Dynamic magnetic information storage or retrieval – Head – Magnetoresistive reproducing head
Reexamination Certificate
2000-01-31
2001-04-17
Evans, Jefferson (Department: 2652)
Dynamic magnetic information storage or retrieval
Head
Magnetoresistive reproducing head
C360S324100, C360S324120
Reexamination Certificate
active
06219211
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to magnetic transducers for reading information signals from a magnetic medium and, in particular, to a soft antiparallel pinned spin valve sensor, and to magnetic recording systems which incorporate such sensors.
2. Description of Related Art
Computers often include auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive read sensors, commonly referred to as MR heads, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an “MR element”) as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization in the MR element and the direction of sense current flow through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers.
GMR sensors using only two layers of ferromagnetic material separated by a layer of non-magnetic material (e.g., copper) are generally referred to as spin valve (SV) sensors manifesting the GMR effect (also referred to as SV effect). In an SV sensor, one of the ferromagnetic layers, referred to as the pinned layer, has its magnetization typically pinned by exchange coupling with an antiferromagnetic (e.g., NiO or Fe—Mn) layer. The magnetization of the other ferromagnetic layer, referred to as the free layer, however, is not fixed and is free to rotate in response to the field from the recorded magnetic medium (the signal field). In SV sensors, the SV effect varies as the cosine of the angle between the magnetization of the pinned layer and the magnetization of the free layer. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in direction of magnetization in the free layer, which in turn causes a change in resistance of the SV sensor and a corresponding change in the sensed current or voltage. IBM's U.S. Pat. No. 5,206,590 granted to Dieny et al. and incorporated herein by reference, discloses an MR sensor operating on the basis of the SV effect.
FIG. 1
shows a prior art SV sensor
100
comprising end regions
104
and
106
separated by a central region
102
. A free layer (free ferromagnetic layer)
110
is separated from a pinned layer (pinned ferromagnetic layer)
120
by a non-magnetic, electrically-conducting spacer
115
. The magnetization of the pinned layer
120
is fixed by an antiferromagnetic (AFM) layer
121
. Free layer
110
, spacer
115
, pinned layer
120
and the AFM layer
121
are all formed in the central region
102
. Hard bias layers
130
and
135
formed in the end regions
104
and
106
, respectively, provide longitudinal bias for the free layer
110
. Leads
140
and
145
formed over hard bias layers
130
and
135
, respectively, provide electrical connections for the flow of the sensing current I
s
from a current source
160
to the MR sensor
100
. Sensing means
170
connected to leads
140
and
145
sense the change in the resistance due to changes induced in the free layer
110
by the external magnetic field (e.g., field generated by a data bit stored on a disk).
Another type of spin valve sensors currently under development is an anti-parallel (AP)-pinned spin valve sensor. IBM's U.S. Pat. No. 5,583,725 granted to Coffey et al. and incorporated herein by reference, describes an AP-pinned SV sensor (
FIG. 2
) wherein the pinned layer is a laminated structure of two ferromagnetic layers separated by a non-magnetic coupling layer such that the magnetizations of the two ferromagnetic layers are strongly coupled together antiferromagnetically in an antiparallel orientation.
Referring to
FIG. 2
, there is shown a prior art AP-Pinned SV sensor
200
having a free layer
210
separated from a laminated AP-pinned layer
220
by a nonmagnetic, electrically-conducting spacer layer
215
. Free layer
210
comprises a Co layer
212
and a Ni—Fe layer
214
. The laminated AP-pinned layer
220
comprises a first ferromagnetic layer
222
and a second ferromagnetic layer
226
separated from each other by an antiparallel coupling (APC) layer
224
of nonmagnetic material. The two ferromagnetic layers
222
and
226
in the laminated AP-pinned layer
220
have their magnetization directions oriented antiparallel, as indicated by the head of the arrow
223
pointing out of the plane of the paper and the tail of the arrow
227
pointing into the plane of the paper. Antiferromagnetic (AFM) exchange biasing layers
230
and
232
formed on the lateral extensions
240
and
242
of the free layer
210
. The AFM layers
230
and
232
longitudinally bias the free layer so its magnetization in the absence of an external field is in the direction of the arrow
250
. Capping layers
260
and
262
provide corrosion resistance for the AFM layers
230
and
232
, respectively. Electrical leads
270
and
272
provide electrical connections to current source
280
and a sensing means
285
.
Coffey does not use a hard bias layer or an AFM layer adjacent to the pinned layer
220
for pinning the magnetization of the pinned ferromagnetic layer
220
. Consequently, Coffey avoids the problems associated with the blocking temperature and/or corrosion of many AFM materials. However, according to Coffey once the sensor geometry is completed the directions of the magnetizations of first and second pinned layers
222
and
226
are set, perpendicular to the air bearing surface (plane of the disk), by applying a sufficiently large magnetic field (about 10 KOe). That is, a large external field is used in order to ensure that the spins are all pinned in the same direction. Once the spins are all pinned in the same direction, Coffey relies on the antiferromagnetic coupling between the two pinned layers
222
and
226
to maintain the pinning.
FIGS. 2
a
and
2
b
are side views showing the spins directions in the pinned layers
222
and
226
before and after applying a large external field. Before applying a large external field, the spins are randomly formed in both pinned layers
222
and
226
(
FIG. 2
a
). After applying a sufficiently large external field, the directions of the magnetizations in both pinned layers
222
and
226
become set meaning that the spins in each pinned layer become uniform in their directions (
FIG. 2
b
).
However, there are two issues present in Coffey's AP-pinned SV sensor. First, if the magnetizations directions in the pinned layers
222
and
226
becomes disoriented due to a thermal asperity (actua
Evans Jefferson
Gill William D.
International Business Machines - Corporation
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