Title: Magnetic recording medium
Abstract: The average major-axis length (L) of a ferromagnetic alloy powder primarily contained together with a binder in a magnetic layer provided on at least one surface of a non-magnetic support is less than 0.10 .mu.m, and the ferromagnetic alloy powder contains Co at a content of 15 to 28 at % relative to Fe, and furthermore, contains Y at a content of 22 .mu.mol/m2 or more with respect to a specific surface area (Sc) calculated from the above-mentioned average major-axis length (L) and the crystallite diameter (d). A coating-type high-recording-density magnetic recording medium has excellent surface properties, an excellent electromagnetic transducing characteristic especially in short wavelength regions and high reliability.
Patent Number: 6,852,404 Issued on 02/08/2005 to Kuwajima, et al.
| Inventors:
|
Kuwajima; Takayoshi (Tokyo, JP);
Inoue; Osamu (Tokyo, JP);
Kaizu; Akimasa (Tokyo, JP)
|
| Assignee:
|
TDK Corporation (Tokyo, JP)
|
| Appl. No.:
|
397519 |
| Filed:
|
March 27, 2003 |
Foreign Application Priority Data
| Mar 29, 2002[JP] | 2002-098031 |
| Current U.S. Class: |
428/839.3; 428/328; 428/336 |
| Intern'l Class: |
G11B 005/70.6 |
| Field of Search: |
428/323,328,336,694 BA
|
References Cited [Referenced By]
| Foreign Patent Documents |
| 9-91697 | Apr., 1997 | JP.
| |
| 10-320759 | Dec., 1998 | JP.
| |
| 11-251122 | Sep., 1999 | JP.
| |
| 2001-81506 | Mar., 2001 | JP.
| |
Primary Examiner: Resan; Stevan A.
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier & Neustadt, P.C.
Claims
What is claimed is:
1. A magnetic recording medium, wherein the average major-axis length (L)
of a ferromagnetic alloy powder primarily contained together with a binder
in a magnetic layer provided on at least one surface of a non-magnetic
support is less than 0.10 .mu.m, and wherein the ferromagnetic alloy
powder contains Co at a content of 15 to 28 at % relative to Fe, and
contains Y at a content of 22 .mu.mol/m.sup.2 or more with respect to a
specific surface area (Sc) calculated from the average major-axis length
(L) and the crystallite diameter (d).
2. The magnetic recording medium according to claim 1, wherein the
ferromagnetic alloy powder contains Al at a content within the range of 27
to 45 .mu.mol/m.sup.2 with respect to the specific surface area (Sc).
3. The magnetic recording medium according to claim 2, wherein the average
thickness of the magnetic layer is 0.03 to 0.30 .mu.m, and wherein a
non-magnetic layer is provided between the non-magnetic support and the
magnetic layer.
4. The magnetic recording medium according to claim 1, wherein the average
thickness of the magnetic layer is 0.03 to 0.30 .mu.m, and wherein a
non-magnetic layer is provided between the non-magnetic support and the
magnetic layer.
5. The magnetic recording medium according to claim 1, wherein the average
major-access length (L) of the ferromagnetic alloy powder is from 0.03 to
0.8 .mu.m.
6. The magnetic recording medium according to claim 1, wherein Y is present
in an amount of 24 .mu.mol/m.sup.2 or more.
7. The magnetic recording medium according to claim 1, wherein Y is present
in an amount of 27 .mu.mol/m.sup.2 or more.
8. The magnetic recording medium according to claim 1, wherein the
ferromagnetic alloy powder contains Al at a content within the range of 27
to 40 .mu.mol/m.sup.2 relative to the specific surface area (Sc).
9. The magnetic recording medium according to claim 1, wherein Co/Fe in the
ferromagnetic alloy powder is from 16 to 24.
10. The magnetic recording medium according to claim 1, wherein Y/(Fe+Co)
is from 7.9 to 9.8.
11. A magnetic recording medium, wherein the average major-axis length (L)
of a ferromagnetic alloy powder primarily contained together with a binder
in a magnetic layer provided on at least one surface of a non-magnetic
support is less than 0.10 .mu.m, and wherein the ferromagnetic alloy
powder contains Co at a content of 15 to 28 at % relative to Fe, and
contains Y at a content of 20 .mu.mol/m.sup.2 or more with respect to a
BET specific surface area (Sbet) of the ferromagnetic alloy powder.
12. The magnetic recording medium according to claim 11, wherein the
ferromagnetic alloy powder contains Al at a content within the range of 22
to 38 .mu.mol/m.sup.2 with respect to the BET specific surface area (Sbet)
of the ferromagnetic alloy powder.
13. The magnetic recording medium according to claim 12, wherein the
average thickness of the magnetic layer is 0.03 to 0.30 .mu.m, and wherein
a non-magnetic layer is provided between the non-magnetic support and the
magnetic layer.
14. The magnetic recording medium according to claim 11, wherein the
average thickness of the magnetic layer is 0.03 to 0.30 .mu.m, and wherein
a non-magnetic layer is provided between the non-magnetic support and the
magnetic layer.
15. The magnetic recording medium according to claim 11, wherein the
average major-access length (L) of the ferromagnetic alloy powder is from
0.03 to 0.8 .mu.m.
16. The magnetic recording medium according to claim 11, wherein Y is
present in an amount of 22 .mu.mol/m.sup.2 or more.
17. The magnetic recording medium according to claim 11, wherein Y is
present in an amount of 24 .mu.mol/m.sup.2 or more.
18. The magnetic recording medium according to claim 11, wherein the
ferromagnetic alloy powder contains Al at a content within the range of 22
to 34 .mu.mol/m.sup.2 with respect to a BET specific surface area (Sbet).
19. The magnetic recording medium according to claim 11, wherein Co/Fe in
the ferromagnetic alloy powder is from 16 to 24.
20. The magnetic recording medium according to claim 11, wherein Y/(Fe+Co)
is from 7.9 to 9.8.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic recording medium. Specifically,
the present invention relates to a coating-type high-recording-density
magnetic recording medium having excellent surface properties and
exhibiting excellent performances in a total recording, playback system
used in short wavelength regions, and in particular, having an
electromagnetic transducing characteristic satisfactorily adaptable to a
high-recording-density medium system using an MR head and the like, as
well as excellent reliability.
2. Description of the Related Art
In recent years, recording densities of magnetic recording media have been
increased, and therefore, further improvements of the electromagnetic
transducing characteristic, e.g. reduction in wavelength (for example, a
shortest recording wavelength specified in DDS-4, which is one standard on
data tapes, is in the order of 0.3 .mu.m), shift toward a digital
recording system and shift toward a system equipped with an MR head for
playback (the shortest recording wavelength is 0.25 .mu.m or less), are
required with respect to magnetic recording media. In order to satisfy
these requirements, reduction in the magnetic layer thickness is under
consideration with respect to the coating-type magnetic recording media.
By reason of the above-mentioned requirements, regarding magnetic recording
media including a magnetic layer which contains a ferromagnetic alloy
powder (hereafter may be referred to simply as "a magnetic powder" or "a
powder") and a binder and which has a reduced layer thickness, so-called
multilayer structures provided with a non-magnetic layer between a
non-magnetic support and a magnetic layer are often adopted as one means
of avoiding problems, such as reduction in an output, noises and head
touches during recording and playback, resulting from a surface condition
on the non-magnetic support up to this time. In this manner, the
electromagnetic characteristic of the magnetic layer having a reduced
layer thickness have been also improved. However, even the magnetic
recording medium having the multilayer structure has been insufficient to
achieve a surface recording density of 0.5 Gbit/inch.sup.2 or more, in
particular 1 Gbit/inch.sup.2 or more.
In order to increase the ratio of a carrier output to a noise output (C/N
or S/N) on a unit volume (unit area) basis for increasing a density,
reduction in noises, especially noises due to particles, is required, and
therefore, reduction in the size of a magnetic powder is attempted. Wh n
the size of the magnetic powder is reduced, the number of the particles on
a unit volume basis is increased, and the particulate noises can be
reduced. Theory holds that the above-mentioned effect can be achieved
because when the number of the particles increases by a factor of n, the
carrier output increases by a factor of n, and the noise output increases
by a factor of the square root of n. Therefore, theoretically, the carrier
output/noise output ratio increases by a factor of the square root of n.
However, uniform dispersion became difficult as the magnetic powder became
small, and when a conventional magnetic powder was used, it was difficult
to achieve the above-mentioned theoretical effect through reduction in the
size of the magnetic powder.
On the other hand, Japanese Unexamined Patent Application Publication No.
9-91697 discloses that a ferromagnetic alloy powder having an average
major-axis length of 0.08 .mu.m or less is used, an ultrasonic treatment
is applied to a magnetic paint, and thereby dispersibility is increased,
the surface properties and magnetic characteristics of the magnetic layer,
in particular the squareness ratio (residual magnetic flux
density/saturation magnetic flux density) and the degree of orientation in
a plane, are improved, and a magnetic recording medium exhibiting a high
electromagnetic transducing charact ristic is produced. Japanese
Unexamined Patent Application Publication No. 11-251122 and Japanese
Unexamined Patent Application Publication No. 2001-81506 disclose
ferromagnetic metal powders produced by gas reduction of a powder composed
of needle particles (average major-axis length: 0.01 to 0.40 .mu.m), in
which iron oxyhydroxide or iron oxide is allowed to contain Co, Al and Y
under a predetermined condition, as ferromagnetic metal powders used for
coating-type magnetic recording media suitable for high-recording-density
recording.
The above-mentioned Japanese Unexamined Patent Application Publication No.
9-91697 discloses a medium having a squareness ratio of 0.88 or more in
the plane of a magnetic layer and the degree of orientation of 3.0 or more
with respect to a ferromagnetic alloy powder having an average major-axis
length of 0.10 .mu.m or more in an embodiment thereof. However, regarding
the ferromagnetic alloy powder having an average major-axis length of less
than 0.10 .mu.m, an upper limit of the squareness ratio was 0.88 in the
plane of the magnetic layer, and the upper limit of the degree of
orientation was 2.8 from the result of calculation based on the data. That
is, the use of the ferromagnetic alloy powder having an average major-axis
length of less than 0.10 .mu.m was insufficient to achieve further
improvement in the electromagnetic transducing characteristic. Regarding
both magnetic tapes described in the above-mentioned Japanese Unexamined
Patent Application Publication No. 11-251122 and Japanese Unexamined
Patent Application Publication No. 2001-81506, target thicknesses of the
magnetic layers were 3 .mu.m in the embodiments, and these thicknesses and
the ferromagnetic metal powders applied to these thicknesses were
insufficient to achieve further improvement in the electromagnetic
transducing characteristics as well.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to overcome the
above-mentioned problems and to provide a coating-type
high-recording-density magnetic recording medium having excellent surface
properties, an excellent electromagnetic transducing characteristic
especially in short wavelength regions and high reliability.
The inventors of the present invention performed earnest research to
overcome the above-mentioned problems, and found out that the
above-mentioned objects was able to achieve by the configuration described
below, so that the present invention was completed.
In a magnetic recording medium of the present invention, the average
major-axis length (L) of a ferromagnetic alloy powder primarily contained
together with a binder in a magnetic layer provided on at least one
surface of a non-magn tic support is less than 0.10 .mu.m, and the
ferromagnetic alloy powder contains Co at a content of 15 to 28 at %
relative to Fe, and furthermore, contains Y (yttrium) at a content of 22
.mu.mol/m.sup.2 or more with respect to a specific surface area (Sc)
calculated from the above-mentioned average major-axis length (L) and the
crystallite diameter (d).
Preferably, the above-mentioned ferromagnetic alloy powder contains Al at a
content within the range of 27 to 45 .mu.mol/m.sup.2 with respect to the
above-mentioned specific surface area (Sc).
In a magnetic recording medium of the present invention, the average
major-axis length (L) of a ferromagnetic alloy powder primarily contained
together with a binder in a magnetic layer provided on at least one
surface of a non-magnetic support is less than 0.10 .mu.m, and the
ferromagnetic alloy powder contains Co at a content of 15 to 28 at %
relative to Fe, and furthermore, contains Y at a content of 20 mol/m.sup.2
or more with respect to a BET specific surface area (Sbet) of the
ferromagnetic alloy powder.
In this case, preferably, the above-mentioned ferromagnetic alloy powder
contains Al at a content within the range of 22 to 38 .mu.mol/m.sup.2 with
respect to a BET specific surface area (Sbet) of the above-mentioned
ferromagnetic alloy powder.
In the magnetic recording medium of the present invention, preferably, the
average thickness of the above-mentioned magnetic layer is 0.03 to 0.30
.mu.m, and a non-magnetic layer is provided between the above-mentioned
non-magnetic support and the above-mentioned magnetic layer.
The magnetic recording medium of the present invention is a coating-type
high-recording-density magnetic recording medium which performs recording
and playback in short wavelength regions and which has a surface recording
density of 0.5 Gbit/inch.sup.2 or more, wherein the ferromagnetic alloy
powder having a small average major-axis length of less than 0.10 .mu.m is
used while the sorts of element contained in the ferromagnetic alloy
powder and the amount thereof are optimized, and thereby, magnetic
characteristics of the powder are improved, and in addition,
dispersibility and orientation property are improved. In this manner,
spacing is reduced by making the surface roughness of the magnetic layer
very small, and thereby, a fine magnetic powder is allowed to exhibit its
effect of reducing noises, so that an excellent electromagnetic
transducing characteristic can be achieved, in which C/N or S/N, that is,
carrier output/noise output, is excellent.
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be specifically described below.
A magnetic recording medium of the present invention includes a magnetic
layer primarily containing a ferromagnetic alloy powder and a binder on at
least one surface of a non-magnetic support, and it is important that the
above-mentioned ferromagnetic alloy powder has characteristics described
below.
That is, in the present invention, the average major-axis length (L) of the
ferromagnetic alloy powder contained in the magnetic layer is less than
0.10 .mu.m, and preferably, is 0.03 to 0.08 .mu.m. A fine powder within
the above-mentioned range must be used in order to prepare a magnetic
recording medium suitable for recording and playback in short wavelength
regions. When this average major-axis length (L) is 0.10 .mu.m or more,
particulate noises are increased, and the C/N or S/N, that is, carrier
output/noise output, is degraded. On the other hand, when the average
major-axis length (L) is too small, although the particulate noises can be
reduced, growth of crystals is likely to become unstable, and undesirable
tendencies are exhibited, for example, magnetic characteristics of the
ferromagnetic alloy powder are degraded, and the specific surface area is
increased, so that the dispersibility is reduced. The average major-axis
length is controlled by the temperature and pH of an aqueous solution
during the preparation of an iron oxyhydroxide powder to become a mat rial
for the ferromagnetic alloy powder in accordance with a conventional
method.
In the present invention, the average major-axis length (L) of the
ferromagnetic alloy powder can be determined by separating the
ferromagnetic alloy powder from a piece of tape, taking a sample
therefrom, and measuring the major-axis lengths of the powder in a
photograph taken with a transmission electron microscope (TEM). An example
of the procedure thereof will be described below. (1) A back coat layer is
removed from a piece of tape by wiping with a solvent. (2) The piece of
tape, in which a non-magnetic layer and an upper magnetic layer are left
on a non-magnetic support, is put into a 5% aqueous solution of sodium
hydroxide, followed by heating and agitating. (3) A coating film fallen
off the non-magnetic support is washed with water, followed by drying. (4)
This coating film is subjected to an ultrasonic treatment in methyl ethyl
ketone (MEK), and the ferromagnetic alloy powder is collected by
adsorption using a magnet stirrer, followed by drying. (5) The powder
resulting from (4) is taken into a specific mesh, a sample for a TEM is
prepared, and a photograph is taken with the TEM. (7) The major-axis
lengths of the powder in the photograph are measured and an average value
thereof is determined (n=100).
The average value of the crystallite diameters (d) is preferably 0.008 to
0.016 .mu.m, and more preferably, is 0.010 to 0.015 .mu.m. When the
crystallite diameter (d) is reduced, particulate noises are reduced.
However, the growth of the crystal becomes unstable, and the saturation
magnetization (.sigma.s) and the magnetic anisotropy of the ferromagnetic
alloy powder tend to decrease. In addition, an increase in the specific
surface area is brought about, and the dispersibility may be reduced. On
the other hand, when the crystallite diameter (d) is more than 0.016
.mu.m, particulate noises are likely to increase.
The ferromagnetic alloy powder primarily contains Fe, and can contain
various elements on an as needed basis. In particular, in the present
invention in which the average major-axis length (L) of the ferromagnetic
alloy powder is less than 0.10 .mu.m, Co must be contained at a content
within the range of 15 to 28 at % relative to Fe. When Co is contained at
a content within this range, the magnetic characteristics of the
ferromagnetic alloy powder can be improved, and a high C/N can be
achieved. When the content of Co is less than 15 at %, the magnetic
characteristics of the powder are likely to degrade, sufficient carrier
output cannot be achieved, and the resulting magnetic characteristics of
the magnetic recording medium are likely to change with time. On the other
hand, when exceeding 28 at %, in the case where another element is further
contained, distortion is lik ly to occur in the crystallite of the
ferromagnetic alloy powder, the magnetic characteristics of the powder are
degraded, the carrier output is reduced, the crystallite diameter is
likely to increase, and particulate noises are likely to increase.
Examples of elements contained in the ferromagnetic alloy powder include,
for example, rare earth elements, Y, Al and Si, other than the
above-mentioned Fe and Co. However, in the present invention, Y must be
contained. In general, these contained elements are present in the
vicinity of the surface layer of the ferromagnetic alloy powder. Most of
all, Y controls the crystal shape without changing the shape of the
crystallite during a heat treatment for oxidation-reduction reaction in
the manufacturing process of the ferromagnetic alloy powder, and
furthermore, improves the dispersibility of the ferromagnetic alloy powder
by covering the ferromagnetic alloy powder surface, and improves the
magnetic characteristics and the surface properties of the magnetic
recording medium.
The ferromagnetic alloy powder used in the present invention can be
produced by a conventional means. A method for adding Y to this
ferromagnetic alloy powder is as described below, for example. A
Co-containing iron oxyhydroxide powder as a material for the ferromagnetic
alloy powder is dispersed in an aqueous solution. Yttrium nitrate is added
thereto, followed by further dispersion. Subsequently, the resulting
suspension is maintained at 50.degree. C. to 60.degree. C., and is
neutralized with sodium hydroxide, so that Y is precipitated on the
surface of the Co-containing iron oxyhydroxide powder. This Co-containing,
Y-containing iron oxyhydroxide powder is separated from the suspension by
filtration, followed by washing with water, drying in air at 300.degree.
C. to 400.degree. C., and firing, so that a Co-containing, Y-containing
iron oxide powder is prepared. The resulting iron oxide powder is
subjected to a reduction treatment at 400.degree. C. to 500.degree. C. for
10 to 15 hours in a rotary oven while a hydrogen gas stream is introduced,
followed by cooling to room temperature while a nitrogen gas is
introduced. Thereafter, a nitrogen gas containing 1% of oxygen is
introduced, and a gradual oxidation treatment is performed for 5 to 10
hours, so that a ferromagnetic alloy powder can be prepared.
In the present invention, it is important to control the content of Y at
the time of above-mentioned addition of Y by, for example, controlling the
amount of addition of the above-mentioned yttrium nitrate in proportion to
the specific surface area determined by calculation based on the average
major-axis length and the crystallite diameter of the above-mentioned
Co-containing iron oxyhydroxide powder. Consequently, it is important that
Y is contained at a content in proportion to the specific surface area of
the ferromagnetic alloy powder rather than the amounts of Co and Fe, and
thereby, the dispersibility and the magnetic characteristics can be
optimized.
The content of Y in the present invention is 22 .mu.mol/m.sup.2 or more,
preferably, is 24 .mu.mol/m.sup.2 or more, and more preferably, is 27
.mu.mol/m.sup.2 or more with respect to a specific surface area (Sc)
calculated from the average major-axis length (L) and the crystallite
diameter (d) when the ferromagnetic alloy powder is assumed to be a
circular cylinder. When less than 22 .mu.mol/m.sup.2, distortion occurs in
the crystal shape of the ferromagnetic alloy powder. In addition, the
amount of Y for covering is reduced, the dispersibility and the magnetic
characteristics of the ferromagnetic alloy powder are degraded, and the
surface properties and the magnetic characteristics of the magnetic
recording medium are degraded. As a result, the carrier output is reduced,
noises are increased, and the C/N is degraded. On the other hand, when
excessive, the coercive force and .sigma.s, which are the magnetic
characteristics of the ferromagnetic alloy powder, may be degraded, and
therefore, preferably, an upper limit of the total content of contained
elements except for Co is specified to be 30 at % relative to Co and Fe.
In the present invention, the content of Y can be specified relative to the
BET specific surface area (Sbet) of the ferromagnetic alloy powder. In
this case, the content of Y is 20 .mu.mol/m.sup.2 or more, preferably, is
22 .mu.mol/m.sup.2 or more, and more preferably, is 24 .mu.mol/m.sup.2 or
more with respect to a BET specific surface area (Sbet).
Here, a method for calculating the above-mentioned content of Y will be
described below in detail.
Regarding Y contained in the ferromagnetic alloy powder, since the amount
of presence of Y atoms with respect to a surface area of the powder
surface (=the content with respect to a specific surface area) is in
particular important, the content of Y relative to the specific area (Sc)
is calculated based on the average major-axis length (L) and the
crystallite diameter (d) in accordance with the following calculation
method.
One particle of the powder is assumed to be a circular cylinder. The volume
V1 (unit: cm.sup.3) is determined by the following equation (1). wherein
the crystallite diameter (d) (unit: .mu.m) is specified to be a diameter
of the base of the circular cylinder, and the average major-axis length
(L) (unit: .mu.m) is specified to be a height of the circular cylinder.
V1=d.sup.2.pi.L.times.1/4.times.1/(1.times.10.sup.12) (1)
If the specific gravity of a Co--Fe alloy is known, the number n of
particles of the powder per gram (unit: particle/g) is determined by the
following equation (2) based on this specific gravity D (unit: g/cm.sup.3)
and the volume V1 of one particle of the powder.
n=1/D.times.1/V1 (2)
Here, regarding the specific gravity (=density) D of the Co--Fe alloy,
every specific gravity of this alloy used in the present invention is
assumed to be the same value, and the value D=6 (unit: g/cm.sup.3)
determined based on the value described in a literature is used for the
calculation.
One particle of the powder is assumed to be a circular cylinder, and the
surface area S1 (unit: m.sup.2 /particle) is determined by the following
equation (3).
S1=.pi..times.d.times.(d+2L).times.1/2.times.1/(1.times.10.sup.12) (3)
In general, a BET specific surface area Sbet (unit: m.sup.2 /g), that is, a
measured value, is used as the surface area of the powder per gram, which
is one of characteristics of the powder. On the other hand, a specific
surface area Sc (unit: m.sup.2 /g) of the powder per gram, which is
determined by calculation, is obtained by the following equation (4)
resulting from the above-mentioned equations (1) to (3).
##EQU1##
The content of Y relative to the total amount (Fe+Co) of F and Co is
represented by p(Y) (wt %). The amount of presence of Y with respect to a
surface area of the magnetic powder surface is represented by Mc(Y, Sbet)
(unit: .mu.mol/m.sup.2) using the BET specific surface area Sbet, and is
determined by the following equation (5). The amount of presence of Y is
represented by Mc(Y, Sc) (unit: .mu.mol/m.sup.2) when determined from the
specific surface area Sc calculated based on the assumption that the
powder is a circular cylinder, and is obtained by the following equation
(6). In the above-mentioned equations, the atomic weight of Y is
represented by A(Y).
Mc(Y, Sbet)=10.sup.4.times.p(Y).times.1/A(Y).times.1/Sbet (5)
Mc(Y, Sc)=10.sup.4.times.p(Y).times.1/A(Y).times.1/Sc (6)
Preferably, the ferromagnetic alloy powder according to the present
invention further contains Al. As described above, in general, the
contained elements are present in the vicinity of the surface layer of the
ferromagnetic alloy powder. In particular, Al is present in the surface
layer of the ferromagnetic alloy powder at a relatively high proportion.
Al suppresses coalescence, keeps the crystal shape uniform, and improves
the dispersibility of the ferromagnetic alloy powder during a heat
treatment for oxidation-reduction reaction in the manufacturing process of
the ferromagnetic alloy powder, and therefore, the magnetic
characteristics and the surface properties of the magnetic recording
medium are improved.
Regarding the content of Al as well, a calculated value thereof can be
determined in a manner similar to that in the above-mentioned case of Y.
The corresponding Mc(Al, Sbet) and Mc(Al, Sc) (each unit: .mu.mol/m.sup.2
) are obtained by the following equations (7) and (8), respectively. In
the following equations, the atomic weight of Al is represented by A(Al).
Mc(Al, Sbet)=10.sup.4.times.p(Al).times.1/A(Al).times.1/Sbet (7)
Mc(Al, Sc)=10.sup.4.times.p(Al).times.1/A(Al).times.1/Sc (8)
Regarding a method for adding Al, Al is added in the above-mentioned
manufacturing process of the iron oxyhydroxide powder as a material for
the ferromagnetic alloy powder. For example, aqueous sodium hydroxide and
sodium aluminate are added into aqueous iron sulfate (FeSO.sub.4), and an
oxidation treatment is performed at 50.degree. C. to 60.degree. C. while
air is blown into for 6 hours. Subsequently, cobalt chloride is added,
followed by standing for 30 hours. The resulting precipitates are
filtrated and separated, followed by washing with water and drying, so
that a Co-containing, Al-containing iron oxyhydroxide powder is prepared.
As another method, in the above-mentioned manufacturing process of the
Co-containing, Al-containing iron oxyhydroxide powder, the timing of
addition of sodium aluminate is changed so that the addition is performed
after the solution containing cobalt chloride is stood for 30 hours,
carbon dioxide is blown into after the addition so as to neutralize, and
thereby, precipitation onto the surface of the iron oxyhydroxide powder
can be brought about.
In the present invention, it is important to control the content of Al at
the time of above-mentioned addition of Al by, for example, controlling
the amount of addition of the above-mentioned sodium aluminate in
proportion to the specific surface area determined by calculation based on
the average major-axis length and the crystallite diameter of the
above-mentioned Co-containing, Al-containing iron oxyhydroxide powder
predicted by a preliminary experiment and the like. Consequently, in a
manner similar to that described above, optimization of the dispersibility
and the magnetic characteristics can be achieved by Al being contained at
a content in proportion to the specific surface area (Sc) of the
ferromagnetic alloy powder as well. In the ferromagnetic alloy powder used
in the present invention, preferably, the content of Al is 27 to 45
.mu.mol/m.sup.2, and more preferably, is 27 to 40 .mu.mol/m.sup.2 relative
to the specific surface area (Sc). When less than 27 .mu.mol/m.sup.2,
coalescence of the ferromagnetic alloy powder is likely to occur, the
dispersibility is likely to degrade, and the surface roughness and the
magnetic characteristics of the magnetic recording medium tend to degrade.
Therefore, the carrier output may be reduced, and noises may be increased.
On the other hand, when exceeding 45 .mu.mol/m.sup.2, the BET specific
surface area (Sbet) of the practical ferromagnetic alloy powder tends to
significantly increase relative to the specific surface area (Sc). In
particular, since the dispersibility is degraded, and the surface
roughness and the magnetic characteristics of the medium tend to degrade,
the carrier output may be reduced, and noises may be increased.
In the present invention, the content of Al can be specified relative to
the BET specific surface area (Sbet) of the ferromagnetic alloy powder. In
this case, preferably, the content of Al is 22 to 38 .mu.mol/m.sup.2 with
respect to a BET specific surface area (Sbet), and more preferably, is 22
to 34 .mu.mol/m.sup.2.
In the present invention, it is essential that the ferromagnetic alloy
powder contained in the magnetic layer satisfies the above-mentioned
condition. In this manner, the effects of the present invention can be
achieved. However, preferably, the following factors are further
satisfied.
Regarding the ferromagnetic alloy powder according to the present
invention, preferably, the saturation magnetization .sigma.s is 90 to 145
Am.sup.2 /kg, and more preferably, the ratio of the residual magnetization
.sigma.r to the saturation magnetization .sigma.s, .sigma.r/.sigma.s, is
0.51 or more. When the values of .sigma.s and .sigma.r/.sigma.s are
specified to be within these range, the magnetic characteristics of the
medium are further improved.
Regarding the surface roughness of the magnetic layer, preferably, the
center line average roughness Ra is 1 to 2.5 nm, the ten-point-average
roughness Rz is 10 to 25 nm, and the maximum height Rmax is 15 to 30 nm.
More preferably, Ra is 1 to 2 nm, Rz is 10 to 20 nm, and Rmax is 15 to 25
nm. When the surface roughness exceeds these range, the spacing tends to
increase, the carrier output tends to decrease, and noises tend to
increase. When the surface roughness is less than these ranges, the
reliability may be reduced.
When the magnetic recording medium of the present invention is a so-called
multilayer medium including the non-magnetic layer between the magnetic
layer and the non-magnetic support, the average thickness of the magnetic
layer is preferably 0.03 to 0.30 .mu.m. By specifying the average
thickness of the magnetic layer within this range, it is advantageous from
the viewpoint of saturation recording and short wavelength recording to
use a ferrite head or an MIG head as a recording, replay head. In general,
when the thickness of the magnetic layer is one-half to one-quarter the
recording wavelength, the saturation recording can be more effectively
performed, and an optimum thickness is further reduced with a decrease in
the recording wavelength. In particular, reduction of the thickness is
effective for reducing noises. When this average thickness is larger than
one-half the recording wavelength, the saturation recording becomes
impossible, and in addition, an influence is exerted on modulation
systems, e.g. PRML (Partial Response Maximum Likelihood) and RLL (Run
Length Limited), so that the carrier output may be reduced. On the other
hand, when the average thickness is less than 0.03 .mu.m, the number of
particles of the ferromagnetic alloy powder is reduced in the thickness
direction of the magnetic layer, the magnetic flux density is reduced, and
the carrier output is unlikely to be achieved.
The squareness ratio (residual magnetic flux density/saturation magnetic
flux density) of the magnetic layer according to the present invention is
preferably 0.88 or more in the direction of magnetic recording by the
recording, replay head, that is, in the longitudinal direction when the
magnetic recording medium of the present invention is a magnetic tape, and
more preferably, is 0.90 or more. At the same time, the degree of
orientation in the longitudinal direction of the magnetic tape (the
squareness ratio in the longitudinal direction/the squareness ratio in the
width direction) is preferably 3.0 or more, and more preferably, is 3.2 or
more. By increasing the squareness ratio and the degree of orientation,
the carrier output can be increased, noises can be reduced, and the
electromagnetic transducing characteristic, e.g. C/N, can be further
improved.
When this squareness ratio is brought close to 1.0, the electromagnetic
transducing characteristic becomes excellent. However, when less than
0.88, the resulting characteristic becomes poor. When the degree of
orientation is increased, excellent effects are exerted, for example, the
carrier output is increased, and noises are reduced. However, when less
than 3.0, the arrangement of the ferromagnetic alloy powder is disturbed,
the carrier output is reduced, noises are increased, and degradation of
the electromagnetic transducing characteristic, e.g. C/N, is likely to
occur. In general, the upper limit is believed to be in the order of 5.
The coercive force in the longitudinal direction of the magnetic recording
medium of the present invention is preferably 165 to 250 kA/m, and more
preferably, is 170 to 250 kA/m. The coercive force within this range is
suitable for high-recording-density recording. When exceeding 250 kA/m,
writing may become insufficient depending on the performance of the
recording head. On the other hand, when less than 165 kA/m, in particular,
the carrier output in short wavelength regions may be reduced.
The residual magnetic flux density in the longitudinal direction of the
magnetic recording medium of the present invention is preferably 200 to
400 mT, and more pr ferably, is 250 to 370 mT. When the ratio of the
ferromagnetic alloy powder to the binder is increased, and this residual
magnetic flux density is increased to more than 400 mT, durability and
running ability may be degraded. On the other hand, when less than 200 mT,
the carrier output may be reduced, and the C/N may be degraded.
When the magnetic recording medium of the present invention is used in a
system equipped with an MR head as a replay head, the residual magnetic
flux density is necessary to some extent. However, when the residual
magnetic flux density is too large, the MR element is saturated, and
becomes less sensitive to change in the magnetic field. The half-value
width (PW50) at the peak portion of an isolated waveform of the replay
signal must be reduced. Consequently, in order to make use of the merits
of the MR head, the average thickness of the magnetic layer is specified
to be 0.03 to 0.30 .mu.m, and preferably, be 0.04 to 0.10 .mu.m, and in
addition, the residual magnetization is specified to be 10 to 35
mT.multidot..mu.m. In this manner, the saturation of this MR element can
be avoided, PW50 of the isolated waveform can be reduced, and therefore,
high-recording-density magnetic recording medium can be provided.
Furthermore, regarding the magnetic recording medium of the present
invention, SFD (Switching Field Distribution) in the longitudinal
direction is also noted. The SFD is a scale for a velocity of the magnetic
flux change (response) relative to the magnetic field strength in the
magnetic recording medium. The SFD is also a scale for variations in the
dimension of the ferromagnetic alloy powder. It is ideal that the SFD gets
closer and closer to zero. When the ferromagnetic alloy powder is uniform,
and response is excellent, the transfer rate of the recording signal can
be further increased. Regarding the measurement of the magnetic
characteristics of the magnetic recording medium, in general, a hysteresis
magnetization curve is provided while the horizontal axis indicates the
magnetic field (Hf), and the vertical axis indicates the magnetic flux
(Mf). Furthermore, Mf is differentiated with respect to Hf, and a graph
having the horizontal axis indicating (Hf) and the vertical axis
indicating the differential value (dMf/dHf) can be provided. The resulting
graph is in the shape of a mountain in which the differential value
(dMf/dHf) is at a maximum in the vicinity of the coercive force (Hc) of
the magnetic recording medium, and the peak spreads toward the bottom.
Regarding this mountain-like portion, the half-value width (.DELTA.Ha) of
(dMf/dHf) at the coercive force (Hc), that is, .DELTA.Hf at 50% of the
maximum differential value, can be determined, and SFD=(.DELTA.Ha/Hc) can
be determined from this half-value width (.DELTA.Ha) and the coercive
force (Hc). This SFD takes on different values depending on the rate of
increase in the magnetic field (sweep rate) during the measurement, and
takes on smaller value with a decrease in the rate of increase. When the
rate of increase is brought close to zero, the SFD converges to a certain
constant value. Consequently, the value of the SFD in the present
invention is an SFD correction value at a sweep rate of zero, determined
by interpolation based on the actually measured SFD values at different
two sweep rates. In the present invention, the SFD correction value
determined by this method is preferably 0.30 or less, and more preferably,
is 0.27 or less. When the SFD correction value exceeds 0.30, response is
poor, and a noise is likely to be high.
Known films of, for example, polyesters, e.g. polyethylene terephthalate
(PET) and polyethylene naphthalate (PEN), polyamide, polyimide and
poly(amide-imide), can be used as the non-magnetic support used in the
present invention.
In the case where the magnetic recording medium of the present invention is
a multilayer medium, the film thickness of the non-magnetic layer is
preferably 0.5 to 2.0 .mu.m in terms of average thickness, and more
preferably, is 0.5 to 1.0 .mu.m. When this average film thickness is less
than 0.5 .mu.m, the surface properties of the magnetic layer is degraded
under the influence of a filler of the non-magnetic support, and at th
same time, friction may be increased, and durability and running ability
may be degraded. On the other hand, when exceeding 2.0 .mu.m, the
thickness is likely to become nonuniform during application of a coating
film, and therefore, the coating condition becomes severe, and the surface
smoothness is likely to degrade.
The non-magnetic layer may contains a resin and, if necessary, a granular
or needlelike non-magnetic powder, e.g. hematite, titanium oxide and
goethite, a lubricant and a conductive material, e.g. carbon black. The
above-mentioned non-magnetic powder is preferably specified to be a
non-magnetic needle powder. This non-magnetic needle powder preferably has
an average major-axis length of 0.05 to 0.10 .mu.m, and preferably has a
needle ratio (ratio of the average major-axis length to the average
minor-axis length) of 3 to 8. When the average major-axis length and the
needle ratio of the non-magnetic needle powder subjected to use is
specified to be within these ranges, the surface properties and the
reliability can become mutually compatible.
The sort of the non-magnetic needle powder in the non-magnetic layer is
appropriately selected on an as needed basis, and preferably, needle
goethite is used. When the average major-axis length thereof is specified
to be within the above-mentioned range of 0.05 to 0.10 .mu.m, the
dispersibility becomes excellent, the surface roughness of the
non-magnetic layer and the magnetic layer of the present invention become
excellent, and an excellent C/N characteristic can be exhibited.
In the case of the magnetic tape, a magnetic layer and, in accordance with
desires, a back coat layer may be provided in addition to the non-magnetic
layer. The back coat layer is provided for improving running stability,
preventing charging of the medium, adjusting lightproof property and the
like. Preferably, the back coat layer contains 20% to 80% by weight of
carbon black. When the content of the carbon black is too low, the effects
of preventing charging and shielding against light tend to be reduced, and
furthermore, the running stability is likely to degrade. When the
light-transmittance is high, a problem occurs in a system in which the
tape end is detected by change in the light-transmittance. On the other
hand, when the content of the carbon black is excessive, strength is
reduced because of the shortage of the binder in the back coat layer, and
running durability are likely to degrade. The carbon black can be
appropriately selected from common carbon black, and preferably, the
average particle diameter thereof is 0.005 to 0.3 .mu.m. More preferably,
an inorganic pigment, an organic pigment, a lubricant and the like is
appropriately contained in order to increase the reliability of the back
coat layer.
Examples of binders used for the magnetic layer, the non-magnetic layer and
the back coat layer of the present invention include, for example, a
thermoplastic resin, a thermosetting or reactive resin, and an
electron-radiation-curing resin. Combinations thereof are appropriately
selected in accordance with the characteristics of media and process
conditions, and are applied to use.
A method for manufacturing the magnetic recording medium of the present
invention will be described in detail.
A manufacturing process for a magnetic paint of the magnetic recording
medium of the present invention includes, on an as needed basis, a step of
kneading all of or a part of materials prepared to be used, a step of
mixing and diluting the resulting kneaded materials with an organic
solvent or a water-based solvent and a part of the above-mentioned
materials on an as needed basis, a step of dispersing with a dispersion
device, and if necessary, a step of adding a part of the above-mentioned
materials during the dispersion, followed by mixing and further
dispersion, and a step of mixing the organic solvent or the water-based
solvent and a part of the above-mentioned materials on an as needed basis.
A series of steps can be constructed depending on the purpose by arranging
these steps in that order or in a different order, while some steps may
overlap.
In the above-mentioned dispersion step, dispersion media selected from
glass beads made of SiO.sub.2 (true specific gravity: 2 to 3.2) or the
like, ceramic beads made of ZrO.sub.2 (true specific gravity; 6),
TiO.sub.2 (true specific gravity: 4) or the like, iron balls (true
specific gravity: 6), etc., are filled in a dispersion device, and the
magnetic paint is introduced into the dispersion device during operation,
so that a dispersion treatment is performed.
In the dispersion step of the magnetic paint according to the present
invention, optimum media can be appropriately selected from the
above-mentioned dispersion media on an as needed basis. However,
preferably, the ceramic beads having an average particle diameter of 0.1
to 0.8 mm and a true specific gravity of 4 or more are used. More
preferably, the ceramic beads are ZrO.sub.2 having an average particle
diameter of 0.1 to 0.5 mm. When the average particle diameter and the true
specific gravity of the dispersion media are specified to be within the
above-mentioned range, the dispersion media are small, the number thereof
is increased, and the weight on a dispersion media particle basis is
increased. As a result, the dispersion strength is improved, the
dispersibility of the ferromagnetic alloy powder in the magnetic paint
becomes excellent, and the electromagnetic transducing characteristic can
be further improved.
Soluble ions in the dispersion media to be used are sodium (Na.sup.+) and
calcium (Ca.sup.2+). Preferably, each of the contents is 100 ppm or less,
and more preferably, the content of the total ions is 100 ppm or less.
When the contents of the soluble ions are specified to be within the
above-mentioned ranges, reactions of the ferromagnetic alloy powder in the
magnetic paint or other materials with the soluble ions become unlikely to
occur, and therefore, improvement of the reliability can be in particular
achieved.
In the dispersion step of the magnetic paint according to the present
invention, the operating condition of the above-mentioned dispersion
device can be appropriately selected in accordance with desires. However,
the operating peripheral velocity of the dispersion device is preferably
specified to be 8 m/s or more, and more preferably, be 8 to 15 m/s.
Regarding the operating condition, when the peripheral velocity is
specified to be 8 m/s or more, movements of the dispersion media are
accelerated, dispersion energy is increased, and therefore, the dispersion
strength can be improved. At this time, the temperature of the magnetic
paint is increased with an increase in the peripheral velocity.
Consequently, even when the peripheral velocity is 15 m/s or less, more
preferably, adjustment of the cooling efficiency and the peripheral
velocity is performed in order to avoid denaturation of the materials
contain d in the magnetic paint. Preferably, the temperature during the
dispersion of the magnetic paint is 0 to 60.degree. C., and more
preferably, is 10.degree. C. to 40.degree. C.
A manufacturing process for a magnetic layer of the present invention can
be composed of appropriately selected steps including, on an as needed
basis, a step of applying the magnetic paint on the non-magnetic support,
although on the nonmagnetic layer after the non-magnetic layer is provided
by coating or at the same time as the provision of the non-magnetic layer
in the case of the multilayer medium, a step of preliminarily orientating
and drying, a step of formally orientating, a step of formally drying, a
step of processing and the like in that order, while some steps may
overlap.
In the above-mentioned application step, preferably, the magnetic paint is
applied by coating after being subjected to an ultrasonic treatment with
an ultrasonic dispersion device. When the volume of the ultrasonic
dispersion device is represented by Mus, and the flow rate of the magnetic
paint passing through the ultrasonic dispersion device is represented by
F, more preferably, the time T elapsed before application of the magnetic
paint after passing through the ultrasonic dispersion device satisfies the
relationship represented by a formula, T<500.times.Mus/F. Since the
ultrasonic dispersion device is used, proper energy is applied to
particles in the magnetic paint, the yield value of the magnetic paint is
reduced, particles are dispersed again immediately before application, and
therefore, smoothing during application can be accelerated. As a result,
the surface properties are improved, and furthermore, the magnetic
characteristics of the medium, in particular, the degree of orientation
and the squareness ratio, are improved.
In the above-mentioned step of formally orientating, a permanent magnet, an
electromagnet or the like can be used as the orientation magnet. However,
preferably, the electromagnet is used, and preferably, orientation is
performed in a magnetic field 2.5 times or larger than the coercive force
of the ferromagnetic alloy powder in the magnetic paint. After the
magnetic paint is applied, more preferably, the resulting coating is
allowed to reach the magnetic field of the electromagnet within 10 seconds
while being in the undried condition. Further preferably, blowing is
performed into the electromagnet, and drying is performed in the magnetic
field. When orientation is performed as described above, the squareness
ratio and the degree of orientation of the magnetic layer are further
improved.
Regarding provision of the nonmagnetic layer by coating in the case where
the magnetic recording medium of th present inventi n is a multilayer
medium, when the magnetic layer is provided on the non-magnetic layer,
either of so-called wet-on-wet coating, in which the non-magnetic layer is
in the wet condition, and the magnetic paint is provided there on, or
so-called wet-on-dry coating, in which the non-magnetic layer is at least
in the dry condition, and the magnetic paint is provided there on, can be
selected. Preferably, the wet-on-dry coating is used because of having an
advantage in preventing interfacial fluctuations between a plurality of
layers.
When the magnetic layer is provided by the wet-on-dry coating, the solvent
in the magnetic paint may attack the non-magnetic layer having been
provided, or may penetrate into the non-magnetic layer and swell the
non-magnetic layer, so that the surface properties of the non-magnetic
layer may be degraded, and the interfacial fluctuation between the
non-magnetic layer and the magnetic layer may be increased. As a method
for overcoming such a problem, preferably, some energy is applied to the
binder contained in the non-magnetic layer, and cross-linking (curing) is
brought about in order to enhance the solvent resistance (flux resistance)
of the non-magnetic layer surface.
A heat curing method, in which a coating film of the non-magnetic layer to
be cured is allowed to contain a heat-reactive cross-linking agent,
followed by drying, and subsequently, standing is performed at a
predetermined temperature for a predetermined time so as to cure, can be
used as the above-mentioned curing means. However, more preferably, an
electron-radiation-curing method described below is adopted as a means for
reducing the curing time to a minimum so that the non-magnetic layer and
the magnetic layer can be provided online and for avoiding heat
deformation due to folding or the weight because the support provided with
the non-magnetic layer by coating is of long lengths or in the shape of a
heavy roll.
That is, more preferably, the non-magnetic layer is allowed to contain an
electron-radiation-curing resin having at least an
electron-beam-functional group, the electron-beam-functional group
undergoes reaction to cross-link the resin by electron beam irradiation,
and therefore, the non-magnetic layer is cured. According to this method,
since the electron beam irradiation is simply required, the non-magnetic
layer can be cured in a short time, and heat deformation can be reduced to
a minimum. Methatrylic double bonds are introduced in thermoplastic resins
by a conventional method, so that the resin is modified to be sensitive to
an electron beam, and the resulting resin can be used as the resin having
an electron-beam-functional group. Examples of thermoplastic resins
include, for example, vinyl chloride copolymers and polyurethane resins,
methacrylic resins, polyester resins, acrylonitrile-butadiene copolymers,
polyamide resins, poly(vinyl butyral), nitrocellulose, styrene-butadiene
copolymers, poly(vinyl alcohol) resins, acetal resins, epoxy resins,
phenoxy resins, polyether resins, multifunctional polyethers, e.g.
polycaprolactone, polyamide resins, polyimide resins, phenol resins,
polybutadiene elastomers, chlorinated rubber, acrylic rubber, isoprene
rubber and epoxy-modified rubber. The content of the
electron-beam-functional group in hydroxy groups thereof is 1% to 40% by
mol, preferably, is 10% to 30% by mol in consideration of stability,
electron-radiation-curing property and the like during preparation. In
particular, regarding the vinyl chloride copolymer, when monomers are
reacted to bring about 1 to 20 functional groups per molecule, and
preferably, 2 to 10 functional groups, an electron-radiation-curing resin
having excellent dispersibility and curing property can be prepared.
Furthermore, it is preferable to have --COOH, --SO.sub.3 M, --OSO.sub.3 M,
--OPO.sub.3 M, --PO.sub.3 M, --PO.sub.2 M, --N.sup.+ R.sub.3 Cl.sup.-,
--NR.sub.2 (where M denotes H or an alkali metal, and R denotes H, methyl
or ethyl) and other acidic polar groups, basic polar groups and the like
at the end and in the side chain. Inclusion of them is suited for
improving the dispersibility.
The amount of irradiation of electron beam is represented by absorbed doses
of the non-magnetic layer, and a larger value indicates a higher degree of
curing. Preferably, the amount of irradiation is 2.5 to 20 Mrad, more
preferably, is 3.5 to 20 Mrad, and further preferably, is 4 to 18 Mrad.
When this amount of irradiation is too small, curing is insufficient, and
when the magnetic layer is provided, the non-magnetic layer is attacked by
the solvent in the magnetic paint. On the other hand, when excessive, the
structure of the resin and the non-magnetic support may be broken, and
therefore, the reliability is likely to degrade.
When the magnetic layer is provided by the wet-on-dry coating, preferably,
the concentration of the solids in the magnetic paint is adjusted to be 5%
to 20% by weight in order to prevent occurrence of coating streaks and to
perform sufficient orientation treatment.
The ferromagnetic alloy powder of the magnetic recording medium according
to the present invention can be prepared in accordance with a conventional
method. In the magnetic recording medium of the present invention, known
materials can be appropriately used in accordance with the purposes
without any restriction except for the above-mentioned materials, and
regarding the manufacturing method therefor, known steps can be
appropriately used in accordance with the purposes without any
restriction.
As described above, accordi
