Coating processes – Magnetic base or coating – Magnetic coating
Reexamination Certificate
2002-09-12
2004-06-29
Pianalto, Bernard (Department: 1762)
Coating processes
Magnetic base or coating
Magnetic coating
C427S131000, C427S132000, C427S404000
Reexamination Certificate
active
06756071
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to magnetic disk drives, and more particularly to spin valve giant magnet or resistive (GMR) thin film read heads.
BACKGROUND OF THE INVENTION
Magnetic disk drives are used to store and retrieve data for digital electronic apparatus 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 arrow 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 sensor. A magnetoresistive (MR) sensor is used to detect magnetic field signals by means of changing resistance in a read element. A conventional MR sensor utilizes the anisotropic magnetoresistive (AMR) effect for such detection, where the read clement 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 MR 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 magnetetoresistance is known as spin valve magnetoresistance or giant magnetoresistance (GMR). In such a spin valve sensor, two ferromagnetic layers are separated by a non-magnetic layer such as copper. One of the ferromagnetic layers is a “free” layer and the other ferromagnetic layer is a “pinned” layer. This pinning is typically achieved by providing an exchange-coupled anti-ferromagnetic layer adjacent to the pinned layer.
More particularly, and with reference to
FIG. 1C
, 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 S is defined by the MRH
10
. The spin valve sensor is preferably centered within 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.
With reference to
FIG. 2A
, a cross-sectional view taken along line
2
—
2
of
FIG. 1C
illustrates the structure of the spin valve sensor
16
of the prior art. The spin valve sensor
16
is built upon a substrate
17
and includes: an anti-ferromagnetic layer
24
; a pinned layer
22
; a first cobalt enhanced layer
19
; a thin copper layer
20
; a second cobalt enhanced layer
23
and a free layer
18
. Ferromagnetic end regions
21
abut the ends of the spin valve sensor
16
. 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 free layer
18
opposite the Co enhanced layer
23
. A current source
29
provides a current I
b
which flows 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
18
and
22
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 80% nickel (Ni) and 20% iron (Fe). While the layer
20
is typically copper, other non-magnetic materials have been used as well. The cobalt enhanced layer can be preferably constructed of Co or more preferably of Co
90
Fe
10
. The AFM layer
24
is used to set the magnetic direction of the pinned layer
22
.
With continued reference to
FIG. 2A
, the spin valve sensor
16
develops a rough interface between the copper layer
20
and the cobalt enhanced layer
23
. This can be understood better with reference to
FIG. 2B
, wherein the interface is shown at the atomic level. Both the copper layer
20
, shown in solid, and the cobalt enhanced layer
23
have face centered cubic (FCC) crystalline structures. However, as the copper is deposited onto the first cobalt enhanced layer
19
, the copper tends to form in groups or “islands” rather than being deposited layer by layer as would be desired. This leads to a rough copper surface upon which the second must subsequently be deposited. Therefore, the interface
30
between the copper spacer layer and the second cobalt enhanced layer
23
takes on this rough texture as shown in FIG.
2
A.
With reference to
FIGS. 3A and 3B
, the free layer
18
can have a magnetization vector
26
which is free to rotate about an angle &agr;, while the pinned layer
22
is magnetized as indicated by the arrow
28
. Absent the influence of a magnetic field, such as that provided by a magnetic recording medium, the magnetization of the free layer, as represented by arrow
30
, would ideally be perpendicular to the direction of the magnetization
28
of the pinned layer
28
. However, when the free layer is subjected to a magnetic field, represented by arrow
32
, the resulting magnetization
26
of the free layer becomes the sum of the magnetic flux magnetization
32
and the magnetization
30
. It is a property of GMR heads that as the angle &agr; changes, the resistance of the sensor
16
will change. The relationship between the angle &agr; and the resistance of the sensor will be essentially linear in the region of &agr;=0 degrees (i.e. when vector
26
is approximately perpendicular to vector
28
. This can be seen with reference to FIG.
3
B.
With reference to
FIG. 3C
, a GMR read element
10
which does not have an initial angle &agr; which is substantially equal to zero in the absence of any external magnetic field will experience errors when reading data. A typical magnetic recording medium records data as a series of magnetic pulses in the form of waves. The sensor reads these waves and generates a signal having a sensor output, i.e. Track Average Amplitude (TAA). As can be seen with reference to
FIG. 3C
, a positive magnetic pulse results in an output amplitude TAA
1
followed by an equivalent negative pulse resulting in a sensor output amplitude TAA
2
of the same absolute value. If a read sensor
10
has an initial magnetization angle &agr; of zero then the read sensor will be able to, detect these opposite pulses as such. However, if the angle is substantially greater than or less than zero the read sensor will impart an offset error which will cause the sensor to read one of the pulses as being larger than it actually is and the other as being smaller. In such a case, the sensor may not register the smaller pulse and may miss that bit of data. The tendency of read heads to impart such an offset error is termed Track Average Amplitude Asymmetry (TAAA) and is defines as (TAA
1
−TAA
2
)/(TAA
1
+TAA
2
). A TAAA of less than 15% is generally required for a read head to function properly.
With brief reference to
3
A, in order for &agr; to equal 0 in the free state, several magnetic forces acting on a magnetization vector
26
of the free layer
18
must balance to 0. In
FIG. 3D
, H
d
repr
Leng Qunwen
Mao Ming
Shi Zhupei
Carr & Ferrell LLP
Pianalto Bernard
Western Digital (Fremont) Inc.
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