring blade basically has a turbine-type or paddle-type structure, and a sharp blade is provided at the end of the blade or at a position in contact with the blade. It is an attached structure and is a stirring device that rotates a blade by a motor. Specifically, devices such as a dissolver (manufactured by Special Machine Industries), an omni mixer (manufactured by Yamato Kagaku), and a homogenizer (manufactured by SMT) are useful. By using these apparatuses, monodispersed alloy particles can be synthesized as a stable dispersion.

  In at least one of the reverse micelle solutions (I) and (II), at least one dispersant having 1 to 3 amino groups or carboxy groups is added in an amount of 0.001 to 1 mol per mole of alloy particles to be produced. It is preferable to add 10 mol.

By adding such a dispersant, it becomes possible to obtain alloy particles that are more monodispersed and have no aggregation.
If the addition amount is less than 0.001 mol, the monodispersity of the alloy particles may not be further improved, and if it exceeds 10 mol, aggregation may occur.

As the dispersant, an organic compound having a group that adsorbs to the surface of the alloy particles is preferable. Specifically, it has 1 to 3 amino groups, carboxy groups, sulfonic acid groups or sulfinic acid groups, and these can be used alone or in combination.
As structural formulas, R—NH ₂ , NH ₂ —R—NH ₂ , NH ₂ —R (NH ₂ ) —NH ₂ , R—COOH, COOH—R—COOH, COOH—R (COOH) —COOH, R —SO ₃ H, SO ₃ H—R—SO ₃ H, SO ₃ H—R (SO ₃ H) —SO ₃ H, R—SO ₂ H, SO ₂ H—R—SO ₂ H, SO ₂ H— R (SO ₂ H) —SO ₂ H, wherein R is a linear, branched or cyclic saturated or unsaturated hydrocarbon.

A particularly preferred compound as a dispersant is oleic acid. Oleic acid is a well-known surfactant in the stabilization of colloids and has been used to protect metal particles such as iron. The relatively long chains of oleic acid (eg, oleic acid has 18 carbon chains and is ˜20 angstroms (˜2 nm). Oleic acid is not aliphatic but has one double bond). This gives an important steric hindrance to counteract strong magnetic interactions.
Similar long chain carboxylic acids such as erucic acid and linoleic acid are used as well as oleic acid (for example, long chain organic acids having between 8 and 22 carbon atoms can be used alone or in combination). Oleic acid is preferred because it is an inexpensive natural resource (such as olive oil) that is readily available. In addition, oleylamine derived from oleic acid is a useful dispersant like oleic acid.

In the reduction process as described above, a metal having a low redox potential such as Co, Fe, Ni, Cr in the CuAu type or Cu ₃ Au type ferromagnetic ordered alloy phase (−0.2 V (vs. NH). E) or less of the metal) is reduced, and is considered to be precipitated in a monodispersed state with a minimum size. Thereafter, in the temperature raising step and the aging step described later, the precipitated base metal is used as a nucleus, and a metal having a redox potential such as Pt, Pd, Rh or the like having a noble reduction potential (−0.2 V (vs. N.H. E) More than about metal) is reduced with a base metal to be substituted and deposited. It is thought that the ionized base metal is reduced again by the reducing agent and deposited. By such repetition, alloy particles capable of forming a CuAu type or Cu ₃ Au type ferromagnetic ordered alloy are obtained.

(2) Aging process:
After completion of the reduction reaction, the temperature of the solution after the reaction is raised to the aging temperature.
The aging temperature is preferably a constant temperature of 30 to 90 ° C., and the temperature is higher than the temperature of the reduction reaction. The aging time is preferably 5 to 180 minutes. If the aging temperature and time deviate from the above range to the high temperature and long time side, aggregation or precipitation tends to occur. Conversely, if the aging temperature and time deviate to the low temperature short time side, the reaction may not be completed and the composition may change. Preferred aging temperature and time are 40 to 80 ° C. and 10 to 150 minutes, and more preferable aging temperature and time are 40 to 70 ° C. and 20 to 120 minutes.

  Here, the “constant temperature” is synonymous with the temperature of the reduction reaction (where “reduction temperature” is the “aging temperature”), but in particular, the range of the above aging temperature (30 It is preferably 5 ° C. or higher, more preferably 10 ° C. or higher, than the temperature of the reduction reaction. If it is less than 5 ° C., the composition as prescribed may not be obtained.

In the aging process as described above, noble metal is deposited on the base metal that has been reduced and deposited in the reduction process.
That is, the reduction of the noble metal occurs only on the base metal, and the base metal and the noble metal do not separate separately, so that the CuAu type or Cu ₃ Au type ferromagnetic ordered alloy is efficiently formed. The obtained alloy particles can be produced in a high yield according to the prescribed composition ratio, and can be controlled to a desired composition. Moreover, the particle diameter of the obtained alloy particle can be made into a desired thing by adjusting suitably the stirring speed of the temperature in the case of ageing | curing | ripening.

After the aging, the solution after aging is washed with a mixed solution of water and a primary alcohol, and then subjected to a precipitation treatment with a primary alcohol to generate a precipitate. It is preferable to provide a washing / dispersing step for dispersing with a solvent.
By providing such a cleaning / dispersing step, impurities can be removed, and coating properties when the magnetic layer of the magnetic recording medium is formed by coating can be further improved.
The washing and dispersion are each performed at least once, preferably twice or more.

The primary alcohol used for washing is not particularly limited, but methanol, ethanol and the like are preferable. The volume mixing ratio (water / primary alcohol) is preferably in the range of 10/1 to 2/1, and more preferably in the range of 5/1 to 3/1.
When the ratio of water is high, the surfactant may be difficult to remove, and conversely, when the ratio of primary alcohol is high, aggregation may occur.

As described above, alloy particles dispersed in the solution (alloy particle-containing liquid) are obtained.
Since the alloy particles are monodispersed, even when applied to a support, they can be kept uniformly dispersed without agglomeration. Therefore, even if the annealing treatment is performed, the alloy particles do not agglomerate, so that the ferromagnetic particles can be efficiently ferromagnetized and the application suitability is excellent.

  The particle size of the alloy particles before the oxidation treatment described later is preferably small from the viewpoint of reducing noise, but if it is too small, it becomes superparamagnetic after annealing and may be unsuitable for magnetic recording. In general, the thickness is preferably 1 to 100 nm, more preferably 1 to 20 nm, and still more preferably 3 to 10 nm.

(Reduction method)
There are various methods for producing alloy particles capable of forming a CuAu type or Cu ₃ Au type ferromagnetic ordered alloy by a reduction method, but at least a metal having a low redox potential (hereinafter simply referred to as “base metal”). A noble metal having a redox potential (hereinafter sometimes simply referred to as “noble metal”), an organic solvent or water, or a mixed solution of an organic solvent and water. It is preferable to apply a reduction method using
The reduction order of the base metal and the noble metal is not particularly limited, and may be reduced simultaneously.

As the organic solvent, alcohol, polyalcohol and the like can be used. Examples of the alcohol include methanol, ethanol and butanol. Examples of the polyalcohol include ethylene glycol and glycerin.
An example of the CuAu type or Cu ₃ Au type ferromagnetic ordered alloy is the same as in the case of the reverse micelle method described above.
Moreover, as a method for precipitating a noble metal first to prepare alloy particles, the method described in paragraphs 18 to 30 of Japanese Patent Application No. 2001-269255 can be applied.

Pt, Pd, Rh and the like can be preferably used as the noble metal having a redox potential. H ₂ PtCl ₆ .6H ₂ O, Pt (CH ₃ COCHCOCH ₃ ) ₂ , RhCl ₃ .3H ₂ O, Pd ( OCOCH ₃ ) ₂ , PdCl ₂ , Pd (CH ₃ COCHCOCH ₃ ) _{2 and the} like can be dissolved in a solvent and used. The concentration of the metal in the solution is preferably 0.1 to 1000 μmol / ml, more preferably 0.1 to 100 μmol / ml.

Further, as the metal having a low redox potential, Co, Fe, Ni, and Cr can be preferably used, and Fe and Co are particularly preferable. Such a metal can be used by dissolving FeSO ₄ .7H ₂ O, NiSO ₄ .7H ₂ O, CoCl ₂ .6H ₂ O, Co (OCOCH ₃ ) ₂ .4H ₂ O, etc. in a solvent. The concentration of the metal in the solution is preferably 0.1 to 1000 μmol / ml, more preferably 0.1 to 100 μmol / ml.

  Moreover, it is preferable to lower the transformation temperature to the ferromagnetic ordered alloy by adding a third element to the binary alloy as in the above-described reverse micelle method. The amount added is the same as in the reverse micelle method.

For example, when reducing and depositing a base metal and a noble metal in this order using a reducing agent, use a reducing agent having a base reduction potential lower than −0.2 V (vs. NHE). In addition to a noble metal source obtained by reducing a base metal or a base metal and a part of the noble metal, a reducing agent having a redox potential higher than -0.2 V (vs. NH) is used. Then, the reduction is preferably performed using a reducing agent having a reduction potential lower than -0.2 V (vs. NHE).
Although the redox potential depends on the pH of the system, the reducing agent having a redox potential nobler than −0.2 V (vs. NHE) includes alcohols such as 1,2-hexadecanediol, and glycerins. , H ₂ and HCHO are preferably used.
S ₂ O ₆ ²⁻ , H ₂ PO ₂ ⁻ , BH ₄ ⁻ , N ₂ H ₅ ⁺ , H ₂ PO ₃ ⁻ is preferable as a reducing agent lower than −0.2 V (vs. N.H.E). Can be used.
In addition, when using with zerovalent metal compounds, such as Fe carbonyl, as a base metal raw material, a base metal reducing agent is not particularly required.

The alloy particles can be stably prepared by allowing an adsorbent to be present when the noble metal is reduced and precipitated. It is preferable to use a polymer or a surfactant as the adsorbent.
Examples of the polymer include polyvinyl alcohol (PVA), poly N-vinyl-2pyrrolidone (PVP), gelatin and the like. Of these, PVP is particularly preferable.
The molecular weight is preferably 20,000 to 60,000, more preferably 30,000 to 50,000. The amount of the polymer is preferably 0.1 to 10 times the mass of the alloy particles to be produced, and more preferably 0.1 to 5 times.

The surfactant preferably used as the adsorbent preferably contains an “organic stabilizer” which is a long-chain organic compound represented by the general formula: R—X. R in the above general formula is a “tail group” which is a linear or branched hydrocarbon or fluorocarbon chain and usually contains 8 to 22 carbon atoms. X in the above general formula is a “head group” which is a moiety (X) that provides a specific chemical bond to the surface of the alloy particles, and is sulfinate (—SOOH), sulfonate (—SO ₂ OH), phosphinate ( -POOH), phosphonate (-OPO (OH) ₂ ), carboxylate, and thiol are preferred.

Examples of the organic stabilizer include sulfonic acid (R—SO ₂ OH), sulfinic acid (R—SOOH), phosphinic acid (R ₂ POOH), phosphonic acid (R—OPO (OH) ₂ ), carboxylic acid (R— COOH) or thiol (R-SH) is preferred. Among these, oleic acid similar to the reverse micelle method is particularly preferable.

  The combination of phosphine and organic stabilizer (such as triorganophosphine / acid) can provide excellent control over particle growth and stabilization. Didecyl ether and didodecyl ether can also be used, but phenyl ether or n-octyl ether is preferably used as a solvent because of its low cost and high boiling point.

  The reaction is preferably performed at a temperature in the range of 80 ° C. to 360 ° C., more preferably 80 ° C. to 240 ° C., depending on the required alloy particles and the boiling point of the solvent. If the temperature is lower than this temperature range, the particles may not grow. If the temperature is above this range, the particles may grow uncontrolled and increase the production of undesirable by-products.

The particle size of the alloy particles is the same as in the reverse micelle method, preferably 1 to 100 nm, more preferably 3 to 20 nm, and still more preferably 3 to 10 nm.
The seed crystal method is effective as a method for increasing the particle size (particle size). For use as a magnetic recording medium, it is preferable to close-pack with alloy particles in order to increase the recording capacity. For this purpose, the standard deviation of the alloy particle size is preferably less than 10%, more preferably 5% or less. is there. The variation coefficient of the particle size is preferably less than 10%, more preferably 5% or less.

  If the particle size is too small, superparamagnetism is undesirable. Therefore, it is preferable to use a seed crystal method as described above in order to increase the particle size. At that time, there are cases in which noble metal is deposited rather than the metal constituting the particle. At this time, since there is a concern about oxidation of the particles, it is preferable to previously hydrotreat the particles.

The particle shape may be spherical, spheroid or complex sugar. Since one of the three equivalent directions at the time of phase transformation is selected and becomes an easy magnetization axis, when the particles are needle-shaped or rod-shaped, particles having the easy magnetization axis on the short axis side or the long axis side are mixed. That is not preferable.
Therefore, the ratio of long axis length / short axis length is preferably 2/1 to 1/1.

It is preferable to remove salts from the solution after the synthesis of the alloy particles from the viewpoint of improving the dispersion stability of the alloy particles. For desalting, there is a method in which alcohol is added excessively to cause light agglomeration, and natural sedimentation or centrifugal sedimentation is performed to remove salts together with the supernatant. However, in such a method, aggregation is likely to occur, so an ultrafiltration method is adopted. It is preferable.
As described above, alloy particles dispersed in the solution (alloy particle-containing liquid) are obtained.

  A transmission electron microscope (TEM) can be used to evaluate the particle size of the alloy particles. Electron diffraction by TEM may be used to determine the crystal system of alloy particles or magnetic particles, but X-ray diffraction is preferred because of its higher accuracy. For analysis of the composition inside the alloy particles or magnetic particles, it is preferable to evaluate by attaching EDAX to an FE-TEM that can narrow the electron beam. Moreover, evaluation of the magnetic property of an alloy particle or a magnetic particle can be performed using VSM.

<Oxidation process>
By subjecting the produced alloy particles to oxidation treatment, magnetic particles having ferromagnetism can be efficiently produced without increasing the temperature when annealing treatment is performed in a non-oxidizing atmosphere later. This is considered to be due to the phenomenon described below.
That is, first, oxygen enters the crystal lattice by oxidizing the alloy particles. When annealing is performed with oxygen entering, oxygen is desorbed from the crystal lattice by heat. Desorption of oxygen causes defects, and the movement of metal atoms constituting the alloy is facilitated through such defects. Therefore, phase transformation is likely to occur even at a relatively low temperature.
Such a phenomenon is inferred, for example, by measuring EXAFS (wide-range X-ray absorption fine structure) of the oxidized alloy particles and the annealed magnetic particles.
For example, in the alloy particles that are not subjected to oxidation treatment with Fe—Pt alloy particles, the existence of bonds between Fe atoms and Pt atoms or Fe atoms can be confirmed.
On the other hand, in the alloy particles subjected to the oxidation treatment, the presence of bonds between Fe atoms and oxygen atoms can be confirmed. However, the bonds with Pt atoms and Fe atoms are almost invisible. This means that Fe—Pt and Fe—Fe bonds are cut by oxygen atoms. As a result, it is considered that Pt atoms and Fe atoms easily move during annealing.
Then, after the alloy particles are annealed, the presence of oxygen cannot be confirmed, and the presence of bonds with Pt atoms and Fe atoms can be confirmed around the Fe atoms.

Considering the above phenomenon, it can be seen that if the oxidation is not performed, the phase transformation is difficult to proceed and the annealing temperature needs to be increased. However, if it is excessively oxidized, it is also conceivable that the interaction between oxygen, such as Fe, which is easily oxidized and oxygen becomes so strong that a metal oxide is generated.
Therefore, it is important to control the oxidation state of the alloy particles. For this purpose, it is necessary to set the oxidation treatment condition to an optimum one.

In the oxidation treatment, for example, when alloy particles are produced by the liquid phase method described above, a gas containing at least oxygen may be supplied to the produced alloy particle-containing liquid.
The oxygen partial pressure at this time is preferably 10 to 100% of the total pressure, and preferably 15 to 50%.
Moreover, it is preferable to set it as 0-100 degreeC, and, as for oxidation treatment temperature, it is preferable to set it as 15-80 degreeC.

The oxidation state of the alloy particles is preferably evaluated by EXAFS or the like, and the number of bonds between a base metal such as Fe and oxygen is 0.5 from the viewpoint of cutting Fe—Fe bond and Pt—Fe bond by oxygen. It is preferable that it is -4, and it is more preferable that it is 1-3.
The oxidation treatment may be performed by exposing the alloy particles to air at room temperature (0 to 40 ° C.) in a state where the alloy particles are applied or fixed on a support or the like. Aggregation of alloy particles can be prevented by carrying out the coating on a support or the like. The oxidation treatment time is preferably 1 hour to 48 hours, more preferably 3 hours to 24 hours.

<Annealing process>
The alloy particles subjected to the oxidation treatment are irregular phases. As described above, ferromagnetism cannot be obtained in the irregular phase. Therefore, in order to obtain a regular phase, it is necessary to perform heat treatment (annealing). The heat treatment needs to be performed at or above that temperature by using a differential thermal analysis (DTA) to obtain a transformation temperature at which the alloy constituting the alloy particles undergoes regular and irregular transformation.
The transformation temperature is usually about 500 ° C., but may be lowered by the addition of the third element. Further, the transformation temperature can be lowered by appropriately changing the atmosphere of the above-described oxidation treatment or annealing treatment. Therefore, the annealing temperature is preferably 150 ° C. or higher, and more preferably 150 to 450 ° C.

  Typical examples of the magnetic recording medium include a magnetic recording tape and a floppy (registered trademark) disk. After forming a magnetic layer in a web state on an organic support, these are processed into a tape in the former, and punched into a disk in the latter. The present invention is effective when an organic support is used in that the transformation temperature to ferromagnetism can be lowered, and application to these is a preferable measure.

In order to perform the annealing process in the web state, it is preferable that the annealing time is short. This is because if the annealing time is long, the apparatus becomes long. For example, when the web conveyance speed is 50 m / min and the annealing time is 30 minutes, the line length is 1500 mm. Therefore, in the method for producing magnetic particles of the present invention, the preferable annealing time is preferably 10 minutes or less, more preferably 5 minutes or less.
Further, in order to shorten the annealing treatment time as described above, the atmosphere of the annealing treatment is preferably a reducing atmosphere as will be described later. This is effective in preventing deformation of the support and is also effective in preventing diffusion of impurities from the support.

In addition, when annealing is performed in a particle state, the particles are likely to move and fusion is likely to occur. For this reason, although a high coercive force can be obtained, it tends to have a disadvantage that the particle size becomes large. Therefore, the annealing treatment is preferably performed in a state where it is applied on a support from the viewpoint of preventing aggregation of alloy particles.
Furthermore, by annealing the alloy particles on the support to obtain magnetic particles, the magnetic particles can be used as a magnetic recording medium having a layer made of such magnetic particles as a magnetic layer.

The support may be either an inorganic substance or an organic substance as long as it is a support used for a magnetic recording medium.
As the inorganic support, Mg alloys such as Al, Al—Mg, and Mg—Al—Zn, glass, quartz, carbon, silicon, ceramics, and the like are used. These supports are excellent in impact resistance and have rigidity suitable for thinning and high-speed rotation. In addition, it has a feature resistant to heat as compared with an organic support.

  Examples of organic supports include polyesters such as polyethylene terephthalate and polyethylene naphthalate; polyolefins; cellulose triacetate, polycarbonate, polyamide (including aromatic polyamides such as aliphatic polyamide and aramid), polyimide, polyamideimide , Polysulfone, polybenzoxazole; and the like can be used.

In order to apply the alloy particles on the support, various additives may be added to the alloy particle-containing liquid after the oxidation treatment, if necessary, and applied on the support.
At this time, the content of the alloy particles is preferably a desired concentration (0.01 to 0.1 mg / ml).

  As a method of applying to the support, air doctor coat, blade coat, rod coat, extrusion coat, air knife coat, squeeze coat, impregnation coat, reverse roll coat, transfer roll coat, gravure coat, kiss coat, cast coat, spray coat, A spin coat etc. can be utilized.

The atmosphere for the annealing treatment is a non-oxidizing atmosphere such as H ₂ , N ₂ , Ar, He, Ne, etc. in order to efficiently advance the phase transformation and prevent oxidation of the alloy.
In particular, from the viewpoint of desorbing oxygen present on the lattice by oxidation treatment, a reducing atmosphere such as methane, ethane, H ₂ or the like is preferable. Further, from the viewpoint of maintaining the particle size, it is preferable to perform the annealing treatment in a magnetic field under a reducing atmosphere. In view of explosion if the atmosphere of H _2, it is preferable to mix an inert gas.
Also, in order to prevent particle fusion during annealing, annealing is performed once in an inert gas below the transformation temperature, carbonizing the dispersant, and then annealing at a temperature above the transformation temperature in a reducing atmosphere. Is preferred. At this time, it is the most preferable aspect that, after the annealing treatment at the transformation temperature or lower, if necessary, a Si-based resin or the like is applied on the layer made of alloy particles and the annealing treatment is performed at the transformation temperature or higher.

  By performing the annealing treatment as described above, the alloy particles are transformed from an irregular phase to an ordered phase, and magnetic particles having ferromagnetism are obtained.

The magnetic particles produced by the method for producing magnetic particles of the present invention described above preferably have a coercive force of 95.5 to 398 kA / m (1200 to 5000 Oe), and when applied to a magnetic recording medium, In consideration of the capability of the head, it is more preferably 95.5 to 278.6 kA / m (1200 to 3500 Oe).
The particle size of the magnetic particles is preferably 1 to 100 nm, more preferably 3 to 20 nm, and even more preferably 3 to 10 nm.

  The magnetic nanoparticles of the present invention have a transition metal oxide layer on the surface thereof. In order to magnetically separate the particles, the oxide layer is preferably thick. On the other hand, if it is too thick, the number of particles present in the film is substantially reduced, which is disadvantageous in terms of electromagnetic conversion characteristics (SNR). Therefore, the thickness of the oxide layer is preferably 0.2 nm to 10 nm, and more preferably 1 nm to 5 nm. If the thickness varies, there is a high possibility that magnetic coupling will occur at a thin thickness. From the viewpoint of stabilizing the performance, the variation coefficient of the thickness of the oxide layer is preferably 15% or less, more preferably 1 to 10%.

In the present invention, in order to form an oxide layer of a transition metal on the surface, it is preferable that the transition metal is more than the noble metal in the particle composition after synthesis, and the transition metal is 51 at% to 65 at%. Preferably, it is 51 at% to 60 at%.
Further, in the magnetic nanoparticles with an oxide layer of a transition metal on the surface of the CuAu type or Cu ₃ Au-type ferromagnetic ordered alloy is a transition metal after synthesis often to noble CuAu type or Cu ₃ Au type ferromagnetic After exposing the alloy particles that can become ordered alloys in the presence of oxygen, specifically in the air to oxidize, annealing and phase transformation to form a CuAu type or Cu ₃ Au type ferromagnetic ordered alloy Although there is a method of performing an oxidation treatment, it is preferable to perform the oxidation treatment after annealing because variation in the thickness of the oxide layer can be reduced. Further, the oxidation treatment may be performed both before and after annealing.

(Oxidation treatment after annealing)
After the above-described annealing treatment, the CuAu type or Cu ₃ Au type ferromagnetic ordered alloy is heated in the presence of oxygen to perform an oxidation treatment for forming a transition metal oxide layer on the surface. If the oxidation treatment is performed too much, there may be a problem that the coercive force is lowered. Therefore, it is preferable to perform the oxidation treatment by heating at 50 ° C. to 250 ° C. in an atmosphere having an oxygen concentration of 0.1% to 20%. Further, it is preferable to perform the treatment under normal pressure or reduced pressure in order to proceed the oxidation treatment slowly. The pressure is preferably 0.005 MPa to 0.1 MPa, more preferably 0.005 MPa to 0.01 MPa.

The evaluation of the composition and film thickness of the oxide on the particle surface was carried out by taking out CuAu type or Cu ₃ Au type ferromagnetic ordered alloy nanoparticles from the medium and setting the beam diameter to 0. 0 with a scanning transmission electron microscope equipped with a composition analyzer. This can be done by scanning to 2 nm and mapping the characteristic X-ray intensities of O, Fe, and Pt.

On the other hand, in the method for producing magnetic nanoparticles of the present invention, a layer containing alloy particles capable of forming a CuAu-type or Cu ₃ Au-type ferromagnetic ordered alloy phase is formed on a support and subjected to an oxidation treatment. Annealing treatment may be performed in a non-oxidizing atmosphere.
The manufacturing method includes the same points as the above-described manufacturing method of magnetic nanoparticles, but is produced by depositing the alloy particles directly on the support and subjecting them to oxidation treatment and annealing treatment. Different.

The deposition method is not particularly limited as long as it can deposit desired alloy particles on a support to form a layer containing alloy particles, but it can be prepared by sputtering film formation. preferable.
The sputtering film forming method includes “RF magnetron sputtering method (hereinafter also referred to as“ RF sputtering method ”)” and “DC magnetron sputtering method”, and either method may be used. Is preferable from the viewpoint of efficiently forming alloy particles capable of forming a CuAu type or Cu ₃ Au type ordered alloy.

It is preferable to segregate Si, Cr, and the like at the grain boundaries from the viewpoint of reducing the unit of magnetization and reducing noise.
The CuAu type and Cu ₃ Au type ordered alloys formed by sputtering are paramagnetic or soft magnetic, and become ferromagnetic after annealing. At this time, it is effective from the viewpoint of lowering the transformation temperature that the present invention is once oxidized and then annealed in a non-oxidizing atmosphere, preferably a reducing atmosphere.
For the oxidation treatment after the film formation, it is preferable to apply the method of exposing to air in the first production method.

  After the oxidation treatment, the alloy particles are preferably annealed by applying the same conditions as in the above-described manufacturing method to obtain magnetic particles having ferromagnetism.

<< Magnetic recording medium >>
The magnetic recording medium of the present invention is characterized in that the magnetic layer contains magnetic nanoparticles, and the magnetic nanoparticles are the magnetic nanoparticles of the present invention described above.
Examples of the magnetic recording medium include magnetic tapes such as video tapes and computer tapes; magnetic disks such as floppy (registered trademark) disks and hard disks.

As described above, when alloy particles (alloy particle-containing liquid) are coated on a support and annealed to form magnetic nanoparticles, a layer made of such magnetic nanoparticles can be used as a magnetic layer.
In addition, when the magnetic nanoparticles are produced by annealing in the state of particles without annealing the alloy particles on the support, the magnetic nanoparticles are kneaded with an open kneader, a three roll mill, etc. A magnetic layer may be formed by finely dispersing with a grinder or the like to prepare a coating solution, which is coated on a support by a known method.

Further, as described above, after forming a layer containing an alloy capable of forming a CuAu type or Cu ₃ Au type ferromagnetic ordered alloy phase on the support by a sputtering film forming method or the like, and performing an oxidation treatment Alternatively, annealing may be performed in a non-oxidizing atmosphere to form a magnetic layer to produce a magnetic recording medium.
In this case, the oxidation treatment can be applied to the exposure at room temperature (0 to 40 ° C.) as described above. Further, it is preferable to apply the above-described processing as the annealing treatment.

  The thickness of the magnetic layer to be formed depends on the type of magnetic recording medium to be applied, but is preferably 4 nm to 1 μm, and more preferably 4 nm to 100 nm.

  The magnetic recording medium of the present invention may have other layers as needed in addition to the magnetic layer. For example, in the case of a disk, it is preferable to further provide a magnetic layer or a nonmagnetic layer on the opposite surface of the magnetic layer. In the case of a tape, it is preferable to provide a back layer on the surface of the insoluble support opposite to the magnetic layer.

  In addition, by forming a very thin protective film on the magnetic layer, the wear resistance is improved, and a lubricant is applied on the protective film to increase slipperiness, thereby providing sufficient reliability. It can be a magnetic recording medium.

  As a material of the protective film, oxides such as silica, alumina, titania, zirconia, cobalt oxide and nickel oxide; nitrides such as titanium nitride, silicon nitride and boron nitride; carbides such as silicon carbide, chromium carbide and boron carbide; Examples of the carbon include carbon and carbon such as graphite and amorphous carbon, and particularly preferable is a hard amorphous carbon generally called diamond-like carbon.

A carbon protective film made of carbon is suitable as a material for the protective film because it has a very thin film thickness and sufficient wear resistance, and hardly causes seizure on the sliding member.
As a method for forming a carbon protective film, sputtering is generally used for hard disks, but many methods using plasma CVD with a higher film formation rate have been proposed for products that require continuous film formation such as video tape. ing. Therefore, it is preferable to apply these methods.
Among them, it has been reported that the plasma injection CVD (PI-CVD) method has a very high film formation rate, and the obtained carbon protective film is hard and has a good quality protective film with few pinholes (for example, JP-A-61-61). No. 130487, JP-A 63-279426, JP-A 3-113824, etc.).

The carbon protective film preferably has a Vickers hardness of 1000 kg / mm ² or more, and more preferably 2000 kg / mm ² or more. The crystal structure is preferably an amorphous structure and non-conductive.
When a diamond-like carbon (diamond-like carbon) film is used as the carbon protective film, this structure can be confirmed by Raman light spectroscopic analysis. That is, when a diamond-like carbon film is measured, it can be confirmed by detecting a peak at 1520 to 1560 cm ⁻¹ . When the structure of the carbon film deviates from the diamond-like structure, the peak detected by Raman light spectroscopic analysis deviates from the above range, and the hardness as a protective film also decreases.

  As the carbon raw material for forming this carbon protective film, it is preferable to use carbon-containing compounds such as alkanes such as methane, ethane, propane, and butane; alkenes such as ethylene and propylene; and alkynes such as acetylene. Further, a carrier gas such as argon or an additive gas such as hydrogen or nitrogen for improving the film quality can be added as necessary.

When the carbon protective film is thick, the electromagnetic conversion characteristics are deteriorated and the adhesion to the magnetic layer is deteriorated. When the carbon protective film is thin, the wear resistance is insufficient. Accordingly, the film thickness is preferably 2.5 to 20 nm, and more preferably 5 to 10 nm.
In order to improve the adhesion between the protective film and the magnetic layer serving as the substrate, the surface of the magnetic layer is preferably etched in advance with an inert gas or surface-modified by exposure to a reactive gas plasma such as oxygen. .

  Further, an inorganic protective film may be formed by a sol-gel method. At this time, the step of forming the inorganic protective film is performed by hydrolysis and dehydration condensation of the starting material in a solution containing the starting material, starting from an alkoxide of any metal of zirconium, silicon, titanium, and aluminum. It is preferable to deposit a mixed phase containing the metal oxide, hydroxide and organic matter.

  Alternatively, preferably, in the step of forming the inorganic protective film, a method in which silicate is used as a starting material and silicon oxide is deposited by dehydration condensation of the starting material in the silicate aqueous solution is also used.

  Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples.

[Preparation of particles 1 to 4]
The following operation was performed in high purity N ₂ gas.
NaBH ₄ (manufactured by Wako Pure Chemical Industries, Ltd.) 0.76 g of H ₂ O (deoxygenated) dissolved in 12 ml of a reducing agent aqueous solution, aerosol OT (manufactured by Wako Pure Chemical Industries, Ltd.) 5.4 g, decane (manufactured by Wako Pure Chemical Industries, Ltd.) 40 ml A reverse micelle solution (I) was prepared by adding and mixing the alkane solution dissolved in 1.

In each of the particles 1 to 4, triammonium iron oxalate (Fe (NH ₄ ) ₃ (C ₂ O ₄ ) ₃ ) (manufactured by Wako Pure Chemical Industries) and potassium chloroplatinate (K ₂ ) in the amounts shown in Table 1 were used. PtCl ₄ ) (manufactured by Wako Pure Chemical Industries, Ltd.) is added to an aqueous metal salt solution in which 24 ml of H ₂ O (deoxygenated) is added, and an alkane solution in which 10.8 g of aerosol OT is dissolved in 80 ml of decane is added and mixed to obtain a reverse micelle solution. (II) was prepared.

While the reverse micelle solution (II) was stirred at a high speed with an omni mixer (manufactured by Yamato Kagaku) at a temperature of 22 ° C., the reverse micelle solution (I) was added instantaneously. Five minutes after the completion of the addition, the stirring was changed to magnetic stirrer and the temperature was raised to 40 ° C., followed by aging for 120 minutes. After cooling to room temperature, 2 ml of oleic acid (manufactured by Wako Pure Chemical Industries) was added, mixed, and taken out into the atmosphere. In order to destroy the reverse micelle, a mixed solution of 200 ml of H ₂ O and 200 ml of methanol was added to separate into an aqueous phase and an oil phase. A state in which the metal nanoparticles were dispersed on the oil phase side was obtained. The oil phase side was washed 5 times with 600 ml of H ₂ O + 200 ml of methanol. Thereafter, 1300 ml of methanol was added to cause flocculation of the alloy nanoparticles and to precipitate them. The supernatant was removed, 20 ml of heptane (manufactured by Wako Pure Chemical Industries) was added and redispersed, and then 100 ml of methanol was precipitated. This was repeated twice, and finally 5 ml of octane (manufactured by Wako Pure Chemical Industries, Ltd.) was added. In each of the particles 1 to 4, the composition (Fe) shown in Table 1 was used, the coefficient of variation was 5%, the particle size was 5 nm, the particle size An FePt nanoparticle dispersion liquid (alloy particle-containing liquid) having a spherical shape with a coefficient of variation of 6% was obtained.

[Example 1]
(Magnetic layer coating solution)
The obtained FePt nanoparticle dispersion was vacuum degassed and concentrated, and decane was added to dilute to 8 mass%.
Then, Toray FilR910 manufactured by Toray as a matrix agent was dissolved in a decane solution to a concentration of 1% by mass, and 80 ml of oleylamine and oleic acid were added in an amount of 0.3 ml per 1 ml of the alloy particle-containing solution. Thereafter, filtration in a clean room was performed to obtain a magnetic layer coating solution.

(Application of magnetic layer coating solution)
While rotating a glass substrate having a thickness of 1 mm at a speed of 500 rpm with a spin coater, 0.12 ml of the magnetic layer coating solution was dropped and rotated at 4000 rpm to blow off the excess coating solution. Thereafter, the particles were dried in air at 150 ° C. and the particles were oxidized.

(Annealing treatment)
Using an image furnace (QH-P610CP manufactured by ULVAC-RIKO) in a 4% H ₂ + N ₂ atmosphere, the temperature was increased to 475 ° C. at a temperature increase rate of 200 ° C./min, held for 30 minutes, and then cooled at 50 ° C./min.

(Oxidation treatment after annealing)
After the annealing treatment, oxidation treatment was performed by heating at 150 ° C. for 30 minutes in an atmosphere having an oxygen concentration of 0.5%.
A magnetic recording medium was manufactured through the above steps.

[Example 2]
A magnetic recording medium was produced in the same manner as in Example 1 except that the oxidation treatment after annealing was not performed.

[Example 3]
A magnetic recording medium was prepared in the same manner as in Example 1 except that the FePt nanoparticle dispersion of particle 1 was changed to the FePt nanoparticle dispersion of particle 2.

[Example 4]
A magnetic recording medium was manufactured in the same manner as in Example 3 except that the oxidation treatment after annealing was not performed.

[Comparative Example 1]
A magnetic recording medium was produced in the same manner as in Example 1 except that the FePt nanoparticle dispersion of particle 1 was changed to the FePt nanoparticle dispersion of particle 3.

[Comparative Example 2]
A magnetic recording medium was produced in the same manner as in Comparative Example 1 except that the oxidation treatment after annealing was not performed.

[Comparative Example 3]
A magnetic recording medium was produced in the same manner as in Example 1 except that the FePt nanoparticle dispersion of particle 1 was changed to the FePt nanoparticle dispersion of particle 4.

[Comparative Example 4]
A magnetic recording medium was produced in the same manner as in Comparative Example 3 except that the oxidation treatment after annealing was not performed.

[Evaluation]
The following evaluations were performed on the magnetic recording media of Examples 1 to 4 and Comparative Examples 1 to 4. The results are shown in Table 2.

(Evaluation of coercive force)
The coercive force was measured using a high-sensitivity magnetization vector measuring device manufactured by Toei Kogyo Co., Ltd. and a DATA processing device manufactured by the same company, with an applied magnetic field of 790 kA / m (10 kOe) and a sweep time of 5 minutes.

(Evaluation of magnetization reversal volume)
The magnetization reversal volume was determined by measuring the coercivity with a high sensitivity magnetization vector measuring machine manufactured by Toei Kogyo Co., Ltd. with a sweep time of 5 minutes and 30 minutes.

(Evaluation of surface transition metal oxide layer)
The magnetic material was scraped off from the medium with tweezers and placed on a Cu grid with C. Oxidation of transition metals by using JEOL JEM-2100FS / CESCOR type Cs-corrected STEM / EDX, scanning with an acceleration voltage of 200 kV and a beam diameter of 0.2 nm, and mapping the characteristic X-ray intensities of O, Fe and Pt The physical layer was evaluated.

(Evaluation of composition)
The composition was measured by ICP spectroscopic analysis (inductively coupled high-frequency plasma spectroscopic analysis) after evaporating and drying the dispersion, decomposing the organic matter with concentrated sulfuric acid, and then dissolving in aqua regia.

From Table 2, it can be seen that in Examples 1 to 4 having an iron oxide layer on the particle surface, a magnetic recording medium having a small magnetization reversal volume can be produced. In Examples 1 and 3 where the oxidation treatment was performed after annealing, particularly excellent results were obtained. Since the magnetization reversal volume is small, it is presumed that the interaction between particles is suppressed.
On the other hand, in any of Comparative Examples 1 to 4, no oxide layer was formed on the particle surface, and the magnetization reversal volume was large as compared with Examples 1 to 4.

Claims (5)

In magnetic nanoparticles of a CuAu type or Cu ₃ Au type ferromagnetic ordered alloy phase composed of a transition metal and a noble metal,
A magnetic nanoparticle having a transition metal oxide layer on a surface thereof.   A magnetic recording medium comprising the magnetic nanoparticles according to claim 1 in a magnetic layer. Alloy particles capable of forming a CuAu type or Cu ₃ Au type ferromagnetic ordered alloy comprising a transition metal and a noble metal, and / or magnetism of a CuAu type or Cu ₃ Au type ferromagnetic ordered alloy phase comprising a transition metal and a noble metal A method for producing magnetic nanoparticles, comprising the step of oxidizing the nanoparticles to form an oxide layer of the transition metal on the surface.   The method for producing magnetic nanoparticles according to claim 3, wherein a ratio of the transition metal in the composition of the alloy particles is larger than a ratio of the noble metal. 5. The method for producing magnetic nanoparticles according to claim 4, wherein the ratio of transition metals in the composition of alloy particles capable of forming the CuAu type or Cu ₃ Au type ferromagnetic ordered alloy is 51 at% to 65 at%. .