Dynamic magnetic information storage or retrieval – Head – Magnetoresistive reproducing head
Reexamination Certificate
2001-01-04
2002-10-29
Cao, Allen (Department: 2652)
Dynamic magnetic information storage or retrieval
Head
Magnetoresistive reproducing head
C360S112000
Reexamination Certificate
active
06473279
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to magnetoresistive sensors. More particularly, it relates to the stabilization of free layers' magnetizations of magnetoresistive sensors.
BACKGROUND ART
Thin film magnetoresistive (MR) sensors or heads have been used in magnetic data storage devices for several years. Physically distinct forms of magnetoresistance such as anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR) and spin tunneling magnetoresistance (TMR) are well-known in the art. Magnetic read-back sensor designs have been built using these principles and other effects to produce devices capable of reading high density data. In particular, three general types of magnetic read heads or magnetic readback sensors have been developed: the anisotropic magnetoresistive (AMR) sensor, the giant magnetoresistive (GMR) sensor or GMR spin valve, and the magnetic tunnel junction (MTJ) sensor. The construction of these sensors is discussed in the literature, e.g., in U.S. Pat. No. 5,159,513 or U.S. Pat. No. 5,206,590.
For maximum spin-valve or tunnel-valve head stability and response linearity without hysteresis, it is generally desired, in the absence of any other source of external magnetic field on the free (or sensing) layer, that the magnetization of the free layer be maintained in a saturated single domain state. In such a state, the local magnetization everywhere in the free layer, up to and including the track-edges, will remain essentially “longitudinal”, i.e., co-linear with the cross-track direction of the head, parallel to the plane of the magnetic recording medium, and orthogonal to the direction of “transverse” magnetic signal fields emanating from a magnetic medium proximate the sensor.
The prior art has used a method of “hard-bias” or edge-coupling-only to stabilize the magnetization of free (or sensing) layers of MR sensors.
FIG. 1
illustrates the basic components of a typical current-in-plane (CIP) GMR sensor
100
with hard bias layers of the prior art. The sensor
100
includes a ferromagnetic reference layer
106
with a fixed transverse magnetic moment (pointing into the page) and a ferromagnetic free layer
110
with a rotatable magnetization vector, which can rotate about the longitudinal direction in response to transverse magnetic signal fields. The direction of the magnetic moment of the reference layer
106
is typically fixed by exchange coupling with an antiferromagnetic layer
104
. Exchange-pinned reference layer
106
and free layer
110
are separated by a thin electrically conductive nonmagnetic layer
108
. Hard bias layers
112
provide a longitudinal biasing magnetic field to stabilize the magnetization of the free layer
110
approximately in a longitudinal orientation in the absence of other external magnetic fields. Sensor
100
further includes top electrical leads
114
in proximity with hard bias layers
112
, and a layer
102
adjacent to the antiferromagnetic layer
104
, which represents a combination of the substrate, undercoat, and seed layers. For a shielded sensor, layer
102
may additionally include the bottom shield and insulation layers (for CIP sensors) or electrical contact layers (for CPP sensors).
FIG. 2
shows a current-perpendicular-to-plane (CPP) sensor
200
with hard bias layers of the prior art. CPP sensor
200
includes a ferromagnetic reference layer
206
with a fixed magnetic moment oriented transversely (into the page) and a ferromagnetic free layer
210
with a rotatable magnetization vector, which can rotate about the longitudinal direction in response to transverse magnetic signal fields. The direction of the magnetic moment of the reference layer
206
is typically fixed by exchange coupling with an antiferromagnetic layer
204
. The exchange-pinned reference layer
206
and free layer
210
are spaced apart by a non-magnetic layer
208
. For MTJ devices, layer
208
includes an electrically insulating tunnel barrier layer. For CPP-GMR devices, layer
208
is electrically conductive, and is analogous to layer
108
of the CIP-GMR sensor of FIG.
1
. Hard bias layers
212
are electrically insulated from the sensor stack and the top electrical lead
216
by insulating layers
214
and
218
respectively. Hard bias layers
212
provide a longitudinal biasing magnetic field to stabilize the magnetization of the free layer
210
. Sensor
200
further includes a layer
202
, which is similar to layer
102
of sensor
100
, in proximity with the antiferromagnetic layer
204
.
An important concern in the design of the sensors of
FIGS. 1 and 2
is the longitudinal bias of the free layers. It is desired that the hard bias layers maintain the free layer's magnetization in a longitudinally oriented, single domain state. In the absence of longitudinal bias, the magnetization of free layer tends to establish a multi-domain state, as is well-known. Free layers in multi-domain states may experience Barkhausen jumps and other domain reorientation phenomena when responding to external magnetic fields from the encoding data bit in a magnetic recording disk. This problem is also known in the art and is highly undesirable as it produces hysteresis noise and worsens the signal-to-noise ratio (SNR) of the sensor.
However, the most common technique of the prior art includes the fabrication of magnetically hard (permanent magnet) bias layers which form an abutted junction with the physical track edges of the GMR sensor. For a CPP sensor, there exists the additional complication of maintaining an insulating spacer layer between the junction of the hard bias layers and the CPP stack. The efficacy of the method of stabilization depends critically on the precise details of the junction geometry, which is difficult to accurately control using present fabrication methods. The main source of this fabrication difficulty is the necessity of depositing and defining the hard-bias junction after the track width of the MR sensor is defined and patterned lithographically, and hence is subjected to the known fabrication and dimensional tolerances associated with this process.
An intrinsic consequence of any form of single domain stabilization of the free layer is the associated magnetic “stiffness” of the free layer, which limits its rotational response to the magnetic signal fields from the recorded bits on the magnetic recording medium. For hard-bias, the stabilization mechanism is magnetostatic coupling to the free layer predominantly at or near the track edges proximate to the hard-bias junction. For edge-coupling-only stabilization in general, the average magnetic stiffness can progressively increase as read track widths shrink and track edges become relatively more proximate, and hence more tightly coupled, to the entire volume of the read head. The stiffness issue will be further exacerbated via the practical necessity of “over-bias”, in which the magnetic moment ratio (M
S
*t)
bias
/(M
s
*t)
free
of the deposited hard-bias layer to that of free layer is designed to be several times greater than the theoretical minimum in order to compensate for the non-idealities (e.g., low coercivity) and geometric fabrication tolerances of actual hard-bias junctions. Further, the degree of required “over bias” is governed by the aforementioned fabrication tolerances, which are hard to control and hence difficult to design for.
U.S. Pat. No. 6,023,395 issued Feb. 8, 2000 to Dill et al. discloses a MTJ sensor, which includes, in addition to the necessary multitude of magnetic and nonmagnetic layers comprising a basic MTJ device known in the art, an extra in-stack ferromagnetic “biasing” layer which is coupled exclusively magnetostatically with the ferromagnetic free layer. When the magnetization of the bias layer is essentially rigidly maintained with approximately longitudinal orientation, the disclosed purpose of the biasing layer is to provide some degree of longitudinal stabilization of the free layer. The form of in-stack stabilization described by Dill avoids some of the fabrication difficulties inherent to co
Smith Neil
Yang Bo
Cao Allen
Lumen Intellectual Property Services Inc.
Nguyen Dzung C.
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