Magnetic recording medium and signal recording system...

Details Magnetic recording medium and signal recording system... Magnetic recording medium and signal recording system...

2001-09-14

2003-11-25

Le, H. Thi (Department: 1773)

Stock material or miscellaneous articles

Composite

Of inorganic material

C428S336000

Reexamination Certificate

active

06652999

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a magnetic recording medium suitable for performing data recording (or information recording) and servo write, and a signal recording system using such a magnetic recording medium.
BACKGROUND OF THE INVENTION
In a magnetic recording system for data storage, a signal for positioning (hereinafter referred also to as “servo signal”) is recorded in a medium to enhance the track density. To this end, there have been proposed a method which comprises making a hole in a magnetic layer so that the difference in light reflectance can be detected, a method which comprises forming a track for servo signal only on the magnetic layer surface, and a method which comprises making a hole in a back layer so that the difference in light reflectance can be detected. However, the two former methods are disadvantageous in that the formation of servo track reduces the area for data recording, resulting in the drop of capacity, or the distance between data track and servo track is increased to reduce the positioning precision. The latter method is disadvantageous in servo precision.
In order to solve these problems, it has been proposed in JP-A-6-139555, which corresponds to U.S. Pat. No. 5,518,804, that different signals are recorded in a magnetic medium containing two or more magnetic layers arranged such that Hc of an upper layer is lower than that of a lower layer (Hc: coercive force, hereinafter referred simply to as “Hc” sometimes). However, within the disclosed range of Hc, Hc of the upper layer is low. Thus, such a system is not suitable for high density recording.
In order to raise S/N ratio of data signal (or information signal) to be recorded by sufficiently raising Hc of the upper layer, Hc of the lower layer must be further enhanced, making it difficult to write servo signal.
SUMMARY OF THE INVENTION
An object of the invention is to provide a magnetic recording medium which has no difficulties described above and enables a high precision servo write and recording at a higher density, and a new recording system using such a magnetic recording medium.
The inventors made extensive studies of magnetic recording medium which allows highly precise servo write and recording at a higher density and as a result have completed the present invention.
The foregoing object of the invention is accomplished by the following means of the invention.
(1) A magnetic recording medium comprising two or more magnetic layers provided on a support, wherein an easily magnetizable axis of an uppermost magnetic layer and an easily magnetizable axis of a lower magnetic layer are oriented in directions crossing each other at right angles.
(2) The magnetic recording medium according to claim (
1
), wherein the easily magnetizable axis of the uppermost magnetic layer is oriented in a width direction and the easily magnetizable axis of the lower magnetic layer is oriented in a length direction.
(3) The magnetic recording medium according to claim (
1
) or (
2
), wherein a thickness of the uppermost magnetic layer is from
{fraction (1/10)} to
1 time the shortest recording wavelength for recording a data signal in the uppermost magnetic layer, a coercive force Hc of the uppermost magnetic layer is from 158 KA/m to 316 KA/m, and a residual magnetic flux density Br of the lower magnetic layer is from 2,500 G to 5,000 G.
(4) The magnetic recording medium according to any one of claims (
1
) to (
3
), wherein a non-magnetic layer containing a non-magnetic powder is provided under the two or more magnetic layers.
(5) A signal recording system which comprises recording a data signal in a width direction in the uppermost magnetic layer of the magnetic recording medium claimed in any one of claims (
1
) to (
4
) and recording a servo signal in a length direction in the lower magnetic layer.
DETAILED DESCRIPTION OF THE INVENTION
Preferred embodiments of the magnetic recording medium according to the invention will be described hereinafter.
[Uppermost Magnetic Layer]
The Hc of the uppermost magnetic layer is preferably from 158 KA/m to 316 KA/m, more preferably from 174 KA/m to 237 K/m. When the HC of the uppermost magnetic layer is too low, C/N of data signal is low. On the contrary, when the Hc of the uppermost magnetic layer is to high, the magnetic intensity of the recording head must be increased, and a servo signal recorded on the lower layer tends to be erased.
The thickness of the uppermost magnetic layer is preferably from
{fraction (1/10)} to
1 time, more preferably from
⅛ to
0.6 times the shortest recording wavelength of data signal. When the thickness of the uppermost magnetic layer is too small, the intensity of data signal is too small, and servo signal can be easily erased during data recording. On the contrary, when the thickness of the uppermost magnetic layer is too great, the intensity of servo signal is too small.
In the case where the magnetic layer consists of two layers, an upper magnetic layer is the uppermost magnetic layer. In the case wherein the magnetic layer consists of three or more layers, a top magnetic layer is the uppermost magnetic layer.
[Lower Magnetic Layer]
The Hc of the lower magnetic layer is preferably almost equal to that of the uppermost magnetic layer.
The residual magnetic flux density Br (hereinafter referred simply to as “Br” sometimes) of the lower magnetic layer is preferably great to increase the output of servo signal and normally is preferably from 2,500 G to 5,000 G.
The thickness of the lower magnetic layer is preferably from 0.05 &mgr;m to 3 &mgr;m, more preferably from 0.05 &mgr;m to 2 &mgr;m, even more preferably from 0.1 &mgr;m to 1 &mgr;m. When the thickness of the lower magnetic layer is too small, the intensity of servo signal is reduced. On the contrary, when the thickness of the lower magnetic layer is too great, the resulting resolution is reduced
By orienting the easily magnetizable axis of the uppermost magnetic layer and the easily magnetizable axis of the lower magnetic layer in directions crossing each other at right angles, e.g., orienting the easily magnetizable axis of the uppermost magnetic layer in a width direction and the easily magnetizable axis of the lower magnetic layer in a length direction, the effect of magnetization of the lower magnetic layer on the uppermost magnetic layer can be lessened.
When at least three magnetic layers exist as magnetic layers, the magnetic layer(s) other than the uppermost magnetic layer and the servo write layer (i.e., lower magnetic layer for recording servo signals) preferably has/have a lower Hc than the servo write layer.
The servo write layer may be provided just under or indirectly under the information recording layer.
[Non-Magnetic Layer]
In the case where the thickness of the lower magnetic layer is reduced, a non-magnetic layer having a thickness of from 0.3 to 3 &mgr;m containing a non-magnetic powder can be provided under the lower magnetic layer to smoothen the magnetic layer, allowing recording at a higher density.
[Magnetic Powder]
As the magnetic powder to be incorporated into the uppermost magnetic layer and lower magnetic layer there may be used a permanent magnet material powder such as powder of acicular alloy mainly composed of Fe, hexagonal ferrite powder, SmCo powder, FePt powder and MnAl powder. Regarding the size of the magnetic powder, if it is acicular, an average major axis thereof is from 0.03 to 0.15 &mgr;m, preferably from 0.04 to 0.1 &mgr;m and a crystallite size thereof is from 70 to 180 angstroms, preferably from 80 to 150 angstroms. If it is hexagonal ferrite powder, a tabular diameter thereof is from 10 to 50 nm, preferably from 15 to 40 nm and a tabular thickness thereof is from 5 to 15 nm, preferably from 7 to 12 nm. Also, if it is granular, a diameter thereof is from 5 to 50 nm, preferably from 8 to 35 nm.
The Hc of the magnetic layer is governed by the magnetic characteristics of the magnetic powder to be incorporated therein. Accordingly, the Hc of the uppermost magnetic

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