U.S. patent application number 10/791669 was filed with the patent office on 2004-11-25 for method of manufacturing magnetic particle, magnetic particle and magnetic recording medium.
This patent application is currently assigned to FUJI PHOTO FILM CO., LTD.. Invention is credited to Hattori, Yasushi, Ichikawa, Yasunori, Shiraishi, Fumiko, Waki, Koukichi.
Application Number | 20040231462 10/791669 |
Document ID | / |
Family ID | 33424758 |
Filed Date | 2004-11-25 |
United States Patent
Application |
20040231462 |
Kind Code |
A1 |
Shiraishi, Fumiko ; et
al. |
November 25, 2004 |
Method of manufacturing magnetic particle, magnetic particle and
magnetic recording medium
Abstract
There is provided a method of manufacturing a magnetic particle,
which comprises: the alloy particle preparation step of preparing
an alloy particle capable of forming a CuAu type or Cu.sub.3Au type
hard magnetic ordered alloy phase and the magnetic particle
formation step; wherein in the alloy preparation formation step, by
using a mixing and reaction device which has a stirring vane
rotating at a high speed in the interior of a mixer, a plurality of
kinds of solutions for preparing the alloy particle are supplied to
the interior of the mixer, where the plurality of kinds of
solutions L1 and L2 are mixed together and caused to react with
each other by a liquid phase process, and at the same time the
plurality of kinds of solutions are mixed together and caused to
react with each other so that the peripheral speed in a leading end
portion of the stirring vane is not less than 10 m/second.
Inventors: |
Shiraishi, Fumiko;
(Minami-Ashigara-shi, JP) ; Ichikawa, Yasunori;
(Minami-Ashigara-shi, JP) ; Waki, Koukichi;
(Minami-Ashigara-shi, JP) ; Hattori, Yasushi;
(Odawara-shi, JP) |
Correspondence
Address: |
SUGHRUE MION, PLLC
2100 PENNSYLVANIA AVENUE, N.W.
SUITE 800
WASHINGTON
DC
20037
US
|
Assignee: |
FUJI PHOTO FILM CO., LTD.
|
Family ID: |
33424758 |
Appl. No.: |
10/791669 |
Filed: |
March 3, 2004 |
Current U.S.
Class: |
75/348 ;
G9B/5.253 |
Current CPC
Class: |
G11B 5/70605 20130101;
B82Y 30/00 20130101; H01F 1/0063 20130101; H01F 1/06 20130101; H01F
1/068 20130101; B82Y 25/00 20130101; B22F 2999/00 20130101; B22F
9/24 20130101; C22C 1/0491 20130101; B22F 1/054 20220101; B22F
2999/00 20130101; C22C 1/0491 20130101; B22F 9/24 20130101 |
Class at
Publication: |
075/348 |
International
Class: |
B22F 009/24 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 5, 2003 |
JP |
2003-059045 |
Mar 7, 2003 |
JP |
2003-062085 |
Feb 10, 2004 |
JP |
2004-033406 |
Claims
What is claimed is:
1. A method of manufacturing a magnetic particle, comprising: the
alloy particle preparation step of preparing an alloy particle
capable of forming a CuAu type or Cu.sub.3Au type hard magnetic
ordered alloy phase and the magnetic particle formation step;
wherein in said alloy preparation formation step, by using a mixing
and reaction device which has a stirring vane rotating at a high
speed in the interior of a mixer, a plurality of kinds of solutions
for preparing said alloy particle are supplied to the interior of
said mixer, where the plurality of kinds of solutions are mixed
together and caused to react with each other by a liquid phase
process, and at the same time the plurality of kinds of solutions
are mixed together and caused to react with each other so that the
peripheral speed in a leading end portion of said stirring vane is
not less than 5 m/second.
2. The method of manufacturing a magnetic particle according to
claim 1, wherein the size of an alloy particle prepared by said
mixing and reaction is 1 to 100 nm and the coefficient of variation
in the particle size is not more than 15%.
3. A method of manufacturing a magnetic particle, comprising: the
alloy particle preparation step of preparing an alloy particle
capable of forming a CuAu type or Cu.sub.3Au type hard magnetic
ordered alloy phase and the magnetic particle formation step;
wherein in said alloy preparation formation step, by using a mixing
and reaction device in which there is provided a reaction vessel
which is filled with a bulk liquid and provided therein with a
mixer which has a stirring vane rotating at a high speed and is
provided with an opening through which said bulk liquid is
circulated to and from the interior of said reaction vessel, a
plurality of kinds of solutions for preparing said alloy particle
are supplied to the interior of said mixer, where the plurality of
kinds of solutions are mixed together and caused to react with each
other by a liquid phase process, and at the same time the plurality
of kinds of solutions are mixed together and caused to react with
each other so that a mixed reaction solution is discharged from
said mixer to said reaction vessel by a circulating stream of said
bulk liquid.
4. The method of manufacturing a magnetic particle according to
claim 3, wherein the size of an alloy particle prepared by said
mixing and reaction is 1 to 100 nm and the coefficient of variation
in the particle size is not more than 15%.
5. The method of manufacturing a magnetic particle according to
claim 3, wherein the peripheral speed in a leading end portion of
said stirring vane is not less than 5 m/second.
6. A method of manufacturing a magnetic particle, comprising: the
alloy particle preparation step of preparing an alloy particle
capable of forming a CuAu type or Cu.sub.3Au type hard magnetic
ordered alloy phase and the magnetic particle formation step;
wherein in said alloy preparation formation step, by using a mixing
and reaction device which is provided with a mixer in the interior
of a reaction vessel and has a microgap formed between an inner
wall of the mixer and a stirring member rotating at a high speed
and in which in order to form this microgap, when the distance from
the center of rotation of said stirring member to a leading end
thereof is put as 1, the distance to said inner wall having the
shortest distance from the center of rotation of said stirring
member is set in the range of 1.001 to 1.200, a plurality of kinds
of solutions for preparing said alloy particle are supplied to said
microgap, where the plurality of kinds of solutions are mixed
together and caused to react with each other by a liquid phase
process, and at the same time the plurality of kinds of solutions
are mixed together and caused to react with each other so that the
mixed reaction solution is discharged from said microgap.
7. The method of manufacturing a magnetic particle according to
claim 6, wherein the size of an alloy particle prepared by said
mixing and reaction is 1 to 100 nm and the coefficient of variation
in the particle size is not more than 15%.
8. The method of manufacturing a magnetic particle according to
claim 3, wherein the peripheral speed in a leading end portion of
said stirring member is not less than 5 m/second.
9. The method of manufacturing a magnetic particle according to
claim 1, wherein said liquid phase process is the reversed micelle
process and wherein as said plurality of kinds of solutions, a
reversed micelle solution (solution L1), which is obtained by
mixing a nonaqueous organic solvent containing a surfactant and an
aqueous reductant solution, and a reversed micelle solution
(solutions L2), which is obtained by mixing a nonaqueous organic
solvent containing a surfactant and an aqueous metallic slat
solution containing a plurality of kinds of metallic atoms
constituting said alloy particle, are prepared, and said solution
L1 and solutions L2 are supplied to said mixer.
10. The method of manufacturing a magnetic particle according to
claim 3, wherein said liquid phase process is the reversed micelle
process and wherein as said plurality of kinds of solutions, a
reversed micelle solution (solution L1), which is obtained by
mixing a nonaqueous organic solvent containing a surfactant and an
aqueous reductant solution, and a reversed micelle solution
(solutions L2), which is obtained by mixing a nonaqueous organic
solvent containing a surfactant and an aqueous metallic slat
solution containing a plurality of kinds of metallic atoms
constituting said alloy particle, are prepared, and said solution
L1 and solutions L2 are supplied to said mixer.
11. The method of manufacturing a magnetic particle according to
claim 6, wherein said liquid phase process is the reversed micelle
process and wherein as said plurality of kinds of solutions, a
reversed micelle solution (solution L1), which is obtained by
mixing a nonaqueous organic solvent containing a surfactant and an
aqueous reductant solution, and a reversed micelle solution
(solutions L2), which is obtained by mixing a nonaqueous organic
solvent containing a surfactant and an aqueous metallic slat
solution containing a plurality of kinds of metallic atoms
constituting said alloy particle, are prepared, and said solution
L1 and solutions L2 are supplied to said mixer.
12. The method of manufacturing a magnetic particle according to
claim 1, wherein said liquid phase process is the reversed micelle
process and wherein as said plurality of kinds of solutions, a
reversed micelle solution (Solution L1), which is obtained by
mixing a nonaqueous organic solvent containing a surfactant and an
aqueous reductant solution, and a reversed micelle solution
(Solution L3), which is obtained by mixing a nonaqueous organic
solvent containing a surfactant and an aqueous metallic salt
solution containing one of a plurality of kinds of metallic atoms
constituting said alloy particle, are prepared, the number of
prepared Solutions L3 being equal to the number of said plurality
of kinds of metallic atoms, and Solution L1 and the plurality of
Solutions L3 are supplied to said mixer.
13. The method of manufacturing a magnetic particle according to
claim 3, wherein said liquid phase process is the reversed micelle
process and wherein as said plurality of kinds of solutions, a
reversed micelle solution (Solution L1), which is obtained by
mixing a nonaqueous organic solvent containing a surfactant and an
aqueous reductant solution, and a reversed micelle solution
(Solution L3), which is obtained by mixing a nonaqueous organic
solvent containing a surfactant and an aqueous metallic salt
solution containing one of a plurality of kinds of metallic atoms
constituting said alloy particle, are prepared, the number of
prepared Solutions. L3 being equal to the number of said plurality
of kinds of metallic atoms, and Solution L1 and the plurality of
Solutions L3 are supplied to said mixer.
14. The method of manufacturing a magnetic particle according to
claim 6, wherein said liquid phase process is the reversed micelle
process and wherein as said plurality of kinds of solutions, a
reversed micelle solution (Solution L1), which is obtained by
mixing a nonaqueous organic solvent containing a surfactant and an
aqueous reductant solution, and a reversed micelle solution
(Solution L3), which is obtained by mixing a nonaqueous organic
solvent containing a surfactant and an aqueous metallic salt
solution containing one of a plurality of kinds of metallic atoms
constituting said alloy particle, are prepared, the number of
prepared Solutions L3 being equal to the number of said plurality
of kinds of metallic atoms, and Solution L1 and the plurality of
Solutions L3 are supplied to said mixer.
15. The method of manufacturing a magnetic particle according to
claim 1, wherein at least two kinds of metallic atoms constituting
the alloy particle capable of forming said CuAu type or Cu.sub.3Au
type hard magnetic ordered alloy phase are selected from the Groups
6, 8, 9 and 10 of the long periodic table and at least further one
kind of metallic atom is selected from the Groups 11, 12, 13, 14
and 15, the content of said one kind of metal atom being 1 to 30
atom % of the whole alloy.
16. The method of manufacturing a magnetic particle according to
claim 3, wherein at least two kinds of metallic atoms constituting
the alloy particle capable of forming said CuAu type or Cu.sub.3Au
type hard magnetic ordered alloy phase are selected from the Groups
6, 8, 9 and 10 of the long periodic table and at least further one
kind of metallic atom is selected from the Groups 11, 12, 13, 14
and 15, the content of said one kind of metal atom being 1 to 30
atom % of the whole alloy.
17. The method of manufacturing a magnetic particle according to
claim 6, wherein at least two kinds of metallic atoms constituting
the alloy particle capable of forming said CuAu type or Cu.sub.3Au
type hard magnetic ordered alloy phase are selected from the Groups
6, 8, 9 and 10 of the long periodic table and at least further one
kind of metallic atom is selected from the Groups 11, 12, 13, 14
and 15, the content of said one kind of metal atom being 1 to 30
atom % of the whole alloy.
18. The method of manufacturing a magnetic particle according to
claim 1, wherein the mixing and reaction temperature in said alloy
particle preparation step is controlled to the range of -5.degree.
C. to 30.degree. C.
19. The method of manufacturing a magnetic particle according to
claim 3, wherein the mixing and reaction temperature in said alloy
particle preparation step is controlled to the range of -5.degree.
C. to 30.degree. C.
20. The method of manufacturing a magnetic particle according to
claim 1, wherein the mixing and reaction temperature in said alloy
particle preparation step is controlled to the range of -5.degree.
C. to 30.degree. C.
21. The method of manufacturing a magnetic particle according to
claim 1, wherein in said magnetic particle formation step of
forming a CuAu type or Cu.sub.3Au type magnetic particle from the
alloy particle prepared in said-alloy particle preparation step,
annealing treatment is performed after the application of an
alloy-particle-containing solution, which contains the alloy
particle prepared in said alloy particle preparation step to a
backing.
22. The method of manufacturing a magnetic particle according to
claim 3, wherein in said magnetic particle formation step of
forming a CuAu type or Cu.sub.3Au type magnetic particle from the
alloy particle prepared in said alloy particle preparation step,
annealing treatment is performed after the application of an
alloy-particle-containing solution, which contains the alloy
particle prepared in said alloy particle preparation step, to a
backing.
23. The method of manufacturing a magnetic particle according to
claim 6, wherein in said magnetic particle formation step of
forming a CuAu type or Cu.sub.3Au type magnetic particle from the
alloy particle prepared in said alloy particle preparation step,
annealing treatment is performed after the application of an
alloy-particle-containing solution, which contains the alloy
particle prepared in said alloy particle preparation step, to a
backing.
24. The method of manufacturing a magnetic particle according to
claim 21, wherein the annealing treatment temperature in said
annealing treatment is controlled in the range of 100.degree. C. to
500.degree. C.
25. The method of manufacturing a magnetic particle according to
claim 22, wherein the annealing treatment temperature in said
annealing treatment is controlled in the range of 100.degree. C. to
500.degree. C.
26. The method of manufacturing a magnetic particle according to
claim 23, wherein the annealing treatment temperature in said
annealing treatment is controlled in the range of 100.degree. C. to
500.degree. C.
27. A magnetic particle manufactured by the method of manufacturing
a magnetic particle according to claim 1.
28. A magnetic recording medium containing the magnetic particle
according to claim 27 in a magnetic layer.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method of manufacturing a
magnetic particle, a magnetic particle manufactured by the method
and a magnetic recording medium containing this magnetic particle
in a magnetic layer.
[0003] 2. Description of the Related Art
[0004] Reducing the particle size of a magnetic particle contained
in a magnetic layer is important for increasing the magnetic
recording density. For example, in magnetic recording media widely
used as video tapes, computer tapes, disks, etc., noise decreases
when the particle size is reduced in a case where the mass of a
hard magnetic material is the same.
[0005] A CuAu type or Cu.sub.3Au type hard magnetic ordered alloy
is attracting attention as a material for a magnetic particle which
is promising for improving the magnetic recording density
(described, for example, in Japanese Patent Application Publication
No. 2003-6830 and Japanese Patent Application Publication No.
2001-256631). Because this hard magnetic ordered alloy has large
crystal magnetic anisotropy because of strains generated during
ordering and it is known that this alloy shows hard magnetism even
when the particle size of a magnetic particle is reduced.
[0006] Although a magnetic particle showing hard magnetism is
prepared by a liquid phase process, a gaseous phase process, etc.,
a magnetic particle immediately after the preparation of a
practical liquid phase process excellent in mass producibility, in
particular, has a structure of an unordered face-centered cubic
crystal. A face-centered cubic crystal usually shows soft magnetism
or paramagnetism and is unsuitable for a magnetic recording medium.
In a magnetic recording medium, therefore, it is necessary to
obtain a hard magnetic ordered alloy having a coercive force of not
less than 95.5 kA/m (1200 Oe). In order to obtain such a hard
magnetic ordered alloy, it is necessary to cause an unordered phase
to transform to an ordered phase. For this reason, the
manufacturing of a magnetic particle comprises the alloy particle
preparation step of preparing an alloy particle capable of forming
a CuAu type or Cu.sub.3Au type hard magnetic ordered alloy phase
and the magnetic particle formation step of forming a CuAu type or
Cu.sub.3Au type magnetic particle from the alloy particle formed in
this alloy particle preparation step, and in the magnetic particle
formation step, usually annealing treatment (heat treatment) is
performed.
[0007] However, whether the performance of a magnetic particle used
in a magnetic recording medium is good is not determined by the
magnetic particle formation step alone, and the key point is how to
prepare, in the alloy particle preparation step, an alloy particle
which has a micro size, is excellent in monodispersibility and
provides an excellent transformation facilitativeness which
facilitates the transformation of an unordered phase to an ordered
phase. Usually, the preparation of an alloy particle is performed
by mixing a plurality of kinds of solutions for alloy particle
preparation by the above-described liquid phase process, and a
device in which a stirring vane is provided in a tank as shown in
FIG. 9 has been proposed as a mixing and reaction device (Japanese
Patent Application Publication No. 5-173267).
[0008] In this mixing and reaction device 1, in order to stir
solutions filled in the interior of a tank 2, a stirring vane 5 is
rotated and driven via transmission device 4 which transmits the
rotational driving force of a motor 3 in a noncontact manner by use
of magnetic force. On the outer circumferential surface of the tank
1 is provided temperature control device 6, which heats or cools
the solutions in order to control the temperature control of the
solutions filled in the interior. A sealing lid 7 of this tank 2 is
provided with an introduction pipe 8 which introduces the solutions
and the lower part of the tank 2 is provided with a discharge pipe
9 which discharges mixed reaction solutions which are mixed and
caused to react within the tank 2.
[0009] However, in the preparation of an alloy particle by mixing
by stirring, a dead space is present within the tank 2, making the
stirring of the solution nonuniform, and particle growth occurs due
to the partial circulation of a formed alloy particle within the
tank 2. Therefore, the conventional preparation of an alloy
particle has the drawback that it is impossible to produce an alloy
particle which has a micro size, is excellent in monodispersibility
and provides an excellent transformation facilitativeness.
[0010] Furthermore, magnetic recording media which have recently
been widely used as video tapes, computer tapes, disks, etc., are
required to meet the requirement for a further noise reduction.
SUMMARY OF THE INVENTION
[0011] The present invention was made in view of such a situation
and has as its object the provision of a method of manufacturing a
magnetic particle, which is capable of manufacturing a magnetic
particle having good performance for achieving a sufficiently low
noise level of a magnetic recording medium because an alloy
particle which meets all of the requirements for a micro size,
monodispersibility and transformation facilitativeness can be
prepared in the alloy particle preparation step, a magnetic
particle manufactured by this method and a magnetic recording
medium containing this magnetic particle in a magnetic layer.
[0012] The inventors of the present invention has obtained the
knowledge that an alloy particle which has a micro size, is
excellent in monodispersibility and provides an excellent
self-assembling property can be obtained by mixing together a
plurality of kinds of solutions for preparing an alloy particle
capable of forming a CuAu type or Cu.sub.3Au type hard magnetic
ordered alloy phase and causing the solutions to react with each
other by an ultrahigh-speed stirring method in which the peripheral
speed at a leading end of a stirring vane is not less than 5
m/second, or an in-tank mixer method in which mixing and reaction
are caused to occur in a mixer provided within a reaction vessel
filled with a bulk liquid and a mixed reaction solution is
discharged to the bulk liquid within the reaction vessel, or a
microgap mixing method in which mixing and reaction are caused to
occur in a microgap formed between an inner wall of a mixer and a
stirring member rotating at a high speed.
[0013] First, the lowering of the noise level of a magnetic
recording medium will be described in order to facilitate the
understanding of the present invention.
[0014] The sputtering method which is known as a method of
preparing a magnetic recording medium for a hard disk forms a
magnetic film in a polycrystal state. A monocrystal which forms the
magnetic film in question has a particle size of about 20 nm and is
in the shape of a cylinder which has a height of about 100 nm. The
coefficient of variation in the particle size (particle diameter)
of a monocrystal is as large as not less than 20%. In the
sputtering method, monocrystals which form the magnetic film in
question adjoin each other and, therefore, the monocrystal in
question, along with the adjoining monocrystals, is magnetized. For
this reason, an actual magnetization unit becomes larger than that
of the monocrystal in question. From this, although attempts have
been made to make the magnetization unit small by causing the
segregation of chromium etc. in a crystal grain field, these
attempts have been insufficient as measures to lower the
magnetization unit. However, in order to lower the noise level of a
recording medium, it is necessary to reduce the particle diameter
and lower the coefficient of variation for further lowering the
magnetization unit.
[0015] In a general manufacturing method of a magnetic recording
medium for magnetic tapes and flexible disks, magnetic particles
formed from iron, an alloy of iron and cobalt, iron oxide or barium
ferrite are first subjected to annealing treatment, kneaded with a
binder and then applied after dispersion. In this method, however,
annealing treatment is performed in the state of the magnetic
particle and hence the particles tend to fuse or coalesce.
Therefore, the coefficient of variation in the particle size is
about 20% at the best and a sufficiently low level noise is not
achieved. Furthermore, the magnetic anisotropy constant of these
magnetic materials is small. Therefore, when the size is not more
than 20 nm, a superparamagnetic state occurs under the influence of
thermal fluctuations and these magnetic materials cannot be used as
a magnetic recording medium.
[0016] Thus, whether a magnetic recording medium for hard disk or a
magnetic recording medium for magnetic tape or flexible disk, it is
important to lower the coefficient of variation in the particle
size to not more than 20% for lowering the noise level
sufficiently.
[0017] The present inventors paid attention to the following
knowledge as measures to lower this coefficient of variation to not
more than 20%:
[0018] (1) Unlike the sputtering method, in the reversed micelle
process, it is possible to prepare within a micelle a magnetic
particle having a size of not more than 20 nm in a condition
separated from other magnetic particles.
[0019] (2) If an alloy particle capable of forming a CuAu type or
Cu.sub.3Au type hard magnetic ordered alloy phase is used as a
precursor for manufacturing a magnetic particle, even in a case
where the size is not more than 20 nm, the magnetic material shows
hard magnetism suitable for a magnetic recording medium and, at the
same time, the particle size of an alloy particle to be prepared is
within the range of 1 to 100 nm and the coefficient of variation in
the particle size of the alloy particle is not more than 15%. This
is advantageous.
[0020] (3) Even when an alloy particle capable of forming a CuAu
type or Cu.sub.3Au type hard magnetic ordered alloy phase is
manufactured by using the reversed micelle process, in the case of
use of a mixing and reaction device in which a stirring vane is
provided within a tank, as described in the prior art, the
coefficient of variation in the particle size does not become 20%
or less and it is difficult to control the particle diameter even
when an alloy particle having a particle size of not more than 20
nm may be prepared. However, an alloy particle which has a micro
size, is excellent in monodispersibility and provides an excellent
self-assembling property can be obtained by mixing a plurality of
kinds of solutions for preparing an alloy particle capable of
forming a CuAu type or Cu.sub.3Au type hard magnetic ordered alloy
phase by an ultrahigh-speed stirring method, a microgap mixing
method or an in-tank mixer method, and it is easy to cause an alloy
phase to undergo transformation for an unordered phase to an
ordered phase in the magnetic particle formation step of forming an
alloy particle to a CuAu type or Cu.sub.3Au type hard magnetic
particle (for example, annealing treatment).
[0021] (4) By appropriately controlling the mixing and reaction
temperature in the ultrahigh-speed stirring method, the microgap
mixing method or the in-tank mixer method, the size control of the
alloy particle to be prepared can be performed with good
accuracy.
[0022] (5) By performing annealing treatment, with alloy particles
applied to a backing and fixed, it is possible to prevent particles
from fusing or coalescing.
[0023] The particle size (particle diameter) in the present
invention is indicated by the diameter of a circle having an area
equal to the projected area of the parallel outer surfaces of a
particle. That is, the projected area of a particle is obtained by
measuring the area on an electron micrograph and correcting the
projection magnification. By supposing a circle having an area
equal to the projected area of a particle, the diameter of this
circle is regarded as the circle-equivalent diameter of an alloy
particle (or a magnetic particle in some cases). Furthermore, the
coefficient of variation in the particle size device a value
obtained by dividing the standard deviation of the
circle-equivalent diameter in all particles by the average particle
diameter.
[0024] On the basis of this knowledge, the present invention was
concretely constituted as a method of manufacturing a magnetic
particle, a magnetic particle and a magnetic recording medium.
[0025] In order to achieve the above object, in the first aspect of
the present invention, there is provided a method of manufacturing
a magnetic particle, which comprises: the alloy particle
preparation step of preparing an alloy particle capable of forming
a CuAu type or Cu.sub.3Au type hard magnetic ordered alloy phase
and the magnetic particle formation step; wherein in the alloy
preparation formation step, by using a mixing and reaction device
which has a stirring vane rotating at a high speed in the interior
of a mixer, a plurality of kinds of solutions for preparing the
alloy particle are supplied to the interior of the mixer, where the
plurality of kinds of solutions are mixed together and caused to
react with each other by a liquid phase process, and at the same
time the plurality of kinds of solutions are mixed together and
caused to react with each other so that the peripheral speed in a
leading end portion of the stirring vane is not less than 5
m/second.
[0026] According to the first aspect of the present invention, in
the alloy preparation formation step, a plurality of kinds of
solutions for preparing the alloy particle are mixed together and
caused to react with each other by an ultrahigh-speed stirring
method at a peripheral speed of not less than 5 n/second at a
leading end of the stirring vane. Incidentally, the peripheral
speed at a leading end of the stirring vane is preferably not less
than 10 n/second.
[0027] As a result of this, it is possible to instantaneously and
efficiently mix, within the mixer, a plurality of kinds of
solutions together and cause these solutions to react and,
therefore, it is possible to form an alloy particle which has a
micro size and is excellent in monodispersibility. Therefore, it is
possible ensure that the particle size of an alloy particle
prepared by the mixing and reaction by this ultrahigh-speed
stirring method is 1 to 100 nm and that the coefficient of
variation in the particle size is not more than 15%. This is
because an alloy particle having a particle size of less than 1 nm
tends to show superparamagnetism and hence is unsuitable for an
alloy particle for manufacturing a magnetic particle used in a
magnetic recording medium and because a sufficiently low noise
level cannot be achieved if the particle size exceeds 100 nm. The
particle size of an alloy particle is more preferably in the range
of 3 to 20 nm and especially preferably in the range is 3 to 10
nm.
[0028] Furthermore, this is because a sufficiently low noise level
cannot be achieved if the coefficient of variation in the particle
size of an alloy particle exceeds 15%. A more preferable
coefficient of variation is not more than 10%.
[0029] In addition, because an alloy particle prepared by this
ultrahigh-speed stirring method is excellent in the self-assembling
property, it is possible to cause the alloy particle to undergo
transformation to a CuAu type or Cu.sub.3Au type magnetic particle
in the magnetic particle forming step.
[0030] The second aspect of the present invention is that in the
first aspect, the particle size of the alloy particle prepared by
the mixing and reaction is 1 to 100 nm and the coefficient of
variation in the particle size is not more than 15%.
[0031] The second aspect of the present invention specifies the
size and coefficient of variation of the alloy particle prepared by
the mixing and reaction in the method of manufacturing a magnetic
particle in the first aspect.
[0032] In order to achieve the above object, in the third aspect of
the present invention, there is provided a method of manufacturing
a magnetic particle, which comprises: the alloy particle
preparation step of preparing an alloy particle capable of forming
a CuAu type or Cu.sub.3Au type hard magnetic ordered alloy phase
and the magnetic particle formation step; wherein in the alloy
preparation formation step, by using a mixing and reaction device
in which there is provided a reaction vessel which is filled with a
bulk liquid and provided therein with a mixer which has a stirring
vane rotating at a high speed and is provided with an opening
through which the bulk liquid is circulated to and from the
interior of the reaction vessel, a plurality of kinds of solutions
for preparing the alloy particle are supplied to the interior of
the mixer, where the plurality of kinds of solutions are mixed
together and caused to react with each other by a liquid phase
process, and at the same time the plurality of kinds of solutions
are mixed together and caused to react with each other so that a
mixed reaction solution is discharged from the mixer to the
reaction vessel by a circulating stream of the bulk liquid.
[0033] According to the third aspect, in the alloy particle
preparation step, a plurality of kinds of solutions for preparing
the alloy particle are mixed together and caused to react with each
other by an in-tank mixer method which involves performing mixing
and reaction in a mixer provided within a reaction vessel filled
with a bulk liquid and discharging a mixed reaction solution to the
bulk liquid within the reaction vessel. As a result of this,
because the plurality of kinds of solutions can be instantaneously
and efficiently mixed together and caused to react with each other,
it is possible to prepare an alloy particle which has a micro size
and an excellent monodispersibility. Therefore, it is possible
ensure that the particle size of an alloy particle prepared by the
mixing and reaction by this in-tank mixer method is 1 to 100 nm and
that the coefficient of variation in the particle size is not more
than 15%. The particle size of an alloy particle is more preferably
in the range of 3 to 20 nm and especially preferably in the range
is 3 to 10 nm. Furthermore, the coefficient of variation in an
alloy particle is preferably not more than 10%. In addition,
because the alloy particle prepared by this in-tank mixer method is
excellent in the self-assembling property, in the magnetic particle
formation step, it is easy to cause the alloy particle to undergo
transformation to a CuAu type or Cu.sub.3Au type hard magnetic
particle.
[0034] In this case, as specified in the fifth aspect, it is
desirable to ensure that the peripheral speed in a leading end
portion of the stirring vane is not less than 5 n/second. In this
manner, by combining the in-tank mixer method and the
ultrahigh-speed stirring method, it is possible to prepare an alloy
particle excellent in a micro size, monodispersibility and the
self-assembling property.
[0035] The fourth aspect is that in the third aspect, the size of
an alloy particle prepared by the mixing and reaction is 1 to 100
nm and the coefficient of variation in the particle size is not
more than 15%.
[0036] The fourth aspect of the present invention specifies the
size and coefficient of variation of an alloy particle prepared in
the mixing and reaction in the method of manufacturing a magnetic
particle of the third aspect.
[0037] In order to achieve the above object, in the sixth aspect of
the present invention, there is provided a method of manufacturing
a magnetic particle, which comprises: the alloy particle
preparation step of preparing an alloy particle capable of forming
a CuAu type or Cu.sub.3Au type hard magnetic ordered alloy phase
and the magnetic particle formation step; wherein in the alloy
preparation formation step, by using a mixing and reaction device
which is provided with a mixer in the interior of a reaction vessel
and has a microgap formed between an inner wall of the mixer and a
stirring member rotating at a high speed and in which in order to
form this microgap, when the distance from the center of rotation
of the stirring member to a leading end thereof is put as 1, the
distance to the inner wall having the shortest distance from the
center of rotation of the stirring member is set in the range of
1.001 to 1.200, a plurality of kinds of solutions for preparing the
alloy particle are supplied to the microgap, where the plurality of
kinds of solutions are mixed together and caused to react with each
other by a liquid phase process, and at the same time the plurality
of kinds of solutions are mixed together and caused to react with
each other so that the mixed reaction solution is discharged from
the microgap.
[0038] According to the sixth aspect of the present invention, in
the alloy particle preparation step, a plurality of kinds of
solutions for preparing the alloy particle are mixed together and
caused to react with each other by a microgap mixing method which
involves performing mixing and reaction in a microgap formed
between an inner wall of the mixer and a stirring member rotating
at a high speed.
[0039] When the distance from the center of rotation of the
stirring member to a leading end thereof is put as 1, the microgap
refers to a gap formed by setting the distance to the inner wall
having the shortest distance from the center of rotation of the
stirring member in the range of 1.001 to 1.200. The stirring member
may have the shape of a cylindrical drum or may be constituted by
disk-shaped stirring members which are provided in multiple stages
on a rotary shaft.
[0040] As a result of this, because due to the shearing force in
the microgap the plurality of kinds of solutions can be
instantaneously and efficiently mixed together and caused to react
with each other, it is possible to prepare an alloy particle which
has a micro size and an excellent monodispersibility. Therefore, it
is possible ensure that the particle size of an alloy particle
prepared by the mixing and reaction by this microgap mixing method
is 1 to 100 nm and that the coefficient of variation in the
particle size is not more than 15%. The particle size of an alloy
particle is more preferably in the range of 3 to 20 nm and
especially preferably in the range is 3 to 10 nm. Furthermore, this
is because a sufficiently low noise level cannot be achieved if the
coefficient of variation in the particle size of an alloy particle
exceeds 15%, and this coefficient of variation is preferably not
more than 10%. In addition, because the alloy particle prepared by
this microgap mixing method is excellent in the self-assembling
property, in the magnetic particle formation step, it is easy to
cause the alloy particle to undergo transformation to a CuAu type
or Cu.sub.3Au type hard magnetic particle.
[0041] In this case, as described in the eighth aspect, it is
preferred that the peripheral speed in a leading end portion of the
stirring member be not less than 5 m/second. In this manner, by
combining the microgap mixing method and the ultrahigh-speed
stirring method, it is possible to prepare an alloy particle
excellent in a micro size, monodispersibility and the
self-assembling property. Incidentally, the mixing method of the
sixth aspect is referred to as the microgap mixing method.
[0042] The ninth aspect of the present invention is that in the
first aspect, the liquid phase process is the reversed micelle
process and that as the plurality of kinds of solutions, a reversed
micelle solution (solution L1), which is obtained by mixing a
nonaqueous organic solvent containing a surfactant and an aqueous
reductant solution, and a reversed micelle solution (solutions L2),
which is obtained by mixing a nonaqueous organic solvent containing
a surfactant and an aqueous metallic slat solution containing a
plurality of kinds of metallic atoms constituting the alloy
particle, are prepared, and the solution L1 and solutions L2 are
supplied to the mixer.
[0043] According to the ninth aspect of the present invention, by
performing the liquid phase process by the reversed micelle method,
it becomes easy to control the particle diameter of a prepared
alloy particle. Furthermore, it is possible to prepare, as the
plurality of kinds of solutions, Solution L1 which is constituted
by a nonaqueous organic solvent containing a surfactant and an
aqueous reductant solution and Solution L2 which is constituted by
a nonaqueous organic solvent containing a surfactant and an aqueous
metallic slat solution containing a plurality of kinds of metallic
atoms constituting the alloy particle. That is, it is possible to
cause all the plurality of kinds of metallic atoms constituting the
alloy particle to be contained in the solution L2 which is mixed
with the solution L1 and caused to react.
[0044] The tenth aspect of the present invention is that in the
third aspect, the liquid phase process is the reversed micelle
process and that as the plurality of kinds of solutions, a reversed
micelle solution (solution L1), which is obtained by mixing a
nonaqueous organic solvent containing a surfactant and an aqueous
reductant solution, and a reversed micelle solution (solutions L2),
which is obtained by mixing a nonaqueous organic solvent containing
a surfactant and an aqueous metallic slat solution containing a
plurality of kinds of metallic atoms constituting the alloy
particle, are prepared, and the solution L1 and solutions L2 are
supplied to the mixer.
[0045] The tenth aspect specifies that in the in-tank mixer method,
the liquid phase process be performed by the reversed micelle
method.
[0046] The eleventh aspect of the present invention is that in the
sixth aspect, the liquid phase process is the reversed micelle
process and that as the plurality of kinds of solutions, a reversed
micelle solution (solution L1), which is obtained by mixing a
nonaqueous organic solvent containing a surfactant and an aqueous
reductant solution, and a reversed micelle solution (solutions L2),
which is obtained by mixing a nonaqueous organic solvent containing
a surfactant and an aqueous metallic slat solution containing a
plurality of kinds of metallic atoms constituting the alloy
particle, are prepared, and the solution L1 and solutions L2 are
supplied to the mixer.
[0047] The eleventh aspect specifies that in the microgap mixing
method, the liquid phase process be performed by the reversed
micelle method.
[0048] The twelfth aspect of the present invention is that in the
first aspect, the liquid phase process is the reversed micelle
process and that as the plurality of kinds of solutions, a reversed
micelle solution (solution L1), which is obtained by mixing a
nonaqueous organic solvent containing a surfactant and an aqueous
reductant solution, and a reversed micelle solution (solution L3),
which is obtained by mixing a nonaqueous organic solvent containing
a surfactant and an aqueous metallic slat solution containing one
of a plurality of kinds of metallic atoms constituting the alloy
particle, are prepared, the number of prepared Solutions L3 being
equal to the number of the plurality of kinds of metallic atoms,
and the solution L1 and the plurality of solutions L3 are supplied
to the mixer.
[0049] In the twelfth aspect of the present invention, a plurality
of the solutions L3 are prepared, each of which contains one kind
selected from the plurality of kinds of metallic atoms constituting
the alloy particle, and the solution L1 and the plurality of
solutions L3 are supplied to the mixer.
[0050] The thirteenth aspect of the present invention is that in
the third aspect, the liquid phase process is the reversed micelle
process and as the plurality of kinds of solutions, a reversed
micelle solution (solution L1), which is obtained by mixing a
nonaqueous organic solvent containing a surfactant and an aqueous
reductant solution, and a reversed micelle solution (solution L3),
which is obtained by mixing a nonaqueous organic solvent containing
a surfactant and an aqueous metallic slat solution containing one
of a plurality of kinds of metallic atoms constituting the alloy
particle, are prepared, the number of prepared Solutions L3 being
equal to the number of the plurality of kinds of metallic atoms,
and the solution L1 and the plurality of solutions L3 are supplied
to the mixer.
[0051] According to the thirteenth aspect of the present invention,
in the in-tank mixer method a plurality of the solutions L3 are
prepared, each of which contains one kind selected from the
plurality of kinds of metallic atoms constituting the alloy
particle, and the solution L1 and the plurality of solutions L3 are
supplied to the mixer.
[0052] The fourteenth aspect of the present invention is that in
the sixth aspect, the liquid phase process is the reversed micelle
process and as the plurality of kinds of solutions, a reversed
micelle solution (solution L1), which is obtained by mixing a
nonaqueous organic solvent containing a surfactant and an aqueous
reductant solution, and a reversed micelle solution (solution L3),
which is obtained by mixing a nonaqueous organic solvent containing
a surfactant and an aqueous metallic slat solution containing one
of a plurality of kinds of metallic atoms constituting the alloy
particle, are prepared, the number of prepared Solutions L3 being
equal to the number of the plurality of kinds of metallic atoms,
and the solution L1 and the plurality of solutions L3 are supplied
to the mixer.
[0053] According to the thirteenth aspect of the present invention,
in the microgap mixing method a plurality of the solutions L3 are
prepared, each of which contains one kind selected from the
plurality of kinds of metallic atoms constituting the alloy
particle, and the solution L1 and the plurality of solutions L3 are
supplied to the mixer.
[0054] The fifteenth aspect of the present invention is that in the
tenth aspect, at least two kinds of metallic atoms constituting the
alloy particle capable of forming the CuAu type or Cu.sub.3Au type
hard magnetic ordered alloy phase are selected from the Groups 6,
8, 9 and 10 of the long periodic table and at least further one
kind of metallic atom is selected from the Groups 11, 12, 13, 14
and 15, the content of the one kind of metal atom being 10 to 30
atom % of the whole alloy.
[0055] According to the fifteenth aspect of the present invention,
by adding at least one kind of metallic atom selected from the
Groups 11, 12, 13, 14 and 15 to at least two kinds of metallic
atoms selected from the Groups 6, 8, 9 and 10 of the long periodic
table, it is possible to lower the transformation temperature in
causing the transformation of the alloy phase prepared in the alloy
particle preparation phase to occur from an unordered phase to an
ordered phase in the magnetic particle formation step.
[0056] The sixteenth aspect of the present invention is that in the
third aspect, at least two kinds of metallic atoms constituting the
alloy particle capable of forming the CuAu type or Cu.sub.3Au type
hard magnetic ordered alloy phase are selected from the Groups 6,
8, 9 and 10 of the long periodic table and at least further one
kind of metallic atom is selected from the Groups 11, 12, 13, 14
and 15, the content of the one kind of metal atom being 10 to 30
atom % of the whole alloy.
[0057] According to the sixteenth aspect of the present invention,
in the in-tank mixer method, at least one kind of metallic atom
selected from the Groups 11, 12, 13, 14 and 15 is added to at least
two kinds of metallic atoms selected from the Groups 6, 8, 9 and 10
of the long periodic table, with the result that it is possible to
lower the transformation temperature in causing the transformation
of the alloy phase prepared in the alloy particle preparation phase
to occur from an unordered phase to an ordered phase in the
magnetic particle formation step.
[0058] The seventeenth aspect of the present invention is that in
the sixth aspect, at least two kinds of metallic atoms constituting
the alloy particle capable of forming the CuAu type or Cu.sub.3Au
type hard magnetic ordered alloy phase are selected from the Groups
6, 8, 9 and 10 of the long periodic table and at least further one
kind of metallic atom is selected from the Groups 11, 12, 13, 14
and 15, the content of the one kind of metal atom being 10 to 30
atom % of the whole alloy.
[0059] According to the seventeenth aspect of the present
invention, in the microgap mixing method, at least one kind of
metallic atom selected from the Groups 11, 12, 13, 14 and 15 is
added to at least two kinds of metallic atoms selected from the
Groups 6, 8, 9 and 10 of the long periodic table, with the result
that it is possible to lower the transformation temperature in
causing the transformation of the alloy phase prepared in the alloy
particle preparation phase to occur from an unordered phase to an
ordered phase in the magnetic particle formation step.
[0060] The eighteenth aspect of the present invention is that in
the first aspect, the mixing and reaction temperature in the alloy
particle preparation step is controlled to the range of -5.degree.
C. to 30.degree. C.
[0061] According to the eighteenth aspect of the present invention,
the mixing and reaction temperature in the alloy particle
preparation step is controlled to the range of -5.degree. C. to
30.degree. C. If the mixing and reaction temperature is less than
-5.degree. C., this poses the problem that a water phase condenses,
making a reduction reaction nonuniform. If the mixing and reaction
temperature exceeds 30.degree. C., coalescence and precipitation
tend to occur and the system may sometimes become unstable. The
mixing and reaction temperature is preferably in the range of
0.degree. C. to 25.degree. C. and especially preferably in the
range of 5.degree. C. to 25.degree. C.
[0062] The nineteenth aspect of the present invention is that in
the third aspect, the mixing and reaction temperature in the alloy
particle preparation step is controlled to the range of -5.degree.
C. to 30.degree. C.
[0063] The nineteenth aspect of the present invention specifies
that in the in-tank mixer method, the mixing and reaction
temperature in the alloy particle preparation step be controlled to
the range of -5.degree. C. to 30.degree. C.
[0064] The twentieth aspect of the present invention is that in the
sixth aspect, the mixing and reaction temperature in the alloy
particle preparation step is controlled to the range of -5.degree.
C. to 30.degree. C.
[0065] The twentieth aspect of the present invention specifies that
in the microgap mixing method, the mixing and reaction temperature
in the alloy particle preparation step be controlled to the range
of -5.degree. C. to 30.degree. C.
[0066] The twenty-first aspect of the present invention is that in
the first aspect, in the magnetic particle formation step annealing
treatment is performed after the application of an
alloy-particle-containing solution, which contains the alloy
particle prepared in the alloy particle preparation step, to a
backing.
[0067] Although the alloy particle prepared in the alloy particle
preparation step has weak magnetism, it is necessary to cause the
alloy phase of the alloy particle to undergo transformation from an
unordered phase to an ordered phase in order to obtain a CuAu type
or Cu.sub.3Au type hard magnetic ordered alloy having a coercive
force of not less than 1200 Oe, which is required in a magnetic
recording medium. However, if this annealing treatment is performed
in the state of a particle, alloy particles are apt to coalesce
together.
[0068] According to the twenty-first aspect of the present
invention, annealing treatment is performed after the application
of an alloy-particle-containing solution, which contains the alloy
particle prepared in the alloy particle preparation step, to a
backing. Therefore, it is possible to prevent the coalescence of
alloy particles and it is possible to form an alloy particle having
a micro size. In this case, also the particle size of the magnetic
particle formed by annealing treatment is preferably in the range
of 1 to 100 nm, more preferably in the range of 3 to 20 nm and
especially preferably in the range of 3 to 10 nm. Furthermore, the
coefficient of variation in the particle size of the magnetic
particle formed by annealing treatment is also preferably not more
than 15% and more preferably not more than 10%.
[0069] The twenty-second aspect of the present invention is that in
the third aspect, in the magnetic particle formation step annealing
treatment is performed after the application of an
alloy-particle-containing solution, which contains the alloy
particle prepared in the alloy particle preparation step, to a
backing.
[0070] The twenty-second aspect of the present invention specifies
that in the in-tank mixer method, annealing treatment be performed
after the application of an alloy-particle-containing solution,
which contains the alloy particle prepared in the alloy particle
preparation step, to a backing.
[0071] The twenty-third aspect of the present invention is that in
the sixth aspect, in the magnetic particle formation step annealing
treatment is performed after the application of an
alloy-particle-containing solution, which contains the alloy
particle prepared in the alloy particle preparation step, to a
backing.
[0072] The twenty-third aspect of the present invention specifies
that in the microgap mixing method, annealing treatment is
performed after the application of an alloy-particle-containing
solution, which contains the alloy particle prepared in the alloy
particle preparation step, to a backing.
[0073] The twenty-fourth aspect of the present invention is that in
the twenty-first aspect, the annealing treatment temperature in the
annealing treatment is controlled to the range of 100.degree. C. to
500.degree. C.
[0074] The twenty-fourth aspect specifies an appropriate range of
the annealing treatment temperature in the ultrahigh-speed stirring
method.
[0075] The twenty-fifth aspect of the present invention is that in
the twenty-second aspect, the annealing treatment temperature in
the annealing treatment is controlled to the range of 100.degree.
C. to 500.degree. C.
[0076] The twenty-fifth aspect specifies an appropriate range of
the annealing treatment temperature in the in-tank mixing
method.
[0077] The twenty-sixth aspect of the present invention is that in
the twenty-third aspect, the annealing treatment temperature in the
annealing treatment is controlled to the range of 100.degree. C. to
500.degree. C.
[0078] The twenty-sixth aspect specifies an appropriate range of
the annealing treatment temperature in the microgap mixing
method.
[0079] In order to achieve the above object, in the twenty-seventh
aspect of the present invention, there is provided a magnetic
particle manufactured by the method of manufacturing a magnetic
particle according to any one of the first to twenty-sixth aspects,
and in the twenty-eighth aspect of the present invention, there is
provided a magnetic recording medium containing the magnetic
particle according to the twenty-seventh aspect in a magnetic
layer.
[0080] As described above, according to the method of manufacturing
a magnetic particle of the present invention, it is possible to
prepare an alloy particle which satisfies all the requirements for
a micro size, monodispersibility and transformation
facilitativeness in the alloy particle preparation step and,
therefore, it is possible to manufacture a magnetic particle of
good performance.
[0081] Furthermore, a magnetic recording medium of the present
invention, which contains the magnetic particle manufactured by the
invention in a magnetic layer, has lower noise level and
high-quality performance of high recording density.
BRIEF DESCRIPTION OF THE DRAWINGS
[0082] FIG. 1 is a desirable conceptual diagram showing the
construction of a mixing and reaction device for carrying out the
ultrahigh-speed stirring method in the alloy particle preparation
step in a method of manufacturing a magnetic particle of the
present invention;
[0083] FIG. 2 is a desirable conceptual diagram showing the
construction of a mixing and reaction device for carrying out the
microgap mixing method in the alloy particle preparation step in a
method of manufacturing a magnetic particle of the present
invention;
[0084] FIG. 3 is a sectional view of the mixing and reaction device
of FIG. 2 which is laterally cut;
[0085] FIG. 4 is a sectional view to explain an example of a
modification of the mixing and reaction device for carrying out the
microgap mixing method;
[0086] FIG. 5 is a sectional view to explain another example of a
modification of the mixing and reaction device for carrying out the
microgap mixing method;
[0087] FIG. 6 is a desirable conceptual diagram showing the
construction of a mixing and reaction device for carrying out the
in-tank mixing method in the alloy particle preparation step in a
method of manufacturing a magnetic particle of the present
invention;
[0088] FIG. 7 is an enlarged view of a mixer in the mixing and
reaction device of FIG. 6; and
[0089] FIG. 8 is an explanatory drawing to explain a flow
regulating valve in the mixer of FIG. 7.
[0090] FIG. 9 is an explanatory diagram to explain a conventional
mixing and reaction device.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0091] Preferred embodiments of a method of manufacturing a
magnetic particle, a magnetic particle and a magnetic recording
medium related to the present invention will be described below
with reference to the attached drawings.
[0092] A method of manufacturing a magnetic particle of the present
invention comprises the alloy particle preparation step of
preparing by a liquid phase process an alloy particle capable of
forming a hard magnetic ordered alloy phase and the magnetic
particle formation step of forming a CuAu type or Cu.sub.3Au type
magnetic particle from the prepared alloy particle.
[0093] A method of manufacturing a magnetic particle and a magnetic
particle of the present invention will be described below while
describing each of the above steps. Incidentally, the magnetic
particle formation step is an example of annealing treatment, which
will be described below. However, the invention is not limited to
this annealing treatment.
[0094] [Alloy Particle Preparing Step]
[0095] Although an alloy particle which becomes a magnetic particle
after annealing treatment can be prepared by the gaseous phase
process other than the liquid phase process, the liquid phase
process is desirable in consideration of the excellent mass
producibility. Although conventionally known various processes can
be applied as the liquid phase process, it is preferred to adopt
reduction processes developed by improving the conventional liquid
phase processes, and among the reduction processes, the reversed
micelle process by which it is easy to control the particle size of
an alloy particle is especially preferable.
[0096] The reversed micelle process comprises the reduction step in
which a reduction reaction is caused to occur by mixing at least
two kinds of reversed micelle solutions and the aging step of aging
at a treatment temperature after the reduction reaction.
[0097] (Reduction Step)
[0098] In the reduction step, a reversed micelle solution (Solution
L1), which is obtained by mixing a nonaqueous organic solvent
containing a surfactant and a reducing aqueous solution, is
prepared. The reversed micelle solution (Solution L1) is
hereinafter referred to simply as the solution L1.
[0099] An oil soluble surfactant is used as the surfactant.
Concretely, the sulfonic acid type (for example, erosol OT (made by
Wako Pure Chemical Industries, Ltd.), the class 4 ammonium salt
type (for example, cetyl trimethylammonium bromide), the ether type
(for example, pentaethylene glycol dodecyl ether), etc. can be
enumerated.
[0100] It is preferred that the amount of the surfactant in the
nonaqueous organic solvent be 20 to 200 g/l.
[0101] Alkanes, ethers, alcohols, etc. are enumerated as those
which are desirable as the nonaqueous organic solvent which
dissolves the surfactant. Alkanes with carbon numbers 7 to 12 are
desirable as alkanes. Concretely, heptane, octane, isooctane,
noane, decane, undecane, dodecane, etc. are desirable. Diethyl
ether, dipropyl ether, dibutyl ether, etc. are desirable as ethers.
Ethoxyethanol, ethoxypropanal, etc. are desirable as alcohols.
[0102] Although, compound containing alcohols, polyalcohols,
H.sub.2, HCHO, S.sub.2O.sub.6.sup.2-, H.sub.2PO.sub.2-,
BH.sub.4-,.cndot.N.sub.2H.- sub.5+,.cndot.H.sub.2PO.sub.3-, etc. as
the reductant in the aqueous reductant solution may be use singly,
it is desirable to use two kinds or more in combination. It is
preferred that the amount of the reductant in the aqueous solution
be 3 to 50 moles to 1 mole of metal salt.
[0103] It is preferred that the mass ratio of the water to the
surfactant in the solution L1 (water/surfactant) be not more than
20. If the mass ratio exceeds 20, this poses the problem that
precipitation is apt to occur and that particles are also apt to
become irregular. The mass ratio is more preferably not more than
15 and especially preferably 0.5 to 10.
[0104] Next, apart from the above-described solution L1, a reversed
micelle solution (Solutions L2), which is obtained by mixing a
nonaqueous organic solvent containing a surfactant and an aqueous
metallic salt solution containing a plurality of kinds of metallic
atoms constituting the alloy particle, are prepared. The reversed
micelle solution (Solution L2) is hereinafter referred to simply as
the solution L2.
[0105] In this case, the solution L1, which is obtained by mixing a
nonaqueous organic solvent containing a surfactant and an aqueous
reductant solution, and a reversed micelle solution (Solution L3),
which is obtained by mixing a nonaqueous organic solvent containing
a surfactant and an aqueous metallic salt solution containing one
of a plurality of kinds of metallic atoms constituting the alloy
particle, are prepared, the number of prepared Solutions L3 being
equal to the number of the plurality of kinds of metallic atoms.
The reversed micelle solution (Solution L3) is hereinafter referred
to simply as the solution L3.
[0106] The conditions (substances to be used, concentrations, etc.)
for the surfactant and the nonaqueous organic solvent are the same
as the solution L1. Incidentally, the same solution as the solution
L1 or dissimilar solutions may be used. Furthermore, the mass ratio
of the water to the surfactant in the solution L2 are also the same
as the solution L1, and the mass ratio may be the same as with the
solution L1 or may be different.
[0107] It is desirable to appropriately select the metallic salt
contained in the aqueous metallic salt solution so that the
magnetic particle to be prepared can form a CuAu type or Cu.sub.3Au
type hard magnetic ordered alloy.
[0108] FeNi, FePd, FePt, CoPt, CoAu, etc. can be enumerated as CuAu
type hard magnetic ordered alloys, and among others, FePd, FePt and
CoPt are desirable. Ni.sub.3Fe, FePd.sub.3, Fe.sub.3Pd, Fe.sub.3Pt,
Co.sub.3Pt, FePt.sub.3, CoPt.sub.3, Ni.sub.3Pt, CrPt.sub.3 and
Ni.sub.3Mn can be enumerated as Cu.sub.3Au type hard magnetic
ordered alloys and, among others, FePd.sub.3, FePt.sub.3,
CoPt.sub.3, Fe.sub.3Pd, Fe.sub.3Pt and Co.sub.3Pt are
desirable.
[0109] As concrete examples of metallic salts, it is possible to
enumerate H.sub.2PtCl.sub.6, K.sub.2PtCl.sub.4,
Pt(CH.sub.3COCHCOCH.sub.3).sub.2, Na.sub.2PdCl.sub.4,
Pd(OCOCH.sub.3).sub.2, PdCl.sub.2, Pd(CH.sub.3
COCHCOCH.sub.3).sub.2, HAuCl.sub.4, Fe.sub.2(SO.sub.4).sub.3,
Fe(NO.sub.3).sub.3, (NH.sub.4).sub.3Fe (C.sub.2O.sub.4).sub.3
COCHCOCH.sub.3).sub.3, NiSO.sub.4, CoCl.sub.2,
Co(OCOCH.sub.3).sub.2, etc.
[0110] The concentration of the aqueous metallic salt solution (as
the metallic salt concentration) is preferably 0.1 to 1000
.mu.mol/ml and more preferably 1 to 100 .mu.mol/ml.
[0111] It is necessary to cause an alloy particle to undergo the
transformation of the alloy phase from an unordered phase to an
ordered phase by the annealing treatment which will be described
later. It is preferred that at least two kinds of metallic atoms
constituting the alloy particle capable of forming the CuAu type or
Cu.sub.3Au type hard magnetic ordered alloy phase be selected from
the Groups 6, 8, 9 and 10 of the long periodic table and that at
least further one kind of metallic atom be selected from the Groups
11, 12, 13, 14 and 15, the content of the one kind of metallic atom
being 1 to 30 atom % of the whole alloy. For example, by adding one
kind of metallic atom (hereinafter referred to as "a third
element") selected from the Groups 11, 12, 13, 14 and 15, such as
Sb, Pb, Bi, Cu, Ag, Zn and In, to a binary alloy constituted by two
kinds of metallic atoms selected from the Groups 6, 8, 9 and 10 of
the long periodic table, it is possible to lower the transformation
temperature in causing the transformation of the alloy phase of the
alloy particle from an unordered phase to an ordered phase.
[0112] The solutions L1 and L2 prepared as described above are
mixed together. In the present invention, an alloy particle which
has a micro size, is excellent in monodispersibility and provides
an excellent self-assembling property can be obtained by mixing
together, as the plurality of solutions L1 and L2, a plurality of
kinds of solutions for preparing an alloy particle capable of
forming a CuAu type or Cu.sub.3Au type hard magnetic ordered alloy
phase and causing the solutions to react with each other by an
ultrahigh-speed stirring method in which the peripheral speed in an
leading end portion of a stirring vane is not less than 5 m/second,
or an in-tank mixer method in which mixing and reaction are caused
to occur in a mixer provided within a reaction vessel filled with a
bulk liquid and a mixed reaction solution is discharged to the bulk
liquid within the reaction vessel, or a microgap mixing method in
which mixing and reaction are caused to occur in a microgap formed
between an inner wall of a mixer and a stirring member rotating at
a high speed.
[0113] The construction of each mixing and reaction device suitable
for carrying out [1] the ultrahigh-speed stirring method, [2] the
microgap mixing method and [3] the in-tank mixer method will be
described below.
[0114] [1] Ultrahigh-Speed Stirring Method
[0115] FIG. 1 is a sectional view of a mixing and reaction device
10 suitable for carrying out the ultrahigh-speed stirring
method.
[0116] As shown in FIG. 1, the mixing and reaction device 10 is
constructed in such a manner that a high-speed stirrer 14 is
provided in a vertical type mixer 12 which is formed in the shape
of a cylindrical vessel. The high-speed stirrer 14 is constructed
in such a manner that the upper portion of a vertical rotational
shaft 16 is rotatably supported by a cover plate 18 of the mixer 12
via bearings 20 and that a motor 22, which is fixed to a seat not
shown in the drawing, is connected to the top end of the rotational
shaft 16. Also, two-tired upper and lower edge turbi type stirring
vanes 24, 24 are installed in the bottom end portion of the
rotational shaft 16 in such a manner that they are immersed in the
water. It is preferred that when the diameter of the stirring vane
24 is denoted by d, the clearance between the stirring vanes 24 be
in the range of 1.0 d to 0.5 d. Furthermore, it is preferred that
the inside diameter D of the mixer 12 with respect to the diameter
d of the stirring vane 24 be in the range of D=1.7 d to 3.7 d.
Furthermore, it is preferred that the static liquid depth is in the
range of 2 d to 3 d. Incidentally, although in FIG. 1, the type of
the stirring vane 24 is shown as the edge turbi type, it is also
possible to use the dissolver type, the paddle type, the propeller
type, the flat disk type, etc. and furthermore, the turbine type,
the disk turbine type, etc. can be used. Even when the stirring
vane 24 of any type is used, the high-speed stirrer 14 is
constructed in such a manner that the peripheral speed at the
leading end of the stirring vane 24 is preferably not less than 5
m/second and more preferably not less than 10 m/second.
[0117] Also, a jacket 13 through which a heating medium having a
relatively large heat capacity such as water and oil flows is wound
around the outer circumference of the mixer 12, and a heating
medium inlet port 13A and a heating medium outlet port 13B are
connected to a heating medium supply device, which is not shown in
the drawing. A heating medium at a temperature which can control
the mixing and reaction temperature of the solutions L1, L2 within
the mixer 12 to the range of -5.degree. C. to 30.degree. C. is
supplied from the heating medium supply device to the jacket 13 and
circulated again to the heating medium supply device. It is
preferred that the mixing and reaction temperature be appropriately
set within the range of -5.degree. C. to 30.degree. C. depending on
the kinds of the solutions L1, L2 etc. The more preferred
temperature range of the mixing and reaction temperature is
0.degree. C. to 25.degree. C. and the especially preferred
temperature range thereof is 5.degree. C. to 25.degree. C.
[0118] Above the mixing and reaction device 10, there are provided
a first preparation tank 26 which prepares the solution L1 and a
second preparation tank 28 which prepares the solution L2. The
first preparation tank 26 prepares the reversed micelle solution of
the solution L1 by mixing a nonaqueous organic solvent containing a
surfactant and an aqueous reductant solution by use of a stirrer
30. The second preparation tank 28 prepares the reversed micelle
solution of the solution L2 by mixing a nonaqueous organic solvent
containing a surfactant and an aqueous metallic slat solution
containing a plurality of kinds of metallic atoms constituting the
alloy particle by use of a stirrer 32. Jackets 27, 29 are provided
along the outer circumferences of the first preparation tank 26 and
the second preparation tank 28, respectively. When it is difficult
to raise the temperature to a set mixing and reaction temperature
by use of the jacket 13 of the mixer 12 alone, though this depends
on the relationship between the volumes of the solutions L1, L2
which are mixed together and caused to react with each other within
the mixer 12, it is possible to heat the solutions L1, L2
beforehand by use of the jackets 27, 29 of the first and second
preparation tanks 26, 28.
[0119] Addition pipes 38, 40 which have valves 34, 36,
respectively, are provided in an extended condition from the
bottoms of the first and second preparation tanks 26, 28 and the
leading ends of the respective addition pipes 38, 40 pierce through
the cover plate 18 of the mixer 12 and are caused to extend along
the rotary shaft 16 of the high-speed stirrer 16 to near the
solution level. In this case, although the leading ends of the
addition pipes 38, 40 may be immersed in the solution, it is
desirable that the leading ends of the addition pipes 38, 40 be not
brought into contact with the solution surface, because it is
feared that alloy particles formed by the reaction of the solutions
L1, L2 within the mixer 12 may adhere to these leading ends of the
addition pipes. A discharge pipe 42 of a mixed solution LM
containing the alloy particles formed by a mixing reaction (an
alloy-particle-containing solution) is connected to a bottom plate
12A of the mixer 12 and a valve 44 is provided in the discharge
pipe 44.
[0120] According to the mixing and reaction device 10 configured as
described above, the solution L1 prepared in the first preparation
tank 26 is supplied in a prescribed amount to the mixer 12 via the
addition pipe 38 and the stirring vane 24 of the ultrahigh-speed
stirrer 14 is caused to perform ultrahigh-speed stirring at an
ultrahigh speed so that the peripheral speed at the leading end of
the stirring vane 24 becomes not less than 5 m/second.
Subsequently, the solution L2 from the second preparation tank 28
is added to the solution L1 within the mixer 12 via the addition
pipe 40. In this case, the temperature of the solutions L1, L2 and
the temperature within the mixer 12 are set at a prescribed
temperature in the range of -5.degree. C. to 30.degree. C. As a
result of this, under appropriate mixing and reaction temperature
conditions, in the solution L1 within the mixer 12 a very strong
V-shaped spiral stream which engulfs the solution from the solution
level into the interior of the solution is formed around the rotary
shaft 16. Therefore, the solution L2 added to the vicinity of the
rotary shaft 16 is instantaneously engulfed by this V-shaped spiral
stream into the solution L1 and uniformly distributed to the whole
solution L1 within the mixer 12. The mixed reaction solution LM
formed by the mixing and reaction in the mixer 12 is discharged
from the discharge pipe 42. The stirring time in the mixer 12,
which is to be appropriately set according to the volumes of the
solutions L1, L2, is preferably in the range of 1 to 30 minutes and
more preferably in the range of 5 to 20 minutes. As a result of
this, it is possible to prepare an alloy particle which has a
particle size range of 1 to 100 nm, whose coefficient of variation
in the particle size is not more than 15%, and which is excellent
in the self-assembling property.
[0121] In this case, although it is also possible to collect and
store the solution L2 in the mixer 12 and to add the solution L1,
in consideration of the uniformity of reduction, it is desirable to
add the solution L2 to the solution L1. Although in FIG. 1 the
description was made by the example in which the solutions L1, L2
are mixed together and caused to react with each other; in the case
where the solution L1 and the plurality of the solutions L3 are
mixed together, this method can be achieved by providing the
preparation tanks and addition pipes in numbers equal to the number
of the solutions.
[0122] Incidentally, the mixing and reaction device 10 for carrying
out the ultrahigh-speed stirring method is not limited to the
mixing and reaction device of FIG. 1, and any mixing and reaction
device can be used so long as the mixing and reaction device is
constructed in such a manner that it can supply a plurality of
kinds of solutions for preparing an alloy particle to the interior
of the mixer 12 and cause the mixing and reaction to occur by the
liquid phase process and at the same time it can ensure that the
peripheral speed at the leading end of the stirring vane 24 is not
less than 5 m/sec.
[0123] [2] Microgap Mixing Method
[0124] FIG. 2 is a sectional view of a mixing and reaction device
50 suitable for carrying out the microgap mixing method.
[0125] As shown in FIG. 2, the mixing and reaction device 50 is
constructed in such a manner that in a vertical type mixer 52 which
is formed in the shape of a cylindrical vessel whose top and bottom
are hermetically closed, there is provided a column-shaped rotary
drum 54 having a diameter a little smaller than the inside diameter
of this mixer 52. A rotary shaft 56 which is perpendicular along
the center of rotation S is provided in the rotary drum 54, and the
upper and lower portions of the rotary shaft 56 are rotatably
supported by the top plate 52A and bottom plate 52B of the mixer 52
via bearings 58, 58. A motor 60, which is fixed to a seat not
shown, is connected the top end of the rotary shaft 56.
[0126] As shown in FIG. 2 and FIG. 3 (a cross-sectional view of the
mixer 52), an annular microgap 62 (a microgap) is formed between
the circumferential surface of the inner wall of the mixer 52 and
the outer circumferential surface of the rotary drum 54. When the
distance D.sub.1 from the center of rotation S of the rotary drum
54 to the leading end thereof is put as 1, this microgap 62 is
formed by setting the distance D.sub.2 to the inner wall of the
mixer 52 having the shortest distance from the center of rotation S
of the rotary drum 54 in the range of 1.001 to 1.200.
[0127] Near the mixing and reaction device 50, there are provided a
first preparation tank 64 which prepares the solution L1 and a
second preparation tank 66 which prepares the solution L2. The
first preparation tank 64 prepares the reversed micelle solution of
the solution L1 by mixing a nonaqueous organic solvent containing a
surfactant and an aqueous reductant solution by use of a stirrer
68. The second preparation tank 66 prepares the reversed micelle
solution of the solution L2 by mixing a nonaqueous organic solvent
containing a surfactant and an aqueous metallic slat solution
containing a plurality of kinds of metallic atoms constituting the
alloy particle by use of a stirrer 70.
[0128] Supply pipes 80, 82 which have valves 72, 74, and supply
pumps 76, 78, respectively, are provided in an extended condition
from the bottoms of the first and second preparation tanks 64, 66
and the leading ends of the respective solution supply pipes 80, 82
are connected to supply ports 84, 86 which are formed in opposed
positions of the side surface of the bottom end portion of the
mixer 52. On the side surface of the top end portion of the mixer
52 is formed a discharge port 88 of the mixed reaction solution LM
and the discharge tube 90 is connected to the discharge port
88.
[0129] Jackets 51, 69, 71, 81, 83 through which a heating medium
having a relatively large heat capacity such as water and oil flows
are wound around the respective outer circumferences of the mixer
52, first and second preparation tanks 64, 66, and supply pipes 80,
82, and the jackets 51, 69, 71, 81, 83 are connected to a heating
medium supply device, which is not shown in the drawing. A heating
medium at a temperature which can control the mixing and reaction
temperature of the solutions L1, L2 within the mixer 52 to the
range of -5.degree. C. to 30.degree. C. is supplied from the
heating medium supply device to the jacket 51, 69, 71, 81, 83 and
circulated again to the heating medium supply device. It is
preferred that the mixing and reaction temperature be appropriately
set within the range of -5.degree. C. to 30.degree. C. depending on
the kinds of the solutions L1, L2 etc. The more preferred
temperature range of the mixing and reaction temperature is
0.degree. C. to 25.degree. C. and the especially preferred
temperature range thereof is 5.degree. C. to 25.degree. C.
[0130] According to the mixing and reaction device 50 configured as
described above, the solution L1 and the solution L2, which have
been prepared in the first preparation tank 64 and the second
preparation tank 66, respectively, under appropriate mixing and
reaction temperature conditions, with the rotary drum 54 kept
rotated at an ultrahigh speed so that the peripheral speed at the
leading end of the rotary drum 54 (in a position of the
circumferential surface at the leading end), are supplied by the
solution supply pumps 76, 78 to the inside of the microgap 62 of
the mixer 52. While being subjected to a uniform shearing force by
the circumferential surface of the inner wall of the fixed mixer 52
and the outer circumferential surface of the rotary drum which is
rotating, the solutions L1, 12 fed into the microgap 62 move in a
spiral form within the microgap 62 from the bottom end to the top
end of the mixer 52 and reaches the discharge port 88 and
discharged from the discharge tube 90. As a result of this, it is
possible to ensure that the solutions L1, L2 are instantaneously
and efficiently mixed together and caused to react with each other
in the microgap 62, an alloy particle which has a micro size and is
excellent in monodispersibility is formed. As a result of this, it
is possible to prepare an alloy particle which has a particle size
range of 1 to 100 nm, whose coefficient of variation in the
particle size is not more than 15%, and which is excellent in the
self-assembling property. In this case, if the above-described
distance D.sub.2 is less than 1.001 and too short, the shearing
force becomes nonuniform due to the effect of the surface roughness
of the circumferential surface of the inner wall of the mixer 52
and the outer circumferential surface of the rotary drum 54, with
the result that the monodispersibility of a prepared alloy particle
becomes apt to worsen. If the distance D.sub.2 exceeds 1.200 and is
too large, the shearing force contributing to the solutions L1, L2
decreases, with the result that the size of a prepared alloy
particle becomes large and that at the same time the
monodispersibility also become apt to worsen.
[0131] FIG. 4 is an example of a modification of a mixing and
reaction device 100 for carrying out the microgap mixing method.
Descriptions are given by assigning like reference numerals to the
same members as in the mixing and reaction device 50 of FIG. 2.
[0132] As shown in FIG. 4, the mixing and reaction device 100 is
constructed in such a manner that the diameter of an inner wall of
a mixer 52 with respect to the outside diameter of a rotary drum 54
decreases from the bottom end to the top end of the mixer 52 and a
microgap 62 becomes narrow from the bottom end to the top end of
the mixer 52. According to this mixing and reaction device 100, the
flow velocity of the solutions L1, L2 supplied into the microgap 62
becomes high as solutions L1, L2 move from the bottom end to the
top end of the mixer 52, with the result that it is possible to
gradually increase the shearing force given to the solutions L1,
L2. As a result of this, it is possible to obtain an alloy particle
which has a micro size, is excellent in monodispersibility and
provides an especially excellent self-assembling property. In this
case, the distance D.sub.1 from the center of rotation S of the
rotary drum 54 to the leading end thereof and the distance D.sub.2
to the inner wall of the mixer 52 having the shortest distance from
the center of rotation S of the rotary drum 54 are the sizes in the
top end position of the mixer 52, as shown in FIG. 4.
[0133] FIG. 5 is another example of a modification of a mixing and
reaction device 110 for carrying out the microgap mixing method.
Descriptions are given by assigning like reference numerals to the
same members as in the mixing and reaction device 50 of FIG. 2.
[0134] As shown in FIG. 5, in the mixing and reaction device 110,
in place of the rotary drum 54 a plurality of flat disks 92, 92 . .
. are radially arranged in prescribed locations at a prescribed gap
from the rotary shaft 56. In this case, it is preferred that the
distance between the flat disks 92 be equal to the microgap 62
formed between the outer circumferential surface of the flat disk
92 and the circumferential surface of the inner wall of the mixer
52. Also in the mixing and reaction device 110 of FIG. 5, it is
possible to obtain an alloy particle which has a micro size, is
excellent in monodispersibility and provides an especially
excellent self-assembling property.
[0135] Incidentally, the mixing and reaction device for carrying
out the microgap mixing method is not limited to those of FIG. 2,
FIG. 4 and FIG. 5. It is possible to use any mixing and reaction
device which has the microgap 62 formed between the inner wall of
the mixer 52 and the stirring member 54, 92 which rotates at a high
speed, and in which by using a device in which when the distance
from the center of rotation S of the stirring member 54, 92 to the
leading end thereof is put as 1, the distance to the inner wall
having the shortest distance from the center of rotation S of the
stirring member 54, 92 is set in the range of 1.001 to 1.200, the
solutions L1, L2 are supplied to the microgap 62 and mixed together
and caused to react with each other and at the same time the mixed
reaction solution LM is discharged from the microgap 62.
[0136] [3] In-Tank Mixer Method
[0137] FIG. 6 and FIG. 7 are each a sectional view of a mixing and
reaction device 120 suitable for carrying out the in-tank mixer
method.
[0138] As shown in these figures, the mixing and reaction device
120 is constructed in such a manner that a mixer 126 whose top end
portion is open and a circular opening 124 for circulation is
formed in the bottom end portion is provided within a reaction
vessel 122 filled with a bulk liquid and the interior of the mixer
126 is also filled with a bulk liquid. The above-described
nonaqueous organic solvent containing a surfactant is used as the
bulk liquid. A jacket 123 through which a heating medium having a
relatively large heat capacity such as water and oil flows is wound
around the outer circumference of the reaction vessel 122, and the
jacket 123 is connected to a heating medium supply device, which is
not shown in the drawings. A heating medium at a temperature which
can control the mixing and reaction temperature of the solutions
L1, L2 within the reaction vessel 122 to the range of -5.degree. C.
to 30.degree. C. is supplied from the heating medium supply device
to the jacket 123 and circulated again to the heating medium supply
device. It is preferred that the mixing and reaction temperature be
appropriately set within the range of -5.degree. C. to 30.degree.
C. depending on the kinds of the solutions L1, L2 etc. The more
preferred temperature range of the mixing and reaction temperature
is 0.degree. C. to 25.degree. C. and the especially preferred
temperature range thereof is 5.degree. C. to 25.degree. C.
[0139] A pair of supply pipes 128, 130 for adding the solutions L1,
L2, which has valves 125, 127, is provided in an extended condition
so that the pair of supply pipes 128, 130 passes through the
reaction vessel 122 from the outside of the reaction vessel 122,
enters drilled passages formed in a bottom plate of the mixer 126
and reaches the edge of an opening for circulation 124. The
addition ports of the supply pipes 128, 130 of the solutions L1, l2
are disposed in the opening for circulation 124 in directions which
are opposite to each other. In the vicinity of the opening for
circulation 124 within the mixer 126, tow-tired upper and lower
stirring vanes 134, 134 which are supported by a rotary shaft 132
are provided, and the rotary shaft 132 is rotated by a motor 138.
Among these two stirring vanes 134, 136, the lower stirring vane
136 is formed so as to be able to cause rapid mixing and reaction
of the solutions L1, L2. Therefore, it is preferred that the
peripheral speed at the leading end of the lower stirring vane 136
be not less than 5 m/second. On the other hand, the upper stirring
vane 134 is formed so as to be able to generate a circulating
current which flows from the open top end portion of the mixer 126
to the reaction vessel 122 and returns from the opening for
circulation 124 to the mixer 126. The circulating flow rate of the
circulating current generated by the upper stirring vane 134 is
designed so that it becomes not less than 500 l/minute in the
position of the opening for circulation 124. An increase in the
circulating flow rate can be achieved by increasing the vane
diameter of the upper stirring vane 134, increasing the opening
diameter of the opening for circulation 124, etc.
[0140] The added flow rates of the solutions L1, l2 added from the
supply pipes 128, 130 are designed so that not less than 20
l/minutes, preferably not less than 30 l/minutes, and especially
preferably not less than 40 l/minutes can be controlled with a good
accuracy. An increase in the added flow rate can be achieved by
increasing the pipe diameter of the supply pipes 128, 130 and by
generating the above-described circulating current of a large flow
velocity near the opening for circulation 124 to which the
solutions L1, L2 are added thereby to generate a suction force near
the addition ports of the supply pipes 128, 130. Furthermore, an
increase in the control accuracy of the added flow rate can be
achieved by installing a flow regulating valve 140 of the
construction shown in FIG. 8 in each of the supply pipes 128,
130.
[0141] As shown in FIG. 8, the valve main body 142 of the flow
regulating valve 140 is constituted by a valve casing 144 and a
valve plate 146, the interior of the valve main body 142 is
provided with a valve chest 150 having an inlet chamber 148, and
the solution L1 or the solution L2 flows in from an inlet port 152.
The valve chest 150 is provided with an outlet port 154 of openings
154a, 154b in the direction orthogonal to the outflow direction of
the fluid. The outlet port 154 is provided with a valve rod 156
which is moved in a sliding manner using a motor (not shown) as the
drive source. The opening area of the outlet port 154 which is
exposed to the valve chamber 150 increases or decreases
proportionally according to the sliding movement distance of this
valve rod 156, and the solution L1 or the solution L2 which has
flown into the inlet chamber 148 of the valve chest 150 is caused
to flow out of the outlet port 154 at a flow rate proportional to
the opening area. This flow regulating valve 140 provides a good
linearity between the valve opening and the flow rate and can
perform flow rate control with good accuracy in a wide flow rate
range.
[0142] According to the mixing and reaction device 120 configured
as described above, the solutions L1, L2 added from the supply
pipes 128, 130 flow into the mixer under appropriate mixing and
reaction temperature conditions 126 while being diluted by the bulk
liquid circulated by the opening for circulation 124 and are mixed
together and caused to react with each other within the mixer 126.
And the mixed reaction solution LM is discharged by the circulating
bulk liquid from the mixer into the reaction vessel 122. In this
manner, by performing mixing and reaction with the solutions L1, L2
and the mixed reaction solution LM kept diluted with the bulk
liquid, the opportunity of contact of generated alloy particles is
reduced and, therefore, the growth of particles can be suppressed.
Furthermore, by ensuring a circulating flow rate of the bulk liquid
of not less than 500 l/minutes in the position of the opening for
circulation 124, the solutions L1, L2 added from the supply pipes
128, 130 can be immediately diluted with the bulk liquid and by
ensuring an added flow rate of the solutions L1, l2 of not less
than 20 l/minute, the reaction can be completed in a short time,
with the result that an alloy particle which has a micro size and
is excellent in monodispersibility is formed. As a result of this,
it is possible to prepare an alloy particle which has a particle
size range of 1 to 100 nm, whose coefficient of variation in the
particle size is not more than 15%, and which is excellent in the
self-assembling property.
[0143] Incidentally, the mixing and reaction device 120 for
carrying out the in-tank mixer method is not limited to the
above-described configurations. It is possible to use any mixing
and reaction device in which, by using a device in which there is
provided the reaction vessel 122 which is filled with a bulk liquid
and provided therein with the mixer 126 which has the stirring vane
136 rotating at a high speed and is provided with the opening 124
through which the bulk liquid is circulated to and from the
interior of the reaction vessel 122, the solutions L1, L2 for
preparing the alloy particle are supplied to the interior of the
mixer 126, where the solutions are mixed together and caused to
react with each other by a liquid phase process, and at the same
time the mixed reaction solution LM can is discharged from the
mixer 126 to the reaction vessel 122 by a circulating stream of the
bulk liquid.
[0144] By mixing the solution L1 and the solution L2 by use of a
mixing and reaction device for carrying out the [1] ultrahigh-speed
stirring method, [2] microgap mixing method and [3] in-tank mixer
method described above, it is possible to prepare an alloy particle
which meets all of the requirements for a micro size,
monodispersibility and transformation facilitativeness in the alloy
particle preparation step.
[0145] It is preferred that the mixing and reaction temperature of
the reduction reaction by the above-described mixing be a constant
temperature in the range of -5.degree. C. to 30.degree. C. If the
mixing and reaction temperature is less than -5.degree. C., this
poses the problem that a water phase condenses, making the
reduction reaction nonuniform. If the mixing and reaction
temperature exceeds 30.degree. C., coalescence and precipitation
tend to occur and the system may sometimes become unstable. The
reduction temperature is preferably in the range of 0.degree. C. to
25.degree. C. and more preferably in the range of 5.degree. C. to
25.degree. C. "A constant temperature" here device that when a set
temperature is T (.degree. C.), this T is in the range of T
.+-.3.degree. C. Incidentally, also in this case, the upper limit
and lower limit to this T are the above-described -5.degree. C. to
30.degree. C. The reduction reaction time, which must be
appropriately selected according to the reversed micelle capacity
etc., is preferably 1 to 30 minutes and more preferably 5 to 20
minutes.
[0146] In the above-described reduction step, it is preferred that
to at lest either of the solutions L1 and L2, at least one kind of
dispersant which contains 1 to 3 amino groups or carboxyl groups be
added in an amount of 0.001 to 10 moles per mole of alloy particle
to be prepared. By adding such a dispersant, it is possible to
obtain an alloy particle which is more monodispersible and free
from coalescence. When the amount of added dispersant is less than
0.001 mole, it may be sometimes impossible to further improve the
monodispersibility of an alloy particle. When the amount of added
dispersant exceeds 10 mole, coalescence may sometimes occur.
[0147] An organic compound having a group which is adsorbed on the
surface of an alloy particle is desirable as a dispersant.
Concretely, dispersants which have 1 to 3 amino groups, carboxyl
groups, sulfonate groups or sulfinate groups are preferred and
these may be used singly or in combination.
[0148] These compounds have the following structural formulas:
R--NH.sub.2, NH.sub.2--R--NH.sub.2, NH.sub.2--R
(NH.sub.2)--NH.sub.2, R--COOH, COOH--R--COOH, COOH--R(COOH)--COOH,
R--SO.sub.3H, SO.sub.3H--R--SO.sub.3H,
SO.sub.3H--R(SO.sub.3H)--SO.sub.3H, R--SO.sub.2H,
SO.sub.2H--R--SO.sub.2H, SO.sub.2H--R (SO.sub.2H)--SO.sub.2H. The R
in the formulas each denotes a linear, branched or cyclic saturated
or unsaturated hydrocarbon.
[0149] A specially desirable compound as a dispersant is oleic
acid. Oleic acid, which is a surfactant which is widely known in
the stabilization of colloids, has been used in protecting metallic
particles of iron etc. The relatively long chain of oleic acid
provides a cubic obstacle important for counteracting the strong
magnetic interaction between particles (for example, oleic acid has
18 carbon chains and its length is up to 20 angstroms (up to 2 nm).
Oleic acid is not a fatty acid and has one double bond).
[0150] As with oleic acid, similar long-chain carboxylic acids,
such as aerucic acid and linoleic acid, are also used (for example,
long-chain organic acids having 8 to 20 carbon atoms can be used
singly or in combination). Oleic acid (for example, olive oil) is
desirable because it is an inexpensive natural resource capable of
being easily obtained. Furthermore, oleylamine derived from oleic
acid is also a dispersant which is as useful as oleic acid.
[0151] It might be thought that in the reduction step as described
above, metals which are base in terms of redox potentials (metals
of not more than -0.2 V (vs. NHE) or so), such as Co, Fe, Ni and
Cr, are reduced in a CuAu type or Cu.sub.3Au type hard magnetic
ordered alloy phase and precipitate in a state of micro size and
monodispersion. It might be thought that after that, in the
temperature rise step and the aging step which will be described
later, with the precipitated base metals serving as nuclei, metals
which are noble in terms of redox potentials (metals of not less
than -0.2 V (vs. NHE) or so), such as Pt, Pd and Rh, are reduced on
the surface, displaced and precipitate by the base metal. It might
be thought that the ionized base metals are reduced again by a
reductant and precipitate. An alloy particle capable of forming
CuAu type or Cu.sub.3Au type hard magnetic ordered alloy is
obtained by repetitions of such reactions.
[0152] Next, a description will be given of the aging step which
raises, after the completion of the reduction step, the mixing and
reaction solution to an aging temperature higher than the mixing
and reaction temperature of -5.degree. C. to 30.degree. C. in the
reduction step as described above.
[0153] (Aging Step)
[0154] It is preferred that the aging temperature be a constant
temperature in the range of 30 to 90.degree. C. and, this
temperature should be higher than the temperature of reduction
reaction. It is preferred that the aging time be 5 to 180 minutes.
If the aging temperature and aging time shift to the high
temperature and long time side from above range, coalescence or
precipitation is apt to occur. Conversely, if the aging temperature
and aging time shift to the low temperature and short time side,
the reaction may not sometimes complete, resulting in a change in
the composition. The aging temperature and aging time are
preferably 40 to 80.degree. C. and 10 to 150 minutes and more
preferably 40 to 70.degree. C. and 20 to 120 minutes.
[0155] "A constant temperature" here is synonymous with the case of
the temperature of reduction reaction (however, in this case, "the
reduction temperature" becomes "the aging temperature"). In
particular, however, "a constant temperature" is preferably higher
than the temperature of reduction reaction by not less than
5.degree. C. within the above-described range of the aging
temperature (30 to 90.degree. C.) and more preferably higher than
the temperature of reduction reaction by not less than 10.degree.
C. In the case of less than 5.degree. C., a prescribed composition
may not sometimes be obtained.
[0156] In the aging step as described above, noble metals
precipitate on the base metals which were reduced and precipitated
in the reduction step. That is, because the reduction of noble
metals occurs only on base metals and base metals and noble metals
dot not separately precipitate, it is possible to prepare an alloy
particle capable of efficiently forming a CuAu type or Cu.sub.3Au
type hard magnetic ordered alloy at a high yield and according to a
prescribed composition and it is possible to control the alloy
particle to a desired composition. Furthermore, by appropriately
adjusting the temperature and the stirring rate of during aging, it
is possible to obtain a desired particle diameter of the obtained
alloy particle.
[0157] It is desirable to provide the cleaning and dispersion step
of cleaning the solution after aging with a mixed solution of water
and first class alcohol, then performing precipitation treatment
with first class alcohol thereby to generate precipitates, and
dispersing the precipitates with an organic solvent.
[0158] Impurities are removed by providing this cleaning and
dispersion step and it becomes possible to further improve
applicability when the magnetic layer of a magnetic recording
medium is formed by application. Cleaning and dispersion are each
performed at least once and preferably each twice or more.
[0159] Although first class alcohol used in cleaning is not
especially limited, methanol, ethanol, etc. are desirable. The
volume mixing ratio (water/first class alcohol) is preferably in
the range of 10/1 to 2/1 and more preferably in the range of 5/1 to
3/1. If the ratio of water is high, the surfactant may not
sometimes be easily removed. Conversely, if the ratio of first
class alcohol is high, coalescence may sometimes occur.
[0160] Alloy particles dispersed in a solution (an
alloy-particle-containi- ng solution) are obtained in a manner as
described above.
[0161] The alloy articles are monodispersed. Therefore, even when
the alloy articles are applied to a backing, these particles do not
coalesce together and can be kept in a uniformly dispersed state.
Therefore, even when annealing treatment is performed, the
respective alloy particles do not coalesce together and hence it is
possible to ensure efficient hard magnetizing, resulting in
excellent applicability. Furthermore, the alloy particle is
excellent in the self-assembling property because it is prepared by
the above-described high-pressure mixing methods, and annealing
treatment enables the alloy particle to undergo transformation from
an unordered phase to an ordered phase easily and positively. This
permits efficient hard magnetizing.
[0162] From the standpoint of lowering the noise level, it is
desirable that the particle size of an alloy particle before
oxidation treatment, which will be described later, be small.
However, if the particle size is too small, supermagnetism occurs
after annealing and the particle may sometimes become unsuitable
for magnetic recording. In general, the particle size is preferably
in the range of 1 to 100 nm, more preferably in the range of 3 to
20 nm, and most preferably in the range of 3 to 10 nm.
[0163] (Reduction Process)
[0164] A general reduction process for preparing alloy particles is
described here.
[0165] Although there are various methods of preparing an alloy
particle capable of forming a CuAu type or Cu.sub.3Au type hard
magnetic ordered alloy in the reduction process, it is desirable to
apply a method of reducing at least a metal which is base in terms
of redox potential (hereinafter may sometimes be referred to simply
as "a base metal") and a metal which is noble in terms of redox
potential (hereinafter may sometimes be referred to simply as "a
noble metal") by use of a reductant etc. in an organic solvent or
water or a mixed solution of an organic solvent and water. The
order of reduction of the base metal and noble metal is not
especially limited and the base metal and noble metal may be
simultaneously reduced.
[0166] Alcohols, polyalcohols, etc. can be used as the
above-described organic agent. Methanol, ethanol, butanol, etc. can
be enumerated as alcohols and ethylene glycerin, glycerol, etc. can
be enumerated as polyalcohols.
[0167] Incidentally, examples of a CuAu type or Cu.sub.3Au type
ferromagnetic ordered alloy are the same as in the case of the
above-described reversed micelle process.
[0168] The methods described in the paragraphs 18 to 30 etc. of the
Japanese Patent Application No. 2001-269255 can be applied as a
method of preparing an alloy particle by precipitating a noble
metal beforehand.
[0169] Pt, Pd, Rh, etc. can be advantageously used as metals which
are noble in terms of redox potential and H.sub.2
PtCl.sub.6.6H.sub.2O, Pt(CH.sub.3COCHCOCH.sub.3).sub.2,
RhCl.sub.3.3H.sub.2O, Pd (OCOCH.sub.3).sub.2, PdCl.sub.2, Pd
(CH.sub.3 COCHCOCH.sub.3).sub.2, etc. can be used by being
dissolving in a solvent. The concentration of a metal in the
solution is preferably in the range of 0.1 to 1000 .mu.mol/ml and
more preferably in the range of 0.1 to 100 .mu.mol/ml.
[0170] Co, Fe, Ni and Cr can be favorably used as metals which are
base in terms of redox potential and Fe and Co are especially
preferably used. Au such metals, FeSO.sub.4. 7H.sub.2O,
NiSO.sub.4.7H.sub.2O, CoCl.sub.2.6H.sub.2O,
Co(OCOCH.sub.3).sub.2.4H.sub.2O, etc. can be used by being
dissolved in a solvent. The concentration of a metal in the
solution is preferably in the range of 0.1 to 1000 .mu.mol/ml and
more preferably in the range of 0.1 to 100 .mu.mol/ml.
[0171] As with the above-described reversed micelle process, it is
desirable to lower the temperature of transformation to a hard
magnetic ordered alloy by adding a third element to a binary alloy.
The amount of an added metal is the same as with the reversed
micelle process.
[0172] For example, in a case where a base metal and a noble metal
are reduced in this order and caused to precipitate, it is
desirable to first reduce the base metal or the base metal and part
of the noble metal by use of a reductant having a more base
reduction potential than -0.2 V (vs. NHE), add the reduced metals
to the source of the noble metal, then perform reduction by use of
a reductant which is nobler in terms of redox potential than -0.2 V
(vs. NHE) and perform reduction by use of a reductant having a
reduction potential which is more base than -0.2 V (vs. NHE).
[0173] Redox potentials depend on the pH of the system. Alcohols,
such as 1,2-hexadecandiol, glycerins, H.sub.2 and HCHO are
advantageously used as reductants which are nobler than -0.2 V (vs.
NHE).
[0174] As reductants which are more base than -0.2 V (vs. NHE),
S.sub.2O.sub.6.sup.2-, H.sub.2PO.sub.2-, BH.sub.4-,
N.sub.2H.sub.5+, H.sub.2PO.sub.3- can be advantageously used.
[0175] Incidentally, when a zero-valent metallic compound, such as
Fe carbonyl, is used as the material for a base metal, it is
unnecessary to use a reductant for the base metal
[0176] By causing an adsorbent to be present during the reduction
and precipitation of a noble metal, it is possible to prepare an
alloy particle in a stable manner. It is desirable to use a polymer
and a surfactant as an adsorbent.
[0177] Polyvinyl alcohol (PVA), poly-N-vinyl-2-pyrrolidone (PVP),
gelatin, etc. can be enumerated as the above-described polymer.
Among others, PVP is particularly preferable.
[0178] The molecular weight is preferably in the range of 20000 to
60000 and more preferably in the range of 30000 to 50000. The
quantity of a polymer is preferably in the range of 0.1 to 10 times
the mass of a generated alloy particle and more preferably in the
range of 0.1 to 5 times.
[0179] It is preferred that a surfactant which is advantageously
used as an adsorbent contain "an organic stabilizer" which is a
long-chain organic compound expressed by the general formula R--X.
The R in the above general formula denotes "a tail group" which is
a straight-chain or branched hydrocarbon or fluorocarbon chain and
usually contains 8 to 22 carbon atoms. The X in the above general
equation denotes "a head group" which is a portion (X) that
supplies a specific chemical bond to the alloy particle surface,
and it is preferred that X be any one of sulfinate (--SOOH),
sulfonate (--SO.sub.2OH), phosfinate (--POOH), phosfonate
(--OPO(OH).sub.2), carboxylate and thiol.
[0180] It is preferred that the above-described organic stabilizer
be any one of sulfonic acid (R--SO.sub.2OH), sulfinic acid
(R--SOOH), phosphinic acid (R.sub.2POOH), phosphonic acid
(R--OPO(OH).sub.2), carboxylic acid (R--COOH), thiol (R--SH), etc.
As with the reversed micelle process, however, the use of oleic
acid is especially preferred than any other above-enumerated
substances.
[0181] Combinations of the above-described phosphine and an organic
stabilizer (triorganophosphine/acid etc.) can provide excellent
controllability for the growth and stabilization of particles.
Although didecyl ether and didodecyl ether can also be used, phenyl
ether or n-octyl ether is advantageously used as a solvent for its
low cost and high boiling point.
[0182] However, a general reduction process for preparing alloy
particles is performed at high temperatures compared to the mixing
and reaction temperature in the range of -5.degree. C. to
30.degree. C. in the mixing and reaction using the high-speed
stirring methods as in the present invention. That is, it is
general practice to cause the reaction to occur at a temperature in
the range of 80.degree. C. to 360.degree. C. owing to necessary
alloy particles and the boiling temperature of a solvent and the
temperature range of 80.degree. C. to 240.degree. C. is more
preferred. In the case of a general reduction process, particles
may sometimes not grow when the reaction temperature is lower than
this temperature range. On the other hand, if the temperature is
higher than this range, particles grow without being controlled and
the generation of undesirable by-products may sometimes
increase.
[0183] As with the reversed micelle process, the particle diameter
of an alloy particle is preferably in the range of 1 to 100 nm,
more preferably in the range of 3 to 20 nm, and further more
preferably in the range of 3 to 10 nm, in the same manner as in the
present invention.
[0184] The seed crystal process is effective as a method of
increasing the particle size (particle diameter). In order to use
alloy particles as a magnetic recording medium, filling alloy
particles at fine densities is preferable for increasing the
recording capacity and, for this purpose, the standard deviation of
an alloy particle size is preferably below 10% and more preferably
not more than 5%. In the present invention, the size of an alloy
particle is specified by the coefficient of variation, and the
coefficient of variation is not more than 15% and preferably not
more than 10%.
[0185] If the particle size is too small, superparamagnetism occurs
and this is undesirable. Therefore, in order to increase the
particle size, it is desirable to adopt the seed crystal process as
described above. On this occasion, there is a case where a metal
which nobler than the metal which constitutes particles is caused
to precipitate. Because the oxidation of particles is feared in
this case, it is desirable that the particles be subjected to
hydrogenation treatment beforehand.
[0186] Although it is desirable that the outermost layer of an
alloy particle be constituted by a noble metal from the standpoint
of the prevention of oxidation, such a noble metal is apt to
coalesce and hence in the present invention, it is desirable that
the outermost layer of an alloy particle be constituted by an alloy
of a noble metal and a base metal. According to the liquid phase
process as described above, such a constitution can be easily and
efficiently realized.
[0187] Removing salts from the solution after the preparation of
alloy particles is desirable from the standpoint of improving the
dispersion stability of alloy particles. In removing salts, there
is a method by which alcohol is excessively added thereby to cause
slight coalescence and the salts are removed together with a
supernatant by causing natural sedimentation or centrifugal
sedimentation. Because coalescence is apt to occur in this method,
it is desirable to adopt the ultrafiltration method.
[0188] Alloy particles dispersed in a solution (an
alloy-particle-containi- ng solution) can be obtained as described
above.
[0189] A transmission electron microscope (TEM) can be used in
evaluating the particle size of an alloy particle. Although
electron diffraction by a TEM may be used in determining the
crystal system of an alloy particle or a magnetic particle, the use
of X-ray diffraction is desirable because of high accuracy. In the
composition analysis of the interior of an alloy particle or a
magnetic particle, it is desirable to perform an evaluation by
adding EDAX to FE-TEM capable of reducing the section of electron
beams. Furthermore, the evaluation of the magnetic properties of an
alloy particle or a magnetic particle can be performed by use of
VSM.
[0190] [Oxidation Treatment Step]
[0191] By subjecting a prepared alloy particle to oxidation
treatment, it is possible to efficiently manufacture a magnetic
particle having hard magnetism without raising the temperature
during annealing treatment, which is performed later in a
nonoxidizing atmosphere. It might be thought that this is due to
the phenomenon which will be described below.
[0192] That is, first, by oxidizing an alloy particle, oxygen
enters the crystal lattice of the alloy particle. When annealing
treatment is performed, with oxygen in the crystal lattice, the
oxygen is released from the crystal lattice due to heat. The
liberation of the oxygen causes a defect. Because the migration of
metallic atoms constituting the alloy becomes easy through this
defect, phase transformation becomes apt occur even at a relatively
low temperature. Therefore, by subjecting an alloy particle having
a good self-assembling property prepared by the above-described
high-pressure mixing method to oxidation treatment, it becomes more
easy to lower the temperature of annealing treatment.
[0193] This phenomenon can be supposed, for example, by measuring
the EXAFS (extended X-ray absorption fine structure) of an alloy
particle after oxidation treatment and an annealed magnetic
particle.
[0194] For example, in an Fe--Pt alloy particle not subjected to
oxidation treatment, the presence of an Fe atom and the bond
between a Pt atom and an Fe atom can be recognized.
[0195] In contrast to this, in an alloy particle subjected to
oxidation treatment, the presence of the bond between an Fe atom
and an oxygen atom can be recognized. However, the bond between a
Pt atom and an Fe atom is scarcely seen. This device that the
Fe--Pt bond and the Fe--Fe bond have been cut by oxygen atoms. It
might be thought that this is the reason why a Pt atom and an Fe
atom can migrate easily during annealing.
[0196] And after this alloy particle is subjected to annealing
treatment, the presence of oxygen cannot be recognized and the
presence of the bond to a Pt atom or an Fe atom can be recognized
around an Fe atom.
[0197] In view of the above phenomenon, it will be understood that
phase transformation does not proceed easily unless oxidation is
performed and that it becomes necessary to raise the temperature of
annealing treatment. However, it might also be though that if
excessive oxidation is performed, the interaction between a metal
which is easily oxidized, such as Fe, and oxygen becomes too
strong, with the result that a metal oxide is formed.
[0198] Therefore, it becomes important to control the oxidation
state of an alloy particle and for this purpose, it is necessary to
set optimum oxidation treatment conditions.
[0199] In oxidation treatment, for example, in a case where alloy
particles are prepared by the above-described liquid phase process,
it is necessary only that a gas containing at least oxygen be
supplied to an alloy-particle-containing solution after the
preparation of the alloy particles.
[0200] The partial pressure of oxygen in this case is preferably in
the range of 10 to 100% of the total pressure and more preferably
in the range of 15 to 50%. The temperature of oxidation treatment
is preferably in the range of 0 to 100.degree. C. and more
preferably in the range of 15 to 80.degree. C.
[0201] It is preferred that the oxidation state of alloy particles
be evaluated by EXAFS etc. and in consideration of that an Fe--Fe
bond and a Pi-Fe bond are cut by oxygen, the number of bonds
between base metals such as Fe and oxygen is preferably in the
range of 0.5 to 4 and more preferably in the range of 1 to 3.
[0202] [Annealing Treatment Step]
[0203] An alloy particle subjected to oxidation treatment forms an
unordered phase. As described above, hard magnetism cannot be
obtained from an unordered phase. In order to form an ordered
phase, therefore, it is necessary to perform heat treatment
(annealing). In this heat treatment, it is necessary to use
differential thermal analysis (DTA) to determine the transformation
temperature at which the alloys constituting an alloy particle
undergoes transformation between an ordered phase and an unordered
phase and perform the heat treatment at a temperature of not less
than this temperature.
[0204] Although the above-described transformation temperature is
usually about 500.degree. C., it is possible to use a temperature
below the usual temperature, because the self-assembling property
of the prepared alloy particle is improved by the mixing by the
high-pressure mixing methods in the above-described reduction step.
Therefore, the temperature of annealing treatment is preferably not
less than 100.degree. C. and more preferably in the range of 100 to
500.degree. C. Furthermore, the temperature may sometimes be fallen
by the addition of a third element.
[0205] When annealing treatment is performed in the state of a
particle, the migration of particles is apt to occur and
coalescence is apt to occur. For this reason, although a high
coercive force is obtained, this tends to pose the problem that the
particle size increases. From the standpoint of the prevention of
the coalescence of alloy particles, it is preferred that alloy
particles applied to a backing etc. be subjected to annealing
treatment.
[0206] Furthermore, by annealing alloy particles on a backing to
form magnetic particles, it is possible to use a layer constituted
by such magnetic particles to be a magnetic layer as a magnetic
recording medium.
[0207] Any backing, whether it is made of an inorganic substance or
an organic substance, may be used so long as it is a backing used
in a magnetic recording medium.
[0208] As the backings of an inorganic substance, Al, Mg-containing
alloys such as Al--Mg and Mg--Al-LMn, glass, quartz, carbon,
silicon, ceramics, etc. are used. These backings are excellent in
impact resistance and has rigidity suitable for thin thickness
design and high speed rotation. These backings have the
characteristic that of high heat resistance as organic
substance.
[0209] As the backings of an organic substance, polyesters such as
polyethylene telephthalate and polyethylene naphthalate,
polyolefins, cellulose triacetate, polycarnonate, polyamids
(including fatty polyamide and aromatic polyamides such as
aramide), polyimide, polyamide-imide, polysulfone,
polybenzooxazole, etc. can be used.
[0210] In applying alloy particles to a backing, it is necessary
only that various additives be added to the above-described
alloy-particle-containi- ng solution subjected to oxidation
treatment as required and then alloy particles be applied to the
backing.
[0211] It is desirable that the content of alloy particles in this
case be a required concentration (0.01 to 0.1 mg/ml).
[0212] Air doctor coating, blade coating, rod coating, extrusion
coating, air knife coating, squeeze coating, impregnation coating,
reverse roll coating, transfer roll coating, offset gravure
coating, kiss coating, cast coating, spray coating, spin coating,
etc. can be used as methods of application to a backing.
[0213] As an atmosphere during annealing treatment, a nonoxidizing
atmosphere of H.sub.2, N.sub.2, Ar, He, Ne, etc. should be used in
order to ensure that phase transformation proceeds efficiently
thereby to prevent the oxidation of alloys.
[0214] Particularly, from the standpoint of causing the oxygen to
remove present on lattices by oxidation treatment, it is desirable
to use a reducing atmosphere of methane, ethane, H.sub.2, etc.
Furthermore, from the standpoint of keeping the particle diameter,
it is desirable to perform annealing treatment in a magnetic field
in a reducing atmosphere. Incidentally, when an H.sub.2 atmosphere
is used, it is desirable that an inert gas be mixed from the
standpoint of explosion protection.
[0215] In order to prevent the coalescence of particles during
annealing, it is desirable first to perform annealing treatment at
a temperature of not more than the transformation temperature in an
inert gas thereby carbonize a dispersant and then to perform
annealing treatment at a temperature of not less than the
transformation temperature in a reducing atmosphere. At this time,
the most desirable mode is first to perform the above-described
annealing treatment at a temperature of not more than the
transformation temperature as required, then to apply an Si-base
resin etc. to a layer constituted by alloy particles, and lastly to
perform annealing treatment at a temperature of not less than the
transformation temperature.
[0216] By performing annealing treatment as described above, it is
ensured that alloy particles undergo phase transformation from an
unordered phase to an ordered phase and magnetic particles having
hard magnetism can be obtained.
[0217] The coercive force of a magnetic particle manufactured by
the above-described method of manufacturing a magnetic particle
according to the present invention is preferably in the range of
95.5 to 955 kA/m (1200 to 12000 Oe). In consideration of that when
this magnetic particle is applied to a magnetic recording medium,
so that a recording head can be compatible with the magnetic
recording medium, this coercive force is more preferably in the
range of 95.5 to 398 kA/m (1200 to 5000 Oe).
[0218] The particle size of the magnetic particle is preferably in
the range of 1 to 100 nm, more preferably in the range of 3 to 20
nm, and most preferably in the range of 3 to 10 nm.
[0219] <<Magnetic Recording Medium>>
[0220] A magnetic recording medium of the present invention
contains magnetic particles in its magnetic layer and the magnetic
particles are those manufactured by the above-described method of
manufacturing a magnetic particle of the present invention.
[0221] As the magnetic recording medium, magnetic tapes such as a
video tape and a computer tape, magnetic disks such as a floppy(R)
disk and a hard disk, etc. can be enumerated. As described above,
in a case where alloy particles (an alloy-particle-containing
solution) are applied to a backing and changed to magnetic
particles by performing annealing treatment, this layer constituted
by magnetic particles can be used as a magnetic layer. Furthermore,
in a case where alloy particles on a backing are not subjected to
annealing treatment and instead magnetic particles are formed by
performing annealing treatment in the state of a particle, the
magnetic particles are kneaded by use of an open kneader, a
three-roll mill, etc. and then finely dispersed by use of a sand
grinder etc. thereby prepare an application solution, and this
solution is applied to a backing by a publicly known method to form
a magnetic layer.
[0222] The thickness of a prepared magnetic layer, which depends on
the types of magnetic recording media to be applied, is preferably
in the range of 4 nm to 1 .mu.m and more preferably in the range of
4 nm to 100 nm.
[0223] A magnetic recording medium of the present invention may
contain other layers as required in addition to the magnetic layer.
For example, in the case of a disk, it is desirable to provide a
further magnetic layer and a nonmagnetic layer on the surface on
the side opposite to the magnetic layer. In the case of a tape, it
is desirable to provide a back layer on the surface of an insoluble
backing on the side opposite to the magnetic layer.
[0224] Furthermore, by forming a very thin protective film on the
magnetic layer thereby to improve wear resistance and in addition,
by applying a lubricant to this protective film thereby to improve
slip properties, it is possible to obtain a magnetic recoding
medium having sufficient reliability.
[0225] As the materials for the protective film, it is possible to
enumerate 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, carbons such as graphite and
amorphous carbide, etc. However, hard amorphous carbon generally
called diamond-like carbon is especially desirable.
[0226] A protective carbon film constituted by carbon, which is a
very thin film having sufficient wear resistance and does not
easily cause sticking to sliding members, is suitable as a material
for the protective film.
[0227] In hard disks, it is general practice to adopt the
sputtering method as a method of forming a protective carbon film.
In products which require continuous film forming as with a video
tape, however, many methods by which plasma CVD having a higher
film forming speed is used have been proposed. Therefore, it is
desirable to apply these methods.
[0228] It has been reported that in the plasma injection CVD
(PI-CVD) method among others, the film forming speed is very high
and that a good protective carbon film which is hard and has few
pinholes is obtained (for example, in Japanese Patent Application
Publication No. 61-130487, Japanese Patent Application Publication
No. 63-279426 and Japanese Patent Application Publication No.
3-113824).
[0229] The Vickers hardness of this protective carbon film is
preferably not less than 1000 kg/mm.sup.2 and more preferably not
less than 2000 kg/mm.sup.2. It is preferred that the crystal
structure of this protective carbon film be an amorphous structure
and that the protective carbon film be electrically
nonconductive.
[0230] In a case where a diamond-like carbon film is used as a
protective carbon film, this structure can be confirmed by a Raman
scattering spectroscopic analysis. That is, when a diamond-like
carbon film is measured, this structure can be confirmed by that a
peak is detected in the range of 1520 to 1560 cm.sup.-1. When the
structure of a carbon film deviates from a diamond-like structure,
the peak detected by a Raman scattering spectroscopic analysis
deviates from the above range and, at the same time, the hardness
as a protective film also decreases.
[0231] As the carbon materials for forming this protective carbon
film, it is desirable to use carbon-containing compounds, including
alkanes such as methane, ethane, propane and butane, alkenes such
as ethylene and propylene, and alkynes such as acetylene.
Furthermore, a carrier gas such as argon and an additive gas for
improving the film quality, such as hydrogen and nitrogen, can be
added as required.
[0232] If the film thickness of the protective carbon film is too
thick, this results in the worsening of the electromagnetic
transducing performance and a decrease in the adhesion to a
magnetic layer. If this film thickness is too thin, the
anti-grindability becomes insufficient. Therefore, the film
thickness is preferably in the range of 2.5 to 20 nm and more
preferably in the range of 5 to 10 nm.
[0233] Furthermore, in order to improve the adhesion of this
protective film to the magnetic layer, which provides a substrate,
it is desirable to improve the surface quality by etching the
surface of the magnetic layer beforehand by using an inert gas or
by exposing the surface to reactive gas plasmas of oxygen etc.
[0234] In order to improve the electromagnetic transducing
performance, the magnetic layer may be of a multi-layered structure
or have a publicly known nonmagnetic substrate layer or
intermediate layer under the magnetic layer. In order to improve
the travel endurance and corrosion resistance, it is desirable to
apply a lubricant or a rust preventive agent to the above-described
magnetic layer or protective film as described above. As a
lubricant to be added, it is possible to use publicly known
hydrocarbon-base lubricants, fluorine-base lubricants,
extreme-pressure additives, etc.
[0235] As the hydrocarbon-base lubricants, it is possible to
enumerate carboxylic acids such as stearic acid and oleic acid,
esters such as butyl stearate, sulfonates such as octadecyl
sulfonate, phosphate esters such as monooctadecyl phosphate,
alcohols such as stearyl alcohol and oleyl alcohol, amides
carboxylate such as amide stearate, amines such as stearylamine,
etc.
[0236] As the fluorine-base lubricants, it is possible to enumerate
lubricants in which part or all of the alkyl groups of the
above-described hydrocarbon-base lubricants are substituted with
fluoroalkyl bases or perfluoro polyether bases.
[0237] The perfluoro polyether bases are a perfluoro methlene oxide
polymer, a perfluoro ethylene oxide polymer, a
perfluoro-n-prolylene oxide polymer (CF.sub.2 CF.sub.2 CF.sub.2
O).sub.n, a perfluoro isopropylene oxide polymer
(CF(CF.sub.3)CF.sub.2O).sub.n or copolymers of these polymers
[0238] Furthermore, compounds in which polar functional groups,
such as a hydroxyl group, an ester group and a carboxyl group, are
present in the terminal or molecules of the alkyl groups of a
hydrocarbon-base lubricant, are effective in reducing the
frictional force and hence suitable.
[0239] The molecular weight of these substances is in the range of
500 to 5000 and preferably in the range of 1000 to 3000. When the
molecular weight is less than 500, the volatility may sometimes be
high and the lubricity may sometimes be low. When the molecular
weight exceeds 5000, the viscosity increases and a slider is apt to
be adsorbed by a disk, with the result that travel stops and head
crushes may sometimes become apt to occur.
[0240] This perfluoro polyether is commercially available under
brand names such as FOMBLIN of Aujimond and KRYTOX of DuPont.
[0241] As the extreme-pressure additives, it is possible to
enumerate esters phosphate such as trilauryl phosphate, esters
phosphite such as trilauryl phosphite, esters trithiophosphite such
as trilauryl trithiophosphite and esters thiophosphate, sulfur-base
extreme-pressure agents such as dibenzyl disulfide, etc.
[0242] The above-described lubricants are used singly or in
combination. In applying these lubricants to the magnetic layer or
the protective film, the lubricants are solved in an organic
solvent and applied by the wire bar method, the gravure coating
method, the spin coating method, the dip coating method, etc. or
the lubricants are caused to adhere by the vacuum evaporation
method.
[0243] As the rust preventive agents, it is possible to enumerate
nitrogen-containing heterocycles, such as benzotriazole,
benzoimidazole, purine and pyrimidine, derivatives obtained by
introducing alkyl side chains etc. into the mother nuclei of these
heterocycles, nitrogen- and Sulfur-containing heterocycles, such as
benzothiazole, 2-mercaptonbenzothiazole, tetrazainden cyclic
compounds and thiouracil compounds, and derivatives of these
heterocycles.
[0244] As described above, when the magnetic recording medium is a
magnetic tape etc., a back coat layer (a backing layer) may be
provided on the surface of the nonmagnetic backing where the
magnetic layer is not formed. The back coat layer is a layer which
is provided by applying a paint for forming a back coat layer,
which is obtained by dispersing granular components, such as an
abrasive material and an antistatic agent, and a binder in a
publicly known organic solvent, to the surface of the nonmagnetic
backing where the magnetic layer is not formed.
[0245] As the granular components, it is possible to use various
kinds of inorganic pigments and carbon black. As the binders,
resins such as cellulose nitrate, phenoxy resin, vinyl chloride
resin and polyurethane can be used singly or in combination.
[0246] Furthermore, a publicly known adhesive layer may be provided
on the surface to which the alloy-particle-containing solution is
applied and the surface on which the back coat is formed.
[0247] When the cut-off value is 0.25 mm, the centerline average
roughness of the surface of a magnetic recording medium thus
manufactured is preferably in the range of 0.1 to 5 nm and more
preferably in the range of 1 to 4 nm. This is because providing a
surface having an excellent smoothness is desirable for a magnetic
recording medium for high-density recording.
[0248] As a method of obtaining such a surface, it is possible to
mention a method which involves performing calendaring treatment
after the formation of the magnetic layer. Also, burnishing
treatment may be performed.
[0249] A magnetic recording medium thus obtained can be
appropriately punched by use of a punching machine or cut to a
desired size by use of a cutting machine so that it can be
used.
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