U.S. patent application number 13/285035 was filed with the patent office on 2012-02-23 for process for producing magnetic metal particles for magnetic recording, and magnetic recording medium.
This patent application is currently assigned to TODA KOGYO CORPORATION. Invention is credited to Toshiharu HARADA, Kazuyuki HAYASHI, Takahiro MATSUO, Mineko OHSUGI, Yosuke YAMAMOTO.
Application Number | 20120042750 13/285035 |
Document ID | / |
Family ID | 40939143 |
Filed Date | 2012-02-23 |
United States Patent
Application |
20120042750 |
Kind Code |
A1 |
OHSUGI; Mineko ; et
al. |
February 23, 2012 |
PROCESS FOR PRODUCING MAGNETIC METAL PARTICLES FOR MAGNETIC
RECORDING, AND MAGNETIC RECORDING MEDIUM
Abstract
The present invention relates to a process for producing
magnetic metal particles for magnetic recording, comprising:
heat-treating goethite particles having an aluminum content of 4 to
50 atom % in terms of Al based on whole Fe to obtain hematite
particles; and heat-reducing the hematite particles at a
temperature of 200 to 600.degree. C., the goethite particles being
obtained by adding a peroxodisulfate to a reaction solution
comprising: a ferrous salt aqueous solution and a mixed alkali
aqueous solution comprising: an alkali hydrogen carbonate aqueous
solution or alkali carbonate aqueous solution and an alkali
hydroxide aqueous solution before initiation of an oxidation
reaction of the reaction solution, and then conducting the
oxidation reaction.
Inventors: |
OHSUGI; Mineko;
(Hiroshima-ken, JP) ; HARADA; Toshiharu;
(Yamaguchi-ken, JP) ; MATSUO; Takahiro;
(Yamaguchi-ken, JP) ; YAMAMOTO; Yosuke;
(Yamaguchi-ken, JP) ; HAYASHI; Kazuyuki;
(Hiroshima-ken, JP) |
Assignee: |
TODA KOGYO CORPORATION
Otake-shi
JP
|
Family ID: |
40939143 |
Appl. No.: |
13/285035 |
Filed: |
October 31, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12364658 |
Feb 3, 2009 |
|
|
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13285035 |
|
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|
Current U.S.
Class: |
75/369 ;
75/370 |
Current CPC
Class: |
B22F 2998/10 20130101;
B22F 2999/00 20130101; H01F 1/112 20130101; H01F 1/113 20130101;
B22F 9/22 20130101; B22F 2201/013 20130101; B22F 1/0088 20130101;
C01B 13/00 20130101; B22F 9/22 20130101; B22F 9/22 20130101; B22F
2999/00 20130101; B22F 1/0085 20130101; B22F 2998/10 20130101; C22C
33/0235 20130101; G11B 5/70642 20130101; H01F 1/11 20130101; C22C
2202/02 20130101; G11B 5/712 20130101 |
Class at
Publication: |
75/369 ;
75/370 |
International
Class: |
B22F 9/20 20060101
B22F009/20 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 6, 2008 |
JP |
2008-26265 |
Claims
1. A process for producing magnetic metal particles for magnetic
recording, comprising: heat-treating goethite particles having an
aluminum content of 4 to 50 atom % in terms of Al based on whole Fe
to obtain hematite particles; and heat-reducing the hematite
particles at a temperature of 200 to 600.degree. C., the goethite
particles being obtained by adding a peroxodisulfate to a reaction
solution comprising: a ferrous salt aqueous solution and a mixed
alkali aqueous solution comprising: an alkali hydrogen carbonate
aqueous solution or alkali carbonate aqueous solution and an alkali
hydroxide aqueous solution before initiation of an oxidation
reaction of the reaction solution, and then conducting the
oxidation reaction.
2. A process according to claim 1, wherein the goethite particles
comprise Co in an amount of 10 to 50 atom % based on whole Fe.
3. A process according to claim 2, wherein a surface of the
goethite particles produced is coated with a rare earth compound
and/or a Co compound, and then the thus coated goethite particles
are subjected to the heat treatment.
4. A process according to claim 3, wherein the rare earth compound
is used such that a coating amount thereof is 10 to 30 atom % in
terms of rare earth element based on whole Fe.
5. A process according to claim 3, wherein the Co compound is used
such that a coating amount thereof is 20 to 200 atom % in terms of
Co based on whole Co contained in the goethite particles before the
coating treatment.
6. A process according to claim 1, wherein the peroxodisulfate is
added in an amount of 0.5 to 5 mol % in terms of peroxodisulfate
based on whole Fe.
7. A process according to claim 1, wherein the peroxodisulfate is
ammonium peroxodisulfate.
8. (canceled)
Description
[0001] This application is a divisional of application Ser. No.
12/364,658, filed Feb. 3, 2009, which in turn claims priority of JP
application Ser. No. 2008-26265 filed Feb. 6, 2008, the entire
content of which is hereby incorporated by reference in this
application.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to magnetic metal particles
which are prevented from suffering from agglomeration thereof and
capable of providing a magnetic coating film having excellent
magnetic properties (coercive force Hc) in spite of fine particles,
in particular, those particles having an average major axis
diameter as small as 5 to 100 nm.
[0003] In recent years, miniaturization, lightening, recording-time
prolongation and high-density recording as well as increase in
memory capacity in magnetic recording and reproducing apparatuses
for computers, etc., have proceeded more rapidly. With such a
recent tendency, it has been increasingly required to provide
magnetic recording media having a high performance and a
high-density recording property, such as magnetic tapes and
magnetic disks.
[0004] Namely, the magnetic recording media have been required to
have high image definition and quality, and high output
characteristics such as, in particular, good frequency
characteristics. For this reason, it has been required that the
magnetic recording media are reduced in noise due to the magnetic
recording media themselves, and exhibit a high coercive force He
and an excellent switching field distribution (S.F.D.).
[0005] These properties of the magnetic recording media have a
close relation to magnetic particles used therein. Therefore, it
has also been strongly required to further improve properties of
magnetic metal particles containing iron as a main component.
[0006] More specifically, in order to obtain magnetic recording
media satisfying various properties mentioned above, the magnetic
metal particles containing iron as a main component which are used
as magnetic particles in the magnetic recording media have been
strongly required to be in the form of fine particles, and to
exhibit a higher coercive force Hc.
[0007] As to the reduction in particle size of the magnetic metal
particles, in order to obtain magnetic recording media having high
output characteristics in a short wavelength region as well as a
lessened noise, it is necessary to reduce the particle size of the
magnetic metal particles, i.e., reduce a major axis diameter
thereof to obtain fine particles.
[0008] Also, in recent years, it has been attempted to use a
magneto-resistance type head as a reproduction head for computer
tapes instead of conventional induction-type magnetic heads. The
magneto-resistance type head can readily produce a high
reproduction output as compared to the conventional induction-type
magnetic heads, and is free from impedance noise due to use of
induction coil. Therefore, the use of the magneto-resistance type
head contributes to reduction in a system noise to a large extent.
In consequence, if such noises due to the magnetic recording media
themselves are reduced, it will be possible to attain a high C/N
ratio. Accordingly, in order to reduce such magnetic recording
media noises, in particular, noises due to particles, it has been
required to further reduce the particle size of the magnetic metal
particles used therein.
[0009] However, since the reduction in particle size of the
magnetic metal particles is accompanied with increase in proportion
of an oxidation layer in the whole particles, the coercive force He
of the magnetic metal particles tends to be deteriorated owing to
formation of the oxidation layer. Therefore, in order to obtain
excellent magnetic recording media, it has been required to provide
magnetic metal particles exhibiting a high coercive force He in
spite of fine particles.
[0010] Conventionally, as the process for producing goethite
particles as a precursor of the magnetic metal particles, there are
known the production process using hydrogen peroxide, etc.,
(Japanese Patent Application Laid-open (KOKAI) Nos. 5-270836
(1993), 5-310431 (1993) and 2007-81227), and the production process
in which various conditions of production reaction of the goethite
particles such as temperature, deaggregation and stirring are
suitably controlled (Japanese Patent Application Laid-open (KOKAI)
No, 2005-277094).
SUMMARY OF THE INVENTION
[0011] Although it has now been required to provide magnetic metal
particles exhibiting a high coercive force He in spite of fine
particles, the magnetic metal particles containing iron as a main
component which fully satisfy various properties mentioned above
are not obtained until now.
[0012] That is, the conventional techniques described in the above
references have failed to obtain the magnetic metal particles
having a high coercive force Hc in spite of fine particles.
[0013] In consequence, an object of the present invention is to
provide magnetic metal particles having excellent magnetic
properties (coercive force Hc) although they are in the form of
fine particles having an average major axis diameter as small as 5
to 100 nm.
[0014] The above object of the present invention can be achieved by
the following subject matters of the present invention.
[0015] In a first invention, there is provided a process for
producing magnetic metal particles for magnetic recording,
comprising:
[0016] heat-treating goethite particles having an aluminum content
of 4 to 50 atom % in terms of Al based on whole Fe to obtain
hematite particles; and
[0017] heat-reducing the hematite particles at a temperature of 200
to 600.degree. C.,
[0018] the goethite particles being obtained by adding a
peroxodisulfate to a reaction solution comprising:
[0019] a ferrous salt aqueous solution and
[0020] a mixed alkali aqueous solution comprising:
[0021] an alkali hydrogen carbonate aqueous solution or alkali
carbonate aqueous solution and
[0022] an alkali hydroxide aqueous solution before initiation of an
oxidation reaction of the reaction solution, and then conducting
the oxidation reaction.
[0023] In a second invention, there is provided a process according
to the first invention, wherein the goethite particles further
comprise Co in an amount of 10 to 50 atom % based on whole Fe.
[0024] In a third invention, there is provided a process according
to the second invention, wherein a surface of the goethite
particles produced is coated with a rare earth compound and/or a Co
compound, and then the thus coated goethite particles are subjected
to the heat treatment.
[0025] In a fourth invention, there is provided a process according
to the third invention, wherein the rare earth compound is used
such that a coating amount thereof is 10 to 30 atom % in terms of
rare earth element based on whole Fe.
[0026] In a fifth invention, there is provided a process according
to the third invention, wherein the Co compound is used such that a
coating amount thereof is 20 to 200 atom % in terms of Co based on
whole Co contained in the goethite particles before the coating
treatment.
[0027] In a sixth invention, there is provided a process according
to the first invention, wherein the peroxodisulfate is added in an
amount of 0.5 to 5 mol % in terms of peroxodisulfate based on whole
Fe.
[0028] In a seventh invention, there is provided a process
according to the first invention, wherein the peroxodisulfate is
ammonium peroxodisulfate.
[0029] In an eighth invention, there is provided a magnetic
recording medium, comprising:
[0030] a non-magnetic substrate;
[0031] a non-magnetic undercoat layer formed on the non-magnetic
substrate, comprising non-magnetic particles and a binder resin;
and
[0032] a magnetic recording layer formed on the non-magnetic
undercoat layer, comprising magnetic particles and a binder
resin,
[0033] the magnetic metal particles for magnetic recording produced
by the process as defined in the first aspect being used as the
magnetic particles.
DETAILED DESCRIPTION OF THE INVENTION
[0034] The present invention is described in detail below.
[0035] First, the process for producing magnetic metal particles
for magnetic recording according to the present invention is
described.
[0036] According to the present invention, in the process for
producing magnetic metal particles which includes the steps of
heat-treating goethite particles having an aluminum content of 4 to
40 atom % in terms of Al based on whole Fe to obtain hematite
particles and then subjecting the resulting hematite particles to
heat reduction at a temperature of 300 to 600.degree. C., the
goethite particles used in the above process are obtained by adding
a peroxodisulfate to a reaction solution comprising a ferrous salt
aqueous solution and a mixed alkali aqueous solution of: an alkali
hydrogen carbonate aqueous solution or an alkali carbonate aqueous
solution, and an alkali hydroxide aqueous solution before
initiation of an oxidation reaction thereof, and then subjecting
the resulting mixture to oxidation reaction.
[0037] The process for producing the goethite particles is
described in more detail below.
[0038] The goethite particles used in the present invention are
obtained by first producing spindle-shaped goethite seed crystal
particles and then growing a goethite layer on the surface of the
respective goethite seed crystal particles. Upon producing the
spindle-shaped goethite seed crystal particles, the peroxodisulfate
is used as an oxidizing agent.
[0039] The spindle-shaped goethite seed crystal particles are
obtained by the steps of reacting a mixed alkali aqueous solution
of an alkali hydrogen carbonate aqueous solution or an alkali
carbonate aqueous solution, and an alkali hydroxide aqueous
solution with a ferrous salt aqueous solution to obtain a water
suspension containing a ferrous-containing precipitate; aging the
water suspension containing the ferrous-containing precipitate in a
non-oxidative atmosphere; and passing an oxygen-containing gas
through the water suspension to conduct an oxidation reaction
thereof, thereby producing the aimed spindle-shaped goethite seed
crystal particles, wherein before initiation of the oxidation
reaction, a Co compound and then the peroxodisulfate as an
oxidizing agent are successively added to the water suspension
containing the ferrous-containing precipitate during aging thereof,
and then the resulting mixture is subjected to oxidation
reaction.
[0040] Thereafter, when an oxidation percentage (Fe.sup.2+/whole
Fe) of the reaction solution in the oxidation reaction reaches 20%
or more, an Al compound is added to the water suspension, and the
resulting mixture is successively subjected to oxidation reaction
as growth reaction, thereby obtaining the goethite particles.
[0041] The aging of the water suspension is suitably conducted at a
temperature of 40 to 80.degree. C. in a non-oxidative atmosphere.
When the aging temperature is less than 40.degree. C., the
resulting particles tend to have a small aspect ratio, and it may
be therefore difficult to attain a sufficient aging effect. When
the aging temperature is more than 80.degree. C., magnetite tends
to be mixed in the obtained particles. The aging time is usually 30
to 300 min. When the aging time is less than 30 min or more than
300 min, it may be difficult to obtain the particles having the
aimed aspect ratio. In order to produce the non-oxidative
atmosphere, an inert gas (such as nitrogen gas) or a reducing gas
(such as hydrogen gas) may be passed and flowed through an inside
of a reactor receiving the above suspension.
[0042] As the ferrous salt aqueous solution used in the production
reaction of the spindle-shaped goethite seed crystal particles,
there may be used a ferrous sulfate aqueous solution, a ferrous
chloride aqueous solution, etc. These ferrous salt aqueous
solutions may be used alone or in the form of a mixture of any two
or more thereof, if required.
[0043] The mixed alkali aqueous solution used in the production
reaction of the spindle-shaped goethite seed crystal particles may
be obtained by mixing an alkali hydrogen carbonate aqueous solution
or an alkali carbonate aqueous solution with an alkali hydroxide
aqueous solution. In this case, the mixing ratio (expressed by % in
terms of normality) between the alkali carbonate aqueous solution
and the alkali hydroxide aqueous solution is controlled such that
the alkali hydroxide aqueous solution is preferably used in the
mixed solution in an amount of 10 to 40% and more preferably 15 to
35% (all expressed by % in terms of normality). When the amount of
the alkali hydroxide aqueous solution used in the mixed solution is
less than 10%, it may be difficult to obtain particles having the
aimed aspect ratio. When the amount of the alkali hydroxide aqueous
solution used in the mixed solution is more than 40%, granular
magnetite tends to be mixed in the obtained particles.
[0044] As the above alkali hydrogen carbonate aqueous solution,
there may be used a sodium hydrogen carbonate aqueous solution, a
potassium hydrogen carbonate aqueous solution, an ammonium hydrogen
carbonate aqueous solution, etc. As the above alkali carbonate
aqueous solution, there may be used a sodium carbonate aqueous
solution, a potassium carbonate aqueous solution, an ammonium
carbonate aqueous solution, etc. As the alkali hydroxide aqueous
solution, there may be used a sodium hydroxide aqueous solution, a
potassium hydroxide aqueous solution, an ammonium hydroxide aqueous
solution, etc. These aqueous solutions may be respectively used
alone or in the form of a mixture of any two or more thereof, if
required.
[0045] The amount of the mixed alkali aqueous solution used is 1.3
to 3.5 and preferably 1.5 to 2.5 in terms of an equivalent ratio
based on whole Fe in the ferrous salt aqueous solution. When the
amount of the mixed alkali aqueous solution used is less than 1.3,
magnetite tends to be mixed in the obtained particles. It may be
industrially undesirable to use the mixed alkali aqueous solution
in an amount of more than 3.5.
[0046] The ferrous concentration in the mixture obtained after
mixing the ferrous salt aqueous solution with the mixed alkali
aqueous solution is preferably 0.1 to 1.0 mol/L and more preferably
0.2 to 0.8 mol/L. When the ferrous concentration is less than 0.1
mol/L, the yield of the aimed product tends to be lowered,
resulting in industrially disadvantageous process. When the ferrous
concentration is more than 1.0 mol/L, the particle size
distribution of the obtained particles tends to be undesirably
broad.
[0047] When the Co compound is added subsequent to the time at
which 50% of the whole aging time elapses, it may be difficult to
obtain particles having the aimed particle size and aspect
ratio.
[0048] Examples of the Co compound to be added in the production
reaction of the spindle-shaped goethite seed crystal particles
include cobalt sulfate, cobalt chloride and cobalt nitrate. These
Co compounds may be used alone or in the form of a mixture of any
two or more thereof, if required.
[0049] The amount of the Co compound added in the production
reaction of the seed crystal particles is preferably 10 to 50 atom
% based on whole Fe.
[0050] The pH value of the reaction solution in the production
reaction of the spindle-shaped goethite seed crystal particles is
preferably in the range of 8.0 to 11.5 and more preferably 8.5 to
11.0. When the pH value is less than 8.0, a large amount of acid
radicals tend to be mixed in the goethite particles. Since such
acid radicals are not easily removed by washing, sintering between
the particles tends to be caused upon producing the aimed magnetic
metal particles therefrom. When the pH value is more than 11.5, the
particles having the aimed coercive force tend to be hardly
produced.
[0051] In the production reaction of the spindle-shaped goethite
seed crystal particles, first, a peroxodisulfate as an oxidizing
agent is added to the reaction solution. When the peroxodisulfate
is added in the course of the oxidation reaction, it may be
difficult to attain the effect of well controlling the particle
size distribution of the obtained particles.
[0052] Examples of the peroxodisulfate include ammonium
peroxodisulfate, potassium peroxodisulfate, sodium peroxodisulfate,
potassium hydrogen peroxodisulfate and sodium hydrogen
peroxodisulfate and ammonium peroxodisulfate. Among these
peroxodisulfates, preferred is ammonium peroxodisulfate. In
particular, combination of ammonium peroxodisulfate as the
oxidizing agent, an ammonium carbonate aqueous solution as the
alkali carbonate aqueous solution and an ammonium hydroxide aqueous
solution as the alkali hydroxide aqueous solution, is preferably
used because no alkali metals, etc., are present in the reaction
solution so that the resulting goethite particles comprise no
impurities such as alkali metals.
[0053] The amount of the peroxodisulfate added as the oxidizing
agent is 0.5 to 5 mol % in terms of peroxodisulfate based on whole
Fe. When the amount of the peroxodisulfate added is less than 0.5
mol %, seed crystals of the particles tend to be unevenly produced.
As a result, growth of the particles tends to become uneven owing
to residual growing components, thereby failing to obtain particles
having a good particle size distribution. When the amount of the
peroxodisulfate added is more than 5 mol %, the effect of addition
of the oxidizing agent tends to be saturated, and therefore the
addition of such a large amount of the oxidizing agent may be
meaningless. The amount of the peroxodisulfate added is more
preferably 1.0 to 4.0 mol %. The peroxodisulfate may be added
either in a form of aqueous solution or as a solid (powder) as it
is.
[0054] Next, an oxygen-containing gas (for example, air) is passed
through the reaction solution to conduct an oxidation reaction
thereof.
[0055] The superficial velocity of the oxygen-containing gas is
preferably 2.3 to 3.5 cm/s. When the superficial velocity of the
oxygen-containing gas is less than 2.3 cm/s, the oxidation reaction
rate tends to be too low, so that granular magnetite tends to be
mixed in the obtained particles, and it may be difficult to control
the particle size thereof to the aimed value. On the other hand,
when the superficial velocity of the oxygen-containing gas is more
than 3.5 cm/s, the oxidation reaction rate tends to be too high, so
that it may be difficult to control the particle size of the
obtained particles to the aimed value. Meanwhile, the superficial
velocity as used herein means an amount of the oxygen-containing
gas passed and flowed per unit sectional area (the bottom sectional
area of a cylindrical column reactor, the pore diameter of a
perforated plate and the number of pores are not taken into
consideration), and its unit is expressed by cm/sec.
[0056] The production reaction of the spindle-shaped goethite seed
crystal particles may be conducted at a temperature of not more
than 80.degree. C. When the production reaction temperature is more
than 80.degree. C., magnetite may be mixed in the obtained goethite
particles. The production reaction temperature is preferably in the
range of 40 to 70.degree. C.
[0057] The pH value in the growth reaction of the goethite layer is
usually in the range of 8.0 to 11.5 and preferably 8.5 to 11.0.
When the pH value is less than 8.0, a large amount of acid radicals
tend to be mixed in the goethite particles and cannot be easily
removed even by washing, so that sintering of the resulting
magnetic metal particles may be caused. On the other hand, when the
pH value is more than 11.5, the resulting magnetic metal particles
may fail to attain the aimed particle size distribution.
[0058] The growth reaction of the goethite layer is conducted
through oxidation reaction by passing an oxygen-containing gas
(e.g. air) through the reaction solution. It is preferred that the
superficial velocity of the oxygen-containing gas passed upon the
growth reaction be larger than that in the production reaction of
the seed crystal particles. When the superficial velocity upon the
growth reaction is not larger than that upon the production
reaction, the viscosity of the water suspension tends to be
increased when adding Al thereto, and the growth in the minor axis
direction of the particles tends to be more promoted, so that the
aspect ratio tends to be decreased, thereby failing to obtain
particles having the aimed aspect ratio. However, when the
superficial velocity in the production reaction of the seed crystal
particles is not less than 2.0 cm/s, it is not required that the
superficial velocity in the growth reaction is lager than that in
the production reaction.
[0059] The temperature used in the growth reaction of the goethite
layer may be not more than 80.degree. C. at which goethite
particles are formed. When the growth reaction temperature is more
than 80.degree. C., magnetite tends to be mixed in the obtained
goethite particles. The growth reaction temperature is preferably
in the range of 40 to 70.degree. C.
[0060] Examples of the Al compound added in the growth reaction of
the goethite layer includes acid salts such as aluminum sulfate,
aluminum chloride and aluminum nitrate, and aluminates such as
sodium aluminate, potassium aluminate and ammonium aluminate. These
Al compounds may be used alone or in the form of a mixture of any
two or more thereof, if required.
[0061] The Al compound may be added at the time at which the
oxidation percentage (Fe.sup.2+/whole Fe) of the reaction solution
lies in the range of 20 to 90%.
[0062] The Al compound may be added at the same time when the
superficial velocity of the oxygen-containing gas is preferably
increased as compared with that in the production reaction of the
seed crystal particles. When the Al compound is added over a
prolonged period of time, the oxygen-containing gas may be replaced
with a nitrogen-containing gas so as not to promote the oxidation
reaction.
[0063] The amount of the Al compound added is 4 to 50 atom % based
on the whole Fe contained in the goethite particles as the final
product. When the amount of the Al compound added is less than 4
atom %, it may be difficult to attain a sufficient anti-sintering
effect, thereby failing to maintain a good shape of the fine
particles. When the amount of the Al compound added is more than 50
atom %, the resulting particles tend to exhibit a relatively small
aspect ratio, so that it may be difficult to well control a
coercive force thereof.
[0064] Next, the coating treatment of the goethite particles is
described.
[0065] In the present invention, the surface of the respective
goethite particles is coated with a rare earth compound and a Co
compound by an ordinary method to produce starting particles for
the subsequent heat treatment.
[0066] As the rate earth compound, there may be suitably used
compounds of at least one rare earth element selected from
scandium, yttrium, lanthanum, cerium, praseodymium, neodymium and
samarium. The rare earth compound may be in the form of a chloride,
a sulfate and a nitrate of these rare earth elements.
[0067] As the Co compound, there may be suitably used the Co
compounds described in the above production reaction of the
spindle-shaped goethite seed crystal particles.
[0068] The coating treatment may be conducted by either a dry
method or a wet method, and is preferably conducted by a wet
method.
[0069] The amount of the rare earth compound added is preferably 10
to 30 atom % in terms of rare earth element based on the whole
Fe.
[0070] The Co compound is preferably added in such an amount that
the coating amount of Co lies in the range of 20 to 200% based on
the total amount of Co contained in the goethite particles produced
(sum of Co in seed crystals and Co in the goethite layer formed in
the growth reaction). When the coating amount of Co is more than
200%, it may be difficult to uniformly coat the particles with Co
owing to excessive amount of Co added, so that the Co compound
tends to be singly precipitated, and further the resulting magnetic
metal particles tend to be deteriorated in magnetic properties.
When the coating amount of Co is less than 20%, it may be difficult
to attain the effects of the addition of Co according to the
present invention owing to the less coating amount of Co.
[0071] When the goethite particles are coated with not only the
rare earth compound but also the Co compound, the resulting coated
goethite particles are prevented from undergoing sintering within
and between the particles upon the heat treatment, so that it is
possible to obtain hematite particles which more closely maintain
and inherit a particle shape and an aspect ratio of the goethite
particles. This allows magnetic metal particles containing iron as
a main component which retain and inherit the particle shape or the
like and are in the form of individual separate particles, to be
readily produced from the hematite particles.
[0072] The surface-coated goethite particles used in the present
invention have a spindle shape, comprise Co in an amount of 10 to
50 atom % (as a total amount of Co contained within the particles
and Co coated thereon) based on the whole Fe, and comprise Al in an
amount of 4 to 50 atom % based on the whole Fe.
[0073] The surface-coated goethite particles used in the present
invention preferably have an average major axis diameter of 0.03 to
0.10 .mu.m and an aspect ratio (average major axis diameter/average
minor axis diameter) of 5 to 10. The goethite particles preferably
have a BET specific surface area of 150 to 300 m.sup.2/g.
[0074] Next, the surface-coated goethite particles are subjected to
heat dehydration treatment in a non-reducing gas atmosphere to
obtain the hematite particles.
[0075] As the non-reducing atmosphere, there may be used a flow of
at least one gas selected from the group consisting of air, oxygen
gas and nitrogen gas. The heat-treating temperature used in the
heat dehydration treatment is usually 100 to 650.degree. C. The
heat-treating temperature may be appropriately varied and selected
from the above-specified range depending upon the kinds of
compounds used upon the coating treatments of the spindle-shaped
goethite particles. When the heat-treating temperature is less than
100.degree. C., the heat dehydration treatment may require a
prolonged period of time. When the heat-treating temperature is
more than 650.degree. C., deformation of the particles and
sintering within or between the particles tend to occur.
[0076] Next, the hematite particles are subjected to heat-reduction
treatment.
[0077] The reducing apparatus preferably used in the present
invention includes those in which a fixed bed is formed. More
specifically, there are preferably used a standing-type reducing
apparatus (batch type) and a movable reducing apparatus (continuous
type) in which the reducing treatment is conducted while
transporting a belt on which the fixed bed is formed.
[0078] The fixed bed used in the present invention preferably has a
height of not more than 30 cm. When the height of the fixed bed is
more than 30 cm, although a remarkable reduction-promoting effect
is obtained because of a large content of Co therein, the water
vapor partial pressure tends to be simultaneously increased owing
to rapid reduction in a lower layer of the fixed bed, thereby
causing problems such as deterioration in coercive force of the
particles present in an upper layer of the fixed bed and,
therefore, deterioration in properties of the particles as a whole.
From the viewpoint of a good industrial productivity, the height of
the fixed bed is more preferably 3 to 30 cm. Meanwhile, since the
batch type (as described in Japanese Patent Application Laid-open
(KOKAI) Nos. 54-62915 (1979) and 4-224609 (1992), etc.) is
different in productivity from the continuous type (as described in
Japanese Patent Application Laid-open (KOKAI) No. 6-93312 (1994),
etc.), the height of the fixed bed formed in the batch type fixed
bed reducing apparatus is preferably more than 4 cm and not more
than 30 cm.
[0079] In the present invention, the heat-reducing temperature is
preferably in the range of 300 to 700.degree. C. When the
heat-reducing temperature is less than 300.degree. C., the
reduction reaction tends to proceed too slowly, resulting in
prolonged reaction time. Further, since crystal growth of the
magnetic metal particles is insufficient, the resulting particles
tend to be deteriorated in magnetic properties such as saturation
magnetization and coercive force. When the heat-reducing
temperature is more than 700.degree. C., the reduction reaction
tends to proceed too rapidly, thereby causing deformation of the
particles as well as sintering within or between the particles.
[0080] The magnetic metal particles according to the present
invention which are obtained after the heat-reduction treatment may
be taken out in air by known methods, for example, by immersing the
particles in an organic solvent such as toluene; by temporarily
replacing the atmosphere existing around the magnetic metal
particle obtained after the reduction reaction, with an inert gas,
and then gradually increasing an oxygen content in the inert gas
until it finally becomes air; by gradually oxidizing the particles
using a mixed gas of oxygen and water vapor; or the like.
[0081] In the present invention, it is preferred that the
heat-reduction treatment and the surface oxidation treatment are
respectively repeated two times.
[0082] More specifically, the hematite particles are subjected to
the first heat-reduction treatment at a temperature of 300 to
650.degree. C. to obtain magnetic metal particles. The thus
obtained magnetic metal particles are then subjected to the first
surface oxidation treatment at a temperature of 60 to 200.degree.
C. in an oxygen-containing inert gas atmosphere to form an
oxidation layer on the surface of the respective magnetic metal
particles. Further, the thus surface-oxidized magnetic metal
particles are subjected to the second heat-reduction treatment at a
temperature of 300 to 700.degree. C., and the resulting magnetic
metal particles are then subjected to the second surface oxidation
treatment to form a surface oxidation layer thereon.
[0083] In the present invention, the atmosphere used during the
period until reacting the respective treating temperatures of the
first and second heat-reduction treatments may be either an inert
gas atmosphere or a reducing gas atmosphere. Examples of the
preferred inert gas atmosphere include nitrogen gas, helium gas and
argon gas. Among these inert gases, more preferred is nitrogen gas.
When the treating temperature is rapidly raised for not more than
40 min and preferably not more than 20 min in the reducing gas
atmosphere, the reducing temperature upon production of the
magnetic metal particles can be kept constant.
[0084] Meanwhile, the temperature rise rate in the first and second
heat-reduction treatments in the reducing atmosphere is preferably
20 to 100.degree. C./min.
[0085] In the present invention, the atmosphere used in the first
and second heat-reduction treatments is preferably a reducing gas.
As the reducing gas, hydrogen is preferably used.
[0086] The heat-reducing temperature used in the first
heat-reduction treatment in the present invention is 300 to
650.degree. C. and preferably 350 to 650.degree. C. The
heat-reducing temperature may be suitably selected from the
above-specified range depending upon kinds and amounts of the
compounds used in the coating treatment of the starting material.
When the heat-reducing temperature is less than 300.degree. C., the
reduction reaction tends to proceed very slowly and, therefore, is
undesirable from the industrial viewpoint, so that the resulting
magnetic metal particles tend to exhibit a low saturation
magnetization. When the heat-reducing temperature is more than
650.degree. C., the reduction reaction tends to proceed too
rapidly, thereby causing destruction of shape of the particles as
well as sintering within or between the particles which results in
deteriorated coercive force of the obtained particles.
[0087] The superficial velocity of the reducing gas used in the
first heat-reduction treatment in the present invention is
preferably 40 to 150 cm/s. When the superficial velocity of the
reducing gas is less than 40 cm/s, the water vapor generated by
reduction of the starting material tends to be discharged out of
the reaction system at a very slow rate, so that the upper layer of
the fixed bed tends to be deteriorated in coercive force and
S.F.D., thereby failing to obtain magnetic metal particles having a
high coercive force as a whole. When the superficial velocity of
the reducing gas is more than 150 cm/s, although the aimed magnetic
metal particles are obtained, the reducing temperature required
tends to be too high, so that there tend to arise problems such as
scattering and breaking of the resulting granulated product.
[0088] In the present invention, the first surface oxidation
treatment may be conducted in an oxygen-containing inert gas
atmosphere. Examples of the preferred inert gas atmosphere include
nitrogen gas, helium gas and argon gas. Among these inert gases,
more preferred is nitrogen gas. The content of oxygen in the inert
gas atmosphere is preferably 0.1 to 5 vol %, and the oxygen content
is preferably gradually increased until reaching a predetermined
amount.
[0089] The treating temperature used in the first surface oxidation
treatment in the present invention is 40 to 200.degree. C. and
preferably 40 to 180.degree. C. When the treating temperature used
in the first surface oxidation treatment is less than 40.degree.
C., it may be difficult to form a surface oxidation layer having a
sufficient thickness. When the treating temperature used in the
first surface oxidation treatment is more than 200.degree. C., the
particles tend to suffer causing deformation of the particles, in
particular, tend to be extremely swelled in the minor axis
direction owing to production of a large amount of oxide, which
tends to result in destruction of skeleton of the particles in the
worse case.
[0090] Meanwhile, when the particles are oxidized as a whole in the
first surface oxidation treatment, the particles tend to undergo
change in skeleton thereof, in particular, growth thereof in the
minor axis direction. As a result, the particles tend to be
extremely swelled in the minor axis direction owing to production
of a large amount of oxide, which tends to result in destruction of
skeleton of the particles in the worse case. The particles which
already suffer from such a destruction of the particle shape, are
no longer improved in coercive force even when subjected to
reduction reaction again.
[0091] The heat-reducing temperature used in the second
heat-reduction treatment in the present invention is 300 to
700.degree. C. When the heat-reducing temperature used in the
second heat-reduction treatment is less than 300.degree. C., the
reduction reaction tends to proceed very slowly and, therefore, is
undesirable from the industrial viewpoint, so that it may be
difficult to reduce the surface oxidation layer formed in the first
surface oxidation treatment and achieve densification of the
particles as a whole. When the heat-reducing temperature used in
the second heat-reduction treatment is more than 700.degree. C.,
destruction of shape of the particles as well as sintering within
or between the particles tend to be caused, resulting in
deteriorated coercive force of the obtained particles. The
heat-reducing temperature used in the second heat-reduction
treatment is preferably 450 to 650.degree. C.
[0092] The superficial velocity of the reducing gas used in the
second heat-reduction treatment in the present invention is
preferably 40 to 150 cm/s similarly to that used in the first
heat-reduction treatment.
[0093] Meanwhile, the second heat-reduction treatment may be
followed by annealing treatment. The treating temperature used in
the annealing treatment is preferably 400 to 700.degree. C. The
atmosphere used in the annealing treatment is preferably hydrogen
gas or an inert gas, in particular, nitrogen gas.
[0094] In the present invention, the second surface oxidation
treatment may be conducted in an inert gas atmosphere comprising 5
to 10 g/m.sup.3 of water vapor and oxygen. When the content of
water vapor in the inert gas atmosphere is less than 5 g/m.sup.3,
it may be difficult to form a dense thin surface oxidation layer,
and the resulting particles may fail to be sufficiently improved in
coercive force. When the content of water vapor in the inert gas
atmosphere is more than 10 g/m.sup.3, the aimed effects tend to be
already saturated, and the inclusion of more than necessary amount
of water vapor is meaningless. The content of water vapor in the
inert gas atmosphere is preferably 2 to 8 g/m.sup.3. The content of
oxygen in the inert gas atmosphere is preferably 0.1 to 5 vol %,
and the oxygen content is preferably gradually increased until
reaching a predetermined amount. Examples of the preferred inert
gas include nitrogen gas, helium gas and argon gas. Among these
inert gases, more preferred is nitrogen gas.
[0095] The treating temperature used in the second surface
oxidation treatment in the present invention is 40 to 160.degree.
C. and preferably 40 to 140.degree. C. Meanwhile, the reaction
temperature used in the second surface oxidation treatment is
preferably lower than that used in the first surface oxidation
treatment.
[0096] Next, various properties of the magnetic metal particles
which are obtained by the process for producing the magnetic metal
particles according to the present invention are described.
[0097] The magnetic metal particles of the present invention are of
a spindle shape and have an average major axis diameter of 5 to 100
nm. When the average major axis diameter is less than 5 nm, the
magnetic metal particles tend to be rapidly deteriorated in
oxidation stability and simultaneously tend to hardly exhibit high
magnetic properties (coercive force Hc). When the average major
axis diameter is more than 100 nm, the resulting magnetic metal
particles tend to be unsuitable as magnetic particles for providing
a magnetic recording medium exhibiting a high output and a reduced
noise in a short wavelength region owing to a large particle size
thereof. The average major axis diameter of the magnetic metal
particles of the present invention is preferably 6 to 80 nm and
more preferably 8 to 60 nm.
[0098] The magnetic metal particles of the present invention
preferably have an aspect ratio of not less than 2.0. When the
aspect ratio is less than 2.0, it may be impossible to obtain the
magnetic metal particles having a high coercive force as aimed. The
aspect ratio of the magnetic metal particles of the present
invention is more preferably 3.0 to 8.0.
[0099] The magnetic metal particles of the present invention as
behavior particles preferably have an average particle diameter of
not more than 90 nm. In particular, when the average major axis
diameter of the magnetic metal particles is 5 to 60 nm, the average
particle diameter of the behavior particles of the magnetic metal
particles is preferably 5 to 50 nm. When the average particle
diameter of the behavior particles is more than 90 nm, the
resulting particles tend to be increased in particle diameter owing
to sintering within or between the particles, thereby failing to
form a coating film having a sufficient surface smoothness. It is
considered that such a problem is caused by sintering of the
particles or stacking between the particles, thereby failing to
attain good magnetic properties by orientation.
[0100] The magnetic metal particles of the present invention
preferably have a BET specific surface area of 40 to 125 m.sup.2/g.
When the BET specific surface area is less than 40 m.sup.2/g, it
may be difficult to obtain magnetic metal particles satisfying a
less noise and a good dispersibility. When the BET specific surface
area is more than 120 m.sup.2/g, the resulting particles tend to be
hardly dispersed upon forming a coating material, so that the
obtained coating material tend to exhibit an undesirably high
viscosity. The BET specific surface area of the magnetic metal
particles is more preferably 70 to 110 m.sup.2/g.
[0101] The magnetic metal particles of the present invention
preferably have a degree of denseness of 0.5 to 2.5. The degree of
denseness of the magnetic metal particles is expressed by a ratio
of a specific surface area S.sub.BET as measured by BET method to a
surface area S.sub.TEM calculated from a major axis diameter and a
minor axis diameter as measured from the particles observed on a
microphotograph (S.sub.BET/S.sub.TEM).
[0102] When the ratio of S.sub.BET/S.sub.TEM is less than 0.5,
although high densification of the particles is achieved, the
resulting particles tend to be increased in particle diameter owing
to sintering within or between the particles, so that it may be
difficult to obtain a coating film having a sufficient surface
smoothness. When the ratio of S.sub.BET/S.sub.TEM is more than 2.5,
densification of the particles tends to be insufficient, so that a
large number of dehydration pores tend to be formed inside of the
particles or on the surface thereof, resulting in poor
dispersibility in a vehicle. From the viewpoints of a good
dispersibility in a vehicle and a good surface smoothness of the
coating film, the ratio of S.sub.BET/S.sub.TEM is preferably 0.7 to
2.0 and more preferably 0.8 to 1.6.
[0103] The content of cobalt in the magnetic metal particles is
preferably 20 to 50 atom % in terms of Co based on the whole Fe.
When the content of cobalt in the magnetic metal particles is less
than 20 atom %, the resulting particles tend to hardly exhibit a
low saturation magnetization while keeping a good switching field
distribution S.F.D., and may fail to exhibit a high coercive force.
When the content of cobalt in the magnetic metal particles is more
than 50 atom %, the resulting particles tend to have a low coercive
force as well as tends to be deteriorated in saturation
magnetization to more than necessary extent. The content of cobalt
in the magnetic metal particles is more preferably 30 to 50 atom
%.
[0104] The content of aluminum in the magnetic metal particles of
the present invention is preferably 4 to 50 atom % in terms of Al
based on the whole Fe. When the content of aluminum in the magnetic
metal particles is less than the lower limit, the anti-sintering
effect in the heat-reduction step tends to be lowered, so that the
resulting particles tend to be deteriorated in coercive force, and
exhibit a broad switching field distribution S.F.D. When the
content of aluminum in the magnetic metal particles is more than
the upper limit, the temperature required for the hydrogen
reduction tends to be considerably high, resulting in undesirable
production process. The content of aluminum in the magnetic metal
particles is more preferably 6 to 40 atom %.
[0105] The content of rare earth element in the magnetic metal
particles of the present invention is preferably 10 to 30 atom % in
terms of rare earth element based on the whole Fe. When the content
of rare earth element in the magnetic metal particles is less than
the lower limit, the anti-sintering effect in the heat-reduction
step tends to be lowered, so that the resulting particles tend to
be deteriorated in coercive force, and exhibit a broad switching
field distribution S.F.D. When the content of rare earth element in
the magnetic metal particles is more than the upper limit, the
temperature required for the hydrogen reduction tends to be
considerably high, resulting in undesirable production process. The
content of rare earth element in the magnetic metal particles is
more preferably 15 to 28 atom %.
[0106] The magnetic metal particles of the present invention
preferably have a crystallite size D.sub.110 of 70 to 170 .ANG..
When the crystallite size D.sub.110 is less than 70 .ANG., although
the magnetic recording medium using such particles is advantageous
from the viewpoints of reduction in noise due to the particles, the
resulting particles tend to be deteriorated in coercive force and
exhibit a broad switching field distribution S.F.D., and further
tend to be deteriorated in oxidation stability. When the
crystallite size D.sub.110 is more than 170 .ANG., the noise due to
the particles tends to be undesirably increased. The crystallite
size D.sub.110 of the magnetic metal particles is more preferably
70 to 150 .ANG..
[0107] The soluble Na content in the magnetic metal particles is
preferably not more than 30 ppm, more preferably not more than 20
ppm and still more preferably not more than 10 ppm. The soluble Ca
content in the magnetic metal particles is preferably not more than
100 ppm, more preferably not more than 80 ppm and still more
preferably not more than 70 ppm. When the contents of the above
each impurity are more than the respective upper limits, the
compounds derived from these impurities tend to be precipitated on
the surface of the obtained magnetic coating film. Also, the
content of, residual sulfur in the magnetic metal particles is
preferably not more than 60 ppm and more preferably not more than
50 ppm.
[0108] The coercive force He of the magnetic metal particles of the
present invention is preferably 95.4 to 278.5 kA/m (1200 to 3500
Oe). When the coercive force He is less than 95.4 kA/m, the
resulting magnetic recording medium tends to hardly exhibit a
sufficient output in a short wavelength region. When the coercive
force He is more than 278.5 kA/m, saturation of a recording head
tends to be caused, thereby failing to attain the aimed high output
in a short wavelength region. The coercive force He of the magnetic
metal particles is more preferably 119.4 to 278.5 kA/m (1500 to
3500 Oe) and still more preferably 143.2 to 278.5 kA/m (1800 to
3500 Oe).
[0109] The saturation magnetization us of the magnetic metal
particles of the present invention is preferably 60 to 160
Am.sup.2/kg (60 to 160 emu/g). When the saturation magnetization as
of the magnetic metal particles is less than 60 Am.sup.2/kg, the
resulting magnetic recording medium tends to hardly exhibit a
sufficiently high output in a short wavelength region owing to
deteriorated residual magnetization, and the resulting magnetic
metal particles may fail to have a high coercive force and a good
switching field distribution S.F.D. When the saturation
magnetization us of the magnetic metal particles is more than 160
Am.sup.2/kg, saturation of a magneto-resistance head tends to be
caused owing to excessive residual magnetization, and the resulting
magnetic recording medium tends to suffer from distortion of
reproduction properties and fail to exhibit a high C/N output in a
short wavelength region. The saturation magnetization as of the
magnetic metal particles of the present invention is more
preferably 60 to 120 Am.sup.2/kg (60 to 120 emu/g) and still more
preferably 70 to 110 Am.sup.2/kg (70 to 110 emu/g).
[0110] The magnetic metal particles of the present invention
preferably have a squareness (.sigma.r/.sigma.s) of 0.48 to 0.55
and more preferably 0.49 to 0.54.
[0111] The magnetic metal particles of the present invention
preferably have an oxidation stability As of not more than 20% and
more preferably not more than 15%.
[0112] The switching field distribution S.F.D. of a magnetic
coating film obtained by using the magnetic metal particles of the
present invention is preferably not more than 0.80. When S.F.D. of
the magnetic coating film is more than 0.80, the region in which
reversal of magnetization occurs tends to be expanded, so that the
resulting magnetic recording medium tends to hardly exhibit a
sufficient output in a short wavelength region. The switching field
distribution S.F.D. of a magnetic coating film obtained by using
the magnetic metal particles is more preferably not more than 0.75
and still more preferably not more than 0.70.
[0113] The coercive force Hc of a magnetic coating film obtained by
using the magnetic metal particles of the present invention is
preferably 111.4 to 278.5 kA/m (1400 to 3500 Oe) and more
preferably 143.2 to 278.5 kA/m (1800 to 3500 Oe). The magnetic
coating film obtained by using the magnetic metal particles of the
present invention has a squareness (Br/Bm) of preferably not less
than 0.65 and more preferably not less than 0.82, a surface
roughness Ra of preferably not more than 4.0 nm and more preferably
not more than 3.5, and an oxidation stability .DELTA.Bm of
preferably less than 15%.
[0114] Next, the magnetic recording medium of the present invention
is described.
[0115] The magnetic recording medium of the present invention
comprises a non-magnetic substrate, and a magnetic recording layer
formed on the non-magnetic substrate which comprises the magnetic
metal particles of the present invention and a binder resin.
[0116] As the non-magnetic substrate, there may be used films of
synthetic resins such as polyethylene terephthalate, polyethylene,
polypropylene, polycarbonates, polyethylene naphthalate,
polyamides, polyamide imides and polyimides, foils and plates of
metals such as aluminum and stainless steel, and various papers
which are presently generally used for production of magnetic
recording media. The thickness of the non-magnetic substrate may
vary depending upon materials thereof, and is usually 1.0 to 300
.mu.m and preferably 2.0 to 50 .mu.m.
[0117] The non-magnetic substrate for magnetic disks is usually
formed from polyethylene terephthalate, and the thickness of the
non-magnetic substrate for magnetic disks is usually 50 to 300
.mu.m. The non-magnetic substrate for magnetic tapes which is
formed from polyethylene terephthalate usually has a thickness of 3
to 100 .mu.m. The non-magnetic substrate for magnetic tapes which
is formed from polyethylene naphthalate usually has a thickness of
3 to 50 .mu.m. The non-magnetic substrate for magnetic tapes which
is formed from polyamides usually has a thickness of 2 to 10
.mu.m.
[0118] Examples of the binder resin include those which are
presently generally used for production of magnetic recording
media, such as vinyl chloride-vinyl acetate copolymers, urethane
resins, vinyl chloride-vinyl acetate-maleic acid copolymers,
urethane elastomers, butadiene-acrylonitrile copolymers, polyvinyl
butyral, cellulose derivatives such as nitrocellulose, polyester
resins, synthetic rubber-based resins such as polybutadiene, epoxy
resins, polyamide resins, polyisocyanates, electron beam-curable
acrylic urethane resins, and mixture thereof.
[0119] Also, the respective binder resins may comprise polar groups
such as --OH, --COOH, --SO.sub.3M, --OPO.sub.2M.sub.2 and
--NH.sub.2 wherein M represents H, Na or K.
[0120] The thickness of the magnetic recording layer as a coating
film formed on the non-magnetic substrate is in the range of 0.01
to 5.0 .mu.m. When the thickness of the magnetic recording layer is
less than 0.01 .mu.m, it may be difficult to obtain a uniform
coating film therefor, resulting in uneven coating thickness. When
the thickness of the magnetic recording layer is more than 5.0
.mu.m, it may be difficult to achieve desired electromagnetic
transfer characteristics owing to influence of diamagnetic
field.
[0121] The mixing ratio between the composite magnetic particles
and the binder resin in the magnetic recording layer is controlled
such that the composite magnetic particles are present in an amount
of 5 to 2000 parts by weight based on 100 parts by weight of the
binder resin.
[0122] Meanwhile, the magnetic recording layer may comprise, if
required, known additives generally used in magnetic recording
media, such as lubricants, abrasives and antistatic agents in an
amount of about 0.1 to 50 parts by weight based on 100 parts by
weight of the binder resin.
[0123] In the magnetic recording medium of the present invention, a
non-magnetic undercoat layer comprising non-magnetic particles and
a binder resin may be formed between the non-magnetic substrate and
the magnetic recording layer.
[0124] As the non-magnetic particles for the non-magnetic undercoat
layer, there may be used non-magnetic inorganic particles which are
usually used for non-magnetic undercoat layer of magnetic recording
media. Specific examples of the non-magnetic particles include
particles of hematite, iron oxide hydroxide, titanium oxide, zinc
oxide, tin oxide, tungsten oxide, silicon dioxide, .alpha.-alumina,
.beta.-alumina, .gamma.-alumina, chromium oxide, cerium oxide,
silicon carbide, titanium carbide, silicon nitride, boron nitride,
calcium carbonate, barium carbonate, magnesium carbonate, strontium
carbonate, calcium sulfate, barium sulfate, molybdenum disulfide
and barium titanate. These non-magnetic particles may be used alone
or in combination of any two or more thereof. Among these
non-magnetic particles, especially preferred are particles of
hematite, iron oxide hydroxide and titanium oxide.
[0125] Meanwhile, in order to improve a dispersibility of the
non-magnetic particles in a vehicle upon production of a
non-magnetic coating material, the surface of the non-magnetic
particles may be coated, if required, with a hydroxide of aluminum,
an oxide of aluminum, a hydroxide of silicon, an oxide of silicon,
etc. In addition, in order to improve various properties of the
resulting magnetic recording medium such as light transmittance,
surface resistivity, mechanical strength, surface smoothness and
durability, Al, Ti, Zr, Mn, Sn, Sb, etc., may be incorporated, if
required, into the non-magnetic particles.
[0126] The non-magnetic particles may have various shapes, and may
include granule-shaped particles having a spherical shape, a
granular shape, an octahedral shape, a hexahedral shape, a
polyhedral shape, etc., acicular particles having an acicular
shape, a spindle shape, a rice grain-like shape, etc., and
plate-shaped particles. From the viewpoint of a good surface
smoothness of the resulting magnetic recording medium, among these
non-magnetic particles, preferred are those particles having an
acicular shape.
[0127] The non-magnetic particles usually have an average particle
diameter of 0.01 to 0.3 .mu.m, and are usually of a granular shape
or, acicular shape a plate shape.
[0128] The acicular non-magnetic particles usually have an aspect
ratio of 2 to 20, whereas the plate-shaped non-magnetic particles
usually have a plate ratio (average plate surface diameter/average
thickness) of 2 to 50.
[0129] The non-magnetic undercoat layer preferably has a thickness
(as a thickness of a coating film) of 0.2 to 10.0 .mu.m. When the
thickness of the non-magnetic undercoat layer is less than 0.2
.mu.m, it may be difficult to improve a surface roughness of the
non-magnetic substrate.
[0130] The binder resin used for production of the non-magnetic
undercoat layer may be the same as that used for production of the
magnetic recording layer.
[0131] The mixing ratio between the non-magnetic particles and the
binder resin in the non-magnetic undercoat layer is controlled such
that the non-magnetic particles are present in an amount of 5 to
2000 parts by weight based on 100 parts by weight of the binder
resin.
[0132] Meanwhile, the non-magnetic undercoat layer may comprise, if
required, known additive used for production of magnetic recording
media, such as lubricants, abrasives and antistatic agents, in an
amount of about 0.1 to 50 parts based on 100 parts by weight of the
binder resin.
[0133] The magnetic recording medium comprising the non-magnetic
undercoat layer according to the present invention exhibits
substantially the same properties as those of the magnetic
recording medium comprising no non-magnetic undercoat layer. In the
magnetic recording medium comprising the non-magnetic undercoat
layer according to the present invention, in particular, the
surface thereof can be easily smoothened and flattened by
calendering, and the running durability thereof can be improved due
to the lubricant supplied from the non-magnetic undercoat
layer.
[0134] <Function>
[0135] The important point of the present invention resides in that
a magnetic tape (magnetic coating film) obtained by using the
magnetic metal particles which are prevented from causing
agglomeration therebetween in spite of fine particles having an
average major axis diameter as small as 5 to 100 nm exhibits
excellent magnetic properties (coercive force Hc).
[0136] Thus, the present invention aims at obtaining fine magnetic
metal particles having an average major axis diameter of 5 to 100
nm. In general, finer particles tend to suffer from sintering and
agglomeration therebetween. In order to prevent occurrence of
sintering between the particles, different kinds of metals such as
aluminum have been conventionally incorporated into the particles.
However, these different kinds of elements incorporated into the
particles are present in a large amount due to the fine particles.
As a result, the resulting magnetic metal particles tend to
comprise some particles which do not contribute to improvement in
magnetic properties of magnetic recording media.
[0137] On the other hand, in the present invention, under the
conditions for production of goethite particles, the oxidizing
agent is used before initiation of the oxidation reaction to
generate uniform goethite seed crystal particles, followed by
growth reaction of the particles. Therefore, the resulting goethite
particles which are minimized in inclusion of ultrafine particles
therein and kept in a more uniformly grown state, can be converted
into hematite particles. Thereafter, the hematite particles are
subjected to heat-reduction treatment to obtain the magnetic metal
particles. For this reason, the resulting magnetic metal particles
are minimized in inclusion of ultrafine particles therein, and as a
result, can exhibit a high coercive force required as magnetic
metal particles.
[0138] The magnetic coating film (magnetic tape) produced by using
the magnetic metal particles of the present invention comprises no
finer particles which do not contribute to improvement in magnetic
properties thereof, and behavior particles thereof have a uniform
particle size distribution, thereby providing a magnetic recording
medium having excellent magnetic properties (coercive force
Hc).
[0139] The magnetic metal particles of the present invention
exhibit a high coercive force He in spite of fine particles having
an average major axis diameter of 5 to 100 nm and are, therefore,
suitable as magnetic particles for production of magnetic recording
media satisfying a high output and a high C/N ratio in a short
wavelength region.
[0140] The magnetic metal particles obtained by the production
process of the present invention can provide a magnetic coating
film having excellent magnetic properties (coercive force Hc) in
spite of fine particles having an average major axis diameter of 5
to 100 nm and are, therefore, suitable as magnetic particles for
production of magnetic recording media capable of exhibiting a high
output and a high C/N ratio in a short wavelength region in which a
magneto-resistance head is used as a reproduction head.
EXAMPLES
[0141] Typical examples and embodiments of the present invention
are as follows.
[0142] The average major axis diameter, average minor axis diameter
and aspect ratio of the goethite particles, hematite particles and
magnetic metal particles as used or produced in the present
invention were respectively expressed by an average of numerical
values measured from a transmission electron micrograph.
[0143] Upon observing a sample by an electron microscope, the
sample was produced by the following method.
[0144] That is, 0.04 parts by weight of the magnetic metal
particles, 0.12 parts by weight of a dispersant and 99.84 parts by
weight of a dispersing medium (dispersing solvent) were treated by
an ultrasonic dispersing apparatus for a period of 30 min to obtain
a dispersion.
[0145] The resulting dispersion was placed on a mesh as a
supporting film for the sample and naturally dried, and then the
dried sample was observed. As a result, the particles thus observed
were uniformly dispersed on the sample supporting film and kept in
a deaggregated state due to the preliminary dispersion.
[0146] As described above, the degree of denseness of the magnetic
metal particles is expressed by the ratio of S.sub.BET/S.sub.TEM
wherein S.sub.BET is a specific surface area as measured by BET
method, and S.sub.TEM is a value calculated according to the
following formula assuming that the particles measured on the
electron micrograph are each of a rectangular parallelepiped shape
having an average major axis diameter of 1 cm and an average minor
axis diameter w cm.
S.sub.TEM(m.sup.2/g)=[(4lw+2w.sup.2)/(lw.sup.2.rho..sub.p)].times.10.sup-
.-4
wherein .rho..sub.p is a true specific gravity of the magnetic
metal particles for which 5.5 g/cm.sup.3 as a value measured by a
multi-volume density meter (manufactured by Shimadzu Seisakusho
Co., Ltd.) is used.
[0147] The contents of Co, Al, rare earth element, Na, Ca and other
metal elements in the goethite particles, hematite particles and
magnetic metal particles as used or produced in the present
invention, were measured using an inductively coupled plasma atomic
emission spectroscope ("SPS4000" manufactured by Seiko Denshi Kogyo
Co., Ltd.).
[0148] The BET specific surface area value of the goethite
particles, hematite particles and magnetic metal particles as used
or produced in the present invention is expressed by the value
measured by BET method using "Monosorb MS-11" (manufactured by
Cantachrom Co., Ltd.).
[0149] The crystallite size D.sub.110 of particles (X-ray crystal
grain size of the magnetic metal particles) is expressed by the
thickness of crystallite in the direction perpendicular to each
crystal plane (110) of the magnetic metal particles as measured by
X-ray diffraction method using a "X-ray diffraction apparatus"
(manufactured by Rigaku Corporation) under the following
conditions: target: Cu; tube voltage: 40 kV; tube current: 40 mA.
The crystallite size D.sub.110 was calculated based on the X-ray
diffraction peak curve prepared with respect to each crystal plane
by using the following Scherrer's formula:
D.sub.110=K.lamda./.beta. cos .theta.
wherein .beta. is a true half-width of the diffraction peak which
was corrected with respect to a width of a machine used (unit:
radian); K is a Scherrer constant (=0.9); .lamda. is a wavelength
of X-ray used (Cu K.alpha.-ray 0.1542 nm); and .theta. is a
diffraction angle (corresponding to a diffraction peak of crystal
plane (110)).
[0150] The magnetic properties of the magnetic metal particles and
magnetic coating film piece were measured using a vibration sample
magnetometer "VSM-3S-15" (manufactured by Toei Kogyo Co., Ltd.) by
applying an external magnetic field of 795.8 kA/m (10 kOe)
thereto.
[0151] The magnetic properties of the magnetic coating film piece
were measured by the following method.
[0152] The respective components as shown below were charged into a
140 mL plastic bottle, and then mixed and dispersed for 8 hr using
a paint shaker (manufactured by Reddevil Co., Ltd.), thereby
preparing a magnetic coating composition. The thus prepared
magnetic coating composition was coated on a 25 .mu.m-thick
polyethylene terephthalate film using an applicator to form a
coating layer having a thickness of 50 .mu.m thereon. The obtained
coating film was then dried in a magnetic field of 500 mT (5
kGauss), thereby obtaining a magnetic coating film piece. The
magnetic properties of the thus obtained magnetic coating film
piece were measured.
[0153] Coating Composition
TABLE-US-00001 Magnetic metal particles: 100 parts by weight Vinyl
chloride-based copolymer resin having a 10 parts by weight
potassium sulfonate group: Polyurethane resin having a sodium
sulfonate 10 parts by weight group: Abrasive (AKP-50): 10 parts by
weight Lubricant (myristic acid/butyl stearate: 1/2): 3 parts by
weight Curing agent (polyisocyanate): 5 parts by weight
Cyclohexanone 65.8 parts by weight Methyl ethyl ketone: 164.5 parts
by weight Toluene: 98.7 parts by weight
[0154] The .DELTA..sigma.s value showing an oxidation stability of
saturation magnetization of the magnetic metal particles, and the
.DELTA.Bm value showing a weather resistance of saturation magnetic
flux density (Bm) of the magnetic coating film were measured as
follows.
[0155] The particles or the magnetic coating film piece were placed
in a thermostatic chamber maintained at 60.degree. and a relative
humidity of 90%, and allowed to stand therein for one week to
conduct an accelerated deterioration test. Thereafter, the
particles or the magnetic coating film piece were measured to
determine the saturation magnetization value (.sigma.s') and
saturation magnetic flux density (Bm'), respectively. The oxidation
stability values .DELTA..sigma.s and .DELTA.Bm were calculated by
dividing the difference (absolute value) between the .sigma.s and
.sigma.s' values measured before and after the one-week accelerated
test, and the difference (absolute value) between the Bra and Bm'
values measured before and after the one-week accelerated test, by
the values us and Bm measured before the accelerated test,
respectively. The closer to zero the .DELTA..sigma.s and .DELTA.Bm
values, the more excellent the oxidation stability.
Example 1
[0156] 28 L of a mixed alkali aqueous solution comprising ammonium
carbonate and aqueous ammonia in amounts of 20 mol and 60 mol
(concentration of the ammonium hydroxide aqueous solution
corresponds to 75 mol % in terms of normality based on mixed
alkalis), respectively, was charged into a reaction tower equipped
with a stirrer having bubble dispersing blades, and heated to
50.degree. C. while rotating the stirrer at 700 rpm and passing a
nitrogen gas at a flow rate of 60 L/min through the reaction tower.
Then, 16 L of a ferrous sulfate aqueous solution comprising 20 mol
of Fe.sup.2+ (concentration of the mixed alkali aqueous solution
corresponds to 3.75 equivalents in terms of normality based on
ferrous sulfate) was charged into the bubble tower, and the
contents of the bubble tower were aged therein for 30 min.
Thereafter, 4 L of a cobalt sulfate aqueous solution comprising 6.0
mol of Co.sup.2+ (corresponding to 30 atom % in terms of Co based
on whole Fe) was added to the bubble tower and the contents of the
bubble tower were further aged for 2.5 hr.
[0157] Next, while rotating the stirrer at 450 rpm, an ammonium
peroxodisulfate aqueous solution as an oxidizing agent (in an
amount of 3.6% based on whole Fe) was added to the reactor, and the
contents in the reactor were allowed to stand for 10 min for
obtaining a uniform mixture. Thereafter, air was passed through the
reactor at a flow rate of 0.821 L/min to conduct the oxidation
reaction until the oxidation percentage of whole Fe.sup.2+ reached
30%.
[0158] Then, 1 L of an aluminum sulfate aqueous solution comprising
1.6 mol of Al.sup.3+ (corresponding to 8 atom % in terms of Al
based on whole Fe) was added to the reactor, and the oxidation
reaction was continued until completion of the reaction while
passing air therethrough at a flow rate of 0.82 L/min. It was
confirmed that the pH value of the reaction solution upon
termination of the reaction was 8.3.
[0159] The thus obtained slurry comprising goethite particles was
filtered by an ordinary method to separate the goethite particles
therefrom, and the thus separated goethite particles were washed
with water and then re-dispersed in water, followed by adding a
cobalt acetate aqueous solution (Co content: 10 atom % based on
whole Fe) to the resulting dispersion and sufficiently stirring the
mixture. Next, while stirring the mixture, a sodium carbonate
aqueous solution was added thereto to adjust a pH value of the
resulting aqueous solution to 8.8. Then, a yttrium nitrate aqueous
solution (yttrium content: 22 atom % based on whole Fe) was added
to the aqueous solution, and the resulting slurry was mixed under
stirring. Further, a sodium carbonate aqueous solution was added to
the slurry to adjust a pH value of the slurry to 9.3. Thereafter,
the slurry was filtered to separate the particles therefrom, and
the thus separated particles were washed with water and then dried,
thereby obtaining a dried solid product of goethite particles.
[0160] It was confirmed that the obtained goethite particles had an
average major axis diameter of 0.077 .mu.m, an aspect ratio of 7.8,
a BET specific surface area of 201.0 m.sup.2/g, a Co content of 40
atom % based on whole Fe, an Al content of 8 atom % based on whole
Fe, and a Y content of 21 atom % based on whole Fe.
[0161] The thus obtained solid product of the goethite particles
was dehydrated in air at 350.degree. C., and then heat-dehydrated
at 500.degree. C. in the same atmosphere, thereby obtaining a solid
product of hematite particles.
[0162] <Heat-Reduction Treatment>
[0163] Next, 100 g of the thus obtained granule-shaped granulated
product (average diameter: 2.6 mm) of the spindle-shaped hematite
particles were charged into a reducing apparatus of a batch fixed
bed type having an inner diameter of 72 mm so as to form a fixed
bed having a height of 7 cm. Thereafter, the heat reduction
reaction was conducted at 350.degree. C. while passing a hydrogen
gas at a superficial velocity of 50 cm/s until the dew point of
exhaust gas reached -30.degree. C., thereby obtaining magnetic
metal particles.
[0164] Then, after replacing the hydrogen gas with a nitrogen gas
again, the obtained particles were cooled to 80.degree. C. and
maintained at that temperature. Successively, air was mixed with
the nitrogen gas, and the amount of air mixed was gradually
increased until the oxygen concentration of the mixed gas reached
0.35 vol %. Under such an atmosphere, the particles were subjected
to surface oxidation treatment until the temperature thereof
reached the retention temperature plus 1.degree. C. (maximum
temperature of particles: 140.degree. C.; treating time: 2 hr),
thereby forming a surface oxidation layer on the surface of the
respective particles.
[0165] Next, the resulting particles were heated to 600.degree. C.
in a hydrogen gas atmosphere, and the heat reduction reaction was
conducted again at 600.degree. C. while passing a hydrogen gas
therethrough at a superficial velocity of 60 cm/s until the dew
point of exhaust gas reached -30.degree. C.
[0166] Then, after replacing the hydrogen gas with a nitrogen gas
again, the obtained particles were cooled to 80.degree. C. and
maintained at that temperature. Successively, 6 g/m.sup.3 of water
vapor and air was mixed with the nitrogen gas, and the amount of
air mixed was gradually increased until the oxygen concentration of
the mixed gas reached 0.35 vol %. Under such an atmosphere, the
particles were subjected to surface oxidation treatment until the
temperature thereof reached the retention temperature plus
1.degree. C. (maximum temperature of particles: 110.degree. C.;
treating time: 3 hr), thereby forming a surface oxidation layer on
the surface of the respective particles and producing a molded
product of magnetic metal particles.
[0167] It was confirmed that the thus obtained magnetic metal
particles had an average major axis diameter of 0.039 .mu.m, an
aspect ratio of 4.2, a BET specific surface area of 79.0 m.sup.2/g
and a crystallite size D.sub.110 of 99.0 .ANG., and were
spindle-shaped particles having a uniform particle size and
comprising no dendritic particles. In addition, it was confirmed
that the magnetic metal particles had a Co content of 40 atom %
based on whole Fe; an Al content of 8 atom % based on whole Fe; and
a Y content of 21 atom % based on whole Fe.
[0168] As to the magnetic properties of the magnetic metal
particles, the coercive force He thereof was 187.0 kA/m (2,350 Oe);
the saturation magnetization value .sigma.s thereof was 98.9
Am.sup.2/kg (98.9 emu/g); the squareness (.sigma.r/.sigma.s)
thereof was 0.535; and the oxidation stability (Aus) of saturation
magnetization thereof was 15.2%.
[0169] As to the magnetic properties of the magnetic coating film,
the coercive force He thereof was 2,573 Oe; the squareness (Br/Bm)
thereof was 0.832; the S.F.D. thereof was 0.57; and the oxidation
stability (.DELTA.Bm) thereof was 6.2%.
[0170] In addition, various properties of the obtained magnetic
metal particles are shown in Table 2, and various properties of the
magnetic tapes produced by using the magnetic metal particles are
shown in Table 3.
Examples 2 and 11 and Comparative Examples 1 to 6
[0171] The same procedure as defined in Example 1 for production of
goethite particles was conducted except that kind and amount of the
oxidizing agent added, neutralization temperature, time of addition
of the Al compound as well as amount of the Al compound added, and
oxidation reaction rate were changed variously, thereby obtaining
goethite particles. Essential production conditions and various
properties of the obtained goethite particles are shown in Table
1.
TABLE-US-00002 TABLE 1 Production conditions Amount of Examples and
oxidizing Neutralization Comparative Kind of agent added
temperature Examples oxidizing agent (mol %) (.degree. C.) Example
1 Ammonium 1.8 50 peroxodisulfate Example 2 Ammonium 0.5 50
peroxodisulfate Example 3 Ammonium 1.3 50 peroxodisulfate Example 4
Ammonium 2.6 50 peroxodisulfate Example 5 Ammonium 3.5 50
peroxodisulfate Example 6 Ammonium 1.6 58 peroxodisulfate Example 7
Ammonium 1.7 40 peroxodisulfate Example 8 Ammonium 1.7 45
peroxodisulfate Example 9 Ammonium 1.6 50 peroxodisulfate Example
10 Ammonium 3.5 40 peroxodisulfate Example 11 Ammonium 1.8 50
peroxodisulfate Comparative None -- 50 Example 1 Comparative Air --
50 Example 2 Comparative Air -- 50 Example 3 Comparative
H.sub.2O.sub.2 1.3 50 Example 4 Comparative H.sub.2O.sub.2 7.5 50
Example 5 Comparative H.sub.2O.sub.2 5.0 50 Example 6 Production
conditions Time of Amount of Al Examples and addition of Al Air
oxidation charged Comparative (oxidation reaction rate (Al/(Fe +
Co)) Examples percentage) (%) (l/min) (atom %) Example 1 30 0.82 8
Example 2 50 0.64 8 Example 3 60 0.85 8 Example 4 40 0.94 8 Example
5 30 3.62 8 Example 6 35 0.82 17 Example 7 35 2.48 19 Example 8 20
0.88 19 Example 9 60 2.48 8 Example 10 40 3.00 8 Example 11 30 0.82
36 Comparative 30 0.82 8 Example 1 Comparative 30 1.00 8 Example 2
Comparative 30 3.62 20 Example 3 Comparative 30 0.82 8 Example 4
Comparative 30 0.82 20 Example 5 Comparative 30 0.82 4 Example 6
Properties of goethite particles Examples and Average major Average
Comparative axis diameter aspect ratio BET Examples (nm) (--)
(m.sup.2/g) Example 1 77.4 7.8 201.0 Example 2 86.8 7.0 167.9
Example 3 82.8 6.9 183.3 Example 4 66.2 8.5 242.8 Example 5 58.0
8.1 238.6 Example 6 77.6 7.9 202.2 Example 7 71.8 8.0 231.0 Example
8 63.5 8.3 230.4 Example 9 77.8 7.4 216.8 Example 10 48.5 8.6 226.4
Example 11 73.6 6.9 267.9 Comparative 104.0 6.2 166.6 Example 1
Comparative 109.2 7.4 170.3 Example 2 Comparative 118.8 7.8 207.9
Example 3 Comparative 108.2 6.4 220.1 Example 4 Comparative 86.9
8.3 267.2 Example 5 Comparative 94.4 7.6 251.0 Example 6
[0172] Next, the same procedure as defined in Example 1 for
production of magnetic metal particles was conducted except that
the kinds of goethite particles used as the raw material were
changed variously, thereby obtaining magnetic metal particles.
[0173] Various properties of the obtained magnetic metal particles
are shown in Table 2, and various properties of the magnetic tapes
produced by using the magnetic metal particles are shown in Table
3.
TABLE-US-00003 TABLE 2 Examples and Properties of magnetic metal
particles Comparative Co/Fe Al/Fe Y/Fe Examples Particle shape
(atom %) (atom %) (atom %) Example 1 Spindle shape 40 8 21 Example
2 Spindle shape 40 8 22 Example 3 Spindle shape 40 8 22 Example 4
Spindle shape 40 8 22 Example 5 Spindle shape 40 8 22 Example 6
Spindle shape 40 17 21 Example 7 Spindle shape 40 19 20 Example 8
Spindle shape 39 19 22 Example 9 Spindle shape 38 8 22 Example 10
Spindle shape 39 8 22 Example 11 Spindle shape 39 36 22 Comparative
Spindle shape 40 8 22 Example 1 Comparative Spindle shape 40 8 8
Example 2 Comparative Spindle shape 40 20 21 Example 3 Comparative
Spindle shape 40 8 21 Example 4 Comparative Spindle shape 40 20 21
Example 5 Comparative Spindle shape 40 4 21 Example 6 Properties of
magnetic metal particles Examples and Average major Average minor
Average aspect Comparative axis diameter axis diameter ratio
Examples (nm) (nm) (--) Example 1 38.7 9.20 4.2 Example 2 52.1 13.4
3.9 Example 3 47.2 11.2 4.2 Example 4 29.8 8.3 3.6 Example 5 25.5
8.2 3.1 Example 6 38.8 9.5 4.1 Example 7 35.9 9.4 3.8 Example 8
30.5 8.0 3.8 Example 9 38.9 9.3 4.2 Example 10 18.9 6.3 3.0 Example
11 36.8 9.5 3.9 Comparative 52.0 12.1 4.3 Example 1 Comparative
53.3 12.4 4.3 Example 2 Comparative 58.4 13.0 4.5 Example 3
Comparative 48.7 12.2 4.0 Example 4 Comparative 39.1 9.3 4.2
Example 5 Comparative 40.6 9.7 4.2 Example 6 Properties of magnetic
metal particles BET Examples specific and Crystallite surface
Degree of Coercive Comparative size D.sub.110 area denseness force
Hc Examples (.ANG.) (m.sup.2/g) (S.sub.BET/S.sub.TEM) (Oe) Example
1 99 79.0 1.39 2350 Example 2 127 72.0 1.42 2550 Example 3 123 76.6
1.28 2525 Example 4 93 85.0 1.35 1975 Example 5 90 93.0 1.41 1887
Example 6 109 91.4 1.37 2417 Example 7 99 78.9 1.40 2290 Example 8
96 101.0 1.38 2317 Example 9 99 100.0 1.36 2275 Example 10 88 99.9
1.35 1689 Example 11 101 121.0 1.36 2310 Comparative 120 79.8 2.68
2430 Example 1 Comparative 121 83.2 2.71 2388 Example 2 Comparative
125 72 3.21 2408 Example 3 Comparative 126 83.2 2.02 2259 Example 4
Comparative 105 82.7 2.95 1850 Example 5 Comparative 99 82.6 2.41
1719 Example 6 Properties of magnetic metal particles Oxidation
Examples and Saturation Squareness stability Comparative
magnetization (r/s) (.DELTA..sigma.s) Examples (.sigma.s) (emu/g)
(--) (%) Example 1 98.9 0.535 15.2 Example 2 118.0 0.539 8.6
Example 3 113.0 0.534 7.3 Example 4 98.4 0.526 13.3 Example 5 96.6
0.513 17.0 Example 6 104.3 0.535 9.3 Example 7 102.8 0.535 16.3
Example 8 97.6 0.523 11.5 Example 9 95.7 0.515 12.0 Example 10 90.1
0.492 19.1 Example 11 93.8 0.515 17.8 Comparative 116.5 0.505 7.4
Example 1 Comparative 112.7 0.516 9.4 Example 2 Comparative 118.6
0.532 19.1 Example 3 Comparative 108.6 0.530 8.9 Example 4
Comparative 104.6 0.470 19.8 Example 5 Comparative 99.7 0.505 11.9
Example 6
TABLE-US-00004 TABLE 3 Properties of magnetic coating film Examples
and Coercive force Comparative Hc Squareness SFD Examples (Oe) (--)
(--) Example 1 2573 0.832 0.57 Example 2 2730 0.832 0.55 Example 3
2690 0.848 0.44 Example 4 2163 0.818 0.66 Example 5 2111 0.790 0.70
Example 6 2632 0.823 0.62 Example 7 2474 0.822 0.56 Example 8 2580
0.838 0.60 Example 9 2517 0.823 0.59 Example 10 1879 0.770 0.74
Example 11 2528 0.828 0.69 Comparative 2620 0.818 0.54 Example 1
Comparative 2303 0.822 0.64 Example 2 Comparative 2490 0.795 0.83
Example 3 Comparative 2454 0.824 0.58 Example 4 Comparative 2109
0.761 0.74 Example 5 Comparative 1898 0.777 0.82 Example 6
Properties of magnetic coating film Examples and Surface roughness
Oxidation stability Comparative Ra (.DELTA.Bm) Examples (nm) (%)
Example 1 3.9 6.2 Example 2 3.6 5.5 Example 3 3.4 5.8 Example 4 4.0
7.9 Example 5 4.2 8.1 Example 6 3.7 7.2 Example 7 4.1 7.6 Example 8
4.0 8.3 Example 9 3.7 6.9 Example 10 4.3 9.9 Example 11 4.1 10.1
Comparative 4.4 6.0 Example 1 Comparative 5.2 7.3 Example 2
Comparative 5.7 9.5 Example 3 Comparative 5.9 6.0 Example 4
Comparative 6.8 9.1 Example 5 Comparative 6.1 8.7 Example 6
[0174] As apparently understood from the results of the above
Examples and Comparative Examples, it was confirmed that the
magnetic metal particles produced by the process of the present
invention exhibited a high coercive force despite of small particle
size thereof. For example, from the comparison between Example 3
and Comparative Example 4 in which the magnetic metal particles
having a similar average major axis diameter were used, it was
apparently recognized that the particles obtained in Example 3
exhibited a higher coercive force than those obtained in
Comparative Example 4.
* * * * *