Dynamic magnetic information storage or retrieval – Head – Magnetoresistive reproducing head
Reexamination Certificate
2001-05-11
2004-01-20
Renner, Craig A. (Department: 2652)
Dynamic magnetic information storage or retrieval
Head
Magnetoresistive reproducing head
C360S319000
Reexamination Certificate
active
06680832
ABSTRACT:
TECHNICAL FIELD
This invention relates in general to magnetoresistive (MR) read sensors or heads for magnetic recording systems, and more particularly to such sensors that operate in the “current-perpendicular-to-the-plane” or CPP mode.
BACKGROUND OF THE INVENTION
In certain types of MR read sensors or heads for magnetic recording systems, the sense current passes perpendicularly through the planes of the layers making up the sensor. Such sensors are called “current-perpendicular-to-the-plane” or CPP sensors. CPP sensors are distinguished from “spin-valve” type MR sensors widely used in commercially available magnetic recording disk drives because spin-valve sensors operate with the sense “current-in-the-plane” of the sensor layers, or in CIP mode.
One type of CPP sensor is a magnetic tunnel junction (MTJ) sensor comprised of two ferromagnetic layers separated by a thin insulating tunnel barrier layer and based on the phenomenon of spin-polarized electron tunneling. The response of a MTJ sensor is determined by measuring the resistance of the MTJ when a sense current is passed perpendicularly through the MTJ from one ferromagnetic layer to the other. The probability of tunneling of charge carriers across the insulating tunnel barrier layer depends on the relative alignment of the magnetic moments (magnetization directions) of the two ferromagnetic layers. In addition to MTJ sensors, giant magnetoresistive (GMR) type of MR sensors have also been proposed to operate in the CPP mode, as described by Rottmayer and Zhu, “A new design for an ultra-high density magnetic recording head using a GMR sensor in the CPP mode”,
IEEE Transactions on Magnetics
, Vol 31, Issue 6, Part: 1, November 1995, pp. 2597-2599; and in U.S. Pat. No. 5,883,763.
One of the problems with CPP MTJ and GMR sensors is the ability to generate an output signal that is both stable and linear with the magnetic field strength from the recorded medium. If some means is not used to stabilize the sensing ferromagnetic layer in the CPP sensor, then magnetic instabilities and hysteresis (Barkhausen noise) will degrade the signal to noise performance of the sensor. The problem of sensor stabilization using a conventional tail stabilization approach is especially difficult in the case of a CPP sensor, like an MTJ MR read head, because the sense current passes perpendicularly through the ferromagnetic layers and the tunnel barrier layer, and thus any metallic materials used in the tails to stabilize the sensing ferromagnetic layer will short circuit the electrical resistance of the MTJ if they come in contact with the ferromagnetic layers.
IBM's U.S. Pat. No. 6,023,395 describes an MTJ MR read head that has a biasing ferromagnetic layer magnetostatically coupled with the sensing ferromagnetic layer of the MTJ to provide longitudinal bias to the sensing ferromagnetic layer. As shown in
FIG. 1
, this MTJ MR head is a sensor structure made up of a stack of layers formed between a bottom shield
10
and a top shield
12
, the shields being typically formed of relatively thick highly magnetically permeable material, such as permalloy (Ni
100−x
Fe
x
, where x is approximately 19). The shields
10
,
12
have generally planar surfaces spaced apart by a gap
53
. The gap material
50
,
52
on the sides of the sensor structure is an insulating material, typically an oxide such as alumina (Al
2
O
3
). The layers in the stack are a bottom electrical lead
20
, the MTJ sensor
30
, the longitudinal bias stack
40
, and top electrical lead
22
. The MTJ sensor
30
is made up of an antiferromagnetic layer
32
, a fixed ferromagnetic layer
34
exchange biased with the antiferromagnetic layer
32
so that its magnetic moment cannot rotate in the presence of an applied magnetic field, an insulating tunnel barrier layer
36
in contact with the fixed ferromagnetic layer
34
, and a sensing or “free” ferromagnetic layer
38
in contact with the tunnel barrier layer
36
and whose magnetic moment is free to rotate in the presence of an applied magnetic field. The longitudinal bias stack
40
includes a nonmagnetic electrically conductive spacer layer
42
, a biasing ferromagnetic layer
44
that has its magnetic moment aligned generally within the plane of the device and is separated from the ferromagnetic layer
38
by the spacer layer
42
, and optionally an antiferromagnetic layer
46
exchange coupled to the biasing ferromagnetic layer
44
. The self field or demagnetizing field from the biasing ferromagnetic layer
44
magnetostatically couples with the edges of the sensing ferromagnetic layer
38
to stabilize its magnetic moment, and, to linearize the output of the device. The electrically conductive spacer layer
42
prevents direct exchange coupling between the biasing ferromagnetic layer
44
and the sensing ferromagnetic layer
38
in the MTJ sensor
30
and allows sense current to flow perpendicularly through the layers in the stack between the two leads
20
,
22
.
The width of the data tracks of the recorded media is determined by the trackwidth (TW) of the MR sensor, as shown in FIG.
1
. The shielding geometry provided by shields
10
,
12
of the MR sensor attenuates the flux coming from adjacent magnetic transitions of the recorded media along the downtrack direction (perpendicular to the layers in the stack) and therefore enhances the sensor's linear resolution. However, it has been discovered as part of the development of the present invention that for very small trackwidths this shielding geometry does not provide adequate suppression of side reading caused by flux coming from adjacent tracks.
What is needed is a CPP sensor with in-stack longitudinal biasing that does not suffer from side reading of adjacent data tracks.
SUMMARY OF THE INVENTION
The invention is a CPP magnetoresistive sensor or read head with a magnetic shield geometry that covers the side walls of the sensor structure to prevent side reading caused by magnetic flux entering from adjacent data tracks. The shield geometry includes a top shield that has nearly vertical portions generally parallel to the side walls of the sensor structure, horizontal side portions over the portions of the bottom shield on either side of the sensor structure, and a top horizontal portion over the top trackwidth region of the sensor. The insulating gap material that separates the bottom and top shields is in contact with the horizontal portions of the bottom shield and the side walls of the sensor structure. Because of the nearly vertical portions of the top shield, the distance from the bottom shield to the sensing layer of the sensor structure is greater than the gap thickness outside the sensor structure so that magnetic flux is generally prevented from entering the sides of the sensing layer.
For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together with the accompanying figures
REFERENCES:
patent: 5883763 (1999-03-01), Yuan et al.
patent: 5898548 (1999-04-01), Dill et al.
patent: 6023395 (2000-02-01), Dill et al.
patent: 6430010 (2002-08-01), Murdock
patent: 6466419 (2002-10-01), Mao
patent: 2002/0030947 (2002-03-01), Chen et al.
R. Rottmayer, et al. “A New Design for an Ultra-High Density Magnetic Recording Head Using a GMR Sensor in the CPP Mode,” IEEE Transactions on Magnetics, vol. 31, No. 6, Nov. 1995, pp.2597-2599.
Fontana, Jr. Robert E.
Ho Kuok San
Katine Jordan A.
Lille Jeffrey S.
Tsang Ching H.
Berthold Thomas R.
Johnson Daniel E.
Renner Craig A.
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