Electricity: measuring and testing – Magnetic – Magnetometers
Reexamination Certificate
2000-03-10
2002-03-05
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Magnetic
Magnetometers
C360S324200
Reexamination Certificate
active
06353318
ABSTRACT:
BACKGROUND
Computer storage devices, such as disk drives, use read/write heads to store and retrieve data. A write head stores data by utilizing magnetic flux to set the magnetic moment of a particular area on a magnetic media. The state of the magnetic moment is later read by a read head which senses the magnetic fields.
Conventional thin film read heads employ magnetoresistive material, generally formed in a layered structure of magnetoresistive and non-magnetoresistive materials, to detect the magnetic moments of the data bits on the media. A sensing current is passed through the magnetoresistive material to detect changes in the resistance of the material induced by the data bits as they pass the read head.
One conventional type of sensor is a current-in-plane or CIP device as shown in FIG.
1
. As can be seen, this sensor
5
has a junction
10
, hard bias
40
and leads
50
. The junction
10
is a stack of film layers which includes, from the bottom, an antiferromagnet layer
12
, a pinned layer
14
, a copper spacer layer
16
and at the top of the stack a free layer
18
. The junction
10
has sloping sides
11
. Typically, the pinned layer
14
is a ferromagnetic layer which, as the name implies, has its magnetization pinned by the antiferromagnetic layer
12
. The free layer
18
, in contrast is a ferromagnetic layer which has its magnetization set perpendicular to the pinned layer
14
, and which is free to change its magnetic orientation in response to a magnetic fields of passing magnetized bits located on an adjacent recording media (not shown).
The hard bias
40
is positioned on both sides of the junction
10
. The hard bias
40
includes an underlayer
42
, which can be chromium (Cr), and a permanent magnet layer
46
, such as cobalt chromium platinum (CoCrPt). The underlayer
42
is laid directly over each side
11
of the junction
10
, and the permanent magnet layer
46
is positioned over the underlayer
42
. Both the underlayer
42
and the permanent magnet layer
46
overhang and contact the upper surface
20
of the free layer
18
. The underlayer
42
contacts the upper surface
20
at end
44
and permanent magnet layer
46
contacts the upper surface
20
at end
48
.
Biasing is critical to the proper operation of the sensor
5
. The hard bias
40
acts to stabilize the response of the sensor
5
and sets the quiescent state of the sensor. That is, the hard bias
40
stabilizes the domain structure of the free layer
18
to reduce noise. In CIP sensors, such as anisotropic magnetoresistive and spin valve devices, the hard bias
40
functions to set the magnetization of the free layer
18
in a longitudinal direction by pinning the magnetization at each end
22
of the free layer
18
. This prevents formation of closure domains at the ends
22
. Without this pinning, movement of the end domains can cause hysteresis in the magnetoresistive response of the device. Typically, in CIP devices the hard bias
40
is formed adjacent to and partially overlying the edges
22
of the free layer
18
.
As can be seen in
FIG. 1
, on top of each permanent magnet layer
46
is a lead
50
. The lead
50
is made of a conductive material, such as, gold, silver or copper. The lead
50
is laid on both sides of the sensor
5
. The lead
50
has ends
52
which each contact the upper surface
20
of the free layer
18
and at or about the edges
22
of the free layer
18
. In this manner, the leads
50
can provide an electrical current to and across the junction
10
.
Flowing a current through the sensor allows changes in the magnetization of the adjacent magnetic media to be detected as changes in the electrical resistance of the sensor
5
. This is because the free layer
18
is free to change its magnetic orientation in response to passing magnetized bits on the recording media. In other words, the magnetized bits on the recording media cause a change in the relative magnetization between the pinned layer
14
and the free layer
18
. The change in magnetization causes the electrical resistance of the layer to change as well. Therefore, data can be read by measuring changes in the current passed through the sensor
5
as the recording media is passed by the sensor
5
.
An improved type of sensor is the current-perpendicular-to-the-plane or CPP sensor. In a CPP sensor, such as a multilayer giant magnetoresistive (GMR) device or a spin dependent tunneling (SDT) device, the quiescent state of the device has antiparallel magnetic alignment of the magnetoresistive element layers for maximum resistance. In a CPP sensor, the current flows perpendicular to the planes of the layers of the sensor and not parallel as is the case with a CIP sensor. The increase in magnetoresistance (MR) values associated with CPP devices make the CPP sensors more sensitive and therefore allow for the use of smaller data bits, which increases the overall data storage of the disk.
Although the layering of the junction of a CPP sensor is similar to a CIP sensor, the positioning of the leads is completely different. Instead of positioning leads on each side of the device, CPP devices use a top lead positioned above the free layer and a bottom lead positioned below the antiferromagnet layer. Current flowing between the leads passes in a perpendicular manner through the layers of the CPP sensor.
Unfortunately, because of the perpendicular current flow of CPP devices, and because hard bias materials are electrically conductive, CPP devices cannot have the hard bias contacting the sides of the layers of the film stack as is the convention with CIP devices. If the hard bias is laid over the sides of the stack, the hard bias will cause electrical shorting between layers of the film stack to occur. Such shorting will dramatically reduce the performance of the CPP device or render it completely useless.
Thus, a CPP device is sought which is hard biased in a manner which will not cause shorting. Likewise, to produce such a hard biased CPP device, a method of fabrication is sought. The device must prohibit shorting and yet provide sufficient bias to properly pin the magnetization at each end of the free layer, so as to prevent formation of closure domains at the ends of the free layer and hysteresis in the magnetoresistive response of the device. The method must provide the fabrication of such a device in a manner which minimizes the cost and time of manufacture.
SUMMARY
The apparatus of the present invention is embodied in a magnetic field sensor having a magnetoresistive element, a magnetic bias layer for biasing the magnetoresistive element with a magnetic field, and an electrical insulator positioned between the magnetic bias layer and the magnetoresistive element. The insulator prevents the flow of electrical current between the magnetoresistive element and the magnetic bias layer and at least a portion of the insulator allows passage of the magnetic field from the magnetic bias layer to the magnetoresistive element.
In at least one embodiment, the electrical insulator has a lower insulator and an upper insulator which are in direct contact with one another, such that the magnetic bias layer is isolated from the magnetoresistive element. The upper and lower insulator are made of either Al
2
O
3
, SiO
2
, Ta
2
O
5
or Si
3
N
4
. The lower insulator has a thickness between 50 Å and 300 Å and the upper insulator a thickness between 300 Å and 1000 Å. The lower insulator is positioned between the magnetoresistive element and the magnetic bias layer and overlays at least a portion of the magnetoresistive element.
The magnetic bias layer overlays the lower insulator and the upper insulator overlays the magnetic bias layer. The magnetoresistive element has a top surface. The magnetic bias layer can have a tapered end. At least a portion of the tapered end overhangs the top surface of the magnetoresistive element. The magnetic bias layer has an underlayer and a magnetic layer which is positioned over the underlayer. The underlayer has a thickness between 50 Å-100 Å and can be
Chen Yingjian
Crue Bill
Sin Kyusik
Zhu Ningja
Carr & Ferrell LLP
Ferrell John S.
Hayden Robert D.
Patidar Jay
Read-Rite Corporation
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