Dynamic magnetic information storage or retrieval – Head – Core
Reexamination Certificate
2001-02-20
2003-12-02
Letscher, George J. (Department: 2653)
Dynamic magnetic information storage or retrieval
Head
Core
Reexamination Certificate
active
06657813
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a perpendicular magnetic recording system using a perpendicular magnetic recording medium (double layered perpendicular magnetic recording medium) having a soft magnetic underlayer.
2. Description of the Related Prior Art
In perpendicular magnetic recording systems using a double layered perpendicular magnetic recording medium, magnetic flux extending from a main pole of a recording head follows a magnetic path that runs through a soft magnetic underlayer of the double layered perpendicular recording medium to enter an auxiliary pole of a magnetic head and then returns to the main pole. Conventional perpendicular magnetic recording media have adopted soft magnetic underlayers of greater design thicknesses so as to avoid magnetic saturation of the soft magnetic underlayers.
FIG. 4
is a diagram explaining the cross section of a path through which magnetic flux flows on a conventional model. As shown in
FIG. 4
, in order to prevent a soft magnetic underlayer
6
of a perpendicular magnetic recording medium from being saturated by the magnetic flux from a main pole
1
of a magnetic head, it is considered that the limit of the prevention is determined by both the area of the cross section
11
of the magnetic flux inside the soft magnetic underlayer, through which the magnetic flux extending from a top face
10
of the main pole passes, and the saturation flux density of the soft magnetic underlayer
6
, and it is necessary to satisfy the following relational expression:
T
WW
×T
b1
×B
S2
>T
WW
×T
m
×B
S1
,
i.e.,
T
b1
×B
S2
>T
m
×B
S1
, (1)
wherein T
WW
is the track width of the main pole
1
, B
S1
the saturation flux density of the main pole
1
, T
m
the thickness of the main pole
1
, B
S2
the saturation flux density of the soft magnetic underlayer
6
, and T
b1
the thickness of the soft magnetic underlayer. In view of this, perpendicular magnetic recording media have been provided with a thick soft magnetic underlayer.
For example, Japanese Patent Laid-Open Publication No. Hei 10-283624 describes a double layered perpendicular magnetic recording medium having a soft magnetic underlayer of 600 nm in thickness.
In conventional ideas, for example, the saturation flux density B
S
of a main pole of 1.6 T, the thickness T
m
of the same of 0.5 &mgr;m, and the saturation flux density B
S
of a soft magnetic underlayer of 1.2 T combine to require, according to the expression (1), the thickness T
b1
of the soft magnetic underlayer as great as 0.67 &mgr;m or more. This is no less than ten times the thickness of the magnetic recording layers of current in-plane magnetic recording media which is no greater than several tens of nanometers. Given here that the growth rates are nearly equal, the growth time becomes more than ten times, causing a drop in production efficiency and a rise in cost. Moreover, the consumption of the target used in the sputtering also increases for a cost increase. Besides, greater thicknesses deteriorate surface roughness because of inhomogeneous grain growth. This causes a problem since high-density magnetic recording media require low surface roughness for the sake of reducing head-medium spacing. Accordingly, conventional double layered perpendicular magnetic recording media were disadvantageous as compared with in-plane magnetic recording media and single layered perpendicular magnetic recording media.
To avoid the saturation of a soft magnetic underlayer without thickening the soft magnetic underlayer, it is necessary to thin the main pole of the magnetic head or raise the saturation magnetization (Bs) of the soft magnetic underlayer significantly. Nevertheless, when the recording layer of the medium has a relatively high coercivity (Hc), the intensity of write magnetic field must be increased, and the thinning of the main pole produces a problem of main pole saturation.
Furthermore, there are other problems including that no material has been found which can increase the saturation magnetization of soft magnetic underlayers considerably.
SUMMARY OF THE INVENTION
In view of such problems in the conventional art, it is an object of the present invention to provide a perpendicular magnetic recording system in which a soft magnetic underlayer of a double layered perpendicular magnetic recording medium is prevented from saturation while the soft magnetic underlayer is designed in a thickness smaller than heretofore.
The present inventor has found that the track width of a recording head and the effect of the track ends can be incorporated into the design of thickness of a soft magnetic underlayer to reduce the soft magnetic underlayer in thickness when the track width is small. With reference to
FIGS. 1 and 2
, description will be given of the relational expressions which the present inventors have found in connection with the thicknesses of soft magnetic underlayers of double layered perpendicular magnetic recording media.
FIG. 1
is a schematic diagram of the top of a recording head as seen from the floating surface, or a diagram explaining the magnetic flux inside a soft magnetic underlayer. Magnetic flux
3
extending from a main pole of a magnetic head for recording magnetization transitions on a magnetic recording medium passes through a soft magnetic underlayer of the magnetic recording medium to return to an auxiliary pole
2
of the magnetic head. The paths through which the magnetic flux
3
flows can be classified into paths A-C. The path A runs at the track center, nearly straight across the opposed faces of the main pole
1
and the auxiliary pole
2
. The paths B return from the sides of the main pole
1
to the auxiliary pole
2
. The paths C return to the auxiliary pole
2
around behind the ends of the main pole
1
. Then, the main pole
1
can be divided into a region I at the track center, provided with the path A alone, and regions II on both sides, each provided with the three paths A, B, and C.
FIG. 2
is a schematic diagram showing the top of the main pole
1
of the magnetic head and a soft magnetic underlayer
6
of a double layered perpendicular magnetic recording medium, or a diagram explaining the cross sections of paths through which magnetic flux flows. Assume here that the width of each region II of the main pole
1
is W
S
. Besides, let T
m
stand for the thickness of the main pole
1
, T
WW
the track width of the main pole
1
, B
S1
the saturation flux density of the main pole
1
, T
b1
the thickness of the soft magnetic underlayer
6
, and B
S2
the saturation flux density of the soft magnetic underlayer
6
. The limit to which the soft magnetic underlayer
6
is not saturated by the magnetic flux extending from the region I of the main pole
1
is reached when the product of the top area of the region I of the main pole
1
and the saturation flux density B
S1
equals to the product of the parallel-to-surface area of the soft magnetic underlayer
6
to make the path A and the saturation flux density B
S2
. In other words, at the time when the following expression holds:
B
S2
×T
b1
×(
T
WW
−2
W
S
)
=B
S1
×T
m
×(
T
WW
−2
W
S
).
The foregoing expression is arranged into the following expression (1′):
B
S2
×T
b1
−B
S1
×T
m
(1′)
Like the region I, the regions II need to satisfy the relations between the products of area and saturation flux density for all the three paths A, B, and C mentioned above. More specifically, the following expression needs to be satisfied:
B
S2
×(
T
b1
×W
S
+T
b1
×W
S
×T
b1
×T
m
)
=B
S1
×T
m
×W
S
Hence
B
S2
×T
b1
×(2
W
S
+T
m
)
=B
S1
×T
m
×W
s
(2)
The expression (2) makes a sufficient condition when the distance between the main pale
1
and the auxiliary pole
2
is nearly equal to or greater than the thickness T
m
of the main pole
1
, and the track width is nearly
Mochizuki Masafumi
Nishida Yasutaka
Takano Hisashi
Beacham C R
Hitachi , Ltd.
Letscher George J.
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