Dynamic magnetic information storage or retrieval – Head – Magnetoresistive reproducing head
Reexamination Certificate
1999-02-10
2004-10-05
Letscher, George J. (Department: 2653)
Dynamic magnetic information storage or retrieval
Head
Magnetoresistive reproducing head
C360S324120
Reexamination Certificate
active
06801411
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to magnetic disk drives, more particularly to spin valve magnetoresistive (MR) read heads, and most particularly to methods and structures for providing a pinning mechanism for spin valve sensors while minimizing pulse amplitude asymmetry.
Magnetic disk drives are used to store and retrieve data for digital electronic apparatuses such as computers. In
FIGS. 1A and 1B
, a magnetic disk drive D of the prior art includes a sealed enclosure
1
, a disk drive motor
2
, a magnetic disk
3
, supported for rotation by a spindle S
1
of motor
2
, an actuator
4
and an arm
5
attached to a spindle S
2
of actuator
4
. A suspension
6
is coupled at one end to the arm
5
, and at its other end to a read/write head, or transducer
7
. The transducer
7
is typically an inductive write element with a sensor read element. As the motor
2
rotates the disk
3
, as indicated by the arrow R, an air bearing is formed under the transducer
7
to lift it slightly off of the surface of the disk
3
. Various magnetic “tracks” of information can be read from the magnetic disk
3
as the actuator
4
is caused to pivot in a short arc, as indicated by the arrows P. The design and manufacture of magnetic disk drives is well known to those skilled in the art.
The most common type of sensor used in the transducer
7
is the magnetoresistive (MR) sensor. An MR sensor is used to detect magnetic field signals by means of a changing resistance in a read element. A conventional MR sensor utilizes the anisotropic magnetoresistive (AMR) effect for such detection, where the read element resistance varies in proportion to the square of the cosine of the angle between the magnetization in the read element and the direction of a sense current flowing through the read element. When there is relative motion between the AMR sensor and a magnetic medium (such as a disk surface), a magnetic field from the medium causes a change in the direction of magnetization in the read element, thereby causing a corresponding change in resistance of the read element. The change in resistance can be detected to recover the recorded data on the magnetic medium.
Another form of magnetoresistive effect is known as the giant magnetoresistive (GMR) effect. A GMR sensor resistance also varies with variation of an external magnetic field, although by a different mechanism than with an AMR sensor. Sensors using the GMR effect are particularly attractive due to their greater sensitivity and higher total range in resistance than that experienced with AMR sensors. One type of GMR sensor is known as a spin valve sensor. In a spin valve sensor, two ferromagnetic (FM) layers are separated by a layer of non-magnetic metal, such as copper. One of the ferromagnetic layers is a “free,” or sensing, layer, with the magnetization generally free to rotate when exposed to external fields. In contrast, the other ferromagnetic layer is a “pinned” layer whose magnetization is substantially fixed, or pinned, in a particular direction. In the prior art, this pinning has typically been achieved with an exchanged-coupled antiferromagnetic (AFM) layer located adjacent to the pinned layer.
More particularly, and with reference to
FIG. 2
, a shielded, single-element magnetoresistive head (MRH)
10
includes a first shield
12
, a second shield
14
, and a spin valve sensor
16
disposed within a gap (G) between shields
12
and
14
. An air bearing surface ABS is defined by the MRH
10
. The spin valve sensor can be centered in the gap G to avoid self-biasing effects. Lines of magnetic flux impinging upon the spin valve sensor create a detectable change in resistance. The design and manufacture of magnetoresistive heads, such as MRH
10
, are well known to those skilled in the art.
In
FIG. 3
a cross-sectional view taken along line
3
—
3
of
FIG. 2
(i.e., from the direction of the air bearing surface ABS) illustrates the structure of the spin valve sensor
16
of the prior art. The spin valve sensor
16
includes a free layer
18
, a copper layer
20
, a pinned layer
22
, and an antiferromagnetic (AFM) layer
24
. The spin valve sensor
16
is supported by an insulating substrate
17
and a buffer layer
19
which can perform as a seed layer for the formation of the free layer
18
during fabrication. Ferromagnetic end regions
21
, which operate as a hard bias, abut the ends of the spin valve sensor
16
and provide stabilization of the free layer
18
. Leads
25
, typically made from gold or another low resistance material, bring the current to the spin valve sensor
16
. A capping layer
27
is provided over the AFM layer
24
. A current source
29
provides a current Ib to flow through the various layers of the sensor
16
, and signal detection circuitry
31
detects changes in resistance of the sensor
16
as it encounters magnetic fields.
The free and pinned layers are typically made from a soft ferromagnetic material such as permalloy. As is well known to those skilled in the art, permalloy is a magnetic material nominally including 81% nickel (Ni) and 19% iron (Fe). The layer
20
is typically copper. The AFM layer
24
is used to set the magnetic direction of the pinned layer
22
, as will be discussed in greater detail below.
The purpose of the pinned layer
22
will be discussed with reference to
FIGS. 4 and 5
. In
FIG. 4
, the free layer
18
can have an actual free magnetization direction
26
, while the pinned layer
22
has a pinned magnetization
28
. Absent the magnetostatic coupling of the pinned layer
22
, the ferromagnetic exchange coupling through the copper layer
20
, and absent the field generated by the sensing current I
S
, the free layer
18
may have an initial free magnetization
30
. The actual free magnetization direction
26
is the sum of the initial free magnetization
30
and the magnetostatic coupling of the pinned layer
22
, the ferromagnetic exchange coupling through the copper layer
20
, and the field generated by the sensing current I
S
. As is known in the art, the magnetization direction of the free layer
18
is preferably variable in response to varying external fields, for example from a nearby magnetic medium.
As seen in
FIG. 5
on the curve of resistance versus magnetic field of the spin valve sensor, the pinned magnetization
28
of the pinned layer
22
at a right angle to the initial free magnetization
30
of the free layer
18
biases the free element to a point
32
on the curve that is relatively linear, and which has a relatively large slope. Linearity is, of course, desirable to provide a linear response, and the relatively large slope is desirable in that it produces large resistance changes in response to the changes in the magnetic field.
The antiferromagnetic material of the AFM layer
24
is typically either a manganese (Mn) alloy such as iron-manganese (FeMn) or an oxide such as nickel-oxide (NiO). The AFM layer
24
prevents the magnetization of the pinned layer
22
from rotating under most operating conditions, with the result that only the magnetic moment of the free layer
18
can rotate in the presence of an external magnetic field.
The spin valve sensor that has the most linear response and the widest dynamic range is one in which the magnetization of the pinned ferromagnetic layer
22
is parallel to the signal field and the magnetization of the free layer
18
is perpendicular to the signal field. However, the use of the AFM layer
24
to pin the pinned layer
22
presents several problems. For one, the exchange field strength generated by the AFM is highly sensitive to temperature. As the temperature increases, the AFM “softens” and its ability to fix the magnetization of the pinned ferromagnetic layer decreases. In consequence, spin valve sensors are highly sensitive to electrostatic discharge (ESD) currents and the resultant heating of the AFM layer
24
. Further, AFM materials such as FeMn are much more susceptible to corrosion than the other materials used in the spin valve sensor. The sensitivity of the AFM mate
Gibbons Matthew R.
Lederman Marcos
Carr & Ferrell LLP
Letscher George J.
Western Digital (Fremont) Inc.
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