Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2001-09-13
2003-12-23
Le, H. Thi (Department: 1773)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S690000, C428S690000
Reexamination Certificate
active
06667119
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a magnetic recording medium for recording information and writing servo signals.
BACKGROUND OF THE INVENTION
In magnetic recording systems for data storage, signals for positioning (which are also referred to as servo signals hereinafter) are recorded on recording media for the purpose of elevating track densities. To the positioning problem, various approaches have been made. For instance, there have been proposed the method of forming holes in a magnetic layer and detecting variations in light reflectivity, the method of forming a track for the exclusive use of servo signals on the magnetic layer surface of a recording medium, and the method of forming holes in the backing layer of a recording medium and detecting variations in light reflectivity. According to the former two methods, however, the servo track formed causes a reduction in data recording area to result in a capacity decrease, and the positioning accuracy decreases due to a distance between the data track and the servo track. The latter method has a drawback of being inferior in accuracy of the servo write.
With the intention of solving those problems, there has been made a proposal to provide at least two magnetic layers, adjust their coercive forces to satisfy the relationship that Hc of an upper layer<Hc of a lower layer (wherein Hc stands for a coercive force), so as to record different signals in these magnetic layers respectively (see JP-A-6-139555, which corresponds to U.S. Pat. No. 5,518,804). However, the upper layer Hc in the range disclosed is too low to attain high-density recording.
SUMMARY OF THE INVENTION
An object of the invention is to provide a magnetic recording medium which is free of the aforementioned drawbacks and enables servo write with high accuracy and recording in higher density.
As a result of our intensive studies on magnetic recording media enabling highly accurate servo write and higher recording densities, we have attained the present invention.
More specifically, the invention solves the foregoing problem by the following magnetic recording media:
(1) A magnetic recording medium having on a support at least two magnetic layers which include an uppermost magnetic layer for recording information and a lower magnetic layer for recording servo signals, wherein the uppermost magnetic layer has a coercive force Hc(U) [KA/m] at most 265 times as much as a saturated magnetic flux density Bs[T] of a recording head used for information recording, and the lower magnetic layer has a coercive force Hc(L) [KA/m] at least 159 times as great as the saturated magnetic flux density Bs[T] of the recording head used for information recording. The Hc(U) and Hc(L) each represents coersive forces of the uppermost magnetic layer and the lower magnetic layer, respectively.
(2) The magnetic recording medium according to the foregoing item (1), wherein the coercive force of the uppermost magnetic layer is 140 KA/m or more.
(3) The magnetic recording medium according to the foregoing item (1), wherein the coercive force of the uppermost magnetic layer is from 158 KA/m to 316 KA/m.
(4) The magnetic recording medium according to any one of the foregoing items (1) to (3), wherein the uppermost magnetic layer has a thickness of from one-tenth to one time the shortest recording wavelength of information signals to be recorded in the uppermost magnetic layer and the lower magnetic layer has a thickness of 0.05 to 3 &mgr;m.
(5) The magnetic recording medium as described in any one of the foregoing items (1) to (4), wherein a non-magnetic layer containing a non-magnetic powder and having a thickness of 0.3 to 3 &mgr;m is provided underneath the magnetic layers and the lower magnetic layer has a thickness of 0.05 to 0.5 &mgr;m.
In order to provide an information recording layer capable of attaining higher recording density and to ensure highly accurate servo write in a magnetic recording system wherein servo signals are recorded, the magnetic recording medium of the invention has the following functions:
1) By forming the layer for servo write just under or indirectly under the information recording layer, the area of information recording is extended and the accuracy of servo is heightened.
2) By designing the magnetic layer so as to have high Hc and a small thickness, the recording density of information recording layer is increased.
3) By specifying the relation between the Hc of information recording layer, the Hc of servo wire layer and the saturated magnetic flux density Bs of a recording head, disappearance of servo signals is prevented and a good servo signal to noise ratio is achieved.
4) By further providing a non-magnetic under layer to reduce a thickness of the servo write layer itself, resolution of servo signals is improved.
DETAILED DESCRIPTION OF THE INVENTION
Suitable embodiments of the magnetic recording medium according to the invention are illustrated below in detail.
The term “saturated magnetic flux density” is also referred to as “Bs” hereinafter.
[Uppermost Magnetic Recording Layer]
The coercive force of the uppermost magnetic layer (Hc(U) [KA/m]) is at most 265 times, preferably at most 199 times, the Bs[T] of a recording head used for information recording. In particular, it is advantageous for the Hc(U) to be 140 KA/m or more, preferably from 158 KA/m (2,000 Oe) to 316 KA/m (4,000 Oe). When the Hc(U) is low, high-density information recording cannot be performed; while too high Hc(U) requires that the magnetic field of a recording head be increased and gives rise to easy erase of the servo signals recorded in the lower magnetic layer.
The suitable thickness of the uppermost magnetic layer is preferably from one-tenth to once, more preferably from one-eighth to three-fifths, the shortest recording wavelength of information signals to be recorded. When the uppermost magnetic layer is too thin, the information signal output becomes too low and servo signals are liable to be erased by a recording head. When the uppermost magnetic layer is too thick, on the other hand, the resolution of information signals is lowered and intensification of the magnetic field of a head becomes necessary for overwrite, under the influence of which servo signals are liable to be erased.
When the magnetic layer is constituted of two layers, the upper magnetic layer is the uppermost magnetic layer; while, when the magnetic layer is constituted of three or more layers, the layer disposed at the top is the uppermost magnetic layer.
[Lower Magnetic Layer]
The coercive force of the lower magnetic layer (Hc(L) [KA/cm]) is at least 159 times, preferably at least 199 times, the Bs[T] of a recording head used for information recording. The upper limit of Hc(L) is determined depending on the servo recording head capacity, but in actuality it is appropriate that the Hc(L) is 553 KA/m (7,000 Oe) at the greatest.
The thickness of the lower magnetic layer is preferably from 0.05 to 3 &mgr;m, more preferably from 0.05 to 2 &mgr;m, still more preferably from 0.1 to 1 &mgr;m. When the lower magnetic layer is too thin, the servo signal intensity is lowered. When the layer is too thick, a drop in resolution is caused.
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.
[Non-magnetic Layer]
When the lower magnetic layer is rendered thin, it is possible to ensure smoothness in the magnetic layer by providing a non-magnetic layer containing a non-magnetic powder and having a thickness of 0.3 to 3 &mgr;m underneath the magnetic layer, and whereby higher-density recording becomes possible.
[Magnetic Powder]
Examples of magnetic powders usable in magnetic layers including the uppermost magnetic layer and the lower magnetic layer include fine powders of
Ejiri Kiyomi
Sueki Minoru
Fuji Photo Film Co. , Ltd.
Le H. Thi
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