Dynamic magnetic information storage or retrieval – Head – Magnetoresistive reproducing head
Reexamination Certificate
2002-03-07
2004-01-06
Evans, Jefferson (Department: 2652)
Dynamic magnetic information storage or retrieval
Head
Magnetoresistive reproducing head
C360S324200
Reexamination Certificate
active
06674617
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the field of magnetoresistive sensors (heads) and more particularly to magnetoresistive heads used in data storages systems and even more particularly to magnetic tunnel junction (MTJ) heads.
BACKGROUND OF THE INVENTION
A typical prior art head and disk system
10
is illustrated in FIG.
1
. In operation the head
20
is supported by the suspension
13
as it flies above the disk
16
. The magnetic sensor, usually called a “head,” is composed of elements that perform the task of writing magnetic transitions (the write head
23
) and reading the magnetic transitions (the read head
12
). The electrical signals to and from the read and write heads
12
,
23
travel along conductive paths (leads)
14
which are attached to or embedded in the suspension
13
. Typically there are two electrical contact pads (not shown) each for the read and write heads
12
,
23
. Wires or leads
14
are connected to these pads and routed in the suspension
13
to the arm electronics (not shown). The disk
16
is attached to a spindle
18
that is driven by a spindle motor
24
to rotate the disk
16
. The disk
16
comprises a substrate
26
on which a plurality of thin films
21
are deposited. The thin films
21
include ferromagnetic material in which the write head
23
records the magnetic transitions in which information is encoded. The read head
12
reads magnetic transitions as the disk rotates under the air-bearing surface of the head
20
.
There are several types of read heads
12
, which are called transducers and sensors interchangably, including those using spin valves and tunnel junctions. Heads using spin valves are called GMR heads. The basic structure of a spin valve sensor (not shown) includes thin films for an antiferromagnetic layer, a pinned layer and a free layer. The spin valve effect is a result of differential switching of two weakly coupled ferromagnetic layers. Magnetic tunnel junction (MTJ) devices. The MTJ device has potential applications as a memory cell and as a magnetic field sensor. The prior art MTJ device shown in
FIG. 2
shows a section of a read head
12
with an MTJ thin film layer structure comprises a pinned ferromagnetic layer (pinned layer)
34
and a free ferromagnetic layer (free layer)
36
separated by a thin, electrically insulating, tunnel barrier layer
35
. The tunnel barrier layer
35
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. When an electric potential is applied between the pinned and free ferromagnetic layers, the sensor resistance is a function of the tunneling current across the insulating layer between the 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.
The magnetization of the pinned layer
34
is fixed through exchange coupling with the antiferromagnetic (AFM) layer
33
. The cap layer
37
separates the free layer
36
from the first lead
31
A. The tunnel barrier layer
35
is a nonmagnetic, electrically insulating material such as aluminum (III) oxide (Al
2
O
3
), aluminum (III) nitride (AIN) and magnesium (II) oxide (MgO). The seed layer
32
is deposited prior to the layers shown above it and is used to establish the growth conditions and control the crystalline characteristics of layers following it. The first and second leads (
31
A,
31
B) provide electrical connections for the flow of sensing current to a signal detector (not shown) that senses the change in resistance in the free layer
36
induced by the external magnetic field that is generated by the magnetic media.
Ferromagnetic materials most suitable for use as the pinned and free layers separated by the insulating tunnel barrier layer are materials with high spin polarization coefficients. Materials with high spin polarization coefficients near the tunneling junction are known to have higher magnetoresistance coefficients in MTJ sensors. A problem arises with some of the known materials that achieve the higher magnetoresistance coefficients is that they also may have high magnetostriction coefficients. When stressed the MTJ sensor layers with high magnetostriction coefficients can result in high uniaxial anisotropy fields in the pinned layer which can act to cancel part of the exchange field from the AFM layer resulting in reduced stability of the MTJ sensor especially at elevated operating temperatures. In addition, the stressed, high magnetostriction materials can result in high anisotropy fields in the free layer which reduces the sensitivity of the free layer to rotate in the presence of the external signal field. In order to eliminate undesirable magnetostriction, previous MTJ sensors have used ferromagnetic materials such as permalloy (Co90Fe10) which have very small magnetostriction coefficients, but which also have smaller magnetoresistance coefficients.
In U.S. Pat. No. 6,127,045 to Gill a magnetic tunnel junction (MTJ) device is described which has a high spin polarization ferromagnetic layer (Ni
40
Fe
60
) is placed near the tunnel barrier layer in both the pinned and free layers to enhance the magnetoresistive effect. The undesirable positive magnetostriction coefficient of the Ni
40
Fe
60
layers is cancelled by placing a negative magnetostriction layer (Ni
90
Fe
10
) of the appropriate thickness adjacent to each Ni
40
Fe
60
layer. The thicknesses of the positive and negative magnetostriction layers are chosen so that the net magnetostriction of the pinned layer and the free layer is approximately zero.
What is needed is a structure for an MTJ sensor which allows the use of materials for free layer that result in the highest magnetoresistive coefficients without degradation in sensitivity and thermal stability due to uncontrolled effects from magnetostrictive properties of these materials.
SUMMARY OF THE INVENTION
A tunnel junction sensor according to the invention replaces the prior art free layer with a free layer structure which allows a wider range of magnetoresistive materials to be used. The preferred free layer structure
40
of the invention includes a negative magnetostriction layer which allows use of magnetoresistive materials which otherwise have unacceptably high magnetostriction values. The materials and thicknesses of the layers result in a total magnetostriction near zero even though ferromagnetic material with high magnetostriction is included. This allows materials with high positive magnetoresistive constants such as Co
50
Fe
50
and iron to be used in the free layer structure in contact with the barrier layer without the deleterious effects of high magnetostriction. In one embodiment, the invention uses bcc iron with a MgO barrier layer, since bcc iron yields particularly high magnetoresistive when used with a MgO barrier. In the preferred embodiment of the invention a layer of material with a negative magnetostriction constant such as nickel or an amorphous cobalt alloy is used to achieve a combined magnetostriction of near zero. The preferred embodiment also includes a softening layer of material such as selected compositions of nickel-iron which maintain the desired magnetic softness of the free layer structure and have a magnetostriction constant near zero.
REFERENCES:
patent: 5595830 (1997-01-01), Daughton
patent: 5648885 (1997-07-01), Nishioka et al.
patent: 5849422 (1998-12-01), Hayashi
patent: 5930085 (1999-07-01), Kitade et al.
patent: 6127045 (2000-10-01), Gill
patent: 6153319 (2000-11-01), Hasegawa
patent: 6168860 (2001-01-01), Daughton
patent: 6301089
Evans Jefferson
International Business Machines - Corporation
Knight G. Marlin
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