Buffer layer of a spin valve structure

Details Buffer layer of a spin valve structure Buffer layer of a spin valve structure

2000-06-05

2002-09-24

Tupper, Robert S. (Department: 2652)

Dynamic magnetic information storage or retrieval

Head

Magnetoresistive reproducing head

Reexamination Certificate

active

06456469

ABSTRACT:

BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention relates generally to spin valve magnetic transducers for reading information signals from a magnetic medium and, in particular, to buffer layers for preventing diffusion in a spin valve sensor, and to magnetic recording systems which incorporate such sensors.
2. The Relevant Art
Computer systems generally utilize 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, such as a 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 carrying 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 now the most common type of read sensors. This is largely due to the capability of MR heads of reading data on a disk of a greater linear density than that which the previously used thin film inductive heads are capable of. An MR sensor detects a magnetic field through a change in resistance in 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 maonetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization of the MR element and the direction of sense current flowing 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 to layers and within the magnetic layers.
GMR sensors using only two layers of ferromagnetic material separated by a layer of non-magnetic electrically conductive material are generally referred to as spin valve (SV) sensors manifesting the GMR 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 causes a change in the 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. It should be noted that the AMR effect is also present in the SV sensor free layer and it tends to reduce the overall GMR effect.
FIG. 1
shows a typical prior art SV sensor
100
comprising a pair of end regions
104
and
106
separated by a central region
102
. The central region
102
is formed by a suitable method such as sputtering and has defined end regions that are contiguous with and abut the edges of the central region. A free layer (free ferromagnetic layer)
110
is separated from a pinned layer (pinned ferromagnetic layer)
120
by a non-magnetic, electrically-conducting spacer layer
115
. The magnetization of the pinned layer
120
is fixed through exchange coupling with an anti ferromagnetic (AFM) layer
121
.
The free layer
110
, spacer layer
115
, pinned layer
120
and 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
. A sensing device
170
connected to the leads
140
and
145
senses the change in the resistance due to changes induced in the free layer
110
by an external magnetic field (e.g., a field generated by a data bit stored on a disk). 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.
Another type of spin valve sensor more recently developed is an anti-parallel (AP)-pinned spin valve sensor.
FIG. 2
shows one representative AP-pinned SV sensor
200
. The AP-pinned SV sensor
200
has a pair of end regions
202
and
204
separated from each other by a central region
206
. The AP-pinned SV sensor
200
is also shown comprising a Ni—Fe free layer
225
separated from a laminated AP-pinned layer
210
by a copper spacer layer
220
. The magnetization of the laminated AP-pinned layer
210
is fixed by an AFM layer
208
which is made of NiO.
The laminated AP-pinned layer
210
includes a first ferromagnetic layer
212
of cobalt and a second ferromagnetic layer
216
of cobalt separated from each other by a ruthenium (Ru) antiparallel coupling layer
214
. The AFM layer
208
, AP-pinned layer
210
, copper spacer
220
, free layer
225
and a cap layer
230
are all formed sequentially in the central region
206
. A pair of hard bias layers
235
and
240
, formed in the end regions
202
and
204
, provide longitudinal biasing for the free layer
225
.
A pair of electrical leads
245
and
250
are also formed in end regions
202
and
204
, respectively, to provide electrical current from a current source (not shown) to the SV sensor
200
. In the depicted example, the magnetization direction of the free layer
225
is set parallel to the air bearing surface (ABS) in the absence of an external field. The magnetization directions of the pinned layers
212
and
214
, respectively, are also set to be perpendicular to the ABS. The magnetization directions of the pinned layers are shown as coming out of the Figure at
260
and going in at
255
. The magnetization of the free layer
225
is shown set to be parallel to the ABS.
The disk drive industry has been engaged in an ongoing effort to increase the overall sensitivity, or GMR coefficient, of the SV sensors in order to permit the drive head to read smaller changes in magnetic flux. Higher GMR coefficients enable the storage of more bits of information on any given disk surface and ultimately provide for higher capacity disk drives without increased size or complexity of the disk drives. The GMR coefficient of an SV sensor is &Dgr;R/R, or the change in resistance of the sensor material, divided by the overall resistance of the material when the sensor material is subjected to a changing magnetic field. The GMR coefficient is dependent on both the “softness” of the material and its overall resistance.
A change in resistance of the sensor material can be easily measured only if the change is large compared to the overall resistance R of the material. Thus, a low overall resistance R, combined with a high change in magnetoresistance, &Dgr;R, will produce a high GMR coefficient.
Other properties relevant to the performance of a GMR head include magnetostriction, exchange coupling between the AFM and the pinned layer or layers, and the electri

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