U.S. patent application number 10/762506 was filed with the patent office on 2004-10-28 for magnetic recording medium.
This patent application is currently assigned to FUJI PHOTO FILM CO., LTD.. Invention is credited to Noguchi, Hitoshi, Saito, Shinji, Yamazaki, Nobuo.
Application Number | 20040213948 10/762506 |
Document ID | / |
Family ID | 32588677 |
Filed Date | 2004-10-28 |
United States Patent
Application |
20040213948 |
Kind Code |
A1 |
Saito, Shinji ; et
al. |
October 28, 2004 |
Magnetic recording medium
Abstract
Provided is a magnetic recording medium exhibiting good
electromagnetic characteristics in high-density recording. The
magnetic recording medium employed for magnetically recording
signals with a track width equal to or less than 2.0 .mu.m and
reproducing the magnetically recorded signals, wherein said
magnetic recording medium comprises a magnetic layer comprising a
hexagonal ferrite ferromagnetic powder and a binder or comprises a
nonmagnetic layer comprising a nonmagnetic powder and a binder and
a magnetic layer comprising a hexagonal ferrite ferromagnetic
powder and a binder in this order on a nonmagnetic support. Said
magnetic layer has a thickness equal to or less than 0.2 .mu.m, and
said hexagonal ferrite ferromagnetic powder has an average plate
diameter being {fraction (1/30)} or less of the magnetically
recorded track width as well as 1/2 or less of the thickness of the
magnetic layer.
Inventors: |
Saito, Shinji; (Odawara-shi,
JP) ; Noguchi, Hitoshi; (Odawara-shi, JP) ;
Yamazaki, Nobuo; (Odawara-shi, JP) |
Correspondence
Address: |
SUGHRUE MION, PLLC
2100 PENNSYLVANIA AVENUE, N.W.
SUITE 800
WASHINGTON
DC
20037
US
|
Assignee: |
FUJI PHOTO FILM CO., LTD.
|
Family ID: |
32588677 |
Appl. No.: |
10/762506 |
Filed: |
January 23, 2004 |
Current U.S.
Class: |
428/842.8 ;
G9B/5.267; G9B/5.277 |
Current CPC
Class: |
G11B 5/70678 20130101;
G11B 5/70 20130101; G11B 5/714 20130101 |
Class at
Publication: |
428/065.3 ;
428/694.0TR |
International
Class: |
B32B 003/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 24, 2003 |
JP |
2003-015601 |
Claims
What is claimed is:
1. A magnetic recording medium employed for magnetically recording
signals with a track width equal to or less than 2.0 .mu.m and
reproducing the magnetically recorded signals, wherein said
magnetic recording medium comprises a magnetic layer comprising a
hexagonal ferrite ferromagnetic powder and a binder on a
nonmagnetic support, said magnetic layer has a thickness equal to
or less than 0.2 .mu.m, and said hexagonal ferrite ferromagnetic
powder has an average plate diameter being {fraction (1/30)} or
less of the magnetically recorded track width as well as 1/2 or
less of the thickness of the magnetic layer.
2. A magnetic recording medium employed for magnetically recording
signals with a track width equal to or less than 2.0 .mu.m and
reproducing the magnetically recorded signals, wherein said
magnetic recording medium comprises a nonmagnetic layer comprising
a nonmagnetic powder and a binder and a magnetic layer comprising a
hexagonal ferrite ferromagnetic powder and a binder in this order
on a nonmagnetic support, said magnetic layer has a thickness equal
to or less than 0.2 .mu.m, and said hexagonal ferrite ferromagnetic
powder has an average plate diameter being {fraction (1/30)} or
less of the magnetically recorded track width as well as 1/2 or
less of the thickness of the magnetic layer.
3. The magnetic recording medium according to claim 1, wherein a
bit length of the magnetically recorded signal ranges from 0.04 to
0.2 .mu.m.
4. The magnetic recording medium according to claim 2, wherein a
bit length of the magnetically recorded signal ranges from 0.04 to
0.2 .mu.m.
5. The magnetic recording medium according to claim 1, wherein the
magnetic layer has a coercive force ranging from 143 to 398 kA/m
(1800 to 5000 Oe).
6. The magnetic recording medium according to claim 2, wherein the
magnetic layer has a coercive force ranging from 143 to 398 kA/m
(1800 to 5000 Oe).
7. The magnetic recording medium according to claim 1, wherein the
magnetic layer has a squareness (SQ) equal to or higher than
0.6.
8. The magnetic recording medium according to claim 2, wherein the
magnetic layer has a squareness (SQ) equal to or higher than
0.6.
9. The magnetic recording medium according to claim 1 for use in a
magnetic recording and reproducing system wherein the magnetically
recorded signal is reproduced with a magnetoresistive magnetic head
(MR head).
10. The magnetic recording medium according to claim 2 for use in a
magnetic recording and reproducing system wherein the magnetically
recorded signal is reproduced with a magnetoresistive magnetic head
(MR head).
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a particulate magnetic
recording medium with high recording density. In particular the
present invention relates to a magnetic recording medium for
high-density recording comprising a hexagonal ferrite ferromagnetic
powder in a magnetic layer.
BACKGROUND OF THE INVENTION
[0002] In the field of magnetic recording disks, 2 MB MF-2HD floppy
disks employing Co-modified iron oxide have been mounted normally
in personal computers. However, today, with the rapid increase in
the volume of data handled, the capacity of such disks is
inadequate and there is a need for the development of high-capacity
floppy disks.
[0003] On the other hand, in the field of magnetic tapes, with the
widespread popularity of office computers such as minicomputers,
personal computers, and work stations in recent years, a large
amount of research has been conducted into magnetic recording tapes
(so-called "back-up tapes") used to record computer data as an
external memory medium. In the practical implementation of magnetic
recording tapes for such applications, particularly as the size of
computers has decreased and information processing capability has
increased, there had been strong demand for an increase in
recording capacity to achieve high-capacity recording and size
reduction.
[0004] Further, with the broadening of the environments in which
magnetic tapes are employed, magnetic tapes have been further
required to have usability under a wide range of conditions
(particularly conditions in which heat and humidity severely
fluctuate), reliability of data storage, and reliability of
performance such as stable recording and reproducing of data when
used repeatedly at high speed with high frequency.
[0005] In recent years, the magnetic tapes employed in digital
signal recording systems have been determined by system. There are
magnetic tapes corresponding to so-called models DLT, 3480, 3490,
3590, QIC, model D8, and model DDS. In the magnetic tape employed
in any system, a nonmagnetic support is provided on one side with a
single-layer structure magnetic layer with a relatively thick film
thickness of about 2.0 to 3.0 .mu.m comprising a ferromagnetic
powder, binder, and abrasives, and on the other side with a
backcoat layer to prevent winding irregularity and ensure good
running durability. However, these comparatively thick,
single-layer-structure magnetic layers have a problem in the form
of thickness loss, where output drops off.
[0006] The technique of thinning the magnetic layer is known to
afford improvement with respect to the drop in reproduction output
caused by thickness loss of the magnetic layer. For example, a
tape-shaped magnetic recording medium (Japanese Unexamined Patent
Publication (KOKAI) Heisei No. 5-182178 has been disclosed in which
on a nonmagnetic support is provided a lower nonmagnetic layer
comprising an inorganic powder dispersed in a binder, on which is
formed, while the nonmagnetic layer is still wet, an upper magnetic
layer equal to or less than 1.0 .mu.m in thickness comprising
ferromagnetic metal powder dispersed in binder. Further, a
disk-shaped magnetic recording medium having a magnetic layer with
a coercive force (Hc) of equal to or greater than 111 kA/m (1,400
Oe) and a thickness of 0.5 .mu.m, and a nonmagnetic layer
comprising electrically conductive particles has been disclosed
(Japanese Unexamined Patent Publication (KOKAI) Heisei
5-109061).
[0007] However, with the rapid development of high-capacity and
high-density disk- and tape-shaped magnetic recording media in
recent years, it has become difficult to achieve sufficient
electromagnetic characteristics, even with the above-described
techniques. In particular, when recording and reproducing are
performed at a recording density of Gbit/in.sup.2 class or higher
with increased linear recoding density and track density, it is
difficult to achieve adequate electromagnetic characteristics with
conventional magnetic recording media.
[0008] The present invention was devised in light of the above
problems. It is an object of the present invention to provide a
magnetic recording medium exhibiting good electromagnetic
characteristics in high-density recording.
[0009] The present inventors conducted extensive research into
obtaining a magnetic recording medium affording good
electromagnetic characteristics even in high-density recording of
Gbit/in.sup.2 class or higher on narrow tracks. As a result, they
discovered that in a magnetic recording medium employing a
hexagonal ferrite ferromagnetic powder, by adjusting the average
plate diameter of the hexagonal ferrite ferromagnetic powder based
on the recording track width and magnetic layer thickness, it was
possible to obtain a magnetic recording medium having good
electromagnetic characteristics even at high density; the present
invention was devised on that basis.
[0010] That is, the object of the present invention is achieved by
a magnetic recording media (1) and (2) below:
[0011] (1) a magnetic recording medium employed for magnetically
recording signals with a track width equal to or less than 2.0
.mu.m and reproducing the magnetically recorded signals,
wherein
[0012] said magnetic recording medium comprises a magnetic layer
comprising a hexagonal ferrite ferromagnetic powder and a binder on
a nonmagnetic support,
[0013] said magnetic layer has a thickness equal to or less than
0.2 .mu.m, and
[0014] said hexagonal ferrite ferromagnetic powder has an average
plate diameter being {fraction (1/30)} or less of the magnetically
recorded track width as well as 1/2 or less of the thickness of the
magnetic layer;
[0015] (2) a magnetic recording medium employed for magnetically
recording signals with a track width equal to or less than 2.0
.mu.m and reproducing the magnetically recorded signals,
wherein
[0016] said magnetic recording medium comprises a nonmagnetic layer
comprising a nonmagnetic powder and a binder and a magnetic layer
comprising a hexagonal ferrite ferromagnetic powder and a binder in
this order on a nonmagnetic support,
[0017] said magnetic layer has a thickness equal to or less than
0.2 .mu.m, and
[0018] said hexagonal ferrite ferromagnetic powder has an average
plate diameter being {fraction (1/30)} or less of the magnetically
recorded track width as well as 1/2 or less of the thickness of the
magnetic layer;
[0019] In the above magnetic recording media, it is preferred that
a bit length of the magnetically recorded signal ranges from 0.04
to 0.2 .mu.m;
[0020] the magnetic layer has a coercive force ranging from 143 to
398 kA/m (1800 to 5000 Oe); and
[0021] the magnetic layer has a squareness (SQ) equal to or higher
than 0.6.
[0022] It is further preferred that the magnetic recording media
are for the use in the magnetic recording and reproducing system
wherein a signal magnetically recorded on the magneric layer is
reproduced with a magnetoresistive magnetic head (MR head).
[0023] Modes of implementing the magnetic recording medium of the
present invention will be described in detail below.
[0024] [Recording Track Width]
[0025] The magnetic recording medium of the present invention is
employed to magnetically record signals at a track width equal to
or less than 2.0 .mu.m and to reproduce the magnetically recorded
signals.
[0026] To achieve magnetic recording at a high density of
Gbit/in.sup.2 class or higher, it is necessary to increase the
track density and the linear recording density. Thus, it is
desirable to make the recording track width as small as possible.
However, when the recording track width is reduced in conventional
magnetic recording media, there is a problem in that adequate
electromagnetic characteristics (S/N ratio) are not achieved. The
present invention is a magnetic recording medium achieving good
electromagnetic characteristics even when reproducing signals that
have been magnetically recorded at a narrow track width of 2.0
.mu.m or less.
[0027] In the present invention, the recording track width is, as
stated above, equal to or less than 2.0 .mu.m, preferably in the
range of 0.5 to 1.5 .mu.m. A recording track width being equal to
or greater than 0.5 .mu.m is desirable in that tracking can be
adequately performed even with existing techniques.
[0028] In this Specification, the phrases "magnetically recorded
track width" and "recording track width" are synonymous, meaning
the width of a single track on which signals are recorded with a
recording head. The reproduction track width with a reproduction
head is generally narrower than this, commonly being about 50 to 95
percent of the track width. The track pitch is broader than this,
usually being about 105 to 140 percent of the recording track
width.
[0029] The linear recording density during magnetic recording is
not specifically limited in the present invention. The recording
bit length is preferably 40 to 200 nm/bit, more preferably 60 to
120 nm/bit. A recording bit length of equal to or greater than 40
nm/bit is desirable in that space loss tends not to have an effect,
and since the size of the plate thickness of the hexagonal ferrite
ferromagnetic powder is small relative to the bit length, little
transition noise is generated and a good S/N ratio is achieved. The
recording bit length is preferably equal to or less than 200 nm/bit
to achieve the desired recording density.
[0030] [Hexagonal Ferrite Ferromagnetic Powder]
[0031] In the present invention, a hexagonal ferrite ferromagnetic
powder is employed as a ferromagnetic powder in the magnetic layer.
Examples of hexagonal ferrite ferromagnetic powders are various
substitution products of barium ferrite, strontium ferrite, lead
ferrite, and calcium ferrite, and Co substitution products.
[0032] Specific examples are magnetoplumbite-type barium ferrite
and strontium ferrite; magnetoplumbite-type ferrite in which the
particle surfaces are covered with spinels; and
magnetoplumbite-type barium ferrite, strontium ferrite, and the
like partly comprising a spinel phase. The following may be
incorporated in addition to the prescribed atoms: Al, Si, S, Sc,
Ti, V, Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, W, Re, Au,
Hg, Pb, Bi, La, Ce, Pr, Nd, P, Co, Mn, Zn, Ni, Sr, B, Ge, Nb, and
the like. Compounds to which elements such as Co--Zn, Co--Ti,
Co--Ti--Zr, Co--Ti--Zn, Ni--Ti--Zn, Nb--Zn--Co, Sb--Zn--Co, and
Nb--Zn have been added may generally also be employed. They may
comprise specific impurities depending on the starting materials
and manufacturing methods, but those can also be employed in the
present invention.
[0033] In the present invention, the average plate diameter of the
hexagonal ferrite ferromagnetic powder is {fraction (1/30)} or less
of the recording track width, and 1/2 or less of thickness of the
magnetic layer.
[0034] When the recording track width is equal to or less than 2.0
.mu.m, nonuniformity in magnetization at both ends of the
reproduction track due to the plate diameter of the hexagonal
ferrite magnetic material is relatively no longer negligible. For
example, if considering the case where hexagonal ferrite
ferromagnetic powders of plate diameter A are disposed in a
magnetic layer of thickness A, there is one magnetic powder
particle in the thickness direction of the magnetic layer.
Therefore, it will be readily understood that uniform film
formation in the direction of thickness of the magnetic layer and
uniform magnetic recording are difficult.
[0035] On the other hand, when conducting high-density recording,
it is necessary to thin the magnetic layer to reduce adjacent
waveform interference. However, when the magnetic layer is thinned,
the S/N ratio ends up dropping abruptly.
[0036] The present inventors conducted extensive research into the
above-stated problems, resulting in the discovery of a significant
relation between the plate diameter of the hexagonal ferrite and
the thickness of the magnetic layer. That is, the present inventors
assumed that in magnetic recording media for in-plane recording,
since there is a large amount of plate-shaped hexagonal ferrite
ferromagnetic powders arranged in a longitudinal direction and thus
the number of hexagonal ferrite ferromagnetic powders arranged in
the thickness direction is limited in a thin magnetic layer,
uniformity in the magnetic layer deteriorates in the thickness
direction, resulting in noise increase.
[0037] Accordingly, the present inventors, based on this
examination, conducted further extensive research into the relation
between the average plate diameter of the hexagonal ferrite
ferromagnetic powder employed and the magnetic layer thickness,
resulting in the discovery that by limiting the average plate
diameter of the hexagonal ferrite ferromagnetic powder to {fraction
(1/30)} or less of the recording track width and adjusting it to
1/2 or less of the thickness of the magnetic layer, it was possible
to adequately reduce the width of the magnetic material relative to
the magnetic layer, nonuniformity in magnetization at both ends of
the track tended not to occur, and noise was reduced.
[0038] The average plate diameter of the hexagonal ferrite
ferromagnetic powder employed in the present invention is not
limited so long as it is within the above-stated range, but is
preferably from 10 to 50 nm, more preferably from 15 to 30 nm. The
average plate diameter is preferably {fraction (1/40)} or less,
more preferably {fraction (1/50)} or less of the recording track
width, as well as preferably 1/3 or less, more preferably 1/4 or
less of the thickness of the magnetic layer. Further, the lower
limit of the average plate diameter is preferably {fraction
(1/150)} of the recording track width as well as {fraction (1/15)}
of the thickness of the magnetic layer. Further, the average plate
thickness is preferably from 5 to 15 nm, more preferably from 7 to
12 nm. An average plate diameter equal to or greater than 10 nm and
an average plate thickness equal to or greater than 5 nm are
desirable because magnetic anisotropy can be maintained and good
coercive force (Hc) and thermal stability can be achieved.
[0039] The plate ratio (plate diameter/plate thickness) of the
hexagonal ferrite ferromagnetic powder is preferably 2 to 5, more
preferably 2.5 to 4. A low plate ratio is desirable because the
fill rate in the magnetic layer increases; however, adequate
orientation cannot be achieved. An excessively high plate ratio is
undesirable because noise increases due to stacking of particles.
From these perspectives, the plate ratio prefeably falls within a
range of 2 to 5.
[0040] The specific surface area by BET method of the hexagonal
ferrite ferromagnetic powder is usually 20 to 200 m.sup.2/g, almost
corresponding to an arithmetic value from the particle plate
diameter and the plate thickness. Narrow distributions of particle
plate diameter and plate thickness are normally preferred. These
can be compared by randomly measuring 500 particles in a TEM
photograph of particles. The distributions are often not a normal
distribution. However, when expressed as the standard deviation to
the average size, .sigma./average size=0.1 to 2.0. The particle
producing reaction system is rendered as uniform as possible and
the particles produced are subjected to a distribution-enhancing
treatment to achieve a sharp particle size distribution. For
example, methods such as selectively dissolving ultrafine particles
in an acid solution are known.
[0041] The coercive force (Hc) of the hexagonal ferrite
ferromagnetic powder is preferably from 143 to 398 kA/m (1,800 to
5,000 Oe), more preferably from 167 to 279 kA/m (2,100 to 3,500
Oe). When the coercive force (Hc) is equal to or greater than 143
kA/m, there tends to be no demagnetization during recording and no
drop in output. At equal to or less than 398 kA/m, recording with
head is possible and output is maintained. The coercive force (Hc)
can be controlled by means of the particle size (plate diameter and
plate thickness), the type and quantities of elements incorporated,
the substitution sites of the elements, the particle generating
reaction conditions, and the like.
[0042] The saturation magnetization (.sigma.s) of the hexagonal
ferrite ferromagnetic powder is preferably 40 to 80
A.cndot.m.sup.2/kg (40 to 80 emu/g). The higher saturation
magnetization (.sigma.s) is preferred, however, it tends to
decrease with decreasing particle size. Known methods of improving
saturation magnetization (.sigma.s) are combining spinel ferrite
with magnetoplumbite ferrite, selection of the type and quantity of
elements incorporated, and the like. It is also possible to employ
W-type hexagonal ferrite. When dispersing the ferromagnetic powder,
the surface of the ferromagnetic powder particles may be processed
with a substance suited to a dispersion medium and a polymer. Both
organic and inorganic compounds can be employed as surface
treatment agents. Examples of the principal compounds are oxides
and hydroxides of Si, Al, P, and the like; various silane coupling
agents; and various titanium coupling agents. The quantity of
surface treatment agent added may range from 0.1 to 10 weight
percent relative to the weight of the ferromagnetic powder. The pH
of the ferromagnetic powder is also important to dispersion. A pH
of about 4 to 12 is usually optimum for the dispersion medium and
polymer. From the perspective of the chemical stability and storage
properties of the medium, a pH of about 6 to 11 can be selected.
Moisture contained in the ferromagnetic powder also affects
dispersion. There is an optimum level for the dispersion medium and
polymer, usually selected from the range of 0.01 to 2.0
percent.
[0043] The hexagonal ferrite ferromagnetic powder may be surface
treated as necessary with Al, Si, P, an oxide thereof, or the like.
The quantity employed desirably ranges from 0.1 to 10 weight
percent of the ferromagnetic powder, and when a surface treatment
is conducted, a lubricant such as a fatty acid is desirably
adsorbed in a quantity of equal to or less than 100 mg/m.sup.2. An
inorganic ion in the form of soluble Na, Ca, Fe, Ni, Sr, or the
like may be contained in the hexagonal ferrite ferromagnetic
powder. These are preferably substantially not contained, but at
levels of equal to or less than 200 ppm, characteristics are seldom
affected.
[0044] Methods of manufacturing the hexagonal ferrite ferromagnetic
powder include: (1) a vitrified crystallization method consisting
of mixing into a desired ferrite composition barium oxide, iron
oxide, and a metal oxide substituting for iron with a glass forming
substance such as boron oxide; melting the mixture; rapidly cooling
the mixture to obtain an amorphous material; reheating the
amorphous material and refining and comminuting the product to
obtain a barium ferrite crystal powder; (2) a hydrothermal reaction
method consisting of neutralizing a barium ferrite composition
metal salt solution with an alkali; removing the by-product;
heating the liquid phase to 100.degree. C. or greater; and washing,
drying, and comminuting the product to obtain barium ferrite
crystal powder; and (3) a coprecipitation method consisting of
neutralizing a barium ferrite composition metal salt solution with
an alkali; removing the by-product; drying the product and
processing it at equal to or less than 1,100.degree. C.; and
comminuting the product to obtain barium ferrite crystal powder.
However, any manufacturing method can be selected in the present
invention.
[0045] [Nonmagnetic Powder]
[0046] The magnetic recording medium of the present invention can
comprise a nonmagnetic layer between a nonmagnetic support and a
magnetic layer. The preferred nonmagnetic layers are those
comprising an inorganic nonmagnetic powder and a binder as main
components. The inorganic nonmagnetic powder employed in the
nonmagnetic layer can be selected from inorganic compounds such as
metal oxides, metal carbonates, metal sulfates, metal nitrides,
metal carbides, metal sulfides and the like. Examples of inorganic
compounds are .alpha.-alumina having an .alpha.-conversion rate
equal to or higher than 90 percent, .beta.-alumina, .gamma.-alumia,
.theta.-alumina, silicon carbide, chromium oxide, cerium oxide,
.alpha.-iron oxide, hematite, goethite, corundum, silicon nitride,
titanium carbide, titanium dioxide, silicon dioxide, tin oxide,
magnesium oxide, tungsten oxide, zirconium oxide, boron nitride,
zinc oxide, calcium carbonate, calcium sulfate, barium sulfite, and
molybdenum disulfide; these may be employed singly or in
combination. Particularly desirable due to their narrow particle
distribution and numerous means of imparting functions are titanium
dioxide, zinc oxide, iron oxide and barium sulfate. Even more
preferred are titanium dioxide and .alpha.-iron oxide.
[0047] The particle size of these nonmagnetic powders preferably
ranges from 0.005 to 2 .mu.m, but nonmagnetic powders of differing
particle size may be combined as needed, or the particle diameter
distribution of a single nonmagnetic powder may be broadened to
achieve the same effect. What is preferred most is a particle size
in the nonmagnetic powder ranging from 0.01 to 0.2 .mu.m.
Particularly when the nonmagnetic powder is a granular metal oxide,
an average particle diameter equal to or less than 0.08 .mu.m is
preferred, and when an acicular metal oxide, the major axis length
is preferably equal to or less than 0.3 .mu.m, more preferably
equal to or less than 0.2 .mu.m.
[0048] The tap density of the nonmagnetic powder preferably ranges
from 0.05 to 2 g/ml, more preferably from 0.2 to 1.5 g/ml. The
moisture content of the nonmagnetic powder preferably ranges from
0.1 to 5 weight percent, more preferably from 0.2 to 3 weight
percent, further preferably from 0.3 to 1.5 weight percent. The pH
of the nonmagnetic powder preferably ranges from 2 to 11, and the
pH between 5.5 to 10 is particular preferred. The specific surface
area of the nonmagnetic powder preferably ranges from 1 to 100
m.sup.2/g, more preferably from 5 to 80 m.sup.2/g, further
preferably from 10 to 70 m.sup.2/g. The crystallite size of the
nonmagnetic powder preferably ranges from 0.004 to 1 .mu.m, further
preferably from 0.04 to 0.1 .mu.m. The oil absorption capacity
using dibutyl phthalate (DBP) preferably ranges from 5 to 100
ml/100 g, more preferably from 10 to 80 ml/g, further preferably
from 20 to 60 ml/100 g. The specific gravity of the nonmagnetic
powder preferably ranges from 1 to 12, more preferably from 3 to
6.
[0049] The shape of the nonmagnetic powder may be any of acicular,
spherical, polyhedral, or plate-shaped. The Mohs' hardness is
preferably 4 to 10. The stearic acid (SA) adsorption capacity of
the nonmagnetic powders preferably ranges from 1 to 20
.mu.mol/m.sup.2, more preferably from 2 to 15 .mu.mol/m.sup.2,
further preferably from 3 to 8 .mu.mol/m.sup.2. The pH between 3 to
6 is preferred. The surface of these nonmagnetic powders is
preferably treated with Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2,
ZrO.sub.2, SnO.sub.2, Sb.sub.2O.sub.3, ZnO and Y.sub.2O.sub.3. The
surface-treating agents of preference with regard to dispersibility
are Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2, and ZrO.sub.2, and
Al.sub.2O.sub.3, SiO.sub.2 and ZrO.sub.2 are further preferable.
These may be used singly or in combination. Depending on the
objective, a surface-treatment coating layer with a coprecipitated
material may also be employed, the coating structure which
comprises a first alumina coating and a second silica coating
thereover or the reverse structure thereof may also be adopted.
Depending on the objective, the surface-treatment coating layer may
be a porous layer, with homogeneity and density being generally
desirable.
[0050] Specific examples of nonmagnetic powders suitable for use in
the nonmagnetic layer in the present invention are: Nanotite from
Showa Denko K. K.; HIT-100 and ZA-G1 from Sumitomo Chemical Co.,
Ltd.; .alpha.-hematite DPN-250, DPN-250BX, DPN-245, DPN-270BX,
DPN-500BX, DBN-SA1 and DBN-SA3 from Toda Kogyo Corp.; titanium
oxide TTO-51B, TTO-55A, TTO-55B, TTO-55C, TTO-55S, TTO-55D, SN-100,
.alpha.-hematite E270, E271, E300 and E303 from Ishihara Sangyo
Co., Ltd.; titanium oxide STT-4D, STT-30D, STT-30, STT-65C, and
.alpha.-hematite .alpha.-40 from Titan Kogyo K. K.; MT-100S,
MT-100T, MT-150W, MT-500B, MT-600B, MT-100F, and MT-500HD from
Tayca Corporation; FINEX-25, BF-1, BF-10, BF-20, and ST-M from
Sakai Chemical Industry Co., Ltd.; DEFIC-Y and DEFIC-R from Dowa
Mining Co., Ltd.; AS2BM and TiO2P25 from Nippon Aerogil; 100A and
600A from Ube Industries, Ltd.; and sintered products of the same.
Particular preferable nonmagnetic powders are titanium dioxide and
.alpha.-iron oxide.
[0051] [Binder]
[0052] Thermoplastic resins, thermosetting resins, and reactive
resins, that are conventionally known as a binder, and mixtures
thereof may be employed in the magnetic layer, the nonmagnetic
layer and the backcoat layer in the present invention.
[0053] The thermoplastic resins having a glass transition
temperature ranging from -100 to 150.degree. C., a number average
molecular weight ranging from 1,000 to 200,000, preferably from
10,000 to 100,000, a degree of polymerization ranging from about 50
to 1,000 can be employed.
[0054] Examples of the thermoplastic resins are polymers and
copolymers comprising structural units in the form of vinyl
chloride, vinyl acetate, vinyl alcohol, maleic acid, acrylic acid,
acrylic acid esters, vinylidene chloride, acrylonitrile,
methacrylic acid, methacrylic acid esters, styrene, butadiene,
ethylene, vinyl butyral, vinyl acetal, and vinyl ether;
polyurethane resins; and various rubber resins.
[0055] Preferred thermosetting resins and reactive resins are those
having a weight average molecular weight equal to or less than
200,000 in a coating liquid. Among such resins, further preferred
are those that are not softened or melted until thermal
decomposition. Specific examples of thermosetting resins and
reactive resins are phenol resins, epoxy resins, polyurethane cured
resins, urea resins, melamine resins, alkyd resins, acrylic
reactive resins, formaldehyde resins, silicone resins, epoxy
polyamide resins, mixtures of polyester resins and isocyanate
prepolymers, mixtures of polyester polyols and polyisocyanates, and
mixtures of polyurethane and polyisocyanates.
[0056] To obtain better dispersibility and durability in all of the
binders set forth above, it is desirable to introduce by
copolymerization or addition reaction one or more polar groups
selected from among --COOM, --SO.sub.3M, --OSO.sub.3M,
--P.dbd.O(OM).sub.2, --O--P.dbd.O(OM).sub.2, (where M denotes a
hydrogen atom or an alkali metal base), OH, NR.sup.1R.sup.2,
N.sup.+R.sup.1R.sup.2R.sup.3 (where R.sup.1 to R.sup.3 denote a
hydrocarbon group), epoxy groups, --SH, and --CN. If such a polar
group is employed, the quantity of the polar group preferably
ranges from 10.sup.-1 to 10.sup.-8 mol/g, more preferably from
10.sup.-2 to 10.sup.-6 mol/g.
[0057] These resins are described in detail in the Handbook of
Plastics published by Asakura Shoten. It is also possible to employ
electron beam-cured resins in individual layers. If employing an
electron beam-cured resin, it is possible to obtain not only
improved coating strength and durability but also improved surface
smoothness and electromagnetic characteristics. Examples thereof
and methods of manufacturing the same are described in detail in
Japanese Unexamined Patent Publication (KOKAI) Showa No.
62-256219.
[0058] For example, the aforementioned binders are commercially
available under the following trade names: VAGH, VYHH, VMCH, VAGF,
VAGD, VROH, VYES, VYNC, VMCC, XYHL, XYSG, PKHH, PKHJ, PKHC, and
PKFE from Union Carbide Corporation; MPR-TA, MPR-TA5, MPR-TAL,
MPR-TSN, MPR-TMF, MPR-TS, MPR-TM, and MPR-TAO from Nisshin Kagaku
Kogyo K. K.; 1000W, DX80, DX81, DX82, DX83, and 100FD from Denki
Kagaku Kogyo K. K.; MR-104, MR-105, MR110, MR100, MR555, and
400X-110A from Nippon Zeon Co., Ltd.; Nippollan N2301, N2302, and
N2304 from Nippon Polyurethane Co., Ltd.; Pandex T-5105, T-R3080,
T-5201, Burnock D-400, D-210-80, Crisvon 6109, and 7209 from
Dainippon Ink and Chemicals Incorporated.; Vylon UR8200, UR8300,
UR-8700, RV530, and RV280 from Toyobo Co., Ltd.; Daipheramine 4020,
5020, 5100, 5300, 9020, 9022, and 7020 from Dainichiseika Color
& Chemicals Mfg. Co., Ltd.; MX5004 from Mitsubishi Chemical
Corporation; Sanprene SP-150 from Sanyo Chemical Industries, Ltd.;
and Saran F310 and F210 from Asahi Chemical Industry Co., Ltd.
[0059] The above-listed resins may be used singly or in
combination. Preferred resins are combinations of polyurethane
resin and at least one member selected from the group consisting of
vinyl chloride resin, vinyl chloride-vinyl acetate copolymers,
vinyl chloride-vinyl acetate-vinyl alcohol copolymers, and vinyl
chloride-vinyl acetate-maleic anhydride copolymers, as well as
combinations of the same with polyisocyanate.
[0060] Known structures of polyurethane resin can be employed, such
as polyester polyurethane, polyether polyurethane, polyether
polyester polyurethane, polycarbonate polyurethane, polyester
polycarbonate polurethane, and polycaprolactone polyurethane.
[0061] For example, the polyurethane resins suitable for use in the
present invention are commercially available under the following
trade names: UR8200, UR8300, and UR8700 from Toyobo Co., Ltd.
[0062] Examples of polyisocyanates suitable for use in the present
invention are tolylene diisocyanate, 4,4'-diphenylmethane
diisocyanate, hexamethylene diisocyanate, xylylene diisocyanate,
napthylene-1,5-diisocyanate, o-toluidine diisocyanate, isophorone
diisocyanate, triphenylmethane triisocyanate, and other
isocyanates; products of these isocyanates and polyalcohols;
polyisocyanates produced by condensation of isocyanates; and the
like.
[0063] These isocyanates are commercially available under the
following trade names, for example: Coronate L, Coronate HL,
Coronate 2030, Coronate 2031, Millionate MR and Millionate MTL
manufactured by Nippon Polyurethane Industry Co. Ltd.; Takenate
D-102, Takenate D-110N, Takenate D-200 and Sakenate D-202
manufactured by Takeda Chemical Industries Co., Ltd.; and Desmodule
L, Desmodule IL, Desmodule N and Desmodule HL manufactured by
Sumitomo Bayer Co., Ltd. They can be used singly or in combinations
of two or more in all layers by exploiting differences in curing
reactivity.
[0064] The quantity of binder employed in the nonmagnetic layer,
magnetic layer, or backcoat layer in the present invention falls
within a range of 5 to 50 weight percent, preferably from 10 to 30
weight percent, of the nonmagnetic powder or hexagonal ferrite
ferromagnetic powder. When employing vinyl chloride resin, the
content in the binder can be from 5 to 30 weight percent. When
employing polyurethane resin, the content of polyurethane resin can
be from 2 to 20 weight percent, and the content of polyisocyanate
resin can fall within a range of from 2 to 20 weight percent. Vinyl
chloride resin, polyurethane resin, and polyisocyanate are
desirably employed in combination from the perspective of
compatibility and crosslinking. However, for example, when head
corrosion occurs due to trace amounts of dechlorination, it is
possible to employ polyurethane alone, or just polyurethane and
isocyanate. When employing polyurethane resin in the present
invention, the glass transition temperature is preferably from -50
to 150.degree. C., more preferably from 0 to 100.degree. C.; the
elongation at break is preferably from 100 to 2,000 percent; the
stress at break is preferably from 0.49 to 98 MPa (0.05 to 10
kg/mm.sup.2), and the yield point is preferably from 0.49 to 98 MPa
(0.05 to 10 kg/mm.sup.2).
[0065] The quantity of binder; the quantity of vinyl chloride
resin, polyurethane resin, polyisocyanate, or some other resin in
the binder; the molecular weight of each of the resins; the
quantity of polar groups; or the physical characteristics or the
like of the above-described resins can naturally be different in
the magnetic layer, the nonmagnetic layer, and the backcoat layer
in the present invention as required. These should be optimized in
each layer. Known techniques for a multilayered magnetic layer may
be applied. For example, when the quantity of binder is different
in each layer, increasing the quantity of binder in the magnetic
layer effectively decreases scratching on the surface of the
magnetic layer. To achieve good head touch, the quantity of binder
in the nonmagnetic layer can be increased to impart
flexibility.
[0066] [Additives]
[0067] Additives may be added as needed to the magnetic layer,
nonmagnetic layer, and backcoat layer in the present invention.
Examples of such additives are carbon black, abrasives, lubricants,
dispersion assistant agents, antimold agents, antistatic agents,
antioxidants, solvents, and the like.
[0068] [Carbon Black]
[0069] Carbon black can be added to the magnetic layer, the
nonmagnetic layer, and the backcoat layer in the present invention
to prevent static, reduce the coefficient of friction, impart
light-blocking properties, enhance film strength in the magnetic
layer and the nonmagnetic layer, and the like. Examples of carbon
black suitable for use are furnace black for rubber, thermal for
rubber, black for coloring, and acetylene black. It is suitable for
carbon black to have a specific surface area of 5 to 500 m.sup.2/g,
a DBP oil absorption capacity of 10 to 400 ml/100 g, a particle
diameter of 5 to 300 nm (5 to 300 m.mu.), a pH of 2 to 10, a
moisture content of 0.1 to 10 percent, and a tap density of 0.1 to
1 g/ml.
[0070] Carbon black suitable for use in the present invention are
commercially available, for example, under the following trade
names: BLACK PEARLS 2000, 1300, 1000, 900, 905, 800, 700 and VULCAN
XC-72 from Cabot Corporation; #80, #60, #55, #50 and #35
manufactured by Asahi Carbon Co., Ltd.; #2400B, #2300, #900, #1000,
#30, #40 and #10B from Mitsubishi Chemical Corporation; CONDUCTEX
SC, RAVEN 150, 50, 40, 15, and RAVEN-MT-P from Columbia Carbon Co.,
Ltd.; and Ketjen Black EC from Nippon EC Co., Ltd. The carbon black
employed may be surface-treated with a dispersant or grafted with
resin, or have a partially graphite-treated surface. The carbon
black may be dispersed in advance into the binder prior to addition
to the magnetic coating material. These carbon blacks may be used
singly or in combination.
[0071] When employing carbon black, the quantity preferably ranges
from 0.1 to 30 weight percent with respect to the weight of the
hexagonal ferrite ferromagnetic powder. As described above, carbon
black works to prevent static, reduce the coefficient of friction,
impart light-blocking properties, enhance film strength, and the
like; the properties vary with the type of carbon black employed.
Accordingly, the type, quantity, and combination of carbon blacks
employed in the present invention may be determined separately for
the magnetic layer and the nonmagnetic layer based on the objective
and the various characteristics stated above, such as particle
size, oil absorption capacity, electrical conductivity, and pH, and
be optimized for each layer. For example, the Carbon Black Handbook
compiled by the Carbon Black Association may be consulted for types
of carbon black suitable for use in the present invention.
[0072] [Abrasive]
[0073] In the present invention, abrasives can be added in order to
improve the durability of the magnetic layer. Known materials,
chiefly with a Mohs' hardness equal to or higher than 6, such as
.alpha.-alumina having an .alpha.-conversion rate equal to or
higher than 90 percent, .beta.-alumina, silicon carbide, chromium
oxide, cerium oxide, .alpha.-iron oxide, corundum, artificial
diamond, silicon nitride, titanium carbide, titanium oxide, silicon
dioxide, and boron nitride, may be used singly or in combination as
abrasives. Further, a composite comprising two or more of these
abrasives (an abrasive obtained by surface-treating one abrasive
with another) may also be used.
[0074] Although these abrasives may contain compounds and elements
other than the main component or element in some cases, there is no
change in effect so long as the main component constitutes equal to
or higher than 90 percent. The particle size of these abrasives
preferably ranges from 0.01 to 2 .mu.m. In particular, in order to
improve electromagnetic characteristics, narrow particle size
distribution is preferred. In order to improve durability,
abrasives of differing particle size may be combined as needed or
the particle size distribution of a single abrasive may be
broadened to achieve the same effect. A tap density of 0.3 to 2
g/mL, a moisture content of 0.1 to 5 percent, a pH of 2 to 11, and
a specific surface area of 1 to 30 m.sup.2/g are desirable. The
abrasive may be any of acicular, spherical, or cubic in shape, but
shapes that are partially angular have good abrasion properties and
are thus preferred.
[0075] Abrasives suitable for use are commercially available, for
example, under the following trade names: AKP-12, AKP-15, AKP-20,
AKP-30, AKP-50, HIT-20, HIT-30, HIT-55, HIT-60, HIT-70, HIT-80 and
HIT-100 from Sumitomo Chemical Co., Ltd.; ERC-DBM, HP-DBM, and
HPS-DBM from Reynolds Co.; WA10000 from Fujimi Abrasives Co.; UB20
from Kamimura Kogyo Co., Ltd.; G-5, Chromex U2, and Chromex U1 from
Nippon Chemical Industrial Co., Ltd.; TF100 and TF-140 from Toda
Kogyo Corp.; Beta Random Ultrafine from Ibidene Co.; B-3 from Showa
Mining Co., Ltd.; MD150 from Tomei Daiya Co., Ltd; and LS-600F from
Lands Co., Ltd.
[0076] The above-mentioned abrasives may be added as needed to the
nonmagnetic layer. By adding them to the nonmagnetic layer, it
becomes possible to control the surface shape and protrusion of
abrasive. The particle diameter and quantity of abrasive added to
the magnetic layer and nonmagnetic layer should be set to optimal
levels.
[0077] [Other Additives]
[0078] Substances having lubricating effects, antistatic effects,
dispersive effects, plasticizing effects, or the like may be
employed as additives in the magnetic layer and nonmagnetic layer
in the present invention.
[0079] Examples of additives are: molybdenum disulfide; tungsten
graphite disulfide; boron nitride; graphite fluoride; silicone
oils; silicones having a polar group; fatty acid-modified
silicones; fluorine-containing silicones; fluorine-containing
alcohols; fluorine-containing esters; polyolefins; polyglycols;
alkylphosphoric esters and their alkali metal salts; alkylsulfuric
esters and their alkali metal salts; polyphenyl ethers;
phenylphosphonic acid; .alpha.-naphthylphosphoric acid;
phenylphosphoric acid; diphenylphosphoric acid;
p-ethylbenzenephosphonic acid; phenylphosphinic acid;
aminoquinones; various silane coupling agents and titanium coupling
agents; fluorine-containing alkylsulfuric acid esters and their
alkali metal salts; monobasic fatty acids (which may contain an
unsaturated bond or be branched) having 10 to 24 carbon atoms and
metal salts (such as Li, Na, K, and Cu) thereof; monohydric,
dihydric, trihydric, tetrahydric, pentahydric or hexahydric
alcohols with 12 to 22 carbon atoms (which may contain an
unsaturated bond or be branched); alkoxy alcohols with 12 to 22
carbon atoms (which may contain an unsaturated bond or be
branched); monofatty esters, difatty esters, or trifatty esters
comprising a monobasic fatty acid having 10 to 24 carbon atoms
(which may contain an unsaturated bond or be branched) and any one
from among a monohydric, dihydric, trihydric, tetrahydric,
pentahydric or hexahydric alcohol having 2 to 12 carbon, atoms
(which may contain an unsaturated bond or be branched); fatty acid
esters of monoalkyl ethers of alkylene oxide polymers; fatty acid
amides with 8 to 22 carbon atoms; and aliphatic amines with 8 to 22
carbon atoms.
[0080] Specific examples of the additives in the form of fatty
acids are: capric acid, caprylic acid, lauric acid, myristic acid,
palmitic acid, stearic acid, behenic acid, oleic acid, elaidic
acid, linolic acid, linolenic acid, and isostearic acid. Examples
of esters are butyl stearate, octyl stearate, amyl stearate,
isooctyl stearate, butyl myristate, octyl myristate, butoxyethyl
stearate, butoxydiethyl stearate, 2-ethylhexyl stearate,
2-octyldodecyl palmitate, 2-hexyldodecyl palmitate, isohexadecyl
stearate, oleyl oleate, dodecyl stearate, tridecyl stearate, oleyl
erucate, neopentylglycol didecanoate, and ethylene glycol dioleyl.
Examples of alcohols are oleyl alcohol, stearyl alcohol and lauryl
alcohol.
[0081] It is also possible to employ nonionic surfactants such as
alkylene oxide-based surfactants, glycerin-based surfactants,
glycidol-based surfactants and alkylphenolethylene oxide adducts;
cationic surfactants such as cyclic amines, ester amides,
quaternary ammonium salts, hydantoin derivatives, heterocycles,
phosphoniums, and sulfoniums; anionic surfactants comprising acid
groups, such as carboxylic acid, sulfonic acid, phosphoric acid,
sulfuric ester groups, and phosphoric ester groups; and ampholytic
surfactants such as amino acids, amino sulfonic acids, sulfuric or
phosphoric esters of amino alcohols, and alkyl betaines. Details of
these surfactants are described in A Guide to Surfactants
(published by Sangyo Tosho K.K.).
[0082] These lubricants, antistatic agents and the like need not be
100 percent pure and may contain impurities, such as isomers,
unreacted material, by-products, decomposition products, and oxides
in addition to the main components. These impurities are preferably
comprised equal to or less than 30 percent, and more preferably
equal to or less than 10 percent.
[0083] The lubricants and surfactants suitable for use in the
present invention each have different physical effects. The type,
quantity, and combination ratio of lubricants producing synergistic
effects should be optimally set for a given objective. It is
conceivable to control bleeding onto the surface through the use of
fatty acids having different melting points in the nonmagnetic
layer and the magnetic layer; to control bleeding onto the surface
through the use of esters having different boiling points, melting
points, and polarity; to improve the stability of coatings by
adjusting the quantity of surfactant; and to increase the
lubricating effect by increasing the amount of lubricant in the
intermediate layer. The present invention is not limited to these
examples. Generally, a total quantity of lubricant ranging from 0.1
to 50 weight percent, preferably from 2 to 25 weight percent with
respect to the hexagonal ferrite ferromagnetic powder or the
nonmagnetic powder is preferred.
[0084] All or some of the additives suitable for use in the present
invention may be added at any stage in the process of manufacturing
the magnetic and nonmagnetic coating liquids. For example, they may
be mixed with the hexagonal ferrite ferromagnetic powder before a
kneading step; added during a step of kneading the ferromagnetic
powder, the binder, and the solvent; added during a dispersing
step; added after dispersing; or added immediately before coating.
Depending on the objective, part or all of the additives may be
applied by simultaneous or sequential coating after the magnetic
layer has been applied to achieve a specific purpose. Depending on
the objective, the lubricant may be coated on the surface of the
magnetic layer after calendering or making slits.
[0085] [Organic Solvent]
[0086] Known organic solvents can be employed in the present
invention. The organic solvent employed in the present invention
may be used in any ratio. Examples are ketones such as acetone,
methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone,
cyclohexanone, isophorone, and tetrahydrofuran; alcohols such as
methanol, ethanol, propanol, butanol, isobutyl alcohol, isopropyl
alcohol, and methylcyclohexanol; esters such as methyl acetate,
butyl acetate, isobutyl acetate, isopropyl acetate, ethyl lactate,
and glycol acetate; glycol ethers such as glycol dimethyl ether,
glycol monoethyl ether, and dioxane; aromatic hydrocarbons such as
benzene, toluene, xylene, cresol, and chlorobenzene; chlorinated
hydrocarbons such as methylene chloride, ethylene chloride, carbon
tetrachloride, chloroform, ethylene chlorohydrin, and
dichlorobenzene; N,N-dimethylformamide; and hexane.
[0087] These organic solvents need not be 100 percent pure and may
contain impurities such as isomers, unreacted materials,
by-products, decomposition products, oxides and moisture in
addition to the main components. The content of these impurities is
preferably equal to or less than 30 percent, more preferably equal
to or less than 10 percent. The amount added may be varied. To
improve dispersion properties, a solvent having a somewhat strong
polarity is desirable. It is desirable that solvents having a
dielectric constant equal to or higher than 15 are comprised equal
to or higher than 50 percent of the solvent composition. Further,
the dissolution parameter is desirably 8 to 11.
[0088] [Nonmagnetic Support]
[0089] The nonmagnetic support employed in the present invention is
not specifically limited. Known films can be employed such as
polyesters such as polyethylene terephthalate and polyethylene
naphthalate, polyolefins, cellulose triacetate, polycarbonates,
polyamides, polyimides, polyamidoimides, polysulfones,
polyaramides, aromatic polyamides, and polybenoooxazoles. Of these,
the use of high-strength supports such as polyethylene naphthalate
and polyamide is preferred. To change the surface roughness of the
magnetic surface and the support surface, a laminated support such
as those described in Japanese Unexamined Patent Publication
(KOKAI) Heisei No. 3-224127 may be employed as required. These
nonmagnetic supports may be subjected beforehand to corona
discharge treatment, plasma treatment, adhesion enhancing
treatment, heat treatment, dust removal, or the like. Aluminum and
glass substrates may be employed as the nonmagnetic support in the
present invention.
[0090] To achieve the objects of the present invention, the center
surface average surface roughness SRa of the nonmagnetic support as
measured by the Mirau method with a TOPO-3D made by WYKO is equal
to or less than 8.0 nm, preferably equal to or less than 4.0 nm,
and more preferably equal to or less than 2.0 nm. Not only does
such a support desirably have a low center surface average surface
roughness, but there are also desirably no large protrusions equal
to or higher than 0.5 .mu.m. The surface roughness shape may be
freely controlled through the size and quantity of filler added to
the support as needed. Examples of such fillers are oxides and
carbonates of elements such as Ca, Si and Ti, and organic
micropowders such as acrylic-based one. The support desirably has a
maximum height SR.sub.max equal to or less than 1 .mu.m, a
ten-point average roughness SR.sub.z equal to or less than 0.5
.mu.m, a center surface peak height SR.sub.P equal to or less than
0.5 .mu.m, a center surface valley depth SR.sub.V equal to or less
than 0.5 .mu.m, a center-surface surface area percentage SSr of 10
percent to 90 percent, and an average wavelength S.lambda..sub.a of
5 to 300 .mu.m. To achieve desired electromagnetic characteristics
and durability, the surface protrusion distribution of the support
can be freely controlled with fillers. It is possible to control
within a range from 0 to 2,000 protrusions of 0.01 to 1 .mu.m in
size per 0.1 mm.sup.2.
[0091] The F-5 value of the nonmagnetic support employed in the
present invention desirably ranges from 49 to 490 MPa (5 to 50
kg/mm.sup.2). The thermal shrinkage rate of the support after 30
min at 100.degree. C. is preferably equal to or less than 3
percent, more preferably equal to or less than 1.5 percent. The
thermal shrinkage rate after 30 min at 80.degree. C. is preferably
equal to or less than 0.5 percent, more preferably equal to or less
than 0.1 percent. The breaking strength of the nonmagnetic support
preferably ranges from 49 to 980 MPa (5 to 100 kg/mm.sup.2). The
modulus of elasticity preferably ranges from 980 to 19600 MPa (100
to 2,000 kg/mm.sup.2). The thermal expansion coefficient of the
nonmagnetic support preferably ranges from 10.sup.-4 to
10.sup.-8/.degree. C., more preferably from 10.sup.-5 to
10.sup.-6/.degree. C. The moisture expansion coefficient is
preferably equal to or less than 10.sup.-4/RH percent, more
preferably equal to or less than 10.sup.-5/RH percent. These
thermal characteristics, dimensional characteristics, and
mechanical strength characteristics are desirably nearly equal,
with a difference equal to less than 10 percent, in all in-plane
directions.
[0092] [Backcoat Layer]
[0093] In a tape-shaped magnetic recording medium, a backcoat layer
may be provided on the opposite surface of the nonmagnetic support
from the surface on which is provided the magnetic layer to achieve
effects such as preventing static and compensating for curl. The
backcoat layer is useful for stable running of the tape-shaped
magnetic recording medium. The backcoat layer normally has a
thickness of about 0.1 to 1 .mu.m, and is desirably electrically
conductive. Carbon black and binder may be incorporated into the
backcoat layer. The same carbon blacks and binders as described
above may be employed.
[0094] The incorporation, into the backcoat layer, of metal oxides
having a Mohs' hardness of 5 to 9, such as .alpha.-alumina and
.alpha.-iron oxide, with an average particle size falling within a
range of 100 to 210 .mu.m, is desirable because it reduces
fluctuation in the coefficient of dynamic friction between the tape
guide of the recording and reproducing device or the tape guide of
the loaded cassette and the backcoat layer as it slides repeatedly
over it, yielding a backcoat layer with good durability. The metal
oxides having a Mohs' hardness of 5 to 9 can be employed in a range
of 3 to 20 weight parts per 100 weight parts of carbon black.
[0095] When the light transmittance of a photosensitive layer is
detected through the backcoat layer, the backcoat layer must be
transparent. Accordingly, in that case, the quantity of carbon
black added is desirably adjusted to conform to the light
transmittance. When detecting reflectance, a reflective film such
as a metal vapor deposition film, can be provided between the
nonmagnetic support and the photosensitive layer.
[0096] [Layer Structure]
[0097] With respect to the thickness structure of the magnetic
recording medium of the present invention, the thickness of the
nonmagnetic support preferably ranges from 2 to 100 .mu.m, more
preferably from 2 to 80 .mu.m. For computer-use magnetic recording
tapes, the nonmagnetic support having a thickness of 3.0 to 6.5
.mu.m, preferably 3.0 to 6.0 .mu.m, more preferably 4.0 to 5.5
.mu.m is suitably employed.
[0098] The thickness of the magnetic layer is equal to or less than
0.2 .mu.m, preferably from 0.03 to 0.15 .mu.m. The rate of
variation in thickness of the magnetic layer is preferably within
.+-.20 percent, more preferably within .+-.5 percent. It is also
possible to separate the magnetic layer into two or more layers
having different magnetic characteristics, and to suitably employ
configurations relating to known multilayer magnetic layers.
[0099] When a nonmagnetic layer is provided on a nonmagnetic
support, the nonmagnetic layer preferably has a thickness ranging
from 0.2 to 5.0 .mu.m, more preferably from 0.3 to 3.0 .mu.m,
further preferably from 1.0 to 2.5 .mu.m.
[0100] The nonmagnetic layer in the present invention is effective
so long as it is substantially nonmagnetic. For example, it
exhibits the effect of the present invention even when it comprises
impurities or trace amounts of magnetic material that have been
intentionally incorporated, and can be viewed as substantially
having the same configuration as the present invention. The term
"substantially nonmagnetic" is used to mean having a residual
magnetic flux density in the nonmagnetic layer of equal to or less
than 50 T.cndot.m (500 G), or a coercive force (Hc) of equal to or
less than 39.8 kA/m (500 Oe), it being preferable not to have a
residual magnetic flux density or coercive force at all.
[0101] An undercoating layer for improving adhesion between the
nonmagnetic support and the nonmagnetic layer or magnetic layer may
be provided. The thickness of the undercoating layer preferably
ranges from 0.01 to 0.5 .mu.m, more preferably from 0.02 to 0.5
.mu.m. When a backcoat layer is provided, the thickness thereof
preferably ranges from 0.1 to 4 .mu.m, more preferably from 0.3 to
2.0 .mu.m.
[0102] The magnetic recording medium of the present invention may
be a disk-shaped medium with double-sided magnetic layers in which
a nonmagnetic layer and magnetic layer are provided on both sides
of the nonmagnetic support, or may be a tape-shaped or disk-shaped
magnetic recording medium having these layers on just one side.
[0103] [Physical Properties]
[0104] The saturation magnetic flux density of the magnetic layer
in the magnetic recording medium of the present invention
preferably range from 100 to 300 T.cndot.m (1,000 to 3,000 G). The
coercive force (Hc) of the magnetic layer preferably ranges from
143 to 398 kA/m (1,800 to 5,000 Oe), more preferably from 167 to
279 kA/m (2,100 to 3,500 Oe). A narrow coercive force distribution
is desirable, with the SFD and SFDr being equal to or less than
0.6, preferably equal to or less than 0.2.
[0105] In tape-shaped magnetic recording media, the squareness (SQ)
is preferably equal to or greater than 0.6 because output
increases. Although the SQ is not specifically limited, a value
equal to or less than 0.90 is desirable because noise decreases due
to prevention of hexagonal ferrite ferromagnetic powders from
stacking. When random orientation is imparted in disk-shaped
magnetic recording media, the SQ is preferably from 0.45 to 0.65,
and the SQ is preferably rendered isotropic within the disk. When
imparting circumferential orientation the SQ is preferably equal to
or greater than 0.6 in a circumferential direction in the same
manner as in tape-shaped magnetic recording media.
[0106] The coefficient of friction of the magnetic recording medium
of the present invention relative to the head is preferably equal
to or less than 0.5 and more preferably equal to or less than 0.3
at temperatures ranging from -10.degree. C. to 40.degree. C. and
humidity ranging from 0 percent to 95 percent, the specific surface
resistivity preferably ranges from 10.sup.4 to 10.sup.12 .OMEGA./sq
on the magnetic surface, and the charge potential preferably ranges
from -500 V to +500 V. The modulus of elasticity at 0.5 percent
extension of the magnetic layer preferably ranges from 980 to 19600
MPa (100 to 2,000 kg/mm.sup.2) in each in-plane direction. The
breaking strength preferably ranges from 98 to 686 MPa (10 to 70
kg/mm.sup.2). The modulus of elasticity of the magnetic recording
medium preferably ranges from 980 to 14700 MPa (100 to 1,500
kg/mm.sup.2) in each in-plane direction. The residual elongation is
preferably equal to or less than 0.5 percent, and the thermal
shrinkage rate at all temperatures below 100.degree. C. is
preferably equal to or less than 1 percent, more preferably equal
to or less than 0.5 percent, and most preferably equal to or less
than 0.1 percent.
[0107] The glass transition temperature (i.e., the temperature at
which the loss elastic modulus of dynamic viscoelasticity peaks as
measured at 110 Hz) of the magnetic layer is preferably 50 to
120.degree. C., and that of the nonmagnetic layer preferably ranges
from 0 to 100.degree. C. The loss elastic modulus preferably falls
within a range of 1.times.10.sup.7 to 8.times.10.sup.8 Pa
(1.times.10.sup.8 to 8.times.10.sup.9 dyne/cm.sup.2) and the loss
tangent is preferably equal to or less than 0.2. Adhesion failure
tends to occur when the loss tangent becomes excessively large.
These thermal characteristics and mechanical characteristics are
desirably nearly identical, varying by 10 percent or less, in each
in-plane direction of the medium. The residual solvent in the
magnetic layer is preferably equal to or less than 100 mg/m.sup.2
and more preferably equal to or less than 10 mg/m.sup.2. The void
ratio in the coated layers, including both the nonmagnetic lower
layer and the magnetic layer, is preferably equal to or less than
30 volume percent, more preferably equal to or less than 20 volume
percent. Although a low void ratio is preferable for attaining high
output, there are some cases in which it is better to ensure a
certain level based on the object. For example, in many cases,
larger void ratio permits preferred running durability in disk
media in which repeat use is important.
[0108] The center surface average surface roughness Ra of the
magnetic layer as measured by Mirau method with a TOPO-3D is
preferably equal to or less than 4.0 nm, more preferably equal to
or less than 3.0 nm, and still more preferably equal to or less
than 2.0 nm. Preferably, the maximum height of the magnetic layer,
SR.sub.max, is equal to or less than 0.5 .mu.m; the ten-point
average roughness SR.sub.z is equal to or less than 0.3 .mu.m; the
center surface peak height SR.sub.p is equal to or less than 0.3
.mu.m, the center surface valley depth SR.sub.v is equal to or less
than 0.3 .mu.m, the center surface area percentage SS.sub.r ranges
from 20 to 80 percent, and the average wavelength S .lambda.a
ranges from 5 to 300 .mu.m. The number of surface protrusions in
the magnetic layer measuring from 0.01 .mu.m to 1 .mu.m in size is
preferably set to within a range of 0 to 2,000, thereby
electromagnetic characteristics and the coefficient of friction are
preferably optimized. These can be easily controlled by varying
surface properties with fillers in the support, as well as by the
particle diameter and quantity of powders added to the magnetic
layer, the surface shape of the rollers employed in calendaring and
the like. Curling is preferably within .+-.3 mm.
[0109] It will be readily deduced that the physical properties of
the nonmagnetic layer and magnetic layer may be varied based on the
objective in the magnetic recording medium of the present
invention. For example, the modulus of elasticity of the magnetic
layer may be increased to improve running durability while
simultaneously employing a lower modulus of elasticity than that of
the magnetic layer in the nonmagnetic layer to improve the head
contact of the magnetic recording medium.
[0110] [Manufacturing Method]
[0111] The process for manufacturing the coating material for
magnetic layer of the magnetic recording medium of the present
invention comprises at least a kneading step, a dispersing step,
and a mixing step to be carried out, if necessary, before and/or
after the kneading and dispersing steps. Each of the individual
steps may be divided into two or more stages. All of the starting
materials employed in the present invention, including the magnetic
material, nonmagnetic powder, binders, carbon black, abrasives,
antistatic agents, lubricants, solvents, and the like, may be added
at the beginning of, or during, any of the steps. Moreover, the
individual starting materials may be divided up and added during
two or more steps. For example, polyurethane may be divided up and
added in the kneading step, the dispersion step, and the mixing
step for viscosity adjustment after dispersion. To achieve the
object of the present invention, conventionally known manufacturing
techniques may be utilized for some of the steps.
[0112] A kneader having a strong kneading force, such as an open
kneader, continuous kneader, pressure kneader, or extruder is
preferably employed in the kneading step. When a kneader is
employed, the hexagonal ferrite ferromagnetic powder or nonmagnetic
powder and all or part of the binder (preferably equal to or higher
than 30 percent of the entire quantity of binder) are kneaded in a
range of 15 to 500 weight parts per 100 weight parts of the
ferromagnetic powder. Details of the kneading process are described
in Japanese Unexamined Patent Publication (KOKAI) Heisei Nos.
1-106338 and 1-79274. Further, glass beads may be employed to
disperse the coating liquid for magnetic layer and the coating
liquid for nonmagnetic layer, with a dispersing medium with a high
specific gravity such as zirconia beads, titania beads, and steel
beads being suitable for use. The particle diameter and fill ratio
of these dispersing media are optimized for use. A known dispersing
device may be employed.
[0113] Methods such as the following are desirably employed when
coating a multilayer-structured magnetic recording medium in the
present invention;
[0114] (1) A method in which the nonmagnetic layer is first applied
with a coating device commonly employed to apply magnetic coating
materials such as a gravure coating, roll coating, blade coating,
or extrusion coating device, and the magnetic layer is applied
while the nonmagnetic layer is still wet by means of a support
pressure extrusion coating device such as is disclosed in Japanese
Examined Patent Publication (KOKOKU) Heisei No. 1-46186 and
Japanese Unexamined Patent Publication (KOKAI) Showa No. 60-238179
and Japanese Unexamined Patent Publication (KOKAI) Heisei No.
2-265672;
[0115] (2) A method in which the upper and lower layers are applied
nearly simultaneously by a single coating head having two built-in
slits for passing coating liquid, such as is disclosed in Japanese
Unexamined Patent Publication (KOKAI) Showa No. 63-88080, Japanese
Unexamined Patent Publication (KOKAI) Heisei No. 2-17971, and
Japanese Unexamined Patent Publication (KOKAI) Heisei No. 2-265672;
and
[0116] (8) A method in which the magnetic and nonmagnetic layers
are applied nearly simultaneously using an extrusion coating
apparatus with a backup roller as disclosed in Japanese Unexamined
Patent Publication (KOKAI) Heisei No. 2-174965.
[0117] To avoid deteriorating the electromagnetic characteristics
or the like of the magnetic recording medium by aggregation of
magnetic particles, shear is desirably imparted to the coating
liquid in the coating head by a method such as disclosed in
Japanese Unexamined Patent Publication (KOKAI) Showa No. 62-95174
or Japanese Unexamined Patent Publication (KOKAI) Heisei No.
1-236968. In addition, the viscosity of the coating liquid must
satisfy the numerical range specified in Japanese Unexamined Patent
Publication (KOKAI) Heisei No. 3-8471.
[0118] Applying the nonmagnetic layer, drying it, and then applying
the magnetic layer thereover in a sequential multilayer coating to
achieve the structure of the present invention is also possible,
and does not compromise the effect of the present invention.
However, to reduce the number of voids in the coating and improve
the quality as regards dropout and the like, the above-describe
simultaneous multilayer coating is preferred.
[0119] In disk-shaped magnetic recording media, it is sometimes
possible to achieve adequately isotropic orientation without
conducting orientation with an orienting device. However, the use
of a known random orientation device in which cobalt magnets are
reciprocally positioned at an angle and an alternating current is
applied with a solenoid is preferred. In the case of hexagonal
ferrite ferromagnetic powder, it is generally easy to achieve
in-plane and vertical three-dimensional randomness, but in-plane
two-dimensional randomness is also possible. Further,
circumferential orientation may also be imparted by spin
coating.
[0120] In tape-shaped magnetic recording media, cobalt magnets and
solenoids are employed to impart orientation in the longitudinal
direction. The temperature and flow volume of the drying air, and
coating rate are desirably controlled to control the drying
position of the coating. The coating rate preferably ranges from 20
to 1,000 m/min, and the temperature of the drying air is preferably
equal to or higher than 60.degree. C. It is also possible to
conduct suitable predrying prior to entering the magnet zone.
[0121] Processing may be conducted with calender rolls in the form
of heat-resistant plastic rolls such as epoxy, polyimide,
polyamide, and polyimidoamide, or metal rolls. When forming
two-surface magnetic layers, treatment with metal rolls is
particularly desirable. The processing temperature is preferably
equal to or higher than 50.degree. C., more preferably equal to or
higher than 100.degree. C. The linear pressure is preferably equal
to or higher than 200 kg/cm, more preferably equal to or higher
than 300 kg/cm.
[0122] Processing the surface of a tape-shaped or disk-shaped
magnetic recording medium with an abrasive tape covered with a
substance such as alumina, chromium oxide, or diamond is desirable
to remove protrusions and foreign matter.
[0123] As we have continued to evolve into a multimedia society,
the need to record images has grown stronger not just in the
industrial world, but also in households. The magnetic recording
medium of the present invention adequately meets the requirements
of image recording media with respect to functions and costs, in
addition to data such as simple text and numbers.
[0124] The magnetic recording medium of the present invention can
be suitably employed in magnetic recording and preproducing systems
using magnetoresistive (MR) reproduction heads. The type of MR head
is not specifically limited; both GMR and TMR heads may be
employed. Although the head employed for recording is not specific
limited, a saturation magnetization level of equal to or greater
than 1.2 T is desirable, with equal to or greater than 2.0 T being
further preferred.
[0125] The magnetic recording medium of the present invention is
suited to use in computer data recording.
EMBODIMENTS
[0126] The specific examples of the present invention will be
described below. However, the present invention is not limited to
the examples. Further, the "parts" given in the embodiments are
weight parts unless specifically stated otherwise.
Embodiment 1
Preparation of Coating Materials for Magnetic Layer and Nonmagnetic
Layer
[0127] Barium ferrite magnetic powders shown in Table 1 below were
employed in the coating liquid for magnetic layer.
1TABLE 1 Average Average Saturati n plate plate Coercive
magnetization Magnetic diameter thickness force (Hc) (.sigma.s)
powder (nm) (nm) (kA/m (Oe)) (A .multidot. m.sup.2/kg) A 18 6 167.2
(2100) 49 B 25 8 183.1 (2300) 50 C 30 10 179.1 (2250) 54 D 35 10
180.7 (2270) 53 E 40 13 186.3 (2340) 55 F 50 10 180.7 (2270) 56 G
70 11 184.7 (2320) 56
[0128]
2 <Composition of magnetic layer coating material> Barium
ferrite magnetic powder 100 parts Polyurethane resin UR8200
(manufactured by Toyobo Co., Ltd.) 8 parts UR8300 (manufactured by
Toyobo Co., Ltd.) 4 parts .alpha.-alumina 4 parts HIT55
(manufactured by Sumitomo Chemical Co., Ltd.) Diamond 1 part MD150
(manufactured by Tomei Daiya Co., Ltd.) Carbon black 1 part #50
(manufactured by Asahi Carbon Co., Ltd.) Phenylphosphorous acid 1
part Butyl stearate 10 parts Butoxyethyl stearate 5 parts
Isohexadecyl stearate 3 parts Stearic acid 2 parts Methyl ethyl
ketone 125 parts Cyclohexanone 125 parts <Composition of
nonmagnetic layer coating material> Nonmagnetic powder
.alpha.-Fe.sub.2O.sub.3 hematite 80 parts Major axis length: 0.08
.mu.m Specific surface area by BET method: 60 m.sub.2/g pH: 9
Surface treatment agent: Al.sub.2O.sub.3, 8 weight percent Carbon
black 15 parts CONDUCTEX SC-U (manufactured by Columbia Carbon Co.,
Ltd.) Polyurethane resin UR8200 (manufactured by Toyobo Co., Ltd.)
12 parts UR8300 (manufactured by Toyobo Co., Ltd.) 6 parts
Phenylphosphorous acid 3 parts Butyl stearate 8 parts Butoxyethyl
stearate 5 parts Isohexadecyl stearate 2 parts Stearic acid 3 parts
Methyl ethyl ketone/cyclohexanone mixed solvent at 8/2 250
parts
Manufacturing the Magnetic Recording Medium
[0129] Each component of the above-described magnetic layer coating
material and nonmagnetic layer coating material was charged to
kneaders and kneaded, and then dispersed in sand mills.
Polyisocyanate was added to the dispersions obtained in the follow
proportions: 10 parts to the nonmagnetic layer coating material,
and 10 parts to the magnetic layer coating material. To each were
further added 40 parts of cyclohexanone. The mixtures were filtered
with a filter having 1 .mu.m average pore diameter to complete the
preparation of coating liquids for magnetic layer and nonmagnetic
layer.
[0130] The nonmagnetic layer coating material obtained was applied
in a quantity calculated to yield a dry thickness of 1.5 .mu.m on a
polyethylene naphthalate support with a thickness of 62 .mu.m and a
center surface average surface roughness of 1.8 nm, and dried.
Immediately thereafter, a blade method was employed to apply the
magnetic layer coating material over the nonmagnetic layer in such
a manner as to yield a magnetic layer of prescribed thickness. The
product was passed through two field intensity alternating current
magnetic field generating devices with a frequency of 50 Hz and a
magnetic field intensity of 25 T.cndot.m (250 Gauss) and 50 Hz, 12
T.cndot.m (120 Gauss), respectively to impart random orientation.
Following drying, the product was processed with a seven-stage
calender at 90.degree. C. and a linear pressure of 300 kg/cm,
punched to 3.7 inches, and the surface was polished. After punching
to disk shape, the disks were thermo-treated at 70.degree. C. and
burnished with an abrasive tape to promote hardening of the coated
surface.
Embodiments 2 to 17
[0131] With the exceptions that the thickness of the magnetic layer
and the recording track width in Embodiment 1 were changed,
magnetic recording media were prepared by the same method as in
Embodiment 1.
Comparative Examples 1 to 14
[0132] With the exceptions that the average plate diameter of the
barium ferrite magnetic powder and the thickness of the magnetic
layer in Embodiment 1 were changed, magnetic recording media were
prepared by the same method as in Embodiment 1.
[0133] <Evaluation of the Magnetic Recording Media>
[0134] The characteristics of the magnetic recording media obtained
in the Embodiments and Comparative Examples were measured by the
methods indicated below.
[0135] (1) Magnetic Characteristics (Hc, .sigma.s)
[0136] Measurements were performed at Hm of 796 kA/m (10 KOe) with
a vibrating sample magnetometer (made by Toei Kogyo).
[0137] (2) Plate Diameter and Plate Thickness of Barium Ferrite
[0138] Five hundred particles were randomly measured by TEM
photography and the average value was calculated.
[0139] (3) Magnetic Layer Thickness
[0140] The magnetic recording medium was cut to a thickness of
about 0.1 .mu.m in a radial direction with a diamond cutter,
observed with a transmission electron microscope at a magnification
of 10,000 to 100,000 fold, preferably 20,000 to 50,000 fold, and a
photograph was taken. The difference in shape of the ferromagnetic
powder and nonmagnetic power in the magnetic layer and nonmagnetic
layers, respectively, was noted, and the interface was visually
evaluated to measure the thickness of the magnetic layer.
[0141] (4) S/N Ratio
[0142] Each of the above-described disk samples was set on a spin
stand on which a head had been mounted and the rotational speed was
adjusted to achieve a speed of the medium relative to the head at a
measured radial position of 4 m/s. A 20 MHz rectangular wave signal
was then recorded on the disk with an inductive head and reproduced
with an AMR head. The signal output was measured, noise falling
within a range of 0 to 40 MHz was integrated, and the ratio was
adopted as the S/N ratio.
[0143] The characteristics of the magnetic disks obtained in (1)
through (4) are given in Table 2.
3 TABLE 2 Average plate Hc of Type of diameter X of Magnetic layer
Recording SN magnetic magnetic magnetic powder thickness track
width Z ratio layer No. powder (nm) Y (nm) (nm) Y/X Z/X (dB) (kA/m
(Oe)) Embodiment 1 A1 A 18 2.0 .times. 10.sup.2 2.0 .times.
10.sup.3 11 111 27 167 (2100) Embodiment 2 A2 B 25 2.0 .times.
10.sup.2 2.0 .times. 10.sup.3 8.0 80 28 183 (2300) Embodiment 3 A3
C 30 2.0 .times. 10.sup.2 2.0 .times. 10.sup.3 6.7 67 27 179 (2250)
Embodiment 4 A4 D 85 2.0 .times. 10.sup.2 2.0 .times. 10.sup.3 5.7
57 26 181 (2270) Embodiment 5 A5 E 40 2.0 .times. 10.sup.2 2.0
.times. 10.sup.3 5.0 50 24 186 (2340) Embodiment 6 A6 F 50 2.0
.times. 10.sup.2 2.0 .times. 10.sup.3 4.0 40 22 181 (2270) Comp.
Ex. 1 B1 G 70 2.0 .times. 10.sup.2 2.0 .times. 10.sup.3 2.9 29 14
185 (2320) Embodiment 7 A7 A 18 0.8 .times. 10.sup.2 2.0 .times.
10.sup.3 4.4 111 26 167 (2100) Embodiment 8 A8 B 25 0.8 .times.
10.sup.2 2.0 .times. 10.sup.3 3.2 80 28 183 (2300) Embodiment 9 A9
C 30 0.8 .times. 10.sup.2 2.0 .times. 10.sup.3 2.7 67 26 179 (2250)
Embodiment 10 A10 D 35 0.8 .times. 10.sup.2 2.0 .times. 10.sup.3
2.3 57 24 181 (2270) Embodiment 11 A11 E 40 0.8 .times. 10.sup.2
2.0 .times. 10.sup.3 2.0 50 22 186 (2340) Comp. Ex. 2 B2 F 50 0.8
.times. 10.sup.2 2.0 .times. 10.sup.3 1.6 40 16 181 (2270)
Embodiment 12 A12 A 18 0.8 .times. 10.sup.2 1.0 .times. 10.sup.3
4.4 56 24 167 (2100) Embodiment 13 A13 B 25 0.8 .times. 10.sup.2
1.0 .times. 10.sup.3 3.2 40 25 183 (2300) Embodiment 14 A14 C 30
0.8 .times. 10.sup.2 1.0 .times. 10.sup.3 2.7 33 23 179 (2250)
Comp. Ex. 3 B3 D 35 0.8 .times. 10.sup.2 1.0 .times. 10.sup.3 2.3
29 17 181 (2270) Comp. Ex. 4 B4 E 40 0.8 .times. 10.sup.2 1.0
.times. 10.sup.3 2.0 25 15 186 (2340) Comp. Ex. 5 B5 F 50 0.8
.times. 10.sup.2 1.0 .times. 10.sup.3 1.6 20 10 181 (2270)
Embodiment 15 A15 A 18 0.5 .times. 10.sup.2 1.0 .times. 10.sup.3
2.7 56 23 167 (2100) Embodiment 16 A16 B 25 0.5 .times. 10.sup.2
1.0 .times. 10.sup.3 2.0 40 23 183 (2300) Comp. Ex. 6 B6 C 30 0.5
.times. 10.sup.2 1.0 .times. 10.sup.3 1.7 33 17 179 (2250) Comp.
Ex. 7 B7 D 35 0.5 .times. 10.sup.2 1.0 .times. 10.sup.3 1.4 29 15
181 (2270) Camp. Ex. 8 B8 E 40 0.5 .times. 10.sup.2 1.0 .times.
10.sup.3 1.3 25 12 186 (2340) Comp. Ex. 9 B9 F 50 0.5 .times.
10.sup.2 1.0 .times. 10.sup.3 1.0 20 8 181 (2270) Embodiment 17 A17
A 18 0.8 .times. 10.sup.2 0.6 .times. 10.sup.3 4.4 33 22 167 (2100)
Comp. Ex. 10 B10 B 25 0.8 .times. 10.sup.2 0.6 .times. 10.sup.3 3.2
24 17 183 (2300) Comp. Ex. 11 B11 C 30 0.8 .times. 10.sup.2 0.6
.times. 10.sup.3 2.7 20 16 179 (2250) Comp. Ex. 12 B12 D 35 0.8
.times. 10.sup.2 0.6 .times. 10.sup.3 2.3 17 14 181 (2270) Comp.
Ex. 13 B13 E 40 0.8 .times. 10.sup.2 0.6 .times. 10.sup.3 2.0 15 11
186 (2340) Comp. Ex. 14 B14 F 50 0.8 .times. 10.sup.2 0.6 .times.
10.sup.3 1.6 12 8 181 (2270)
[0144] Table 2 reveals that when the average plate diameter (X) of
barium ferrite magnetic powder was 1/2 or less of the magnetic
layer thickness (Y), that is, when the ratio of Y/X was equal to or
greater than 2, as well as when the average plate diameter (X) was
{fraction (1/30)} or less of the recording track width (Z), that
is, when the ratio of Z/X was equal to or greater than 30, a good
S/N ratio of equal to or greater than 20 dB was achieved
(Embodiments 1 to 17).
[0145] By contrast, when the ratio of Y/X was less than 2
(Comparative Examples 2, 6), the ratio of Z/X was less than 30
(Comparative Examples 1, 3, 4, 10 to 13), or the ratio of Y/X was
less than 2 and the ratio of Z/X was less than 30 (Comparative
Examples 5, 7 to 9, 14), the S/N ratio dropped sharply and an S/N
ratio of less than 20 dB was obtained (Comparative Examples 1 to
14).
[0146] From these results, it can he understood that good
electromagnetic characteristics are obtained when the average plate
diameter of barium ferrite is {fraction (1/30)} or less of the
recording track width and is 1/2 or less of the magnetic layer
thickness.
[0147] In the magnetic recording medium of the present invention as
set forth above, the average plate diameter of the hexagonal
ferrite ferromagnetic powder employed in the magnetic layer is set
to {fraction (1/30)} or less of the recording track width and 1/2
or less of the magnetic layer thickness. Thus, the present
invention provides a magnetic recording medium exhibiting good
electromagnetic characteristics in high-density recording.
[0148] The present disclosure relates to the subject matter
contained in Japanese Patent Application No. 2003-15601 filed on
Jan. 24, 2003, which is expressly incorporated herein by reference
in its entirety.
* * * * *