Dynamic magnetic information storage or retrieval – Head – Magnetoresistive reproducing head
Reexamination Certificate
2001-03-27
2004-04-20
Cao, Allen (Department: 2652)
Dynamic magnetic information storage or retrieval
Head
Magnetoresistive reproducing head
Reexamination Certificate
active
06724586
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates in general to magnetic tunnel junction transducers for reading information signals from a magnetic medium and, in particular, to a magnetic tunnel junction magnetoresistive sensor with an improved bias layer structure.
2. Description of the 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 (MR) read sensors, commonly referred to as MR sensors, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater track and 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 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 layers and within the magnetic layers.
GMR sensors using only two layers of ferromagnetic material (e.g., Ni—Fe) separated by a layer of non-magnetic material (e.g., copper) are generally referred to as spin valve (SV) sensors manifesting 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 first ferromagnetic layer, referred to as a pinned layer
120
, has its magnetization typically fixed (pinned) by exchange coupling with an antiferromagnetic (AFM) layer
125
. The magnetization of a second ferromagnetic layer, referred to as a free layer
110
, is not fixed and is free to rotate in response to the magnetic field from the recorded magnetic medium (the signal field). The free layer
110
is separated from the pinned layer
120
by a non-magnetic, electrically conducting spacer layer
115
. Leads
140
and
145
formed in the end regions
104
and
106
, respectively, provide electrical connections for sensing the resistance of SV sensor
100
. IBM's U.S. Pat. No. 5,206,590 granted to Dieny et al., incorporated herein by reference, discloses a SV sensor operating on the basis of the GMR effect.
Another type of SV sensor is an antiparallel (AP)-pinned SV sensor. In AP-pinned SV sensors, 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. The AP-pinned SV sensor provides improved exchange coupling of the antiferromagnetic (AFM) layer to the laminated pinned layer structure than is achieved with the pinned layer structure of the SV sensor of FIG.
1
. This improved exchange coupling increases the stability of the AP-pinned SV sensor at high temperatures which allows the use of corrosion resistant and electrically insulating antiferromagnetic materials such as NiO for the AFM layer.
Another type-of magnetic device currently under development is a magnetic tunnel junction (MTJ) device. The MTJ device has potential applications as a memory cell and as a magnetic field sensor. The MTJ device comprises two ferromagnetic layers separated by a thin, electrically insulating, tunnel barrier layer. The tunnel barrier layer is sufficiently thin that quantum-mechanical tunneling of charge carriers occurs between the ferromagnetic layers. The tunneling process is electron spin dependent, which means that the tunneling current across the junction depends on the spin-dependent electronic properties of the ferromagnetic materials and is a function of the relative orientation of the magnetic moments, or magnetization directions, of the two ferromagnetic layers. In the MTJ sensor, one ferromagnetic layer has its magnetization fixed, or pinned, and the other ferromagnetic layer has its magnetization free to rotate in response to an external magnetic field from the recording medium (the signal field). When an electric potential is applied between the two ferromagnetic layers, the sensor resistance is a function of the tunneling current across the insulating layer between the ferromagnetic layers. Since the tunneling current that flows perpendicularly through the tunnel barrier layer depends on the relative magnetization directions of the two ferromagnetic layers, recorded data can be read from a magnetic medium because the signal field causes a change of direction of magnetization of the free layer, which in turn causes a change in resistance of the MTJ sensor and a corresponding change in the sensed current or voltage. IBM's U.S. Pat. No. 5,650,958 granted to Gallagher et al., incorporated in its entirety herein by reference, discloses an MTJ sensor operating on the basis of the magnetic tunnel junction effect.
FIG. 2
shows a prior art MTJ sensor
200
comprising a first electrode
204
, a second electrode
202
, and a tunnel barrier
215
. The first electrode
204
comprises a pinned layer (pinned ferromagnetic layer)
220
, an antiferromagnetic (AFM) layer
230
, and a seed layer
240
. The magnetization of the pinned layer
220
is fixed through exchange coupling with the AFM layer
230
. The second electrode
202
comprises a free layer (free ferromagnetic layer)
210
and a cap layer
205
. The free layer
210
is separated from the pinned layer
220
by a non-magnetic, electrically insulating tunnel barrier layer
215
. In the absence of an external magnetic field, the free layer
210
has its magnetization oriented in the direction shown by arrow
212
, that is, generally perpendicular to the magnetization direction of the pinned layer
220
shown by arrow
222
(tail of the arrow that is pointing into the plane of the paper). A first lead
260
and a second lead
265
formed in contact with first electrode
204
and second electrode
202
, respectively, provide electrical connections for the flow of sensing current I
s
from a current source
270
to the MTJ sensor
200
. A signal detector
280
, typically including a recording channel such as a partial-response maximum-likelihood (PRML) channel, connected to the first and second leads
260
and
265
senses the change in resistance due to changes induced in the free layer
210
by the external magnetic field.
As mentioned earlier, an MR sensor exhibits a change in resistance when in the presence of a changing magnetic field. This resistance change is transformed into a voltage signal by passing a constant sense current through the MR element. The value of the DC voltage for a given MR sensor is the product of the constant sense current and the total resistance between the MR sensor leads. Since the change in the resistance is the princi
Cao Allen
Feece Ronald B.
Gill William D.
Hitachi Global Storage Technologies - Netherlands B.V.
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