U.S. patent application number 11/434529 was filed with the patent office on 2006-09-14 for magnetic material, method for producing the same, and magnetic recording medium.
This patent application is currently assigned to Sony Corporation. Invention is credited to Yoh Iwasaki, Mikihisa Mizuno, Yuichi Sasaki.
Application Number | 20060204789 11/434529 |
Document ID | / |
Family ID | 29195503 |
Filed Date | 2006-09-14 |
United States Patent
Application |
20060204789 |
Kind Code |
A1 |
Iwasaki; Yoh ; et
al. |
September 14, 2006 |
Magnetic material, method for producing the same, and magnetic
recording medium
Abstract
A magnetic material having a structure of a material having a
ferromagnetic phase at ordinary temperature as a core and a
material having an antiferromagnetic phase at ordinary temperature
surrounding the periphery of the core in the form of a shell,
wherein a ratio between a volume of the ferromagnetic phase
material and the volume of the antiferromagnetic phase material in
the magnetic material is in a range where no exchange biasing field
of the magnetic material appears and a rotational hysteresis loss
of the magnetic material is made the maximum, a method of producing
the same, and a magnetic recording medium using the same.
Inventors: |
Iwasaki; Yoh; (Miyagi,
JP) ; Sasaki; Yuichi; (Miyagi, JP) ; Mizuno;
Mikihisa; (Miyagi, JP) |
Correspondence
Address: |
ROBERT J. DEPKE;LEWIS T. STEADMAN
ROCKEY, DEPKE, LYONS AND KITZINGER, LLC
SUITE 5450 SEARS TOWER
CHICAGO
IL
60606-6306
US
|
Assignee: |
Sony Corporation
|
Family ID: |
29195503 |
Appl. No.: |
11/434529 |
Filed: |
May 15, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10230625 |
Aug 29, 2002 |
|
|
|
11434529 |
May 15, 2006 |
|
|
|
Current U.S.
Class: |
428/692.1 ;
252/62.51C; 252/62.51R; 252/62.55; 252/62.56; G9B/5.276 |
Current CPC
Class: |
Y10T 428/2991 20150115;
G11B 5/712 20130101; Y10T 428/32 20150115 |
Class at
Publication: |
428/692.1 ;
252/062.55; 252/062.56; 252/062.51R; 252/062.51C |
International
Class: |
H01F 1/00 20060101
H01F001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 3, 2001 |
JP |
JP2001-265938 |
Claims
1-12. (canceled)
13. A magnetic material formed as an aggregate of a plurality of
antiferromagnetic phases and a plurality of ferromagnetic phases
wherein the phases are maintained at a temperature of 300.degree. K
or higher and wherein a ratio between a volume of said
ferromagnetic phase material and a volume of said antiferromagnetic
phase material is in a range where substantially no exchange
biasing field appears and a rotational hysteresis loss becomes
maximum or less than but substantially close to said maximum.
14. A magnetic material as set forth in claim 13, wherein said
ferromagnetic phase material comprises at least Fe, Co, or an alloy
having Fe or Co as an ingredient.
15. A magnetic material as set forth in claim 13, wherein said
ferromagnetic phase material comprises an alloy including at least
Pt, Cr, or an Fe oxide-based magnetic material.
16. A magnetic material as set forth in claim 13, wherein said
antiferromagnetic phase material includes at least an oxide of one
of Ni, Co, Fe, and Cr, or a fluoride of an alloy of one of Fe, Mn,
and Ni with K.
17. A magnetic material as set forth in claim 13, wherein said
antiferromagnetic phase material comprises an alloy including at
least Mn or Cr.
18-30. (canceled)
31. A magnetic material formed as an aggregate of a plurality of
antiferromagnetic phases and a plurality of ferromagnetic phases
wherein the phases are maintained at a temperature of 300.degree. K
or higher; and wherein a ratio between a volume of said
ferromagnetic phase material and a volume of said antiferromagnetic
phase material is in a range where substantially no exchange
biasing field appears and a rotational hysteresis loss becomes a
maximum or less than but substantially close to said maximum;
wherein said ferromagnetic phase material comprises at least Fe,
Co, or an alloy having Fe or Co as an ingredient, wherein said
ferromagnetic phase material comprises at least Fe, Co, or an alloy
having Fe or Co as an ingredient; and
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a magnetic material formed
by magnetically coupling a ferromagnetic material and an
antiferromagnetic material, a method of producing the same, and a
magnetic recording medium using the magnetic material.
[0003] 2. Description of the Related Art
[0004] As magnetic recording media, magnetic tape formed by coating
magnetic particles on a substrate, floppy discs, and magnetic disc
devices enabling random access as external storage devices of
computers, that is, so-called "hard discs"; are being widely
used.
[0005] A magnetic disc is comprised of a substrate formed on one
surface with a layer-like recording medium comprised of a magnetic
material. Fine magnetic particles are filled in the thin film
recording medium uniformly at a high density and in a good
dispersion state. A magnetic head moving above the recording medium
along a predetermined track magnetizes a group of fine magnetic
particles of the magnetic material corresponding to one bit or
determines a magnetization state of the group of magnetic particles
to record or reproduce one bit of data.
[0006] One of the methods for raising the recording density of a
magnetic disc device is reducing the thickness of the recording
medium and increasing the fineness of the magnetic particles of the
magnetic material forming the recording medium.
[0007] When magnetic particles of a magnetic material are increased
in fineness, however, the effect of so-called "thermal
fluctuaction" appears. If the effect of the thermal fluctuaction
exerted upon the magnetization becomes large, the direction of the
recorded magnetization of the fine magnetic particles is reversed
by the surrounding thermal energy and the direction of the
magnetization lost, or the residual magnetization or reproduction
output declines along with the elapse of time. Due to this, the
magnetic recording medium gradually becomes unable to stably
maintain a recorded state over a long time.
[0008] The stability of a magnetic particle against the thermal
fluctuaction may be expressed by K.sub.uV/kT
[0009] where,
[0010] K.sub.u is the magnetic anisotropy energy per volume of a
magnetic particle,
[0011] V is the volume of a magnetic particle,
[0012] k is the Boltzmann constant, and
[0013] T is the absolute temperature.
[0014] The smaller the value of K.sub.uV/kT, the larger the
influence of the thermal fluctuaction. The higher the recording
density, the smaller the volume V of a magnetic particle, so the
smaller the K.sub.uV/kT and the weaker the resistance to the
thermal fluctuaction.
[0015] The smaller the volume V of a magnetic particle, the smaller
the magnetic anisotropy energy of the magnetic particle and the
shallower the potential energy barrier for stabilizing the
magnetization direction, so the magnetization vector of the
magnetic particles easily escapes from that potential energy
barrier even due to the energy of the thermal fluctuaction and the
magnetization state becomes unstable.
[0016] The magnetic field necessary for changing the magnetization
direction of a magnetized magnetic particle is referred to as the
"coercive force". If the coercive force of the magnetic recording
medium is low, the magnetization state of the magnetic particles
will change and become unstable even by a small external effect
such as the thermal fluctuaction. Conversely, if the coercive force
is high, the magnetization state will be difficult to change by the
thermal fluctuaction, so resistance to the thermal fluctuaction
will be strong and the stability of the recorded state can be
secured.
[0017] Accordingly, in order to achieve high density recording, it
is necessary to increase the magnetic anisotropy energy of the
magnetic particles, and consequently raise the coercive force of
the magnetic material and overcome the influence of the thermal
fluctuaction.
[0018] Further, the magnetic material forming the magnetic
recording medium is being required to be more excellent in
corrosion resistance, smoothness, abrasion resistance, the ability
to secure a low process temperature, and various other
characteristics relating to ease of production and convenience in
usage. However, it is not easy to simultaneously satisfy the
requirements of magnetic anisotrophy energy, coercive force, and
the above characteristics.
[0019] One method being experimented with to increase the magnetic
anisotrophy energy of a magnetic material to raise the coercive
force is the method of utilizing magnetic coupling of a
ferromagnetic material and an antiferromagnetic material. Such a
magnetic material is attracting attention as a magnetic material
able to provide a large magnetic anisotrophy energy and a high
coercive force while largely reducing the thickness of the
ferromagnetic thin film.
[0020] For example, the thin film-shaped magnetic material
disclosed in Japanese Unexamined Patent Publication (Kokai) No.
11-296832 utilizes magnetic coupling of a ferromagnetic material
and antiferromagnetic material to raise the magnetic anisotrophy
energy and the coercive force while sufficiently satisfying the
reduction of the thickness of the recording medium and is therefore
suitable for a high recording density.
[0021] FIG. 1 shows the configuration of a magnetic recording
medium using such a thin film-shaped magnetic material.
[0022] The magnetic recording medium shown in FIG. 1 is comprised
of a substrate 101 on one surface of which an antiferromagnetic
layer 102 and a ferromagnetic recording layer 103 are stacked. The
antiferromagnetic layer 102 acts as an underlying layer of the
ferromagnetic recording layer 103. A nonmagnetic underlying layer
or the like may also be formed between the substrate 101 and the
antiferromagnetic layer 102.
[0023] Such a foil-like magnetic recording medium has a high
anisotrophy energy and coercive force, so is suitable for reduction
of thickness of the recording medium and a high recording
density.
[0024] Turning now to the problem to be solved by the invention, to
deal with future advances in magnetic recording devices, magnetic
materials and magnetic recording media having more excellent
properties are being demanded.
[0025] A magnetic fine particle having an antiferromagnetic phase
and a ferromagnetic phase is one leading candidate. Such a magnetic
fine particle would enable further fineness of the magnetic
material, would enable a good dispersion state of the magnetic
material to be secured, and could be expected to overcome the
problem of the thermal fluctuaction and raise the coercive
force.
[0026] The magnetic characteristics of a magnetic particle having
an antiferromagnetic phase and a ferromagnetic phase, for example,
a ferromagnetic particle having an antiferromagnetic shell, have
been reported in W. H. Meiklejohn and C. P. Bean, Phys. Rev., vol.
102 (1956) 1413 and Phys. Rev., vol. 105 (1957) 904.
[0027] However, the phenomena reported there were observed in the
case of cooling to a low temperature of 77 K, so the Co particles
with CoO shells used there are not suitable for use for a magnetic
recording medium at room temperature. Also, the reports dealt with
"asymmetry of magnetization hysteresis curves" as the effect of the
antiferromagnetic shell and not an increase of the coercive force
important for magnetic recording.
[0028] As the material of fine magnetic particles having a
sufficient coercive force, needle-like fine iron particles and
barium ferrite fine particles having been considered to be
candidates.
[0029] Iron-based metal particles, however, increase in surface
area along with increased fineness and suffer from a severe problem
of corrosion. Also, a barium ferrite fine particle has a plate-like
shape, so they solidify by superposing the fine particles,
therefore, no method has been established for realizing a good
dispersion state for coating as a medium.
SUMMARY OF THE INVENTION
[0030] A first object of the present invention is to provide an
increased fineness magnetic material having a ferromagnetic phase
and an antiferromagnetic phase suppressing the thermal
fluctuaction, further raising the magnetic anisotrophy energy and
coercive force, excellent in corrosion resistance and thermal
stability, having a good dispersion state, and able to cope with
further higher density recording while sufficiently satisfying the
requirements of a reduction of thickness of the magnetic recording
medium and increased fineness of the magnetic particles, and a
method for producing the same.
[0031] A second object of the present invention is to provide a
magnetic recording medium comprised of such an increased fineness
magnetic material.
[0032] To attain the first object, according to a first aspect of
the present invention, there is provided a magnetic material
comprising a core formed by a material having a ferromagnetic phase
at ordinary temperature and a shell formed by surrounding a
periphery of the core by a material having an antiferromagnetic
phase at ordinary temperature.
[0033] Preferably, a ratio between a volume of the ferromagnetic
phase material and a volume of the antiferromagnetic phase material
is in a range where no exchange biasing field appears in the
magnetic material and a rotational hysteresis loss becomes
maximum.
[0034] To attain the above first object, according to a second
aspect of the present invention, there is provided a method for
producing a magnetic material having a core formed by a material
having a ferromagnetic phase at ordinary temperature and having a
shell formed by coating a periphery of the core by a material
having an antiferromagnetic phase at ordinary temperature,
comprising the steps of forming the ferromagnetic core from a
predetermined ferromagnetic material, surrounding the ferromagnetic
core by a predetermined material for forming the antiferromagnetic
shell, and causing a reaction in an outer periphery of the
predetermined material for forming the antiferromagnetic shell in a
predetermined atmosphere to form the antiferromagnetic shell.
[0035] Preferably, in the step of forming the ferromagnetic core,
the method comprises heating the predetermined ferromagnetic
material for forming the ferromagnetic core and cooling the heated
ferromagnetic material to increase a component of a hexagonal
closest-packed structure in the crystal structure of the
ferromagnetic material.
[0036] To attain the above first object, according to a third
aspect of the present invention, there is provided a magnetic
material formed by aggregates each having a plurality of
antiferromagnetic phases formed at ordinary temperature and a
plurality of ferromagnetic phases formed at ordinary temperature
and magnetically coupled with the antiferromagnetic phases or by
composite particles each including a plurality of the
aggregates.
[0037] To attain the above second object, according to a fourth
aspect of the present invention, there is provided a magnetic
recording medium having a substrate and a magnetic layer arranged
on the substrate, wherein the magnetic layer includes a plurality
of magnetic units each having a core formed by a material having a
ferromagnetic phase at ordinary temperature and a shell formed by
coating the periphery of the core by a material having an
antiferromagnetic phase at ordinary temperature.
[0038] To attain the above second object, according to a fifth
aspect of the present invention, there is provided a magnetic
recording medium having a substrate and a magnetic layer arranged
on the substrate, wherein the magnetic layer includes a plurality
of magnetic units each formed from aggregates each having a
plurality of antiferromagnetic phases formed at ordinary
temperature and a plurality of ferromagnetic phases formed at
ordinary temperature magnetically coupled with the
antiferromagnetic phases, or by composite particles each including
a plurality of the aggregates.
[0039] According to the present invention, the magnetic material
includes a magnetic particle comprising a ferromagnetic phase core
and an antiferromagnetic phase shell. In a magnetic particle having
a ferromagnetic phase and antiferromagnetic phase, the
magnetizations of magnetic elements in the magnetic particle can be
easily aligned, the volume of the ferromagnetic phase relatively
increases due to the antiferromagnetic shell, and the
antiferromagnetic shell has the effect of maintaining the magnetic
order of the ferromagnetic phase. Therefore, even if small in
particle size, the particle holds a high coercive force and is
strong against the thermal fluctuaction.
[0040] The strength of the magnetic coupling of the ferromagnetic
phase core and the antiferromagnetic phase shell is dependennt upon
the ratio of volume between the ferromagnetic core and
antiferromagnetic shell. In the present invention, preferably the
value of the ratio of volume is in a range where no exchange
biasing field appears and the rotational hysteresis loss becomes
maximum in the magnetic particle.
BRIEF DESCRIPTION OF THE DRAWINGS
[0041] These and other objects and features of the present
invention will become clearer from the following description of the
preferred embodiments given with reference to the attached
drawings, wherein:
[0042] FIG. 1 is a sectional view of a thin film-shaped recording
medium of the related art utilizing magnetic coupling between a
ferromagnetic material and an antiferromagnetic material;
[0043] FIG. 2 is a sectional view of a magnetic material comprised
of a coexisting ferromagnetic phase and antiferromagnetic phase
according to a first embodiment of the present invention;
[0044] FIGS. 3A to 3C are views for explaining the enhancement of
the magnetic anisotrophy energy and coercive force caused by
magnetic coupling of a ferromagnetic material and an
antiferromagnetic material in a magnetic material comprised of a
coexisting ferromagnetic phase and the antiferromagnetic phase
according to the first embodiment of the present invention;
[0045] FIGS. 4A to 4C are views showing magnetization curves of the
magnetic materials shown in FIG. 3;
[0046] FIG. 5 is a graph of changes of a rotational hysteresis loss
and an exchange biasing field in a case where a thickness of the
antiferromagnetic shell in the magnetic material shown in FIG. 2 is
changed;
[0047] FIGS. 6A to 6C are views showing magnetization curves in a
case where the thickness of the antiferromagnetic shell in the
magnetic material shown in FIG. 2 is changed;
[0048] FIGS. 7A to 7C are views showing different types of
particles having the same magnetization;
[0049] FIG. 8 is a sectional view of a structure wherein a coupling
enhancement layer is provided in the magnetic material according to
the first embodiment;
[0050] FIG. 9 is a view showing experimental results on the
relationship between a diameter and coercive force in the magnetic
material according to the first embodiment;
[0051] FIG. 10 is a view showing experimental results on a
temperature dependency of the coercive force in the magnetic
material according to the first embodiment;
[0052] FIG. 11 is a view showing experimental data on corrosion
resistance of the fine particle magnetic material shown in the
first embodiment of the present invention;
[0053] FIG. 12 is a sectional view of a magnetic material comprised
of a coexisting ferromagnetic phase and antiferromagnetic phase
according to a second embodiment of the present invention;
[0054] FIG. 13 is a sectional view of a structure wherein a
coupling enhancement layer is provided in the magnetic material
according to the second embodiment;
[0055] FIG. 14 is a view showing experimental results on the
relationship between the diameter and the coercive force in the
magnetic material according to the second embodiment;
[0056] FIG. 15 is a perspective view of a disk-like magnetic
particle comprised of a coexisting ferromagnetic phase and
antiferromagnetic phase according to a third embodiment of the
present invention;
[0057] FIG. 16 is a sectional view of a magnetic particle comprised
of a coexisting ferromagnetic phase and antiferromagnetic phase
according to a fifth embodiment of the present invention;
[0058] FIG. 17 is a sectional view of a magnetic particle comprised
of a coexisting ferromagnetic phase and antiferromagnetic phase
according to a sixth embodiment of the present invention;
[0059] FIG. 18 is a sectional view of a magnetic particle comprised
of a coexisting ferromagnetic phase and antiferromagnetic phase
according to a seventh embodiment of the present invention; and
[0060] FIG. 19 is a sectional view of a magnetic recording medium
using a magnetic material comprised of a coexisting ferromagnetic
phase and antiferromagnetic phase according to an eighth embodiment
of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0061] Below, preferred embodiments of the magnetic material of the
present invention, the method for producing the same, and a
magnetic recording medium using the same will be explained with
reference to the attached drawings.
First Embodiment
[0062] FIG. 2 shows a magnetic material comprised of a coexisting
ferromagnetic phase and antiferromagnetic phase of the present
embodiment, that is, a magnetic particle 10. The magnetic particle
10 has a so-called spherical core-shell structure comprised of a
ferromagnetic phase 1 forming a schematically spherical core and an
antiferromagnetic phase 2 surrounding it in the form of a
shell.
[0063] A ferromagnetic material having a "ferromagnetic phase"
means a material exhibiting a net magnetization able to interact
with an external magnetic field. In this sense, it does not mean
just a ferromagnetic material in the strict sense, but includes
also a ferrimagnetic material such as spinel ferrite.
[0064] In a recording medium including a plurality of magnetic
particles 10 having such a core-shell structure, even if two or
more magnetic particles contact each other, since the ferromagnetic
phases 1 are isolated by the shells of the antiferromagnetic phases
so that they will not directly touch each other, the energy for
gathering together of a plurality of particles by magnetic force to
form secondary particles is small. Accordingly, it is easy to
obtain a magnetic powder coating film with a good dispersion state
of magnetic particles.
[0065] By providing a coexisting ferromagnetic phase 1 and
antiferromagnetic phase 2 in each magnetic particle 10, a high
coercive force can be maintained even with increased fineness of
the magnetic particles 10 due to the magnetic coupling of the
ferromagnetic phase 1 and the antiferromagnetic phase 2.
[0066] In the case of a recording medium of a ferromagnetic
material alone, the case of a thin film magnetic recording medium
utilizing the magnetic coupling of a ferromagnetic material and
antiferromagnetic material, or the case of a large magnetic
particle having a ferromagnetic phase and an antiferromagnetic
phase, the magnetic material exhibits a spatial distribution of the
magnetization of magnetic elements and the magnetizations are not
all aligned, so the coercive force ends up being lost by that
amount.
[0067] Contrary to this, in the case of a small magnetic material
having a ferromagnetic phase and an antiferromagnetic phase, the
magnetizations of the magnetic elements in the ferromagnetic
material 1 can be easily aligned due to the magnetic coupling of
the ferromagnetic phase 1 and the antiferromagnetic phase 2, the
volume of the ferromagnetic material 1 relatively increases due to
the antiferromagnetic material shell 2, and the antiferromagnetic
material shell 2 maintains the internal magnetic order of the
ferromagnetic material 1, so even with a small particle size, a
high coercive force can be held and resistance to the thermal
fluctuaction is strong.
[0068] The reason why the coercive force can be raised in the
magnetic particle 10 will be explained in more detail by using
FIGS. 3A to 3C.
[0069] FIGS. 3A to 3C show disc-like magnetic elements magnetized
by a magnetic head. Reference numeral 13 shows a ferromagnetic
element forming the ferromagnetic phase core 1, reference numeral
12 shows an antiferromagnetic element forming the antiferromagnetic
phase shell 2, and the arrows show the direction of
magnetization.
[0070] An antiferromagnetic material has a regular arrangement of
spin in its crystal lattice. For example, rightward spins and
leftward spins alternately occupy the lattice points. In the
crystal as a whole, the sum of the magnetic moments of the spins
becomes zero. Taking note of just one group of spins aligned in the
same direction, the broken arrows shown in the antiferromagnetic
elements 12 in FIGS. 3A to 3C show the magnetic moments
(sub-lattice magnetizations) of such a group of spins.
[0071] Also, in FIGS. 3A to 3C, the ferromagnetic element 13 and
the antiferromagnetic element 12 are single domains and are
oriented in magnetization to either the left or right. Due to the
magnetic field of the magnetic head, the magnetization direction of
the ferromagnetic element 13 is reversed and thus magnetic
recording is achieved.
[0072] FIGS. 4A to 4C show magnetization curves of the magnetic
elements shown in FIGS. 3A to 3C. In FIGS. 4A to 4C, the abscissas
indicate the external magnetic fields, and the ordinates indicate
the measured magnetizations.
[0073] As explained above, the antiferromagnetic element 12 has an
overall magnetic moment of zero and does not receive any torque
from the external magnetic field, so in the measurement of the
magnetization curve of the magnetic material element comprised of
the ferromagnetic element 12 and the antiferromagnetic element 13,
the magnetization observed is that of the ferromagnetic element 13
only.
[0074] As shown in FIG. 3B, when the antiferromagnetic element 12
and the ferromagnetic element 13 contact each other, an exchange
interaction acts between spins at the interface of the two
elements, and the magnetic coupling effect arises that aligns
magnetization of the ferromagnetic element 13 and sub-lattice
magnetization of the antiferromagnetic element 13 in parallel (or,
anti-parallel depending on the material) to each other. The
strength of the magnetic coupling is determined by the structure of
the interface.
[0075] For this reason, when the magnetization direction of the
ferromagnetic element 13 is changed by the external magnetic field,
the magnetization of the sub-lattice in the antiferromagnetic
element 12 tries to change orientation together. FIG. 3B expresses
this mode of magnetization reversal by changing the orientations of
the magnetization vector (solid arrow) of the ferromagnetic element
13 and the sub-lattice magnetization (broken arrow) of the
antiferromagnetic element 12 while leaving them aligned in the same
direction as each other.
[0076] As shown in FIG. 3B, in a magnetic material element where
the magnetization of the ferromagnetic element 13 and the
sub-lattice magnetization of the antiferromagnetic element 12
change in orientation while being aligned by the exchanging
coupling at the interface, there is an effect of increase of the
coercive force.
[0077] The height of the potential energy barrier which must be
overcome in order to change the orientation of the magnetization is
given by the sum of the anisotrophy energy of the ferromagnetic
element 13 and the anisotrophy energy of the antiferromagnetic
element 12 of this magnetic material element. Particularly, if the
axes of easy magnetization of the two coincide, it is clear that
the potential energy barrier becomes higher by taking the sum of
the same.
[0078] On the other hand, the torque applied by the external
magnetic field so that the magnetization rotates is proportional to
the product of a saturation magnetization M of the ferromagnetic
element 13, a volume V of the ferromagnetic element 13, and the
strength of the magnetic field H. In order to produce the
magnetization reversal, this torque must exceed the above slope of
the potential energy.
[0079] Namely, when the ferromagnetic element 13 is coupled with
the antiferromagnetic element 12, in order to produce the
magnetization reversal, the amount of increase of the torque acting
upon the ferromagnetic element 13 must be larger than the amount of
increase of the potential energy and therefore greater magnetic
field H becomes necessary.
[0080] FIG. 3A shows a magnetic material element comprised of an
antiferromagnetic element 12 and a ferromagnetic element 13 which
is not coupled with but is isolated from the antiferromagnetic
element 12. The magnetization curve thereof is shown in FIG.
4A.
[0081] FIG. 3B shows a magnetic material element where the
ferromagnetic element 13 is coupled with the antiferromagnetic
element 12. The magnetization curve thereof is shown in FIG.
4B.
[0082] In the magnetization curves shown in FIGS. 4A to 4C, the
value of the magnetic field H necessary for producing the
magnetization reversal is the magnetic field where magnetization
curve cuts across the abscissa.
[0083] Accordingly, as shown in FIGS. 4A to 4C, when the
ferromagnetic element 13 is coupled with the antiferromagnetic
element 12 (FIG. 4B), the magnetic field necessary for the
magnetization reversal increases and the coercive force (magnetic
field corresponding to distance between two positions where the
magnetization curve cuts across the abscissa) increases from the
case where the ferromagnetic element 13 and the antiferromagnetic
element 12 are not coupled (FIG. 4A).
[0084] The strength of the exchange coupling at the interface of
the ferromagnetic element 13 and the antiferromagnetic element 12
is determined just by the structure of the interface, so does not
change and maintains a constant strength even if the volume of the
parts away from the interface increases or decreases. It is seen
from this how the magnetization curve of the ferromagnetic element
13 coupled with the antiferromagnetic element 12 changes according
to the volume of the antiferromagnetic element.
[0085] In the case where an antiferromagnetic element 12 thicker
than the one shown in FIG. 3B is used, as shown in FIG. 3C, the
entire magnetic anisotropy of the antiferromagnetic element 12
becomes greater in proportion to the volume, so enough of a torque
to overcome this and cause rotation of the sub-lattice
magnetization cannot be given by the exchange coupling of the
interface. Namely, when the magnetization of the ferromagnetic
element 13 rotates, the action of the coupling of trying to align
the spins at the two sides of the interface in the same orientation
is still maintained, but the restraint requiring that the spins on
two sides be aligned in the same orientation disappears. In this
way, as shown in FIG. 3C, only the magnetization of the
ferromagnetic element 13 reverses without a change of orientation
of the sub-lattice magnetization of the antiferromagnetic element
12.
[0086] FIG. 4C shows the magnetization curve in this case. The
volume of the antiferromagnetic element 13 is large, therefore even
if the ferromagnetic element 13 and the antiferromagnetic element
12 are coupled at the interface, in order to reverse the
magnetization M from positive to negative, a stronger magnetic
field than that in the case of the antiferromagnetic element 12
having a small volume as in FIG. 3B becomes necessary. Namely, in
comparison with FIG. 4B, the position where the left side of the
magnetization curve of FIG. 4C cuts across the abscissa appears at
a point moved in the negative direction of the H-axis of the
magnetic field.
[0087] Conversely, when reversing the magnetization from negative
to positive, coupling trying to align the magnetization of the
ferromagnetic element 13 in the same orientation as that of the
sub-lattice magnetization of the antiferromagnetic element 12
originally directed to the positive direction is maintained,
therefore when the external magnetic field directing the
magnetization to the negative direction is slightly weakened, the
orientation of the magnetization is reversed early before the
magnetic field direction changes to the positive direction. The
value of this magnetic field appears at the position where the
right side of the magnetization curve of FIG. 4C cuts across the
abscissa.
[0088] Namely, when the volume of the antiferromagnetic element 12
is large, the coercive force does not increase in comparison with
the case where there is no antiferromagnetic element 12, but the
magnetization curve is offset to the negative direction on the axis
of the magnetic field H. This offset will be referred to as the
"exchange biasing field (Hex)".
[0089] In the asymmetric magnetization curve of FIG. 4C, only one
magnetization state can be taken when the external magnetic field
is zero, so this material cannot be utilized for a magnetic storage
wherein the magnetization direction is changed to the intended
direction.
[0090] As seen from the above explanation, in order to couple an
antiferromagnetic element 12 with a ferromagnetic element 13 to
realize an increase of the coercive force, the volume of the
antiferromagnetic element 12 must be made in an appropriate
range.
[0091] Hereinafter, the properties of the magnetic particle 10 of
the present embodiment will be explained by taking as an example
one using Co as the material of the ferromagnetic core 1 shown in
FIG. 2 and using NiO as the material of the antiferromagnetic shell
2, that is, surrounding the Co core by NiO.
[0092] For a magnetic particle 10 of the core-shell structure to
give the largest coercive force, there is an optimum combination of
volume of the ferromagnetic phase 1 and the antiferromagnetic phase
2.
[0093] The change of the magnetic characteristics of a magnetic
particle 10 when holding a radius r.sub.FM of the ferromagnetic
phase core 1 constant and gradually changing a radius r.sub.AF of
the outside of the antiferromagnetic shell 2 larger was checked for
a core-shell magnetic particle 10 in a state of a substantially
concentric spherical shell shown in FIG. 2, FIG. 5 and FIGS. 6A to
6C show the results thereof.
[0094] FIG. 5 plots the change of the rotational hysteresis loss Wr
(amount determined by torque measurement) and the change of the
exchange biasing field Hex in the case where the radius r.sub.FM of
the Co core 1 in the core-shell magnetic particle 10 of the
concentric spherical shell state shown in FIG. 2 is held at 10 nm
and the thickness t.sub.AF of the NiO shell 2 is changed. Here, use
is made of an amount Wr/Wr.sub.MAX and Hex/Hex.sub.MAX normalized
by the maximum value Wr.sub.MAX of the rotational hysteresis loss
and the maximum value Hex.sub.MAX of the exchange biasing field
within the range where t.sub.AF is changed.
[0095] FIGS. 6A to 6C show magnetization curves of magnetic
particles 10 of the concentric spherical shell state shown in FIG.
2. They show the change of the coercive force due to a change of
the volume (here, thickness) of the antiferromagnetic phase 2. In
FIGS. 6A to 6C, the abscissas indicate an external magnetic field,
and the ordinate indicates the magnetization by the ferromagnetic
phase 1 of the magnetic particles 10.
[0096] When the thickness T.sub.AF of the antiferromagnetic shell 2
is 5 nm, as shown in FIG. 5 and FIG. 6A, no exchange biasing field
Hex appears, but the rotational hysteresis loss is small, and the
coercive force of the magnetic particle 10 is small.
[0097] When the thickness T.sub.AF of the antiferromagnetic shell 2
is increased, the rotational hysteresis loss and the coercive force
increase. As shown in FIG. 5 and FIG. 6B, near a thickness T.sub.AF
of the antiferromagnetic shell 2 of 10 nm, the rotational
hysteresis loss becomes the maximum and the coercive force becomes
the maximum.
[0098] When the thickness T.sub.AF of the antiferromagnetic shell 2
is increased more than that, as shown in FIG. 5 and FIG. 6C, the
magnetization curve is laterally shifted asymmetrically, that is,
an exchange biasing field Hex appears, and the coercive force
becomes small.
[0099] Accordingly, in the above example, the magnetic particle 10
gives the maximum coercive force at the ratio of volume of the
ferromagnetic phase 1 and the antiferromagnetic phase 2 at the time
when T.sub.AF=10 nm.
[0100] However, the thickness of the antiferromagnetic shell 2
required for obtaining an increase of the coercive force becomes a
thickness not negligible in comparison with the size of the
ferromagnetic core 1, so the average magnetization M.sub.AV of the
entire core-shell magnetic particle 10 is represented as follows by
using the magnetization M.sub.FM of the ferromagnetic phase 1:
M.sub.AV=M.sub.FM.times.(r.sub.FM/r.sub.AF).sup.3 Therefore, it is
diluted to become considerably smaller than the magnetization of
the ferromagnetic phase 1. Accordingly, in order to realize a
magnetic recording medium able to provide to a reproduction head a
magnetic field equivalent to that of the case of use of barium
ferrite magnetic particles having for example a saturation flux
density of 5 kG by core-shell structure particles of the present
invention, a ferromagnetic core 1 having a saturation flux density
considerably larger than 5 kG must be used.
[0101] FIGS. 7A to 7C are views of different aspects of particles
having the same magnetization, wherein FIG. 7A shows a case of a
particle 11a having a high saturation flux density, FIG. 7B shows a
case of a high saturation flux density particle 11b having an
antiferromagnetic material shell, and FIG. 7C shows a case of a
particle 11c having a relatively low saturation flux density.
[0102] As ferromagnetic materials having a large saturation flux,
other than Co (18 kG), there are Fe (22 kG) or alloys having these
as main ingredients. In the present embodiment, these are
preferably used for the ferromagnetic core 1.
[0103] Of course, a magnetic material having a relatively large
magnetic anisotropy and coercive force should be employed for the
ferromagnetic core 1 itself. With this plus the help of the
antiferromagnetic shell 2, a further improvement of characteristics
can be achieved. For example, as the material of the ferromagnetic
phase core 1, it is possible to use a high coercive force alloy
containing at least Pt, Cr, or the like, or an Fe oxide-based
magnetic material.
[0104] Note that such high coercive force alloys include ones
having a relatively low ratio of magnetic atoms in the composition.
Sometimes, the probability of existence at adjacent positions where
the spin of the antiferromagnetic phase 2 and the spin of the
ferromagnetic phase 1 can interact at the contact interface with
the antiferromagnetic shell 2 becomes low.
[0105] As a means for preventing a drop in the interaction of the
two phases occurring in this way, it is effective to place a
magnetic metal layer 3 having a high spin density between the high
coercive force alloy core 1 and the antiferromagnetic shell 2.
[0106] FIG. 8 is a sectional view of the structure where a coupling
enhancement layer 3 is provided in the core-shell magnetic particle
10. For this coupling enhancement layer 3, a ferromagnetic metal
layer containing for example Fe, Co, or Ni as a main ingredient is
used.
[0107] A ferromagnetic metal like Fe and Co having a large magnetic
moment per atom is apt to form a strong exchange coupling with a
different type of material, so by interposing this, the coupling
between the ferromagnetic phase 1 and the antiferromagnetic phase 2
will be raised.
[0108] For the ferromagnetic core 1, Co or another hard to oxidize
metal is preferably used.
[0109] When use is made of a ferromagnetic material easily
oxidized, along with the elapse of time, near the interface between
an antiferromagnetic shell 2 made of for example NiO and the
ferromagnetic core 1, the NiO is reduced and an oxide of the
ferromagnetic material is produced. When the oxide is nonmagnetic
at room temperature, the magnetic coupling between the
ferromagnetic core 1 and the antiferromagnetic shell 2 becomes weak
and a sufficiently large coercive force is no longer obtained.
[0110] The crystal structure of Co that forms the ferromagnetic
phase core 2 is comprised of a mixture of hexagonal closest packed
(hcp) planes packed in an fcc (face centered cubic) structure and
packed in an hcp structure. An hcp-enriched one is advantageous
since the magnetic anisotropy derived from the crystal structure
becomes large.
[0111] The relationship between the size of various types of Co
particles and the coercive force given by the magnetic particles
for Co particles forming the ferromagnetic core 1 will be explained
next based on the experimental results of FIG. 9.
[0112] In FIG. 9, the data a, data b, and data c show the coercive
force at room temperature (300 K) of a spherical Co particle with
surface oxidation, the coercive force at 77 K of a spherical Co
particle with surface oxidation, and the coercive force at room
temperature (300 K) of a hexagonal closest packed structure
(hcp)-enriched spherical Co particle with a shell of NiO having a
thickness of 10 nm. Note that, in FIG. 9, the abscissa shows the
diameter of the Co core 1, and the ordinate shows the coercive
force of the magnetic material comprised of the Co core 1 and the
shell 2.
[0113] The coercive force at room temperature (300 K) of a Co
particle with surface oxidation, as shown by the data a, becomes
extremal near a diameter of 30 nm.
[0114] When the same specimen is cooled to the liquid nitrogen
temperature (77 K) and measured, as shown by the data b, the
coercive force is not that much different from the coercive force
at room temperature when the particle size is 10 nm or more, while
the coercive force becomes maximum when the particle size is 20
nm.
[0115] When a slight hcp-enriched Co particle has a shell of NiO
having a thickness of 10 nm, at room temperature (300 K), as shown
by the data c, the coercive force is larger than that of a Co
particle with surface oxidation in entire range of particle size
measured.
[0116] Due to this, it was seen that by covering the ferromagnetic
Co core 1 by a shell of the antiferromagnetic NiO having a
thickness of 10 nm, a large coercive force was obtained by a small
size of this magnetic particle having a core-shell structure.
[0117] The effect of the antiferromagnetic shell 2 becomes
conspicuous when the size of the CoO particle is 30 nm or less. The
increase in the coercive force at a particle size more than that is
attributable to the fact that the crystal magnetic anisotropy
becomes large since the particle is hcp-enriched.
[0118] The fact that the coercive force does not increase so much
in the particle size range of 60 nm or more is understood to be due
to the following. Namely, magnetization reversal of the fine
particles occurs in the uniform rotation mode. The increase in the
magnetic anisotropy (including also contribution of the magnetic
anisotropy sensed by the spin group of the antiferromagnetic
material exchange coupled and rotating together) is reflected in
the coercive force in a substantially direct proportional
relationship. This is because when the particle size becomes large,
a curling mode, fanning mode, or other mode of magnetization
reversal where the coercive force is not always proportional to the
magnitude of the magnetic anisotropy becomes the main mode.
[0119] In order to use this for a magnetic coating for a super-high
density magnetic recording medium, a particle size of 10 to 60 nm
is preferred. The present embodiment has a valid effect within this
range of particle size.
[0120] The function of the antiferromagnetic shell 2 made of NiO
surrounding the ferromagnetic core 1 made of Co will be explained
next based on experimental data of the relationship between the
coercive force and temperature with reference to FIG. 10. The
diameter of the ferromagnetic core 1 is set at 10 nm.
[0121] In FIG. 10, the data a, data b, and the data c show the
temperature dependency of coercive force in the cases of a Co
particle with surface oxidation, a Co particle with a shell of NiO
having a thickness of 10 nm, and an hcp-enriched Co particle with a
shell of NiO having a thickness of 10 nm.
[0122] A Co particle with surface oxidation (data a) and a Co
particle with a shell of NiO having a thickness of 10 nm (data b)
exhibit equivalent coercive forces at a low temperature near 100 K.
When the temperature rises to 150 K or more, the coercive force of
the Co particle with surface oxidation is conspicuously lowered,
but, as shown by the data b, the coercive force of the Co particle
with a shell of NiO is not reduced so much over a temperature range
up to 400 K. Namely, a Co particle with a shell of NiO is suitable
for a magnetic recording medium used in a room temperature
environment.
[0123] When using an hcp-enriched Co particle, as shown by the data
c, the crystal magnetic anisotropy of the Co particle itself
becomes large, therefore a high coercive force is obtained over a
wide temperature range by a synergistic effect with the function of
the NiO shell and, in addition, the fluctuation of the coercive
force is small.
[0124] The antiferromagnetic material loses antiferromagnetic order
at a certain temperature or more. This critical temperature is
referred to as the "Neel temperature" and defined as for example
T.sub.N.
[0125] Also, the highest temperature at which a contacting
ferromagnetic material exhibits a restraining force on
magnetization will be referred to as the "blocking temperature" and
defined as for example T.sub.B.
[0126] The high coercive force of the Co particle with surface
oxidation at a low temperature is due to the fact that the Neel
temperature of the CoO phase of the surface is near 300 K, so the
CoO phase of the surface has a stable antiferromagnetic spin order
in a low temperature region.
[0127] Since the Neel temperature of NiO is near 520 K, or far
higher, with a particle with an NiO shell, this action is
maintained up to room temperature or more. Also, fabrication of NiO
is easy. For this reason, a core-shell structure of a metal Co core
1 surrounded by an NiO shell 2 is the most practical structure of
the present embodiment.
[0128] While NiO was used as an example as the material of the
antiferromagnetic shell 2 above, it is also possible to use other
materials.
[0129] Next, a more detailed description will be given of the
material of the antiferromagnetic phase material of the present
invention.
[0130] The magnetic material of the present embodiment includes a
ferromagnetic phase 1 able to exchange information with a magnetic
head via a magnetic field and an antiferromagnetic phase 2 which
does not generate a magnetic field in space and does not receive
torque from the magnetic field while holding magnetic order inside
the material.
[0131] As usable antiferromagnetic materials, a wide range of
materials such as alloys, oxides, and fluorides can be utilized so
long as they exhibit antiferromagnetic order at a temperature more
than the operating temperature of the magnetic recording
device.
[0132] As materials maintaining antiferromagnetic order up to a
temperature higher than room temperature (more than 300 K), there
are Ni--Mn, Pt--Mn, Pt--Cr--Mn, and Pd--Pt--Mn. As materials which
can be prepared without necessity of heat treatment, there are
Fe--Mn, Ir--Mn, Rh--Mn, Ru--Mn, Pt--Cr--Mn, Cr--Al, and the
like.
[0133] Other than alloys, it is also possible to use the
antiferromagnetic materials of oxides or fluorides. Oxide materials
are excellent in the point of resistance to corrosion.
[0134] Mn exhibits antiferromagnetism in the .gamma. phase (gamma
phase) and exhibits antiferromagnetism in a considerably wide range
of composition even alloyed with a 3d transition metal or precious
metal, so is a constituent element of an alloy suited to the
present embodiment. Particularly, a composition wherein the ratio
of the number of atoms of Mn is 40% or more can give good
characteristics.
[0135] As examples of antiferromagnetic materials of fluorides,
there are FeF.sub.2, MnF.sub.2, K.sub.2NiF.sub.4, and the like.
Some of them lose antiferromagnetic order at a temperature lower
than room temperature, but these can be utilized in combination
with different types of substances sustaining antiferromagnetism up
to a high temperature.
[0136] As materials having properties resembling antiferromagnetic
materials (small magnetization and large magnetic anisotropy) and
suitable for the present embodiment, ferrimagnetic materials having
compositions close to compensation compositions can be utilized.
Alloys of rare earth metals and transition metals, for example,
Tb--Co alloys, are candidates.
[0137] As mentioned above, CoO has a Neel temperature near room
temperature and does not strongly hold antiferromagnetic order at
room temperature, so cannot be utilized alone in the present
embodiment. With a solid solution or a multilayer laminate
(artificial lattice film) of CoO and NiO, however, a Neel
temperature between the Neel temperatures of CoO and NiO can be
realized. At this time, when compared with pure NiO, the Neel
temperature is lowered, but CoO has a larger magnetic anisotrophy
energy than NiO, so the contribution thereof can be utilized. There
are also some materials mentioned above which have low Neel
temperatures, but these can be used by forming such complexes.
[0138] The optimum thickness of the antiferromagnetic shell 2 for
obtaining the maximum coercive force in the magnetic particle 10 of
the present embodiment was investigated for various
antiferromagnetic materials. Table 1 lists the results thereof.
[0139] The thickness of the antiferromagnetic shell 2 preferred for
obtaining an increase of the coercive force by coupling with a
ferromagnetic particle 1 is near the maximum of the rotational
hysteresis loss as mentioned above. This is determined by both of
the magnetic anisotropy of the antiferromagnetic material 2 and the
strength of coupling between the antiferromagnetic phase 2 and the
ferromagnetic phase 1, so the optimum thickness of the
antiferromagnetic shell changes according to the material and
preparation conditions of the ferromagnetic phase 1.
[0140] When aiming at an increase in the coercive force at room
temperature, there are the experimental results as shown in Table 1
for the range of the optimum value of the antiferromagnetic shell
thickness. TABLE-US-00001 TABLE 1 Optimum Value of
Antiferromagnetic Shell Thickness Preferred Material thickness (nm)
Fe--Mn 2-4 N--Mn 3-10 Ir--Mn 2-5 Pt--Mn 3-8 Pd--Pt--Mn 7-15 Rh--Mn
4-7 Pt--Cr--Mn 4-10 Cr--Al 5-20 Tb--Co 2-10 NiO 4-20
[0141] One property generally required for a recording medium is
that it does not corrode in the usage or storage environment.
Particularly, corrosion becomes the serious problem along with
increased fineness. This will be explained using results of an
experiment on the corrosion resistance of an Fe powder magnetic
material not having an antiferromagnetic shell.
[0142] FIG. 11 shows experimental data of the corrosion resistance
of an Fe powder magnetic material containing 30% of Co in terms of
number of atoms. The data a shows the sheet squareness ratio of the
powder magnetic material, while the data b shows the dependency of
the change .DELTA..sigma.s of the relative saturation magnetization
of the powder magnetic material upon the long axis length of
granules of the powder.
[0143] The squareness ratio is the ratio between the magnetization
of the magnetic material when the external magnetic field is zero,
that is, the residual magnetization, and the saturation
magnetization of the magnetic material.
[0144] The sheet squareness ratio shown in FIG. 11 was found by
adding 20 parts by weight of a vinyl chloride resin to 100 parts by
weight of Fe powder, dispersing the mixture together with a 1/1
mixture of methylethylketone and cyclohexanon, and drying the
resultant coating by a doctor blade under an orientation magnetic
field of 2 kOe to prepare a coating film on a PET film and finding
a residual flux density/saturation flux density ratio in the
orientation direction by a vibrating-sample magnetometer (VSM).
[0145] The change .DELTA..sigma.s of the relative saturation
magnetization shown in FIG. 11 expresses the amount of reduction of
the saturation magnetization from an initial value after the Fe
powder is held under an environment of 60.degree. C./90% RH for one
week.
[0146] As shown by the data a and b, as the size (here, long axis)
of the particles of the Fe powder magnetic material becomes
smaller, the sheet squareness ratio and the magnetization are
conspicuously lowered. This is caused by the oxidation of the
particles of the Fe powder magnetic material.
[0147] Accordingly, FIG. 11 shows the problem that the corrosion
resistance of the magnetic material is conspicuously lowered along
with the increased fineness of the magnetic material.
[0148] In the present embodiment, desirably an antiferromagnetic
material not corroding under the usage or storage environment is
used.
[0149] An oxide antiferromagnetic material is excellent in
corrosion resistance in comparison with a metal. NiO is a typical
oxide exhibiting antiferromagnetic order at the operating
environment of a magnetic recording device, that is, room
temperature or more. Also the .alpha.-Fe.sub.2O.sub.3 and
Cr.sub.2O.sub.3 exhibit antiferromagnetism at room temperature. The
properties can also be adjusted by a solid solution comprised
mainly of NiO plus other materials. One example of this would be to
prepare a solid solution of NiO having a Neel temperature T.sub.N
near 520 K and CoO having a Neel temperature of 293 K or lower than
room temperature, but having a magnetic anisotropy larger than NiO
so as to obtain a material having a larger magnetic anisotropy than
that of NiO at room temperature. Also, an antiferromagnetic
material comprised of a complex of NiO and CoO can be utilized for
the same purpose.
[0150] Further, even with a metal, the corrosion resistance is
often enhanced with an alloy of a precious metal such as Ir--Mn,
Pt--Mn, Pd--Pt--Mn, Rh--Mn, and Pt--Cr--Mn.
[0151] The heat resistance of an antiferromagnetic material will be
explained next.
[0152] The antiferromagnetic material used as a component of the
magnetic particle of the present embodiment is preferably one
maintaining its antiferromagnetic magnetization order and
maintaining coupling with magnetization of the ferromagnetic
material at room temperature and an environment where the
temperature rises. Particularly, when looking at the region where
the recording medium and the magnetic head face each other in a
magnetic recording device, there is a possibility that the
temperature will locally become much higher on the medium than the
ambient temperature due to friction by head contact or discharge of
charged static electricity. In order to hold the recording against
this, an antiferromagnetic material having a high heat resistance
is necessary.
[0153] As already mentioned, at the Neel temperature T.sub.N or
more, the antiferromagnetic material loses its antiferromagnetic
order. Also, at the blocking temperature T.sub.B or more, the
contacting ferromagnetic material loses its restraining force on
magnetization. From the standpoint of thermal stability, an
antiferromagnetic material having a high T.sub.B is desirable as
the antiferromagnetic material utilized in the present embodiment.
T.sub.B is ordinarily much lower than T.sub.N, but roughly
speaking, there is a tendency that a material having a higher
T.sub.N will have a higher T.sub.B. Materials known to have a high
T.sub.B are listed in Table 2. Among these materials having a high
T.sub.N, many materials are given the antiferromagnetism by their
components being regularly aligned and regular crystals being
formed. Normalization does not occur unless heat treatment is
carried out at a certain heat treatment temperature Ta or more.
This can restrict production, but when a treatment temperature of
300.degree. C. or less is sufficient, it can be reached by
substrate heating ordinarily carried out at a time when the
medium's thin films are stacked and does not especially become an
obstacle. TABLE-US-00002 TABLE 2 Characteristic Temperature of
Antiferromagnetic Material Material name T.sub.B (.degree. C.)
T.sub.N (.degree. C.) Ta (.degree. C.) Ni--Mn 450 797 280 Pt--Mn
380 702 280 Pt--Cr--Mn 380 Pd--Pt--Mn 300 230
[0154] According to the present embodiment, by coating a material
having a ferromagnetic phase at ordinary temperature by a material
having an antiferro-magnetic phase at ordinary temperature, the
thermal fluctuaction is suppressed and a higher coercive force is
obtained while satisfying the further increased fineness of the
magnetic particles.
[0155] The ferromagnetic phase is isolated by the antiferromagnetic
phase shell and does not contact others, therefore a good
dispersion state of the magnetic material can be held in a magnetic
recording medium comprised of a plurality of magnetic
particles.
[0156] By providing a shell of a material having an
antiferromagnetic phase at ordinary temperature, a high coercive
force is obtained in a very wide temperature range (for example, in
a range of at least about 100 K to about 400 K) and the corrosion
resistance and the thermal stability are excellent.
[0157] The particle size preferred so as to obtain the best effect
of the present invention by the shell of the antiferromagnetic
phase is a small 10 nm to 60 nm, so this is advantageous for the
increased fineness of magnetic particles.
Second Embodiment
[0158] In the present embodiment, the magnetic material comprised
of a coexisting ferromagnetic phase and antiferromagnetic phase has
a core-shell structure shown in FIG. 12 wherein a magnetic particle
20, that is, a ferromagnetic phase 1, forms a core exhibiting a
spindle shape and the periphery thereof is surrounded by an
antiferromagnetic phase material 2 in the form of a shell.
[0159] In the present embodiment, the same reference numerals are
used for the same components as in the first embodiment, and
overlapping explanations will be omitted.
[0160] In a recording medium comprised of magnetic particles 20
having such a core-shell structure, even if two or more particles
contact, since the ferromagnetic phases 1 are isolated by the
shells of the antiferromagnetic phase 2 so that they do not
directly touch each other, the energy for gathering together a
plurality of particles by magnetic force to form secondary
particles is small. Accordingly, it is easy to obtain a magnetic
powder coating film with a good dispersion state of magnetic
particles.
[0161] By providing a coexisting ferromagnetic phase 1 and
antiferromagnetic phase 2 in each magnetic particle 20, the
magnetic particles 20 are resistant to the thermal fluctuaction and
a high coercive force can be maintained even with increased
fineness of the magnetic particles 20.
[0162] In the same way as the first embodiment, there is an optimum
combination of volumes of the ferromagnetic phase 1 and the
antiferromagnetic phase 2 for obtaining the maximum coercive force.
Namely, the coercive force becomes the maximum near the thickness
of the antiferromagnetic shell 2 by which the rotational hysteresis
loss becomes the maximum. When the thickness of the
antiferromagnetic shell 2 is increased more than that, the
magnetization curve is laterally shifted asymmetrically, an
exchange biasing field Hex appears, and the coercive force becomes
small.
[0163] Accordingly, the ratio of volumes of the ferromagnetic phase
1 and the antiferromagnetic phase 2 at the time when the rotational
hysteresis loss becomes maximum and the exchange biasing field has
not appeared enables the magnetic particle 20 to give the maximum
coercive force.
[0164] Also, the thickness of the antiferromagnetic shell 2
required for obtaining an increase of the coercive force becomes a
thickness not negligible in comparison with the size of the
ferromagnetic core 1, therefore the magnetization of the
ferromagnetic phase 1 is diluted considerably small. Accordingly, a
ferromagnetic core having a considerably large saturation flux
density must be used.
[0165] As a core material having a large saturation flux, other
than Co (18 kG), Fe (22 kG) or alloys containing them as main
ingredients are preferred.
[0166] It is also possible to employ a magnetic recording medium
material having a relatively large magnetic anisotropy and coercive
force for the ferromagnetic core 1 itself and with this plus the
help of the antiferromagnetic material achieve a further
improvement of the characteristics. For example, as the material of
the ferromagnetic phase 1, it is also possible to utilize a high
coercive force alloy containing at least Pt, Cr, or the like or an
Fe oxide-based magnetic material.
[0167] Note that some high coercive force alloys have a relatively
low ratio of magnetic atoms in the composition. Sometimes, the
probability of existence at adjacent positions where the spin of
the antiferromagnetic phase and the spin of the ferromagnetic phase
can interact at the contact interface with the antiferromagnetic
material becomes low. As a means for preventing the drop in the
interaction of the two phases in this way, it is effective to place
a magnetic metal layer having a high spin density between the high
coercive force alloy core and the antiferromagnetic shell.
[0168] FIG. 13 is a sectional view of a structure wherein a
coupling enhancement layer 3 is provided in a core-shell magnetic
particle 20. As this coupling enhancement layer 3, it is also
possible to use a ferromagnetic metal layer containing for example
Fe, Co, or Ni as a main ingredient.
[0169] A ferromagnetic metal like Fe and Co where the magnetic
moment per atom is large is apt to form strong exchange coupling
with a different type of material, so by interposing this, coupling
between the ferromagnetic phase 1 and the antiferromagnetic phase 2
will be raised.
[0170] FIG. 14 is a view showing comparison of the characteristics
of the magnetic particles 20 of the present embodiment and the
experimental results of the characteristics of the magnetic
particles according to the first embodiment.
[0171] In FIG. 14, the data a, data b, and the data c show the
coercive force at room temperature (300 K) of a spherical Co
particle with surface oxidation, the coercive force at 77 K of a
spherical Co particle with surface oxidation, and the coercive
force at room temperature (300 K) of a spherical hcp-enriched Co
particle with an NiO shell having a thickness of 10 nm similar to
FIG. 9, while the data d shows the coercive force at room
temperature (300 K) in an hcp-enriched spindle-state Co particle
having a short diameter of 20 nm and a long diameter of 40 nm with
an NiO shell having a thickness of 10 nm. Note that, the abscissa
indicates the length of the diameter or short axis of the Co core
1, and the ordinate indicates the coercive force of the magnetic
material comprised of the Co core 1 and the shell 2.
[0172] As shown by the data d, an hcp-enriched spindle-state Co
particle having a short diameter of 20 nm and a long diameter of 40
nm with an NiO shell having a thickness of 10 nm gives the highest
coercive force, i.e., 3.52 kOe, at room temperature in comparison
with the data a, data b, and data c in the case of spherical Co
particles.
[0173] The crystal structure of a ferromagnetic phase core 2 made
of Co is comprised of a mixture of hexagonal closest packed planes
packed in an fcc structure and packed in an hcp structure. An
hcp-enriched structure is advantageous since the magnetic
anisotropy derived from the crystal structure becomes large. When
the c-axis of this hcp component coincides with the long axis
direction of the particle outer shape, the contribution of the
shape anisotropy and the crystal magnetic anisotropy can be
additionally utilized.
[0174] When covering a ferromagnetic core 1 made of Co by an NiO
shell to form a spindle-shaped magnetic particle 20 such as shown
in FIG. 12, shape magnetic anisotropy is obtained, so this is
advantageous so as to obtain a high coercive force. A particularly
preferred range of the shape ratio is a range of 1.2<(long
axis/short axis)<5. When this ratio is smaller than 1.2, the
effect of producing the anisotropy according to the shape is small,
while if it is 5 or more, agglomeration of the particles becomes
apt to occur.
[0175] As an example of the preferred size of the magnetic particle
20 of the present embodiment, the size of the core shown in FIG. 12
is 20 nm in the short axis direction and 40 nm in the long axis
direction, while the size of the entire magnetic particle becomes
40 nm in the short axis direction and 60 nm in the long axis
direction.
[0176] Regarding the relationship between the coercive force and
the temperature, in the same way as the first embodiment, even if
the temperature changes in a wide range, the coercive force of a
magnetic particle obtained by covering a Co core by an NiO shell is
not reduced that much, so this is suitable for a magnetic recording
medium used in a usage environment of room temperature or more.
Particularly, when an hcp-enriched Co particle is used, the crystal
magnetic anisotropy of the Co particle itself becomes large,
therefore a high coercive force is obtained over a wide temperature
range by the synergistic effect with the function of the NiO
shell.
[0177] In the present embodiment, the selection of the
ferromagnetic material and the antiferromagnetic material is
basically the same as that of the first embodiment.
[0178] According to the present embodiment, the thermal
fluctuaction is suppressed and a higher coercive force is obtained
while satisfying the need for further fineness of the magnetic
particles.
[0179] The ferromagnetic phase is isolated by the antiferromagnetic
phase shell and does not contact others, so a good dispersion state
of the magnetic material can be held.
[0180] By forming a shell of a material having an antiferromagnetic
phase at ordinary temperature, a high coercive force is obtained
over a very wide temperature range and the thermal stability is
excellent.
[0181] The preferred particle size for obtaining the best effect of
the present invention by the shell of the antiferromagnetic phase
is a small 10 nm to 60 nm. This is advantageous for the increased
fineness of the magnetic particles.
Third Embodiment
[0182] In the present embodiment, the magnetic material comprised
of the coexisting ferromagnetic phase and the antiferromagnetic
phase has a core-shell structure where a magnetic particle 30 shown
in FIG. 15, that is, the ferromagnetic phase 1, becomes a core of a
disk-like shape and the periphery thereof is surrounded by an
antiferromagnetic phase material 2 in the form of a shell.
[0183] Even in a recording medium comprised of magnetic particles
30 having such a core-shell structure, it is easy to obtain a
magnetic powder coating film in the good dispersion state of
magnetic particles.
[0184] In the present embodiment, the same reference numerals are
used for same components as in the above embodiments, and
overlapping explanations will be omitted.
[0185] Also, by providing the coexisting ferromagnetic phase 1 and
antiferromagnetic phase 2 in each magnetic particle 30, even with
further fineness of the magnetic particles 30, the magnetic
particles 30 are resistant to the thermal fluctuaction and can hold
a high coercive force. The reason is similar to that of the first
embodiment.
[0186] The characteristics of the magnetic particle 30 of the
present embodiment will be explained by taking as an example one
obtained by surrounding a core of a ferromagnetic phase material Co
by an antiferromagnetic phase material NiO.
[0187] When the magnetic particle 30 and the ferromagnetic phase Co
core 1 thereof are formed in a disk shape as shown in FIG. 15, by
coating a paste containing the particles while applying a force
pressing against it, there is the advantage that a coating film
having surfaces of the disks arranged parallel to the substrate
surface can be formed.
[0188] Investigation revealed that as an example of the preferred
size of the magnetic particle 30 of the present embodiment, the
diameter of the core shown in FIG. 15 is about 40 nm and the height
of the disk is about 20 nm.
[0189] In the same way as the first and second embodiments, in the
magnetic particle 20 of the core-shell structure described above,
there is an optimum combination of volumes of the ferromagnetic
phase 1 and the antiferromagnetic phase 2 in order to obtain the
maximum coercive force. Namely, the magnetic particle 30 gives the
maximum coercive force with a ratio of volumes of the ferromagnetic
phase 1 and antiferromagnetic phase 2 at the time when the
rotational hysteresis loss becomes maximum and no exchange biasing
field appears.
[0190] Also, the magnetization of the ferromagnetic phase 1 is
diluted considerably small by the thick antiferromagnetic shell 2,
therefore one having a fairly large saturation flux density of the
ferromagnetic core must be used. As a core material having a large
saturation flux, other than Co (18 kG), Fe (22 kG) or alloys
containing them as main ingredients are preferred.
[0191] It is also possible to employ a magnetic material having a
relatively large magnetic anisotropy and coercive force for the
ferromagnetic core 1 itself and, with this plus the help of the
ferromagnetic material, achieve a further improvement of the
characteristics. For example, as the material of the ferromagnetic
phase 1, it is also possible to utilize a high coercive force alloy
containing at least Pt, Cr, or the like or an Fe oxide-based
magnetic material.
[0192] Also, although not illustrated, by providing a magnetic
metal layer having a high spin density, for example, a
ferromagnetic metal layer containing Fe, Co, or Ni as a main
ingredient as a coupling enhancement layer between the high
coercive force alloy core and the antiferromagnetic shell, the
coupling between the ferromagnetic phase and the antiferromagnetic
phase will be raised.
[0193] In the present embodiment, the selection of the
ferromagnetic material and the antiferromagnetic material is
basically the same as that of the first embodiment.
[0194] According to the present embodiment, a thin film state
magnetic medium can be easily formed. The rest of the effects are
the same as those of the first and second embodiments.
Fourth Embodiment
[0195] The method of production of the magnetic particles 10, 20,
and 30 having the core-shell structure shown in FIG. 2, FIG. 12,
and FIG. 15 will be explained next.
[0196] A particle of a structure of a Co core surrounded by NiO can
be produced by oxidizing the outer periphery of a structure of Co
coated by Ni metal in an oxygen atmosphere. The oxide of the Co,
that is, the CoO, hinders the coupling of Co and NiO, so the
oxidation region in the oxygen atmosphere is kept to the range not
producing CoO. Also, a particle of a structure of the Co core
surrounded by Ni, which is targeted by the oxidation treatment, is
obtained by supplying Co and Ni one after another by vapor
deposition in an inert gas. Concretely, a metal cluster/fine
particles are grown while generating a metallic vapor under
conditions of a relatively high pressure of 100 mTorr or more and a
mean free path of a 20th, 30th, etc. of a vacuum vapor deposition
tank dimension and repeating collision in the inert gas.
[0197] The size of the core and the thickness of the shell are
determined based on the principle that the rotational hysteresis
loss is made the maximum and no exchange biasing field appears.
[0198] When the core is formed by a high coercive force alloy, a
magnetic metal layer having a high spin density containing for
example Fe, Co, or Ni as its main ingredient is formed between the
high coercive force alloy core and the antiferromagnetic shell.
[0199] By supplying heat energy to the particle and gradually
cooling it in the step of preparing the Co particle, the Co crystal
can be hcp enriched and a spindle-like particle extending in the
c-axis direction thereof can be grown. As the method of supplying
the energy, use can be made of electron beam impact, infrared ray
irradiation, and induction heating by AC or microwaves.
[0200] The materials of the ferromagnetic material and the
antiferromagnetic material are selected as mentioned in the first
embodiment.
[0201] In the selection of the material of the antiferromagnetic
material, the ease of production should also be considered.
[0202] A disordered alloy giving antiferromagnetism does not
require heat treatment or substrate heating up to a high
temperature and can easily give an antiferromagnetic thin film by a
thin film preparation process such as sputtering, so is suitable
for production. For example, alloys of compositions of Fe--Mn,
Ir--Mn, Rh--Mn, Ru--Mn, Pt--Cr--Mn, and Cr--Al can be utilized.
[0203] According to the method of production of a magnetic particle
comprised of a ferromagnetic phase core and antiferromagnetic phase
shell of the present embodiment, a magnetic material sufficiently
satisfying the requirement of increased fineness of magnetic
particles, holding a good dispersion state of magnetic particles,
further raising the magnetic anisotrophy energy and coercive force,
excellent in corrosion resistance and thermal stability, and able
to cope with further higher density recording can be realized.
Fifth Embodiment
[0204] In the above embodiments, as a magnetic particle comprised
of a coexisting ferromagnetic phase and antiferromagnetic phase, a
magnetic particle of the core-shell structure wherein the
ferromagnetic phase core was surrounded by an antiferromagnetic
phase material shell was explained, but in the present embodiment,
another form of a magnetic particle comprised of a coexisting
ferromagnetic phase and antiferromagnetic phase will be
explained.
[0205] Both of the ferromagnetic phase material and the
antiferromagnetic phase material which must be coupled with each
other in the present embodiment do not have to be in the form of
layers. For example, as shown in FIG. 16, it is also possible for a
plurality of particles to be gathered and form a larger complex
particle 40 and for antiferromagnetic phases 2 to be included
between ferromagnetic phases 1 or conversely ferromagnetic phases 1
to be included between antiferromagnetic phases 2.
[0206] In order to utilize this as a high density magnetic
recording medium, the size of a composite particle coupled by
magnetic coupling is preferably several tens of nm.
[0207] In the same way as a magnetic particle of the core-shell
structure, in a magnetic particle 40 of the structure described
above, there is an optimum combination of volumes of the
ferromagnetic phases 1 and the antiferromagnetic phases 2 in order
to obtain the maximum coercive force. Namely, in the composite
magnetic particle 40, the ratio of the volume of the ferromagnetic
phases 1 and the volume of the antiferromagnetic phases 2 is a
range where no exchange biasing field appears and the rotational
hysteresis loss is made the maximum.
[0208] In the present embodiment as well, in the same way as the
first embodiment, the materials of the ferromagnetic phase 1 and
the antiferromagnetic phase 2 are selected and the sizes are
determined so that a high coercive force and magnetic anisotrophy
energy and a good corrosion resistance and thermal stability are
achieved.
[0209] According to the present embodiment, by bringing a substance
having a ferromagnetic phase at ordinary temperature into contact
with a substance having an antiferromagnetic phase at ordinary
temperature, a higher coercive force is obtained without the
thermal fluctuaction while satisfying the requirement of increased
fineness of the complex magnetic particles.
[0210] By coating the substance having the antiferromagnetic phase
at ordinary temperature, a high coercive force is obtained in a
very wide temperature range and the thermal stability is
excellent.
Sixth Embodiment
[0211] In the present embodiment, as a magnetic particle comprised
of a coexisting ferromagnetic phase and antiferromagnetic phase, a
composite particle 50 comprised of antiferromagnetic precipitates 4
coated by a ferromagnetic phase 1 is formed as shown in FIG.
17.
[0212] As mentioned above, in order to utilize it as for a high
density magnetic recording medium, the size of one composite
particle coupled by magnetic coupling is preferably several tens of
nm.
[0213] In a magnetic particle 50 having the structure described
above, there is an optimum combination of volumes of the
ferromagnetic phase 1 and the antiferromagnetic precipitates 4 in
order to obtain the maximum coercive force. Namely, in the
composite magnetic particle 50, the ratio between the volume of the
ferromagnetic phase 1 and the volume of the antiferromagnetic
precipitates 4 is a range where no exchange biasing field appears
and the rotational hysteresis loss is made the maximum.
[0214] In the present embodiment as well, in the same way as the
first embodiment, the materials of the ferromagnetic phase 1 and
the antiferromagnetic precipitates 4 are selected and the sizes are
determined so as to achieve a high coercive force and a good
corrosion resistance and thermal stability.
[0215] The present embodiment exhibits similar effects to those by
the above embodiments.
Seventh Embodiment
[0216] In the present embodiment, as a magnetic particle comprising
a coexisting ferromagnetic phase and antiferromagnetic phase, as
shown in FIG. 18, a composite particle 60 comprised of
ferromagnetic inclusions 6 surrounded by an antiferromagnetic
matrix 5 is formed.
[0217] As mentioned above, in order to utilize this for a high
density magnetic recording medium, the size of one composite
particle coupled by the magnetic coupling is preferably several
tens of nm.
[0218] In the magnetic particle 60 having the structure described
above, there is an optimum combination of volumes of the
ferromagnetic inclusions 6 and the antiferromagnetic matrix 5 in
order to obtain the maximum coercive force. Namely, in the
composite magnetic particle 60, the ratio between the volume of the
ferromagnetic inclusions 6 and the volume of the antiferromagnetic
matrix 5 is a range where no exchange biasing field appears and the
rotational hysteresis loss is made the maximum.
[0219] In the present embodiment as well, the materials of the
ferromagnetic inclusions 6 and the antiferromagnetic matrix 5 are
selected and the sizes are determined so as to achieve a high
coercive force and a good corrosion resistance and thermal
stability.
[0220] The present embodiment exhibits similar effects to those by
the above embodiments.
Eighth Embodiment
[0221] As shown in FIG. 19, magnetic particles 70 comprised of
magnetic particles 10, 20, 30, 40, 50, or 60 of the present
invention mentioned above may be filled in a layer region on a
substrate 11 of a hard or flexible non-magnetic material by coating
or another method to form a magnetic filled layer 12 and thereby
achieve a magnetic recording medium suitable for high density
recording and excellent in thermal stability.
[0222] In FIG. 19, the thickness of the magnetic filling layer 12
is for example 0.5 .mu.m.
[0223] In each magnetic material 70, the ratio of volumes of the
ferromagnetic phase and the antiferromagnetic phase is determined
by the principle that the rotational hysteresis loss be made the
maximum and no exchange biasing field appears.
[0224] In the case of the magnetic materials 10, 20, and 30 of the
core-shell structure, when the core is formed by a high coercive
force alloy, a magnetic metal layer having a high spin density
containing for example Fe, Co, or Ni as its main ingredients is
formed between the high coercive force alloy core and the
antiferromagnetic shell.
[0225] The materials of the ferromagnetic material and the
antiferromagnetic material are selected as mentioned in the first
embodiment.
[0226] The present invention is not limited to the embodiments
explained above. Various modifications are possible in a range not
out of the gist of the present invention.
[0227] For example, the materials of the ferromagnetic phase
material and the antiferromagnetic phase material, the mode of
providing a coexisting ferromagnetic phase and antiferromagnetic
phase, and the shape, size, etc. of the magnetic material of the
core-shell structure are not particularly limited so far as the
effects of the present invention are obtained.
[0228] Summarizing the effects of the present invention, according
to the present invention, by surrounding a material having a
ferromagnetic phase at ordinary temperature by a shell of a
material having an antiferromagnetic phase at ordinary temperature,
the thermal fluctuaction is suppressed and a higher coercive force
is obtained while satisfying the requirement for increased fineness
of the magnetic material.
[0229] Since the ferromagnetic phases of different particles are
isolated by the antiferromagnetic phase shells and do not contact
each other, a good dispersion state of the magnetic particles can
be held when preparing a magnetic coating. By providing a shell of
a material having an antiferromagnetic phase at ordinary
temperature, a high coercive force is obtained in a very wide
temperature range and the thermal stability is excellent.
[0230] By providing a shell of an antiferromagnetic phase, the
particle size suitable for obtaining the best effect of the present
invention is small. This is advantageous for increased fineness of
the magnetic particles.
[0231] Due to the above, a magnetic material and magnetic recording
medium able to cope with a further higher recording density can be
realized.
[0232] While the invention has been described with reference to
specific embodiments chosen for purpose of illustration, it should
be apparent that numerous modifications could be made thereto by
those skilled in the art without departing from the basic concept
and scope of the invention.
* * * * *