U.S. patent application number 10/234841 was filed with the patent office on 2003-03-27 for material coated with dispersion of ferromagnetic nanoparticles, and magnetic recording medium using the material.
This patent application is currently assigned to FUJI PHOTO FILM CO., LTD.. Invention is credited to Hattori, Yasushi, Masaki, Kouichi, Waki, Koukichi.
Application Number | 20030059604 10/234841 |
Document ID | / |
Family ID | 19095109 |
Filed Date | 2003-03-27 |
United States Patent
Application |
20030059604 |
Kind Code |
A1 |
Hattori, Yasushi ; et
al. |
March 27, 2003 |
Material coated with dispersion of ferromagnetic nanoparticles, and
magnetic recording medium using the material
Abstract
Ferromagnetic nanoparticles which are produced by reducing, in
the presence of a polymer, two or more metals having different
reduction potentials twice or more, using two or more reducing
agents having different reduction potentials, a material coated
with a dispersion of ferromagnetic nanoparticles in which the
ferromagnetic nanoparticles are dispersed, and a magnetic recording
medium which has a magnetic layer consisting of the material. The
ferromagnetic nanoparticles having a holding power Hc of 95.5 kA/m
or more, the material coated with the dispersion of ferromagnetic
nanoparticles having excellent industrial coatability, and the
magnetic recording medium using the dispersion are provided.
Inventors: |
Hattori, Yasushi; (Kanagawa,
JP) ; Masaki, Kouichi; (Kanagawa, JP) ; Waki,
Koukichi; (Kanagawa, JP) |
Correspondence
Address: |
SUGHRUE MION, PLLC
2100 Pennsylvania Avenue, NW
Washington
DC
20037-3213
US
|
Assignee: |
FUJI PHOTO FILM CO., LTD.
|
Family ID: |
19095109 |
Appl. No.: |
10/234841 |
Filed: |
September 5, 2002 |
Current U.S.
Class: |
428/329 ;
106/638; 428/843.3; G9B/5.252 |
Current CPC
Class: |
B32B 27/18 20130101;
Y10T 428/257 20150115; H01F 1/0063 20130101; G11B 5/706 20130101;
Y10T 428/25 20150115; H01F 10/123 20130101; Y10T 428/2982 20150115;
B82Y 25/00 20130101 |
Class at
Publication: |
428/329 ;
428/694.0BA; 106/638; 428/694.0MT |
International
Class: |
B32B 005/16 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 5, 2001 |
JP |
2001-269256 |
Claims
What is claimed is:
1. Ferromagnetic nanoparticles produced by reducing, in the
presence of a polymer, at least two metals having different
reduction potentials at least twice, using at least two reducing
agents having different reduction potentials.
2. The ferromagnetic nanoparticles of claim 1, wherein the at least
two of the metals are a noble metal and a poor metal, and the noble
metal is reduced before the poor metal.
3. The ferromagnetic nanoparticles of claim 2, wherein the noble
metal is reduced with a reducing agent having a reduction potential
higher than -0.2 V and the poor metal is reduced with a reducing
agent having a reduction potential lower than -0.2 V.
4. The ferromagnetic nanoparticles of claim 1, wherein at least one
of the metals is selected from the group consisting of Co, Fe, Ni,
Mn, Cr, Pr, Pt, Au, Ag, Ir, and Rh.
5. The ferromagnetic nanoparticles of claim 2, wherein at least one
of the metals is selected from the group consisting of Co, Fe, Ni,
Mn, Cr, Pr, Pt, Au, Ag, Ir, and Rh.
6. The ferromagnetic nanoparticles of claim 3, wherein at least one
of the metals is selected from the group consisting of Co, Fe, Ni,
Mn, Cr, Pr, Pt, Au, Ag, Ir, and Rh.
7. The ferromagnetic nanoparticles of claim 1, wherein the
ferromagnetic nanoparticles are ferromagnetic regular alloys of
CuAu or Cu.sub.3Au.
8. The ferromagnetic nanoparticles of claim 2, wherein the
ferromagnetic nanoparticles are ferromagnetic regular alloys of
CuAu or Cu.sub.3Au.
9. A material comprising a non-magnetic support coated with a
dispersion containing at least ferromagnetic nanoparticles and a
polymer, wherein the ferromagnetic particles are produced by
reducing, in the presence of a polymer, at least two metals having
different reduction potentials at least twice, using at least two
reducing agents having different reduction potentials.
10. The material of claim 9, wherein the at least two of the metals
are a noble metal and a poor metal, and the noble metal is reduced
before the poor metal.
11. The material of claim 10, wherein the noble metal is reduced
with a reducing agent having a reduction potential higher than -0.2
V and the poor metal is reduced with a reducing agent having a
reduction potential lower than -0.2 V.
12. The material of claim 9, wherein at least one of the metals is
selected from the group consisting of Co, Fe, Ni, Mn, Cr, Pr, Pt,
Au, Ag, Ir, and Rh.
13. The material of claim 9, wherein the ferromagnetic
nanoparticles are ferromagnetic regular alloys of CuAu or
Cu.sub.3Au.
14. The material of claim 9, wherein the material is formed by
coating the dispersion onto the support and then annealing the
coated support at a temperature that exceeds a transformation
temperature between regular phase and irregular phase.
15. A magnetic recording medium comprising a magnetic layer that
consists of a material comprising a non-magnetic support coated
with a dispersion containing at least ferromagnetic nanoparticles
and a polymer, wherein the ferromagnetic nanoparticles are produced
by reducing, in the presence of a polymer, at least two metals
having different reduction potentials at least twice, using at
least two reducing agents having different reduction
potentials.
16. The magnetic recording medium of claim 15, wherein at least two
of the metals are a noble metal and a poor metal, and the noble
metal is reduced before the poor metal.
17. The magnetic recording medium of claim 16, wherein the noble
metal is reduced with a reducing agent having a reduction potential
higher than -0.2 V and the poor metal is reduced with a reducing
agent having a reduction potential lower than -0.2 V.
18. The magnetic recording medium of claim 15, wherein at least one
of the metals is selected from the group consisting of Co, Fe, Ni,
Mn, Cr, Pr, Pt, Au, Ag, Ir, and Rh.
19. The magnetic recording medium of claim 15, wherein the
ferromagnetic nanoparticles are ferromagnetic regular alloys of
CuAu or Cu.sub.3Au.
20. The magnetic recording medium of claim 15, wherein the material
is formed by coating the dispersion onto the support and then
annealing the coated support at a temperature that exceeds a
transformation temperature between regular phase and irregular
phase.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to ferromagnetic nonoparticles
which are inexpensive and can be used in an MRAM and a magnetic
recording medium, a material coated with a dispersion containing
the ferromagnetic nanoparticles, and a magnetic recording medium
having a magnetic layer consisting of the material coated with the
dispersion containing the ferromagnetic nanoparticles.
[0003] 2. Description of the Related Art
[0004] Magnetic recording media widely used as recording tapes,
video tapes, computer tapes, disks, and the like are denser and
have shorter recording wavelengths every year. Increasing the S/N
ratio in such magnetic recording media is important. The smaller
the particle size of ferromagnetic particles, the lower the noise
when the total weight of the particles is the same. However, when
the particle size of iron particles, which are generally used as
ferromagnetic particles, decreases, a phenomenon
(superparamagnetism) where magnetization cannot be maintained is
caused due to heat fluctuation. Accordingly, there are limits to
reducing noise using the conventional iron particles.
[0005] In order to overcome this problem, use of ferromagnetic
nanoparticles having a particle size of 50 nm or less has been
proposed. As for a unitary system, ferromagnetic nanoparticles of
Co, Fe, and Ni are described in J. Appl. Phys., Vol. 80, No. 1, pp.
103-108, 1996, and ferromagnetic nanoparticles of Co are described
in J. Appl. Phys., Vol. 79, No. 8, Part 2A, pp. 5312-5314, 1996.
However, the ferromagnetic nanoparticles have a holding power Hc of
only 47.76 kA/m (600 Oe), and there ferromagnetic nanoparticles
having a holding power Hc of 95.5 kA/m (1200 Oe), which is required
for the magnetic recording media, have not been obtained.
[0006] Japanese Patent Application Laid-Open (JP-A) No. 2000-54012
discloses a method for forming ferromagnetic nanoparticles,
comprising the steps of: forming a solution of a metal precursor
from a transition metal; adding a coagulant to the solution so that
nanoparticles are separated from the metal precursor solution
without forming permanent agglomeration; and adding a solution of
hydrocarbon to the metal precursor solution so that the
nanoparticles are recombined or become colloids again.
[0007] Further, U.S. Pat. No. 6,162,532 discloses particles
containing a magnetic material selected from the group consisting
of elements Co, Fe, Ni, Sm, Nd, Pr, Pt, and Gd, intermetallic
compounds of the aforementioned elements, binary and ternary alloys
of the elements, an Fe oxide further containing at least one of the
elements other than Fe, barium ferrite, and strontium ferrite. U.S.
Pat. No. 6,162,532 and Science, Vol. 287, 1989 (2000) disclose a
method for forming particles by adding a solution of a metal
precursor to a surfactant solution.
[0008] Science, Vol. 287, 1989 (2000) also discloses a production
method of ferromagnetic FePt nanoparticles. A coated product of
ferromagnetic nanoparticles is obtained by sedimenting
nanoparticles, which are protected only by a surfactant, on the
surface of a support in a stationary manner. However, the
stationary sedimentation method takes time and is not preferable
from an industrial point of view.
[0009] Studies in Surface and Catalysis, 132, 243 (2001) suggest
synthesis of magnetic nanoparticles in a binder. However,
ferromagnetic nanoparticles having a holding power Hc of 95.5 kA/m
(1,200 Oe) or more, which are required for the magnetic recording
media, cannot be obtained by mere synthesis of magnetic
nanoparticles in a binder.
SUMMARY OF THE INVENTION
[0010] In view of the above-described conventional problems, the
present invention is intended to achieve the following object.
[0011] Namely, the object of the present invention is to provide
ferromagnetic nanoparticles having a holding power Hc of 95.5 KA/m
or more, a material coated with a dispersion of the ferromagnetic
nanoparticles excellent in industrial coatability, and a magnetic
recording medium using the same.
[0012] The above object can be achieved by the following means.
Namely, the present invention provides ferromagnetic nanoparticles
which are produced by reducing, in the presence of a polymer, at
least two types of metals having different reduction potentials at
least twice, using at least two reducing agents having different
reduction potentials.
[0013] An aspect of the present invention is ferromagnetic
nanoparticles in which at least two of the metals are a noble metal
and a poor metal, and the noble metal is reduced before the poor
metal.
[0014] Another aspect of the present invention is ferromagnetic
nanoparticles produced by reducing the noble metal with a reducing
agent having a reduction potential higher than -0.2 V, and reducing
the poor metal with a reducing agent having a reduction potential
lower than -0.2 V.
[0015] Still another aspect of the present invention is
ferromagnetic nanoparticles in which at least one of the metals is
selected from the group consisting of Co, Fe, Ni, Mn, Cr, Pr, Pt,
Au, Ag, Ir, and Rh.
[0016] Yet another aspect of the present invention is ferromagnetic
nanoparticles which are ferromagnetic regular alloys of either CuAu
or Cu.sub.3Au.
[0017] Further, the present invention provides a material
comprising a non-magnetic support that is coated with a dispersion
of ferromagnetic nanoparticles containing at least the above
ferromagnetic nanoparticles and a polymer.
[0018] The material is formed by coating the dispersion onto the
support and then annealing the coated support at a temperature that
exceeds a transformation temperature between regular phase and
irregular phase.
[0019] Furthermore, the present invention provides a magnetic
recording medium, which has a magnetic layer consisting of the
above material.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0020] Ferromagnetic nanoparticles, a material coated with a
dispersion of ferromagnetic nanoparticles, and a magnetic recording
medium of the present invention will be described below.
[0021] Ferromagnetic Nanoparticles
[0022] Ferromagnetic nanoparticles of the present invention are
produced by reducing, in the presence of a polymer, two or more
metals having different reduction potentials twice or more, using
two or more reducing agents having different reduction potentials.
The "ferromagnetic nanoparticles" described herein refer to
nanoparticles having ferromagnetic characteristics or nanoparticles
having an ability to gain ferromagnetic characteristics through
processes such as annealing.
[0023] A liquid phase method, a gaseous phase method, and a
mechanochemical synthesis method can be used to form the
ferromagnetic nanoparticles. However, in the present invention, the
ferromagnetic nanoparticles are preferably synthesized using the
liquid phase method, which is advantageous for mass production. In
the liquid phase method, an organic solvent, water, or a mixture of
an organic solvent and water may be used as a solvent. Preferable
examples of the solvent include water, alcohol, and polyalcohol.
Methanol, ethanol, butanol, or the like can be used as the alcohol,
and ethylene glycol or glycerine can be used as the
polyalcohol.
[0024] In the present invention, a polymer is used as an adsorbent
in the reduction and separation of the metals. The polymer serves
as a binder when a coated product is obtained, whereby a
nanoparticle-containing layer is formed regardless of stationary
sedimentation of nanoparticles. Examples of the polymer, which can
be preferably used in the present invention, include polyvinyl
alcohol (PVA), poly N-vinyl-2 pyrrolidone (PVP), gelatin, sodium
polyacrylate, polyacrylic acid amide, and the like. PVP is
particularly preferable. The polymer has a weight average molecular
weight of preferably 20,000 to 60,000, and more preferably 30,000
to 50,000. The polymer preferably weighs 0.1 to 10 times, and more
preferably 0.1 to 5 times as much as the ferromagnetic
nanoparticles.
[0025] At least one of the two or more metals is preferably
selected from the group consisting of Co, Fe, Ni, Mn, Cr, Pr, Pt,
Au, Ag, Ir, and Rh. Further, at least two of the two or more metals
are preferably a noble metal and a poor metal.
[0026] In the present invention, the poor metal may be separated
first, the noble metal may be separated first, or the poor metal
and the noble metal may be separated at the same time. A reducing
agent having a reduction potential lower than -0.2 V (vs. N. H. E)
is preferably used to reduce the poor metal and the noble metal at
the same time. When an alloy is reduced and separated in a liquid,
it is preferable in the present invention that the noble metal is
separated first, and then the poor metal is separated independently
or together with the noble metal so that a particle size becomes
uniform. In order to separate the poor metal, a reducing agent may
be used, or a compound having zero poor metal value may be added.
An example of the compound having zero poor metal value is iron
carbonyl. In order to separate the noble metal and the poor metal
in this order using reducing agents, it is preferable that a
reducing agent having a reduction potential lower than -0.2 V (vs.
N. H. E) is used after a reducing agent having a reduction
potential higher than -0.2 V (vs. N. H. E) has been used.
[0027] The reduction potential depends on the pH level of the
system. Preferable examples of the reducing agent having a
reduction potential higher than -0.2 V (vs. N. H. E) include
alcohols, H.sub.2, and HCHO. Preferable examples of the reducing
agent having a reduction potential lower than -0.2 V (vs. N. H. E)
include SO.sub.2O.sub.6.sup.2-, H.sub.2PO.sub.2.sup.-,
BH.sub.4.sup.-, N.sub.2H.sub.5.sup.+, H.sub.2PO.sub.3.sup.-, and
glycols (such as ethylene glycol).
[0028] Pt, Pd, Rh, or the like can be preferably used as the noble
metal. H.sub.2PtCl.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.3COCHCOCH.sub.3).sub.2, or the like can be dissolved in a
solvent and used. The concentration of the resulting solution is
preferably 0.1 to 1,000 .mu.mol/ml, and more preferably 0.1 to 100
.mu.mol/ml.
[0029] Co, Fe, Ni or Cr can be preferably used as the poor metal.
Fe and Co are particularly preferable. Such metals can be used by
dissolving 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, or the like
in a solvent. The concentration of the resulting solution is
preferably 0.1 to 1000 .mu.mol/ml, and more preferably 0.1 to 100
.mu.mol/ml.
[0030] In the present invention, a binary alloy is preferably a
ferromagnetic regular alloy of either CuAu or CU.sub.3Au. Examples
of the alloy preferably used as a ferromagnetic regular alloy of
CuAu include FeNi, FePd, FePt, CoPt, and the like. FePd, FePt, and
CoPt are particularly preferable.
[0031] Examples of the alloy preferably used as a ferromagnetic
regular alloy of Cu.sub.3Au include but are not limited to
Ni.sub.3Fe, FePd.sub.3, Fe.sub.3Pt, FePt.sub.3, CoPt.sub.3,
Ni.sub.3Pt, CrPt.sub.3, and Ni.sub.3Mn. FePd.sub.3, FePt.sub.3,
CoPt.sub.3, and Fe.sub.3Pt are particularly preferable.
[0032] A reduction reaction for separating the noble metal can be
carried out at a temperature of 80 to 360.degree. C., and more
preferably 80 to 240.degree. C., according to the required boiling
points of the ferromagnetic nanoparticles and solvent. When the
temperature is less than 80.degree. C., the particles do not grow.
When the temperature exceeds 360.degree. C., the particles grow
without being controlled, thereby increasing undesirable
by-products.
[0033] The temperature at which a reduction reaction for separating
the poor metal is carried out is not particularly limited and can
be room temperature. However, the temperature at which reduction is
carried out using a compound having zero poor metal value is the
same as the temperature at which the noble metal is separated.
[0034] The reaction is preferably carried out in an inert gas in
order to prevent oxidation of the ferromagnetic nanoparticles.
Preferable examples of the inert gas include He, Ar, N.sub.2, and
the like.
[0035] When the ferromagnetic nanoparticles produced from a
solution have an irregular phase, the ferromagnetic nanoparticles
are preferably annealed in order to obtain a regular phase. The
annealing temperature must exceed a transformation temperature
between the regular phase and the irregular phase, which
transformation temperature is determined using differential thermal
analysis (DTA). The annealing time is preferably 1 minute to 24
hours, and more preferably 1 to 30 minutes.
[0036] In order to enhance the phase transformation of the
ferromagnetic particles, it is effective to facilitate diffusion of
atoms by oxidizing the ferromagnetic particles before annealing and
introducing oxide deficiency by annealing the ferromagnetic
particles under a non-oxidative atmosphere, and particularly under
a reductive atmosphere.
[0037] Preferable examples of gases used for the non-oxidative
atmosphere condition include N.sub.2, Ar, He and Ne. Preferable
examples of gases used for the reductive atmosphere condition
include methane, ethane and H.sub.2. In view of preventing
explosions, mixtures of reductive atmospheric gases and
non-oxidative atmospheric gases are preferably used. Specifically,
a temperature for the phase transformation is 300 to 600.degree.
C., and preferably 300 to 500.degree. C.
[0038] In order to enhance the phase transformation of the
ferromagnetic particles, causing a stress-induced phase
transformation by pressure annealing is also effective.
Specifically, the pressure for the stress-induced phase
transformation is preferably 2 to 10 atmospheres and more
preferably 2 to 5 atmospheres, and a temperature for the
stress-induced phase transformation is preferably 150 to
450.degree. C. and more preferably 200 to 350.degree. C. The time
the annealing is maintained at a maximum temperature is preferably
1 to 60 minutes, and more preferably 1 to 30 minutes.
[0039] It is preferable to conduct the pressure annealing under a
reductive atmosphere in view of using the effects of both together.
However, filling reductive gases under high pressure and processing
under high temperature requires safety precautions, and thus is not
always industrially advantageous. Therefore, it is preferable to
introduce oxide deficiency by annealing the ferromagnetic particles
under a reductive atmosphere and then conduct pressure annealing
under a non-oxidative atmosphere in view of obtaining the effects
of both.
[0040] The holding power of the ferromagnetic nanoparticles is
preferably 95.5 to 398 kA/m (1,200 to 5,000 Oe). When the
ferromagnetic nanoparticles are applied to a magnetic recording
medium, the holding power of the ferromagnetic nanoparticles is
preferably 95.5 to 278.6 kA/m (1,200 to 3,500 Oe) in view of
compatibility with a recording head. The particle size of the
ferromagnetic nanoparticles is preferably 1 to 100 nm, more
preferably 3 to 20 nm, and most preferably 3 to 10 nm. Various
crystallization methods are effective for increasing the particle
size. In order to use the ferromagnetic nanoparticles for a
magnetic recording medium, closest possible packing of the
ferromagnetic nanoparticles is preferable in view of increasing
recording capacity. For this reason, the standard deviation in the
size of the ferromagnetic nanoparticles is preferably less than
10%, and more preferably 5% or less.
[0041] When the particle size is too small, the ferromagnetic
nanoparticles become superparamagnetism and thus undesirable.
Therefore, various crystallization methods arc preferably used to
increase the particle size. A metal nobler than a metal forming the
particles might be separated depending on the crystallization
method. In such a case, the particles are preferably hydrotreated
in advance to prevent oxidation of the particles.
[0042] Although outermost layers of the ferromagnetic nanoparticles
are preferably formed of a noble metal in view of preventing
oxidation, ferromagnetic nanoparticles having an outermost layer of
a noble metal easily agglomerate. Therefore, in the present
invention, the outermost layers of the ferromagnetic nanoparticles
are formed of an alloy of a noble metal and a poor metal.
[0043] Removing salts from the solution after synthesis of the
ferromagnetic nanoparticles is preferable in terms of improving
dispersion stability of the particles. There are desalting methods
in which an excessive amount of alcohol is added to the solution to
form light aggregates, which are sedimented naturally or
centrifugally, and salts are removed together with a supernatant
liquid. However, aggregates are easily formed in these methods.
Thus, ultrafiltration is preferably used in the present
invention.
[0044] A transmission electron microscope (TEM) can be used to
evaluate the particle size of the ferromagnetic nanoparticles
according to the present invention. Although electron diffraction
by the TEM can be used to determine a crystal system of the
ferromagnetic nanoparticles, X-ray diffraction provides higher
accuracy. The component analysis of an inside of the ferromagnetic
nanoparticles is preferably evaluated using an FE-TEM with an EDAX
attached thereto, which FE-TEM can narrow electron beams. A
magnetic property of the ferromagnetic nanoparticles can be
evaluated using a VSM.
[0045] The ferromagnetic nanoparticles of the present invention can
be preferably used for video tapes, computer tapes, recordable
floppy disks, and hard disks. Use of the ferromagnetic
nanoparticles for MRAMs is also preferable.
[0046] Material Coated with a Dispersion of ferromagnetic
Nanoparticles
[0047] A dispersion of ferromagnetic nanoparticles of the present
invention is formed by dispersing the above-described ferromagnetic
nanoparticles and polymer. In one embodiment, material of the
present invention comprises a non-magnetic support that is coated
with the dispersion. In another embodiment, the material of the
present invention is formed by coating the dispersion onto the
support and then annealing the coated support a temperature that
exceeds a transformation temperature between regular phase and
irregular phase The dispersion has high coatability because the
polymer serves as a binder, as described above. The polymer weighs,
as described above, preferably 0.1 to 10 times, and more preferably
0.1 to 5 times as much as the ferromagnetic nanoparticles. In any
embodiment of the present invention, the presence of the polymer
works to prevent adherence of the ferromagnetic particles and to
provide industrial coatability.
[0048] Magnetic Recording Medium
[0049] Hereinafter, description will be given of a method for
manufacturing a magnetic recording medium and the material coated
with the dispersion in which the ferromagnetic nanoparticles of the
present invention can be preferably used, and a magnetic recording
medium of the present invention will be described in detail via the
manufacturing method thereof. The magnetic recording medium of the
present invention includes a magnetic layer and, if necessary,
other layers, and the magnetic layer consists of the material
coated with the dispersion. Namely, the magnetic recording medium
of the present invention includes, on the surface of the base
(non-magnetic support), a magnetic layer containing ferromagnetic
nanoparticles and, if necessary, a non-magnetic layer disposed
between the magnetic layer and the base. If the magnetic recording
medium is a disk, a magnetic layer and, if necessary, a
non-magnetic layer can be provided on the rear surface of a support
as well. If the magnetic recording medium is a tape, a backing
layer is preferably provided on a surface of a support opposite
from a surface thereof on which a magnetic layer is provided.
[0050] In the magnetic recording medium of the present invention, a
non-magnetic layer comprising inorganic powder and a binder is
provided between the base and the magnetic layer, if necessary.
Examples of the inorganic powder used in the non-magnetic layer
include metal oxides, metal carbonates, metal sulfates, metal
nitrides, metal carbides, and metal sulfides. Among these, titanium
oxide or .alpha.-iron oxide is particularly preferable in view of
availability, cost, ease of control of particle size distribution,
and the like. It is preferable that the .alpha.-iron oxide is
needle-shaped or spindle-shaped, and has an average major axis
length of 0.05 to 0.3 .mu.m, a major axis length/a minor axis
length ratio of 3 to 10, and a pH level of 8 to 11. It is
preferable that the titanium oxide is spherical, and has a specific
surface area of 50 to 80 m.sup.2/g and a pH level of 8 to 11. A
particle size of the titanium oxide is preferably 0.01 to 0.1
.mu.m.
[0051] Polyurethane resin; polyester-based resin, polyamide-based
resin; vinyl chloride-based resin; acrylic-based resin in which
styrene, acrylonitrile, methyl methacrylate, and the like are
copolymerized; cellulose-based resin such as nitrocellulose; epoxy
resin; phenoxy resin; and polyvinyl alkylal resin, such as
polyvinylacetal or polyvinylbutyral can be used, alone or in
combination, as the binder for the non-magnetic layer. Polyurethane
resin, vinyl chloride-based resin, and acrylic-based resin are
preferable.
[0052] In order to improve dispersibility of the ferromagnetic
nanoparticles and the non-magnetic particles, the binder preferably
has a functional group (polar group), which adheres to the surfaces
of the particles. Preferable examples of the functional group
include --SO.sub.3M, --SO.sub.4M, --PO(OM).sub.2, --OPO(OM).sub.2,
--COOM, >NSO.sub.3M, >NRSO.sub.3M, --NR.sup.1R.sup.2, and
--N.sup.+R.sup.1R.sup.2R.sup.3X.sup.-, wherein M is hydrogen or an
alkali metal, such as Na or N; R is an alkylene group; R.sup.1,
R.sup.2, and R.sup.3 are alkyl groups, hydroxyalkyl groups, or
hydrogen; and X is a halogen such as Cl or Br. An amount of the
functional group in the binder is preferably 10 to 200 .mu.eq/g,
and more preferably 30 to 120 .mu.eq/g. When the amount exceeds 200
.mu.eq/g or is less than 10 .mu.eq/g, dispersibility deteriorates.
In addition to the adhering functional group, the binder preferably
has a functional group having active hydrogen, such as a --OH
group, such that the functional group reacts with an isocyanate
hardening agent to form a crosslinking structure, thereby improving
film strength. A preferable amount of the functional group is 0.1
to 2 meq/g. The binder has a weight average molecular weight of
preferably 10,000 to 200,000, and more preferably 20,000 to
100,000. When the weight average molecular weight is less than
10,000, film strength becomes insufficient, and durability
deteriorates. When the weight average molecular weight exceeds
200000, dispersibility deteriorates.
[0053] Polyurethane resin, which is a preferable binder, is
described in detail in Poriuretane Jushi Handbukku ("Polyurethane
Resin Handbook"), edited by Keiji Iwata, 1986, The Nikkan Kogyo
Shimbun, Ltd. Polyurethane resin is generally obtained by addition
polymerization of long-chain diol, short-chain diol (sometimes
referred to as a "chain extending agent"), and a diisocyanate
compound. Polyester diol, polyether diol, polyether ester diol,
polycarbonate diol, or polyolefin diol having a molecular weight of
500 to 5,000 is used as the long-chain diol. The polyurethane resin
is called polyester urethane, polyether urethane, polyether ester
urethane, polycarbonate urethane, or the like, according to the
type of the long-chain diol.
[0054] A glass-transition temperature of the polyurethane resin is
preferably 0 to 200.degree. C., and more preferably 40 to
160.degree. C. When the temperature is less than 0.degree. C.,
durability deteriorates. When the temperature exceeds 200.degree.
C., calender moldability deteriorates, whereby magnetic parametric
performance deteriorates.
[0055] Examples of a method of introducing the adhering functional
group (polar group) described above into the polyurethane resin
include a method in which the functional group is used for a
position of monomers of the long-chain diol, a method in which the
functional group is used for a position of the short-chain diol,
and a method in which the polar group is introduced by a polymer
reaction after polymerization of polyurethane.
[0056] A copolymer of vinyl chloride monomers and various monomers
is used as the vinyl chloride-based resin. Examples of the monomer
for copolymerization include fatty acid vinyl esters, such as vinyl
acetate and vinyl propionate; acrylates and methacrylates, such as
methyl(metha)acrylate, ethyl(metha)acrylate, isopropyl(metha)
acrylate, butyl(metha)acrylate, and benzyl(metha)acrylate;
alkylallylethers, such as allylmethylether, allylethylether,
allyipropylether, and allylbutylether; styrene;
.alpha.-methylstyrene; vinylidene chloride; acrylonitrile;
ethylene; butadiene; and acrylamide. Examples of the monomer for
copolymerization having a functional group include vinyl alcohol,
2-hydroxyethyl (metha)acrylate, polyethylene glycol
(metha)acrylate, 2-hydroxypropyl (metha)acrylate, 3-hydroxypropyl
(metha)acrylate, polypropyrene glycol (metha)acrylate,
2-hydroxyethyl allyl ether, 2-hydroxypropyl allyl ether,
3-hydroxypropyl allyl ether, p-vinylphenol, maleic acid, maleic
anhydride, acrylic acid, methacrylic acid, glycidyl
(metha)acrylate, allyl glycidyl ether, phosphoethyl
(metha)acrylate, sulfoethyl(metha)acrylate, p-styrene sulfonic
acid, and Na salts and K salts thereof. Herein, "(metha)acrylate"
means a monomer containing at least one of acrylate and
methacrylate.
[0057] A composition of vinyl chloride monomers in the vinyl
chloride-based resin is preferably 60 to 95% by mass. When the
amount is less than 60% by mass, dynamic strength decreases. When
the amount exceeds 95% by mass, solubility of the monomers in a
solvent deteriorates, the viscosity of the solution becomes high,
and dispersibility deteriorates. Preferable amounts of the adhering
functional group (polar group) and the functional group for
increasing hardening ability with a polyisocyanate-based hardening
agent are as described above. These functional groups may be
introduced by copolymerizing monomers containing the aforementioned
functional groups or by conducting a polymer reaction after
copolymerization of the vinyl chloride-based resin. A preferable
polymerization degree is 200 to 600, and more preferably 240 to
450. When the polymerization degree is less than 200, dynamic
strength deteriorates. When the polymerization degree exceeds 600,
the viscosity of the solution becomes high, and dispersibility
deteriorates.
[0058] An amount of the binder in the non-magnetic layer is
preferably 5 to 25% by mass with respect to the non-magnetic
particles. Carbon black is preferably included in the non-magnetic
layer for various purposes, such as for decreasing surface
electrical resistance Rs of the magnetic recording medium,
decreasing light transmittance of the magnetic recording medium in
a direction perpendicular to a magnetic surface thereof, and
setting a micro Vickers hardness of the non-magnetic layer to
within a desirable range (preferably 30 to 50 kg/mm.sup.2). Carbon
black is preferably included in an amount of 1 to 50% by mass with
respect to the non-magnetic particles.
[0059] Further, the non-magnetic layer preferably includes fatty
acid as a lubricant. Fatty acid gradually migrates into the surface
of the magnetic layer and provides a constant coefficient of
dynamic friction. Preferable fatty acid is saturated or unsaturated
monobasic fatty acid having 12 to 24 carbon atoms, such as lauric
acid, myristic acid, palmitic acid, stearic acid, behenic acid,
oleic acid, linoleic acid, linolenic acid, or elaidic acid. Lauric
acid, myristic acid, palmitic acid, stearic acid, behenic acid, and
oleic acid are particularly preferable. An amount of fatty acid
added to an underlying coating layer is selected from a range of
0.3 to 3% by mass with respect to the non-magnetic particles.
[0060] Other additives exhibiting lubricating effects, antistatic
effects, dispersing effects, and plasticizing effects are used for
the magnetic or non-magnetic layer in the present invention.
Molybdenum disulfide; tungsten disulfide; graphite, boron nitride;
graphite fluoride; silicone oil; silicone having a polar group;
fatty acid-denatured silicone; fluorine-containing silicone;
fluorine-containing alcohol; fluorine-containing ester; polyolefin;
polyglycol; alkyl phosphate and alkali metallic salt thereof; alkyl
sulfate and alkali metallic salt thereof; polyphenylether;
fluorine-containing alkyl sulfate and alkali metallic salt thereof;
monobasic fatty acid having 10 to 24 carbon atoms, which may
include an unsaturated bond or may be branched, and metallic salts
thereof (such as Li, Na, K, and Cu), or monovalent, bivalent,
trivalent, quadrivalent, quinquevalent, and hexavalent alcohols
having 12 to 22 carbon atoms, which may include an unsaturated bond
or may be branched; alkoxy alcohol having 12 to 22 carbon atoms
which may include an unsaturated bond or may be branched; mono-
fatty acid ester, di-fatty acid ester, or tri-fatty acid ester
formed by one of monobasic fatty acid having 10 to 24 carbon atoms,
which may include an unsaturated bond or may be branched; and
monovalent, bivalent, trivalent, quadrivalent, quinquevalent, and
hexavalent alcohols having 2 to 12 carbon atoms, which may include
an unsaturated bond or may be branched; fatty acid ester of
monoalkylether of an alkylene oxide polymer; fatty acid amide
having 2 to 22 carbon atoms; and aliphatic amine having 8 to 22
carbon atoms can be used. Specific examples thereof include lauric
acid, myristic acid, palmitic acid, stearic acid, behenic acid,
butyl stearate, oleic acid, linoleic acid, linolenic acid, elaidic
acid, octyl stearate, amyl stearate, isooctyl stearate, octyl
myristate, butoxyethyl stearate, anhydrosorbitan monostearate,
anhydrosorbitan distearate, anhydrosorbitan tristearate, oleyl
alcohol, and lauryl alcohol.
[0061] Further, nonionic surfactants, such as alkylene oxide-based,
glycerine-based, and glycidol-based surfactants and an addition
product of alkylphenol ethylene oxide; cationic surfactants, such
as cyclic amine, ester amide, quaternary ammonium salts, hydantoin
derivatives, heterocyclic rings, phosphoniums, or sulfoniums;
anionic surfactants containing carboxylic acid, sulfonic acid,
phosphoric acid, or an acidic group, such as a sulphate group or a
phosphate group; and amphoteric surfactants, such as amino acids,
aminosulfonic acids, sulfic acids or phosphates of amino alcohol,
or alkyl betaine type surfactants can also be used. These
surfactants are described in detail in Kaimen Kasseizai Binran
("Surfactant Handbook") published by Sangyo Tosho Publishing
Company. These lubricants and surfactants need not be pure and may
include impurities such as isomers, unreactant products, side
reaction products, resolvents, oxides, and the like besides the
main components. An amount of the impurities is preferably 30% by
weight or less, and more preferably 10% by weight or less.
[0062] Types and amounts of the lubricants and surfactants used for
the non-magnetic layer and the magnetic layer in the present
invention can be selected as necessary. For example, fatty acids
having different boiling points may be used for the non-magnetic
layer and the magnetic layer so that exudation of the fatty acids
on the surface is controlled, the amount of the surfactant may be
adjusted to improve coating stability, or the amount of the
lubricant added to the non-magnetic layer may be increased to
improve lubricating effect. However, selection of the type or the
amount of the lubricants and the surfactants is not limited to the
above-described cases. All or a portion of the additives used in
the present invention may be added to a coating solution for the
magnetic layer or the underlying layer at any step of the
manufacturing process thereof.
[0063] Specific examples of the lubricants used in the present
invention include (all trade names): NAA-102, castor oil hardened
fatty acid, NAA-42, CATION SA, NIMINE L-201, NONION E-208, ANON BF,
ANON LG, butyl stearate, butyl laurate, and erucic acid, all of
which are produced by NOF Corp.; oleic acid produced by Kanto
Kagaku; FAL-205 and FAL-123 produced by Takemoto Oil & Fat Co.,
Ltd.; NGELB OL produced by New Japan Chemical Co., Ltd.; TA-3
produced by Shin-Etsu Chemical Co., Ltd.; ARMIDE P produced by
Lion-Armer Co., Ltd.; DUOMINE TDO produced by Lion Corp.; BA-41G
produced by The Nisshin Oil Mills, Ltd.; PROFUAN 2012 E, NEW POLE
PE61, and IONET MS-400 produced by Sanyo Chemical Industries,
Ltd.
[0064] A coating solution (dispersion) prepared using the above
materials is coated onto the non-magnetic support to form the
underlying coating layer or the magnetic layer. The magnetic
recording medium of the present invention is manufactured by, for
example, coating the coating solution for the magnetic layer onto
the surface of the non-magnetic support, which is being conveyed,
so that the magnetic layer after being dried has a thickness of
preferably 5 nm to 5 .mu.m, and more preferably 5 nm to 0.2 .mu.m.
Multiple coating solutions for magnetic layers may be successively
or simultaneously coated one over another, and the coating solution
for the underlying layer and the coating solution for the magnetic
layer may be successively or simultaneously coated. Examples of a
coating machine for coating the coating solution for the magnetic
layer or the underlying layer include machines for air doctor
coating, blade coating, rod coating, extrusion coating, air knife
coating, squeeze coating, impregnation coating, reverse-roll
coating, transfer roller coating, gravure coating, kiss-roll
coating, cast coating, spray coating, spin coating, and the like.
As for the above coating techniques, refer to, for example, Saishin
Kochingu Gijutsu ("New Coating Techniques") published by Sogo
Gijutsu Senta Co., Ltd. on May 31, 1983.
[0065] When the dispersion of ferromagnetic nanoparticles according
to the present invention is applied to a magnetic recording medium
having two or more layers, the following coating machines and
coating methods are recommended as examples.
[0066] (1) First, a lower layer is formed by coating the dispersion
using a coating machine that is generally used for coating a
dispersion for a magnetic layer, such as a gravure coater, a roll
coater, a blade coater, or an extrusion coater. Before the lower
layer dries, the dispersion is coated thereon to form an upper
layer using a support-pressing extrusion coater as disclosed in
Japanese Patent Application Publication (JP-B) No. 1-46186 and JP-A
Nos. 60-238179 and 2-265672.
[0067] (2) An upper layer and a lower layer are formed almost at
the same time using a coating head having two slits for passing
coating solutions therethrough as disclosed in JP-A Nos. 63-88080,
2-17971, and 2-265672.
[0068] (3) An upper layer and a lower layer are formed almost at
the same time using an extrusion coater having a backup roll as
disclosed in JP-A No. 2-174965.
[0069] A back coating layer (backing layer) may be formed on the
surface of the base used in the present invention on which the
magnetic layer is not formed. The back coating layer is formed on
the surface of the base on which no magnetic layer is formed, by
applying a coating solution for a back coating layer in which
granular components, such as an abrasive and an antistatic agent,
and a binder are dispersed in an organic solvent. Various inorganic
pigments and carbon black can be used as the granular components.
Further, resins, such as nitrocellulose, phenoxy resin, vinyl
chloride-based resin, and polyurethane, can be used as the binder
independently or in the form of a mixture. An adhesive layer may be
formed on the surface of the base on which the dispersion of
ferromagnetic nanoparticles and the coating solution for a back
coating layer have been applied.
[0070] The magnetic layer is dried after the ferromagnetic
nanoparticles included in the dispersion of ferromagnetic
nanoparticles have been subjected to a magnetic field orientation
process. Subsequently, the coated layer is subjected to a surface
smoothing process. A supercalender roll, for example, is used in
the surface smoothing process. The surface smoothing process
eliminates holes formed by the solvent being removed by drying the
magnetic layer, thereby improving the fill factor of the
ferromagnetic particles in the magnetic layer. Therefore, a
magnetic recording medium having high magnetic parametric
performance can be obtained. A heat-resistant roll formed of
plastic such as epoxy, polyimide, polyamide, polyamideimide, or the
like is used as the calender roll. Alternatively, a metallic roll
can be used.
[0071] The magnetic recording medium of the present invention has a
very smooth surface whose center line average height is 0.1 to 5
nm, and preferably 1 to 4 nm at a cut off value of 0.25 mm, and is
thus preferable for high-density recording. The very smooth surface
is obtained by, as described above, calendering the magnetic layer
formed by the particular ferromagnetic particles and binder.
Preferable calendering conditions are a calender roll temperature
of 60 to 100.degree. C., preferably 70 to 100.degree. C., and
particularly preferably 80 to 100.degree. C. and a calender roll
pressure of 100 to 500 kg/cm, preferably 200 to 450 kg/cm, and
particularly preferably 300 to 400 kg/cm. The magnetic recording
medium thus obtained can be cut into desired size by a cutter and
used.
[0072] The higher the recording density becomes, the more important
it is to control the running position of a recording head against a
track. Thus, it is preferable to control the running position of
the recording head by magnetically recording, on the magnetic
recording medium of the present invention, signals showing tracking
position. Specifically, this can be accomplished by writing the
magnetic recording medium with a commonly used magnetic head,
however, this method has a drawback in that recording time become
longer as recording density increases. A method has therefore been
proposed in which the magnetic recording medium is written one time
by magnetic transfer, and it is particularly preferable to apply
this magnetic transfer method to high-density magnetic recording
media such as the magnetic recording medium of the present
invention.
[0073] Method of magnetic transfer include contacting magnetic
transfer and non-contacting magnetic transfer using laser heating.
Examples of contacting magnetic transfer are specifically disclosed
in Japanese Patent Application Laid-Open (JP-A) Nos. 2000-331341,
2000-14667, 2001-143257, 2001-143258, 2001-243624, 2001-307324,
2001-312821, and 2001-325724, and it is preferable to apply
contacting magnetic transfer to the magnetic recording medium of
the present invention. Examples of non-contacting magnetic transfer
are specifically disclosed in JP-A Nos. 2001-312808, 2001-331902,
2001-338419, 2002-50036, 2002-74605, and 2002-197647, and it is
particularly preferable to apply non-contacting magnetic transfer
to the magnetic recording medium of the present invention since it
is non-contacting and thus does not contaminate or produce
scratches in the medium.
EXAMPLES
Examples 1-2 and Comparative Example 1
[0074] 1. Production of Ferromagnetic Nanoparticles
[0075] (1) Preparation of Dispersion 1 of Ferromagnetic
Nanoparticles (CoPt)
[0076] The following process was carried out in an N.sub.2
atmosphere.
[0077] H.sub.2PtCl.sub.6.6H.sub.2O was dissolved in a solution
(distilled water: ethanol=1:1) to obtain 100 ml of a solution
having a concentration of 2 .mu.mol/ml. 0.75 g of PVP having a
weight average molecular weight of 40,000 was dissolved in the
solution, and the resulting mixture was maintained under reflux at
100.degree. C. Thereafter, 10 ml of a 12 .mu.mol/ml aqueous
solution of CoCl.sub.2.6H.sub.2O was added to the mixture. A
product obtained by dissolving 1 g of NaBH.sub.4 in 15 ml of
distilled water was added to the mixture. The resulting mixture was
pressurized under an N.sub.2 atmosphere and subjected to
ultrafiltration using a membrane filter. The mixture was filtered
until a volume thereof became 1/3 of its original volume.
Subsequently, deaerated distilled water was added to the mixture so
that the volume thereof reached its original volume, and the
mixture was filtered until the volume thereof became 1/3 of its
original volume. Again, deaerated distilled water was added to the
mixture so that the volume thereof reached its original volume, and
the mixture was filtered until the volume thereof became 1/3 of its
original volume. As a result, a dispersion 1 of ferromagnetic
nanoparticles of Example 1 was obtained.
[0078] (2) Preparation of Dispersion 2 of Ferromagnetic
Nanoparticles (FePt)
[0079] The following process was carried out in an N.sub.2
atmosphere.
[0080] H.sub.2PtCl.sub.6.6H.sub.2O was dissolved in a solution
(distilled water: ethanol=1:1) to obtain 100 ml of a solution
having a concentration of 2 .mu.mol/ml. 0.3 g of PVP having a
weight average molecular weight of 40,000 was dissolved in the
solution, and the resulting mixture was maintained under reflux at
100.degree. C. Thereafter, 10 ml of a 12 .mu.mol/ml aqueous
solution of FeSO.sub.4.7H.sub.2O was added to the mixture. A
product obtained by dissolving 1 g of NaBH.sub.4 in 15 ml of
distilled water was added to the mixture. The resulting mixture was
pressurized under an N.sub.2 atmosphere and subjected to
ultrafiltration using a membrane filter. The mixture was filtered
until a volume thereof became 1/3 of its original volume.
Subsequently, deaerated distilled water was added to the mixture so
that the volume thereof reached its original volume, and the
mixture was filtered until the volume thereof became 1/3 of its
original volume. Again, deaerated distilled water was added to the
mixture so that the volume thereof reached its original volume, and
the mixture was filtered until the volume thereof became 1/3 of its
original volume. As a result, a dispersion 1 of ferromagnetic
nanoparticles of Example 2 was obtained.
[0081] (3) Preparation of Dispersion 3 of Ferromagnetic
Nanoparticles (FePt)
[0082] The following process was carried out in a high purity Ar
gas.
[0083] 0.5 mmol of platinum (II) acetylacetonate
[CH.sub.3COCH.dbd.C(O--)C- H.sub.3].sub.2Pt, 1.5 mmol of
1,2-hexadecanediol, and 20 ml of dioctylether were mixed and heated
at 100.degree. C. Next, 0.5 mmol of oleic acid, 0.5 mmol of
oleilamine, and 1 mmol of Fe(CO).sub.5 were added to the resulting
mixture, and the mixture was maintained under reflux at 297.degree.
C. for 30 minutes. 40 ml of ethanol was added to the mixture after
cooling thereof such that a deposit was precipitated, and a
supernatant liquid was removed. After 0.16 mmol of oleic acid and
0.15 mmol of oleilamine were added to the mixture, 25 ml of hexane
was added to the mixture and dispersed therein. 20 ml of ethanol
was added such that a deposit was precipitated, and a supernatant
liquid was removed. After 0.16 mmol of oleic acid and 0.15 mmol of
oleilamine were added to the mixture, 20 ml of hexane was added to
the mixture and dispersed therein. 15 ml of ethanol was added such
that a deposit was precipitated, and a supernatant liquid was
removed. 0.16 mmol of oleic acid and 0.15 mmol of oleilamine were
added to the mixture, and 20 ml of hexane was added to the mixture
and dispersed therein, whereby a dispersion 3 of ferromagnetic
nanoparticles of Comparative Example 1 was obtained.
[0084] 2. Confirmation of Nanoparticles
[0085] A sample for a transmission electron microscope (TEM) was
produced by placing each of the prepared dispersions of
ferromagnetic nanoparticles on a mesh for observation with a TEM
and drying each dispersion. A particle size was measured for each
dispersion using a TEM (manufactured by Hitachi, Ltd.) having an
acceleration voltage of 300 KV. As a result, nanoparticles having a
diameter of 5 nm were confirmed.
[0086] 3. Elemental Analysis
[0087] A Co/Fe ratio and a Pt/Fe ratio were determined using an
ICP.
[0088] 4. Preparation of Nanoparticle-containing Coating
Solution
[0089] a) Coating by Spin Coater
[0090] The dispersion of each of Examples 1-2 and Comparative
Example 1 including ferromagnetic nanoparticles at a concentration
of 5 mg/ml was dropped, in an amount of 0.04 ml/cm.sup.2, onto an
ARAMICA base (manufactured by Asahi Kasei Corp.) having a thickness
of 40 .mu.m, and the base was spun at 2000 rpm for 30 seconds and
dried. Whether the coating layer formed on the base was uniform or
not was visually observed. Thereafter, the base with the coating
layer formed thereon was heated in an N.sub.2 atmosphere with an Ar
laser having the following characteristics.
[0091] (Ar laser)
[0092] Wavelength: 488 nm
[0093] Laser beam diameter: 25 .mu.m
[0094] Linear velocity: 2 m/ sec
[0095] Recording power: 20 W
[0096] 5. Annealing
[0097] A freeze-dried product of the dispersion of each of Examples
1-2 and Comparative Example 1 was heated in an electric furnace
from room temperature to 600.degree. C., at a programming rate of
50.degree. C./min using a differential thermal analyzer (trade
name: TGD7000, manufactured by ULVAC-RIKO, Inc.). X-ray diffraction
and magnetic property of the heated samples were evaluated.
[0098] 6. X-ray Diffraction
[0099] Samples for X-ray diffraction were produced by placing each
of the prepared solutions on a crystal, reflection-free sample
holder and drying each solution. A Cu K.alpha. ray was generated by
an X-ray diffractometer (manufactured by Rigaku Corp.) under
conditions of a tube voltage of 50 KV and a tube current of 300 mA,
and X-ray diffraction was carried out by a powder method using a
goniometer. An irregular phase and a regular phase were
distinguished based on the crystal structure.
[0100] 7. Magnetic Property
[0101] Magnetic property was measured at an applied magnetic field
of 796 kA/m (10 kOe) using a sensitive magnetiziation vector
measuring apparatus and a DATA processor (both manufactured by Toei
Industry Co., Ltd.).
[0102] The results of the above evaluations are given in Table
1.
1 TABLE 1 Pt/ Hc Co Pt/Fe Coatability Structure (kA/m) Example 1
Disper- NA 0.98 Uniform Regular 222.88 sion 1 layer phase Example 2
Disper- 2.8 NA Uniform Regular 91.54 sion 2 layer phase Compara-
Disper- NA 0.97 Ununiform Regular 226.86 tive sion 3 layer phase
Example 1
[0103] It can been seen from Table 1 that the materials coated with
dispersions of ferromagnetic nanoparticles having excellent
industrial coatability obtained in Examples 1 and 2.
[0104] According to the present invention, ferromagnetic
nanoparticles having a holding power Hc of 95.5 kA/m or more, a
material coated with a dispersion of ferromagnetic nanoparticles
having excellent industrial coatability, and a magnetic recording
medium using the material can be provided.
* * * * *