Dynamic magnetic information storage or retrieval – Head – Magnetoresistive reproducing head
Reexamination Certificate
2000-03-02
2002-10-22
Renner, Craig A. (Department: 2652)
Dynamic magnetic information storage or retrieval
Head
Magnetoresistive reproducing head
Reexamination Certificate
active
06469879
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magneto-resistive tunnel junction head for reading the magnetic field intensity from a magnetic recording medium or the like as a signal and, in particular, to a magneto-resistive tunnel junction head which has a new design of biasing means for improving an output for adaptation to ultra-high density recording and is excellent in flexibility of selection of the biasing means.
2. Description of the Prior Art
MR sensors based on the anisotropic magneto-resistance (AMR) or spin-valve (SV) effect are widely known and extensively used as read transducers in magnetic recording. MR sensors can probe the magnetic stray field coming out from transitions recorded on a recording medium by the resistance changes of a reading portion formed of magnetic materials. AMR sensors have quite a low resistance change ratio &Dgr;R/R, typically from 1 to 3%, whereas the SV sensors have a &Dgr;R/R ranging from 2 to 7% for the same magnetic field excursion. The SV magnetic read heads showing such high sensitivity are progressively supplanting the AMR read heads to achieve very high recording density, namely over several Giga bits per square inch (Gbits/in
2
).
Recently, a new MR sensor has attracted attention for its application potential in ultra-high density recording. Magneto-resistive tunnel junctions (MRTJ, or synonymously referred to as TMR) are -reported to have shown a resistance change ratio &Dgr;R/R over 12%. Although it has been expected that TMR sensors replace SV sensors in the near future as the demand for ultra-high density is ever growing, an application to the field of the magnetic heads has just started, and one of the outstanding objects is to develop a new head structure which can maximize the TMR properties. Great efforts of developments are still needed to design a new head structure since TMR sensors operate in CPP (Current Perpendicular to the Plane) geometry, which means that TMR sensors requires the current to flow in a thickness direction of a laminate film.
In a basic SV sensor which has been developed for practical applications, two ferromagnetic layers are separated by a non-magnetic layer, as described in U.S. Pat. No. 5,159,513. An exchange layer (FeMn) is further provided so as to be adjacent to one of the ferromagnetic layers. The exchange layer and the adjacent ferromagnetic layer are exchange-coupled so that the magnetization of the ferromagnetic layer is strongly pinned (fixed) in one direction. The other ferromagnetic layer has its magnetization which is free to rotate in response to a small external magnetic field. When the magnetization's of the ferromagnetic layers are changed from a parallel to an antiparallel configuration, the sensor resistance increases and a &Dgr;R/R in the range of 2 to 7% is observed.
In comparison between the SV sensor and the TMR sensor, the structure of the TMR is similar to the SV sensor except that the non-magnetic layer separating the two ferromagnetic layers is replaced by a tunnel barrier layer being an insulating layer and that the sense current flows perpendicular to the surfaces of the ferromagnetic layers. In the TMR sensor, the sense current flowing through the tunnel barrier layer is strongly dependent upon a spin-polarization state of the two ferromagnetic layers. When the magnetization's of the two ferromagnetic layers are antiparallel to each other, the probability of the tunnel current is lowered, so that a high junction resistance is obtained. On the contrary, when the magnetization's of the two ferromagnetic layers are parallel to each other, the probability of the tunnel current is heightened and thus a low junction resistance is obtained.
U.S. Pat. No. 5,729,410 discloses an example wherein a TMR sensor (element) is applied to a magnetic head structure. The TMR sensor is sandwiched between two parallel electrical leads (electrodes), that are in turn sandwiched between first and second insulating gap layers to form a read gap. A pair of permanent magnets are formed to secure a single magnetic domain structure of a free layer so as to suppress generation of the Barkhausen noises. In this case, attention is paid to avoiding a contact between the pair of permanent magnets and the TMR sensor portion so as to prevent an electrical short circuit of an insulating barrier.
However, the TMR head structure proposed in U.S. Pat. No. 5,729,410 has problems that since the permanent magnet and the free layer are formed with a given distance therebetween, the bias effect is reduced, and that the biasing means are limited to permanent magnets due to magnetic separation between the biasing means and the free layer.
For solving the foregoing problems, the present inventors have attempted to design one head structure which is shown in
FIG. 6
in section. The TMR head
100
shown in
FIG. 6
is provided with a TMR element
200
in the form of a laminate body comprising a ferromagnetic free layer
120
, a tunnel barrier layer
130
, a ferromagnetic pinned layer
140
and an antiferromagnetic pinning layer
150
, and further provided with insulating layers
191
and
191
formed at opposite ends (left and-right sides in
FIG. 6
) of the element
200
. Magnetization of the ferromagnetic pinned layer
140
is fixed in one direction (depth direction of the drawing sheet) by the antiferromagnetic pinning layer
150
, while magnetization of the ferromagnetic free layer
120
can be rotated freely in response to an external signal magnetic field.
Further, on upper surfaces at opposite ends of the ferromagnetic free layer
120
located at the top of the TMR element
200
, bias layers
161
and
161
in the form of permanent magnets are formed for applying a bias magnetic field in a direction of arrow &agr;. Therefore, at portions of the ferromagnetic free layer
120
where the bias layers
161
and
161
abut the upper surfaces of the ferromagnetic free layer
120
, the magnetization of the ferromagnetic free layer
120
is pinned in the direction of arrow a due to an exchange-coupling magnetic field. In
FIG. 6
, numerals
171
and
175
denote a pair of upper and lower electrodes, and numerals
181
and
185
denote a pair of upper and lower shield layers.
By adopting the head structure shown in
FIG. 6
, the problems generated in U.S. Pat. No. 5,729,410 could be solved. However, it was confirmed by the present inventors that new problems were generated in the head structure shown in FIG.
6
.
Now, the ferromagnetic magneto-resistive tunnel effect (spin tunneling magneto-resistive effect) will be briefly explained. As the sense current is flowing perpendicularly to the surfaces of the TMR multilayered film
200
, the conduction electrons are spin-polarized when they experienced the first ferromagnetic layer (
20
or
40
depending on the current flowing direction). The probability of tunneling through the tunnel barrier layer is thus spin-dependent and depends upon the relative orientation of the two ferromagnetic layers
20
and
40
sandwiching the tunnel barrier layer. As illustrated in
FIG. 5A
, when the ferromagnetic layers
20
and
40
are parallel in magnetization to each other (or the relative magnetization angle therebetween is small), the density of states of majoritary spins is high in both layers, resulting in a high probability of electron tunneling through the tunnel barrier layer and a low junction resistance R
p
. In constrast with this, as illustrated in
FIG. 5C
, when the ferromagnetic layers
20
and
40
are antiparallel in magnetization to each other (or the relative angle of magnetization therebetween is large), the density of states of majoritary spins is very different in each ferromagnetic layer, resulting in a low probability of electron tunneling through the tunnel barrier layer and a high junction resistance R
ap
. In the intermediate state between the state shown in FIG.
5
A and the state shown in
FIG. 5C
, i.e. when both ferromagnetic layers are orthogonal in magnetization to each other, a resistance valu
Araki Satoru
Kasahara Noriaki
Redon Olivier
Shimazawa Koji
Oblon, Spivak, McClelland, Maier & Nuestadt, P.C.
Renner Craig A.
TDK Corporation
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