Dynamic magnetic information storage or retrieval – Head – Magnetoresistive reproducing head
Reexamination Certificate
1999-11-18
2003-04-01
Cao, Allen (Department: 2652)
Dynamic magnetic information storage or retrieval
Head
Magnetoresistive reproducing head
Reexamination Certificate
active
06542341
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to magnetic sensors for detecting external magnetic fields using a ferromagnetic free layer, and in particular to magnetic sensors such as spin valves or tunnel valves in which the free layer is exchanged coupled with an antiferromagnetic layer.
BACKGROUND OF THE INVENTION
Thin film magnetoresistive heads have been used in magnetic data storage devices for several years. The fundamental principles of magnetoresistance including anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR) and spin tunneling have been well-known in the art for some time. Magnetic read heads, e.g., those used in the field of magnetic recording, use magnetic sensors built on these principles and other effects to produce devices capable of reading high density magnetically recorded 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 tunnel valve 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.
Magnetoresistive sensors rely on a ferromagnetic free layer to detect an external magnetic field, e.g., the field produced by data stored in the form of magnetic domains in a magnetic storage medium. The free layer typically has a low coercivity and low anisotropy and thus an easily movable or rotatable magnetic moment which responds to the external field. The rotation of the free layer's magnetic moment causes a change in the resistance of the device by a certain value &Dgr;R (measured between electrical contacts). (In general, the larger the value of &Dgr;R in relationship to total resistance R, i.e., the larger &Dgr;R/R the better the sensor.) This change in resistance due to rotation of the magnetization of the free layer can thus be electronically sensed and used in practical applications such as reading of magnetic data.
An important concern in the design of the sensor is the longitudinal bias of the free layer. In particular, the free layer must be biased by a hard bias so that it is essentially in a single domain state. Deviations from a single domain state are mostly due to edge effects and corners and demagnetizing field effects as the sensor is excited by the external magnetic field. Also, the free layer has to be properly biased in the quiescent state to ensure a linear or essentially linear response with maximum dynamic range. When the free layer is allowed to have more than one magnetic domain, then the free layer experiences Barkhausen jumps and other domain reorientation phenomena, as is known in the art. This is highly undesirable as it produces noise and worsens the signal-to-noise ratio (SNR) of the sensor.
In order to provide the biasing field and prevent noise some of the prior art sensors deploy a longitudinal biasing scheme or a hard bias layer having a high coercivity. Typically, such scheme uses a magnetic material placed essentially in the same plane as the free layer next to and close to it. For example, a hard bias material such as CoPt hard magnet alloy can be used in the form of a hard bias tab. This biasing scheme ensures that the free layer has a single magnetic domain. For more details on longitudinal biasing the reader is referred to U.S. Pat. No. 5,729,410 to Fontana, Jr. et al.
To properly bias the free layer other prior art solutions employed in spin valve and tunnel valve sensors balance the forces of the magnetostatic field H
m
set up by the pinned layer, the interlayer coupling field H
i
between the free layer and the pinned layer (due to Neél orange peel, pinholes, oscillatory coupling etc.) and the current-induced field H
j
caused by current flowing through the sensor structure. This approach is illustrated in
FIG. 1
in a typical spin valve
12
with a seed layer
14
on one side and a cap layer
24
on the other side. Sandwiched between layers
14
,
24
are a ferromagnetic free layer
16
, a spacer layer
18
, and a pinned layer
20
which is exchange-coupled with an antiferromagnetic layer
22
. The arrows indicate the overall magnetizations of layers
16
,
20
and
22
. A current j flowing through spin valve
12
between electrical contacts (not shown) is indicated by an arrow. For optimal performance free layer
16
has to be properly magnetically biased so that its response to an external magnetic field, e.g., a field created by a magnetic recording medium, is highly linear and so that there is maximum dynamic range (i.e., so that the responses to a positive and negative signals are both as large as possible before there is signal saturation). This is accomplished by maintaining the magnetization of free layer
16
substantially at 90° to the magnetization of pinned layer
20
in the absence of a signal or external magnetic field. Thus, the forces of fields H
m
, H
i
and H
j
as well as any other forces (e.g., due to uniaxial anisotropy, shape anisotropy, etc.) acting on free layer
16
have to be balanced such that the transverse component of the sum of the forces acting on the free layer cancel:
H
i
+H
j
+H
m
=0 (Eq. 1)
Thus the transverse components of these vectors add to zero. In practice, these vectors are aligned along a transverse direction as shown and that is why vector addition can be replaced by simple addition. Under ideal conditions equation 1 is satisfied over entire free layer
16
such that free layer
16
experiences zero field and is highly sensitive to the external magnetic field.
The problem with balancing the transverse components of H
i
, H
j
and H
m
is that in a practical device such balance is hard to achieve. Generally, magnetostatic field H
m
is spatially non-uniform in free layer
16
with substantial fields of 100-200 Oe present at a bottom surface
26
(typically the air-bearing surface) and at a top surface
28
, and substantially lower fields in the interior of free layer
16
. The result is a spatially non-uniform orientation of the magnetization in free layer
16
. Field H
i
is uniform across sensor
12
but is not easily controlled over a wide range and can not be always made small. Also, H
i
and H
m
depend on a height of free layer
16
or the stripe height between bottom surface
26
and top surface
28
. This height can not be easily controlled in practice. Field H
j
is nearly uniform except for variations caused by current bunching near the leads.
Thus, equation 1 is typically constraining since the values of H
m
, H
i
, and H
j
cannot be independently optimized, especially if large magnetoresistance is to be obtained because the optimization of magnetoresistance often requires layer thicknesses incompatible with the constraint of equation 1. The result is a non-optimal compromise.
In particular, it would be desirable to make H
m
as small as possible so that the H
m
-related nonuniformities are minimized. It would also be desirable to make H
j
relatively large to be able to use a large bias current for increased sensitivity. To satisfy the equation, then, H
i
must be made relatively large to help balance H
j
. This poses problems because H
i
is sensitively dependent upon the surface textures of the layers. It is difficult to fabricate the layers so that a large, well-defined value of H
i
is provided consistently. Therefore, there exists a practical limit on the magnitude of H
i
.
Consequently, H
m
and H
j
cannot have vastly different magnitudes. At best, state of the art GMR sensors compromise between the competing benefits of low H
m
values, high H
j
values and low H
i
values.
There thus exists a need for developing a proper scheme for longitudinal biasing of the free layer of a magnetic sensor. In particular, there exists a need for balancing fields H
m
, H
i
and H
j
acting on the free layer without sacrificing the ability to optimize the values of these fields for good sensor performance. More precisely, there exists a need for magnetic sensors suc
Carey Matthew Joseph
Childress Jeffrey Robinson
Gurney Bruce Alvin
Cao Allen
International Business Machines - Corporation
Lumen Intellectual Property Services Inc.
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