Dynamic magnetic information storage or retrieval – Head – Magnetoresistive reproducing head
Reexamination Certificate
2001-08-30
2003-12-02
Ometz, David L. (Department: 2653)
Dynamic magnetic information storage or retrieval
Head
Magnetoresistive reproducing head
C360S324200
Reexamination Certificate
active
06657823
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a differential detection read sensor for perpendicular magnetic recording suitable for high-density magnetic recording, a thin film head for perpendicular recording using the same and a magnetic recording apparatus using the head.
2. Description of the Related Art
As personal computers and workstations have been widespread rapidly, magnetic disk units as magnetic recording apparatuses forming the core of a nonvolatile file system have been required to increase the capacity quickly than ever. Increase of the capacity of the magnetic disk unit basically enhances a recording bit density, i.e., areal recording density.
The recording system in magnetic disk units currently commercially used is generally called an longitudinal recording method. This is a system in which a ferromagnetic film with high coercive force in the direction in parallel with a disk substrate surface is used as a recording medium, and then, the recording medium is magnetized in the substrate longitudinal direction so as to record information. In this case, a magnetization reversal part in which in-plane magnetizations are opposite to each other at a 180 angle corresponds to bit
1
. To increase the areal recording density, it is necessary to increase the bit density in the disk circumferential direction (linear recording density) and the bit density in the disk radius direction (track density). The track density is currently limited by the forming process of geometrical track width and accuracy of head following of a recording/reproducing head. These are thus considered to be mainly the problems of processing and control system techniques. On the contrary, the linear recording density is thought to be limited in principle in that in light of the fact that the recording medium is an aggregate of ferromagnetic material crystalline particles, the linear recording density is associated with the magnetic stability of the aggregate. In the lomgitudinal recording system, magnetizations are opposite to each other around the magnetization reversal. A large inner magnetic field called a demagnetizing field in the direction to reduce the magnetization is generated around the magnetization reversal. The demagnetizing field forms, in the magnetization reversal part, a transition region with a finite width, that is, a region in which magnetization does not reach a sufficient value. When the bit intervals are narrowed and the adjacent magnetization transition regions are interfered with each other, there arises the disadvantage that the position of the magnetization reversal is shifted substantially. To increase the linear recording density, there is required a construction such that the medium is magnetized by overcoming the demagnetizing field. More specifically, the coercive force of the medium must be improved and the thickness of the recording magnetic film must be reduced to suppress the demagnetizing field. For this reason, the linear recording density is strongly limited by the construction and the magnetic property of the medium. In a standard longitudinal magnetic recording system, the ratio of the linear recording density to the track density is desirably about 10 to 15. When a recording density of 100 Gb/in
2
is realized under the conditions, the bit interval in the circumferential direction is about 25 nm. When the necessary magnetic property of the medium in which the magnetization reversal width is below 25 nm is estimated by a simple model, the medium film thickness is below 15 nm and the coercive force is above 5 kOe.
When the coercive force exceeds 5 kOe, it is difficult to ensure the recording magnetic field enough to magnetize the medium. When the thickness of the Co alloy magnetic film is below 15 nm, the substantial volume of the medium crystalline particles is reduced. As compared with the magnetic anisotropic energy of the particles (i.e., the energy to stabilize the magnetization in the constant direction), the magnitude of the heat energy (i.e., the energy to disturb the magnetization) cannot be ignored. Thermal fluctuation of the magnetization is significant, so that there arises the problem of thermal signal loss in which the magnitude of the recording magnetization is reduced with time. To suppress the thermal signal loss, it is necessary to increase the coercive force or the volume of the crystalline particles. When the head magnetic field is limited as described above, the allowable coercive force has an upper limit. In addition, increase of the film thickness to increase the volume of crystalline particles means increase of the demagnetizing field. When attempting to ensure the crystalline particle volume of crystalline size in the longitudinal direction, the randomness of the magnetization distribution in the medium is large, resulting in increase of the medium noise. A sufficient signal S/N cannot be thus obtained. To realize the areal recording density exceeding 100 Gb/in
2
while resisting the thermal signal loss and reducing the noise in the areal magnetic recording system, it is expected to be difficult in principle.
The perpendicular magnetic recording system is a system for forming the magnetization of a thin film medium so as to be perpendicular to the film surface, in which the recording principle is different from that of the prior art longitudinal magnetic recording medium. In other words, in the perpendicular magnetic recording system, since the adjacent magnetizations are not opposite to each other and are arranged in antiparallel, they are not affected by the demagnetizing filed. The magnetization transition region is expected to be very small to easily increase the linear recording density. The require to reduce the medium thickness is not as strong as that of the longitudinal recording. It is thus possible to ensure high resistance to the thermal signal loss. The perpendicular magnetic recording system is focused as a system essentially suitable for high-density magnetic recording. Various medium materials and constructions are proposed.
The perpendicular magnetic recording system has a system for employing a single-layer perpendicular magnetization film and a system for providing a soft magnetic underlayer adjacent to the disk substrate side of a perpendicular magnetization film. Using a two-layer perpendicular magnetic recording medium having a soft magnetic underlayer, there are considered the advantages: (1) a demagnetizing field generated on the surface of the recording layer can be reduced; and (2) the medium can be combined with a single pole type recording element to generate a large recording magnetic field having a steep distribution as compared with the ring head in the longitudinal recording. The technique is described in, for example, IEEE Transactions on Magnetics, Vol. MAG-20, No. 5, September 1984, pp. 657-662, Perpendicular Magnetic Recording—Evolution and Future. As the perpendicular magnetic recording medium of this system, there is studied a medium in which a perpendicular magnetization film made of a CoCr alloy is provided on a soft magnetic underlayer made of a soft magnetic film layer such as permalloy or Fe amorphous alloy.
Corresponding to a difference in the medium magnetization state between the longitudinal recording and the perpendicular recording, it is expected that the space distribution of a magnetic field applied from the medium to the reproducing sensor and the reproduced signal waveform of the perpendicular recording are different from those of the in-plane recording. Generally used as a reproducing sensor in the current longitudinal recording system is a so-called a shield type GMR (Giant Magnetoresistive) reproducing sensor. As shown in the upper part of
FIG. 1
, this is constructed such that one GMR reproducing element
12
is disposed between a pair of magnetic shields
11
a
and
11
b
made of soft magnetic materials. In the in-plane recording, a static magnetic field leaks from the reversal part of a medium magnetization
13
. The GMR reproducing
Hitachi , Ltd.
Ometz David L.
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