System for resetting sensor magnetization in a spin valve magnetoresistive sensor

Sensors based on the giant magnetoresistance effect, specifically "spin valve" (SV) magnetoresistive sensors, have applications as external magnetic field sensors and as read heads in magnetic recording systems, such as rigid disk drives. These sensors have a ferromagnetic layer whose magnetization orientation is fixed or pinned by being exchange coupled to an antiferromagnetic layer. The magnetization of the pinned layer will become misaligned and the sensor will experience an abnormal response to the field being sensed, i.e., the external magnetic field or the recorded data in the magnetic media, if an adverse event elevates the antiferromagnetic layer above its blocking temperature. A pinned layer mangetization reset system is incorporated into systems that use SV sensors. The reset system generates an electrical current waveform that is directed through the SV sensor with an initial current value sufficient to heat the antiferromagnetic layer above its blocking temperature, and a subsequent lower current value to generate a magnetic field around the pinned layer sufficient to properly orient the magnetization of the pinned layer while the antiferromagnetic layer is cooling below its blocking temperature. This process resets the magnetization of the pinned layer to its preferred orientation and returns the SV sensor response substantially back to its desired state.

TECHNICAL FIELD 
This invention relates in general to "spin valve" (SV) magnetoresistive 
(MR) sensors that may be subjected to adverse temperature effects during 
operation, and more particularly to magnetic recording disk drives that 
use such sensors as read heads. 
BACKGROUND OF THE INVENTION 
An MR sensor detects magnetic field signals through the resistance changes 
of a magnetoresistive element, fabricated of a magnetic material, as a 
function of the strength and direction of magnetic flux being sensed by 
the element. The conventional MR sensor operates on the basis of the 
anisotropic magnetoresistive (AMR) effect in which a component of the 
element resistance varies as the square of the cosine of the angle between 
the magnetization in the element and the direction of sense or bias 
current flow through the element. 
MR sensors have application in magnetic recording systems because recorded 
data can be read from a magnetic medium when the external magnetic field 
from the recorded magnetic medium (the signal field) causes a change in 
the direction of magnetization in an MR read head. This in turn causes a 
change in electrical resistance in the MR read head and a corresponding 
change in the sensed current or voltage. 
A different and more pronounced magnetoresistance, called giant 
magnetoresistance (GMR), has been observed in a variety of magnetic 
multilayered structures, the essential feature being at least two 
ferromagnetic metal layers separated by a nonferromagnetic metal layer. 
This GMR effect has been found in a variety of systems, such as Fe/Cr or 
Co/Cu multilayers exhibiting strong antiferromagnetic coupling of the 
ferromagnetic layers, as well as in essentially uncoupled layered 
structures in which the magnetization orientation in one of the two 
ferromagnetic layers is fixed or pinned. The physical origin is the same 
in all types of GMR structures: the application of an external magnetic 
field causes a variation in the relative orientation of the magnetizations 
of neighboring ferromagnetic layers. This in turn causes a change in the 
spin-dependent scattering of conduction electrons and thus the electrical 
resistance of the structure. The resistance of the structure thus changes 
as the relative alignment of the magnetizations of the ferromagnetic 
layers changes. 
A particularly useful application of GMR is a sandwich structure comprising 
two essentially uncoupled ferromagnetic layers separated by a nonmagnetic 
metallic spacer layer in which the magnetization of one of the 
ferromagnetic layers is "pinned". The pinning may be achieved by 
depositing the ferromagnetic layer to be pinned onto an antiferromagnetic 
layer, such as an iron-manganese (Fe-Mn) layer, to create an interfacial 
exchange coupling between the two layers. The spin structure of the 
antiferromagnetic layer can be aligned along a desired direction (in the 
plane of the layer) by heating beyond the "blocking" temperature of the 
antiferromagnetic layer and cooling in the presence of a magnetic field. 
The blocking temperature is the temperature at which exchange anisotropy 
vanishes because the local anisotropy of the antiferromagnetic layer, 
which decreases with temperature, has become too small to anchor the 
antiferromagnetic spins to the crystallographic lattice. The unpinned or 
"free" ferromagnetic layer may also have the magnetization of its 
extensions (those portions of the free layer on either side of the central 
active sensing region) also fixed, but in a direction perpendicular to the 
magnetization of the pinned layer so that only the magnetization of the 
free-layer central active region is free to rotate in the presence of an 
external field. The magnetization in the free-layer extensions may be 
fixed by longitudinal hard biasing or exchange coupling to an 
antiferromagnetic layer. However, if exchange coupling is used the 
antiferromagnetic material is different from the antiferromagnetic 
material used to pin the pinned layer, and is typically nickel-manganese 
(Ni-Mn). This resulting structure is called a "spin valve" (SV) MR sensor. 
In a SV sensor only the free ferromagnetic layer is free to rotate in the 
presence of an external magnetic field. U.S. Pat. No. 5,159,513, assigned 
to IBM, discloses a SV sensor in which at least one of the ferromagnetic 
layers is of cobalt or a cobalt alloy, and in which the magnetizations of 
the two ferromagnetic layers are maintained substantially perpendicular to 
each other at zero externally applied magnetic field by exchange coupling 
of the pinned ferromagnetic layer to an antiferromagnetic layer. U.S. Pat. 
No. 5,206,590, also assigned to IBM, discloses a basic SV sensor wherein 
the free layer is a continuous film having a central active region and end 
regions. The end regions of the free layer are exchange biased by exchange 
coupling to one type of antiferromagnetic material, and the pinned layer 
is pinned by exchange coupling to a different type of antiferromagnetic 
material. 
SV sensors are a replacement for conventional MR sensors based on the AMR 
effect. They have special potential for use as external magnetic field 
sensors, such as in anti-lock braking systems, and as read heads in 
magnetic recording systems, such as in rigid disk drives. However, the SV 
sensor, which is typically fabricated by depositing an antiferromagnetic 
layer of Fe-Mn onto the ferromagnetic pinned layer of cobalt (Co) or 
permalloy (Ni-Fe), suffers from the problem that the range of blocking 
temperature for this interface is relatively low, i.e., it extends only 
from approximately 130 deg. C. to approximately 160 deg. C. These 
temperatures can be reached by certain thermal effects during operation of 
the disk drive, such as an increase in the ambient temperature inside the 
drive, heating of the SV sensor due to the bias current, and rapid heating 
of the SV sensor due to the head carrier contacting asperities on the 
disk. In addition, during assembly of the disk drive the SV sensor can be 
heated by current resulting from an electrostatic discharge. If any of 
these thermal effects cause the SV sensor to exceed the antiferromagnet's 
blocking temperature the magnetization of the pinned layer will no longer 
be pinned in the desired direction. This will lead to a change in the SV 
sensor's response to an externally applied magnetic field, and thus to 
errors in data read back from the disk. 
What is needed is a recovery system and process to reset the magnetization 
of the SV sensor's pinned layer to the desired orientation with minimal 
changes in the SV sensor's magnetoresistive response. 
SUMMARY OF THE INVENTION 
The invention is a system for re-setting or re-pinning the magnetization 
orientation of an SV sensor's pinned layer if an adverse event elevates 
the SV sensor's antiferromagnetic layer above its blocking temperature. 
The invention has application to field sensors and to magnetic recording 
systems, with special application to magnetic recording rigid disk drives. 
In the case of a rigid disk drive, if the magnetization of the SV read 
head's pinned layer becomes partially or totally unpinned, so that its 
magnetization direction becomes misaligned from its preferred direction, 
the SV read head will experience an abnormal response to magnetically 
recorded user data and/or servo head positioning data on the disk. This 
will be reflected in the data readback channel as user data errors and/or 
servo errors. The user data error detection/correction circuitry in the 
disk drive provides error signals to a digital processor, such as a 
microprocessor. The digital processor runs an algorithm that checks for 
the type and frequency of errors to determine that the errors are caused 
by the misalignment of the magnetization of the SV read head's pinned 
layer. The digital processor provides an output signal to turn on an 
electrical current waveform generator. The electrical current waveform is 
directed through the SV read head with an initial current value sufficient 
to heat the antiferromagnetic layer above its blocking temperature, and a 
subsequent lower current value sufficient to generate a correctly oriented 
magnetic field around the pinned layer while the antiferromagntic layer is 
cooling below its blocking temperature. This process resets the 
magnetization of the SV read head's pinned layer to its preferred 
orientation and returns the SV read head response substantially back to 
its desired state. Depending on the specific type of SV read head, the 
current waveform can be applied by the same current source that applies 
the normal sense or bias current to the SV read head. 
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.

DETAILED DESCRIPTION OF THE INVENTION 
Magnetic Recording Systems 
The invention will be described and illustrated in terms of its application 
to magnetic recording rigid disk drives. However, the invention is also 
applicable to and can be implemented into other types of magnetic 
recording systems, such as tape drives, tape cassettes and flexible 
diskette drives. These types of data storage systems may also use SV read 
heads that sense magnetically recorded data from movable magnetic media. 
The magnetic recording disk drive operable with an SV sensor as the read 
head and the SV sensor reset mechanism of the present invention is shown 
in FIG. 1. The disk drive comprises a base 10 to which are secured a disk 
drive motor 12 and an actuator 14, and a cover 11. The base 10 and cover 
11 provide a substantially sealed housing for the disk drive. Typically, 
there is a gasket 13 located between base 10 and cover 11 and a small 
breather port (not shown) for equalizing pressure between the interior of 
the disk drive and the outside environment. A magnetic recording disk 16 
is connected to drive motor 12 by means of hub 18 to which it is attached 
for rotation by the drive motor 12. A thin lubricant film 50 is maintained 
on the surface of disk 16. A read/write head or transducer 25 is formed on 
the trailing end of a carrier, such as an air-bearing slider 20. 
Transducer 25 is typically an inductive write element with a SV sensor 
read element (not shown in FIG. 1, but shown as item 30 in FIG. 3). The 
slider 20 is connected to the actuator 14 by means of a rigid arm 22 and a 
suspension 24. The suspension 24 provides a biasing force that urges the 
slider 20 onto the surface of the recording disk 16. During operation of 
the disk drive, the drive motor 12 rotates the disk 16 at a constant 
speed, and the actuator 14, which is typically a linear or rotary voice 
coil motor (VCM), moves the slider 20 generally radially across the 
surface of the disk 16 so that the read/write head may access different 
data tracks on disk 16. As is well known in the art the read element reads 
not only data but also servo positioning information pre-recorded on the 
disk, typically in servo sectors angularly spaced around the disk and 
located in the data tracks. The servo information is read and processed by 
a digital control system to control the amount of current sent to the VCM. 
In this manner the head is maintained on track during read and write 
operations and accurately moved across the tracks to read and write on all 
the tracks. 
FIG. 2 is a top view of the interior of the disk drive with the cover 11 
removed, and illustrates in better detail the suspension 24 that provides 
a force to the slider 20 to urge it toward the disk 16. The suspension may 
be a conventional type of suspension such as the well-known Watrous 
suspension, as described in IBM's U.S. Pat. No. 4,167,765. This type of 
suspension also provides a gimbaled attachment of the slider which allows 
the slider to pitch and roll as it rides on the air bearing. The data 
detected from disk 16 by the transducer 25 is processed into a data 
readback signal by signal amplification and processing circuitry in the 
integrated circuit arm electronics (AE) module 15 located on arm 22. The 
signals from transducer 25 travel via flex cable 17 to module 15, which 
sends its output signals via cable 19. 
The above description of the magnetic recording disk drive incorporating 
the present invention, and the accompanying FIGS. 1 and 2, are for 
representation purposes only. Disk drives may contain a large number of 
disks and actuators, and each actuator may support a number of sliders. 
The SV sensor 30 that forms a part of read/write transducer 25 is shown in 
FIG. 3. The films forming the completed sensor are supported on a suitable 
substrate 31. The SV sensor 30 forms part of transducer 25 in the disk 
drive system of FIGS. 1 and 2, and the substrate 31 may be the trailing 
end of the head carrier or slider 20. 
An underlayer or buffer layer 33 is deposited on substrate 31, followed by 
a first thin layer 35 of soft ferromagnetic material that serves as the 
"free" layer. A thin nonferromagnetic metallic spacer layer 37, a second 
thin layer 39 of ferromagnetic material that serves as the "pinned" layer, 
and a thin layer 41 of an exchange biasing material having relatively high 
electrical resistance and being in direct contact with the ferromagnetic 
layer 39, are successively deposited over free ferromagnetic layer 35. 
Other types of SV sensors described in the prior art, such as those in 
which the pinned layer is deposited on the substrate before the free 
layer, may have layers different from or arranged differently from layers 
33-41. Layers 33, 35, 37, 39, 41 are then etched away at their end regions 
to have a predetermined width generally corresponding to the width of the 
data track on the magnetic medium, such as disk 16. An additional 
ferromagnetic layer is then formed directly on the substrate 31 to form 
ferromagnetic end regions 42, 43 that abut the ends of the active sensing 
"free" ferromagnetic layer 35. Antiferromagnetic material is then 
deposited as layers 62, 63 over the end regions 42, 43, respectively, to 
provide exchange coupling with end regions 42, 43 to longitudinally bias 
the magnetizations of the end regions 42, 43. As previously mentioned, the 
longitudinal biasing of the magnetization of the free layer can also be 
accomplished by hard biasing. For example, layers 42, 43 and 62, 63 can be 
replaced with a chromium (Cr) underlayer and a cobalt-platinum-chromiun 
(CoPtCr) magnetic layer, respectively. Not shown in FIG. 3 are the capping 
layer for corrosion protection and the electrical leads that are patterned 
on layers 62, 63. 
In the absence of an externally applied magnetic field from the recorded 
magnetic disk 16, the magnetizations of the two layers 35, 39 of 
ferromagnetic material are oriented at an angle, preferably of about 90 
degrees, with respect to each other, as indicated by arrows 32 and 38, 
respectively. The ferromagnetic layer 35 is called the "free" 
ferromagnetic layer because its magnetization is free to rotate its 
direction in response to an externally applied magnetic field (such as 
magnetic field h from the magnetically recorded disk 16, as shown in FIG. 
3). This rotation of the free layer is shown by the dashed arrows on layer 
35. The ferromagnetic layer 39 is called the "pinned" ferromagnetic layer 
because its magnetization direction is fixed or pinned in a preferred 
orientation, as shown by arrow 38. Layer 41 provides a biasing field by 
exchange coupling and thus pins the magnetization of the ferromagnetic 
layer 39 in a preferred direction (arrow 38) so that it cannot rotate its 
direction in the presence of an applied external magnetic field having a 
strength in the range of the signal field h from disk 16. Similarly, the 
layers 62, 63 provide longitudinal biasing by exchange coupling to the end 
regions 42, 43 that abut the free ferromagnetic layer 35. This assures 
that the magnetization of the free ferromagnetic layer 35 is maintained 
generally perpendicular to the magnetization of pinned ferromagnetic layer 
39 in the absence of an externally applied magnetic field. 
FIG. 4 is a view of the structure of FIG. 3 as it would appear looking up 
from the surface of disk 16 and shows the SV sensor 30 with its free layer 
35 and pinned layer 39 abutted against ferromagnetic layers 42, 43 of 
Ni-Fe and antiferromagnetic layers 62, 63 of Ni-Mn. Lead metallurgy 92 for 
making electrical contact to the sensor is deposited on layers 62, 63, and 
a capping layer 67 of tantalum (Ta) is formed over antiferromagnetic layer 
41. 
The SV sensor 30 is electrically connected to AE module 15. In addition to 
driving the inductive write coil portion of the read/write transducer 25, 
the AE module 15 provides the SV sensor 30 with a sense or bias current 
I.sub.DC from current source 82. 
Referring again to FIG. 3, the magnetization direction 32 of free layer 35 
is perpendicular to the magnetization direction 38 of pinned layer 39 with 
zero applied external field h. This is because of equal and opposite 
contributions of the pinned layer average magnetostatic field H.sub.d and 
the sum of the coupling field H.sub.i and the field H.sub.ips. The field 
H.sub.ips is the field induced in the free layer by the current outside 
the free layer, i.e., approximately the current flowing through the pinned 
and spacer layers. This amount of current is a factor (f1) of 
approximately 1/2 to 2/3 of the total bias current I.sub.DC. This 
relationship can be expressed as follows: 
EQU H.sub.d =H.sub.i +H.sub.ips (1) 
where H.sub.ips is related to the bias current through the sensor I.sub.DC 
(FIG. 4). There is an optimum bias current I.sub.DC and bias current 
direction that satisfies the following equation: 
EQU H.sub.ips =f1*(2.pi.)*I.sub.DC /S (2) 
where s is the stripe height (shown in FIG. 3) of the sensor. A typical 
stripe height is in the range of 0.7-1.5 microns. 
Shown in FIG. 5A is the normal magnetoresistance transfer curve 110 for the 
SV sensor. FIG. 5B is normal resultant readback waveform 111 from the SV 
sensor due to two consecutive magnetic transitions on the disk. An 
external positive applied field, as shown by h in FIG. 3, aligns the 
magnetizations of the free layer 35 and pinned layer 39, resulting in a 
low resistance state (the right side of curve 110), while an external 
negative applied field results in the higher resistance state (the left 
side of curve 110). 
The pinned layer magnetization 38 is held perpendicular to the 
magnetization 32 of the free layer 35 at zero applied field. The 
ferromagnetic exchange provided by the antiferromagnetic Fe-Mn layer 41 
holds the pinned layer 39 against its own demagnetizing field H.sub.d and 
the effective field H.sub.ifs. The field H.sub.ifs is the field induced in 
the pinned layer by the current outside the pinned layer, i.e., the 
current flowing through the free and spacer layers. However, if the SV 
sensor gets heated above its blocking temperature, exchange pinning is 
temporarily lost and the magnetization of pinned layer 39 will align in 
the direction of the net field acting on it. This net field would be 
dominated by H.sub.ifs, the field associated with current outside the 
pinned layer, as long as it exceeds H.sub.d. This amount of current is a 
factor (f2) of approximately 1/2 to 2/3 of the total bias current 
I.sub.DC. This results in the pinned layer 39 having an orientation of its 
magnetization antiparallel (rotated 180 degrees) to its original desired 
orientation 38. As the SV sensor cools below its blocking temperature, the 
new undesired magnetization orientation is retained. This results in the 
transfer curve 112 shown in FIG. 5C. In this transfer curve, the SV sensor 
is now suboptimally biased, which means that the magnetization of free 
layer 35 is not perpendicular to the magnetization of pinned layer 39 at 
zero applied field. This is due to the bias current polarity now being 
wrong for the direction of the new demagnetizing field. Furthermore, this 
transfer curve 112 has the wrong sensitivity to the applied field, as 
shown by curve 113 in FIG. 5D. This state will result in a high error rate 
due to the asymmetry between the positive and negative pulses, as shown by 
the readback signal waveform 113 in FIG. 5D associated with the transfer 
curve 112. If the waveform 113 occurs, it will result in catastrophic 
servo system failure because in most types of disk drives the pre-recorded 
servo patterns are polarity sensitive, i.e., the servo control system 
requires detection of a successive pair of positive and negative magnetic 
transitions. If the disk drive included some means for switching the 
direction of the bias current, then the adverse response curve 112 of FIG. 
5C could be corrected by switching the bias current direction. This would 
recenter the curve 112 so that it would be essentially the reverse of 
curve 110 in FIG. 5A. In the absence of the ability to switch the bias 
current direction some means is required to reset the magnetization 
direction of the pinned layer to return the SV sensor to the desired 
response curve 110 of FIG. 5A. 
If the SV sensor is heated above its blocking temperature while the sensor 
is not powered on, or only parts of the SV sensor are heated above the 
blocking temperature, the result can be a state where the SV sensor 
exhibits a reduced amplitude transfer curve 114, such as shown in FIG. 5E. 
This is because the component of the average magnetization of the pinned 
layer along the initial set direction is still positive, although there is 
a substantial angle between the average magnetization of the pinned layer 
and the initial set direction. The SV sensor with this undesired transfer 
curve will result in poor signal to noise ratio because a significant 
amount of sensor sensitivity is lost, as shown by curve 115 in FIG. 5F. 
In the preferred embodiment of the present invention errors in the servo 
data and user data are used to detect that the signal amplitude of the SV 
sensor readback waveform has deviated from the desired response and signal 
represented by FIGS. 5A-5B. The detected errors trigger a head reset 
circuit that generates current pulses of a specific waveform to reset the 
SV sensor to a state as close to the original state as possible. 
Referring now to FIG. 6, a magnetic recording disk drive with a SV 
magnetoresistive read head or sensor 30 is coupled to AE module 15 and a 
combination data channel/servo module 134. The AE module 15 includes the 
conventional current source 82 that provides the bias current I.sub.DC to 
the SV sensor 30 and an amplifier 84. AE module 15 also includes a reset 
pulse generator 132 that forms a part of the present invention and whose 
operation will be explained below. In channel module 134 the recorded 
magnetic transitions that are sensed by the SV sensor 30 and amplified by 
amplifier 84 are converted to readback data for recovery of user data and 
servo data. The servo data provides the necessary head positioning 
information for the read/write transducer 25 (which includes SV read 
sensor 30) to remain on the designated track center. The output of AE 
module 15 is first put through automatic gain control (AGC) circuitry 120 
where the readback waveform for a particular head is amplified to 
approximately the same average amplitude for all the heads at all times. 
The output of AGC 120 is sent through a finite impulse response (FIR) 
filter 122 for weighted sampling. The output from FIR 122 is fed into a 
dibit detector 202 that detects dibits representing servo timing marks 
(STM) and track identification marks (TID) that are present in the 
readback signal. The same FIR 122 output is also fed into an A/D converter 
204 whose output is fed into a partial-response maximum-likelihood (PRML) 
data detector 126 for recovery of user data. The output of A/D 204 is also 
sent to servo data generator 124 for coding the servo burst amplitude that 
provides the actual head position information for the disk drive servo 
control system. The servo data generator 124 codes and clocks the dibits 
of the STM and TID as well as the servo burst amplitude and sends this to 
the hard disk controller (HDC) 200. The user data from PRML data detector 
126 is also fed into the HDC 200. 
The HDC 200 includes data error detection/correction circuitry 130 that 
checks the validity of the user data and its parity bits. In the event of 
errors, error correction is invoked. In the present invention the 
detection that a user data error has occurred is also used to signal 
microprocessor 206 with a data error signal 127. 
The HDC 200 also includes a servo position error signal (PES) generator 128 
that converts the output from servo data generater 124 into signals (PES) 
representative of the head position error. The PES is used to control the 
VCM actuator 14 to position the read/write transducer 25 (FIGS. 1 and 2). 
The HDC 200 also includes a STM/TID detector 131 to control the timing of 
the PES information. STM/TID detector 131 provides a servo data error 
output signal 129 to microprocessor 206. This signal 129 indicates the 
absence or invalidity of STM/TID information at the expected positions on 
the disk. 
In most disk drives the servo patterns are pre-recorded on the disk as 
dibits, i.e., where a positive-going transition is immediately followed by 
a negative-going transition. This is done to remove the low frequency 
content from the servo data. The electronic dibit detector 202 is polarity 
sensitive, although reversal of the detection polarity is allowed. This 
can be accomplished by inverting FIR 122 filter taps, as represented by 
polarity reversal signal 123. 
A failure in the SV sensor 30 that results in the sensor response and data 
signal represented by FIGS. 5C-5D would result in servo data errors and 
thus the generation of servo error signals 129. A failure in the SV sensor 
30 that results in the sensor response and data signal represented by 
FIGS. 5E-5F would result in user data errors, and thus the generation of 
user data error signals 127, but without any noticeable disk drive servo 
performance problems because there has been no polarity reversal. In the 
present invention the microprocessor 206 receives as input from HDC 200 
data error signals 127 and servo error signals 129 and outputs a polarity 
reversal signal 123 to FIR filter 122 and a reset head signal 208 to reset 
pulse generator 132. 
Referring now to the flowchart of FIG. 7, the procedure for resetting the 
SV sensor 30 will be explained. An increase in the data error rate is 
detected (150) by counting data error signals 127. If the count is above a 
predetermined threshold then microprocessor 206 checks at block (152) to 
determine if the increased data error rate is caused by servo errors. If a 
servo error signal 129 is also present (YES) then at block (154) the 
microprocessor 206 outputs a polarity reversal signal 123 to the analog 
FIR filter 122. A check is then made (158) to see if there has been a 
servo recovery, i.e., do servo error signals 129 continue to be received. 
If polarity reversal has been successful and there are no servo errors, 
then at block (162) the polarity is reset back to normal. Then at block 
(170) a reset head signal 208 is sent to reset pulse generator 132 to 
reset the SV sensor 30. If at block (158) polarity reversal does not 
result in servo error recovery so that servo error signals 129 continue to 
be received (YES), then the polarity is reset to normal at block (160). 
Then at block (166) the amplitude of the servo signal is checked and 
compared with its prior value. If the AGC gain is more than 20% (or some 
other preselected percentage) greater than its prior value (166), then at 
block (170) a reset head signal 208 is sent by microprocessor 206 to reset 
pulse generator 132. If at block (166) there is no change in servo signal 
amplitude, as determined by measuring the AGC increase, then it is assumed 
that any errors are not a result of the SV sensor losing its magnetization 
orientation. At this point standard error detection and recovery proceeds 
(168). 
If at block (152) there are no servo errors detected, even though an 
increase in data error rate has been detected (150), the increase in AGC 
gain is checked (156). If(YES) the head is reset (170). Then standard 
error detection and error recovery proceeds (168). Alternatively, 
referring again to the start of the flow chart, if the microprocessor 206 
receives no data error signals 127 but does receive servo error signals 
129, then block (152) is entered. 
The microprocessor 206 is the preferred device for providing digital 
processing of the data and/or servo error signals to activate the pulse 
generator 132. In the present invention the microprocessor 206 is the 
microprocessor that is already present in the disk drive to control the 
operation of the components making up the data channel/servo module 134. 
However, the function of microprocessor 206 may also be performed by a 
separate dedicated microprocessor or other digital processors that are 
present in the disk drive to control other functions. 
The SV sensor 30 is reset when reset pulse generator 132 receives a reset 
head signal 208 from microprocessor 206 and applies a current waveform to 
the SV sensor 30. Referring to FIG. 8, the reset pulse generator 132 
applies a waveform that takes the current across the head to the opposite 
polarity of the value of its operating bias current I.sub.DC provided by 
the current source 82. The first stage of the waveform is a short duration 
(t1) but high amplitude pulse with a value I1. This current pulse is 
designed to heat the SV sensor from its ambient temperature T.sub.a to 
above the blocking temperature T.sub.b of the Fe-Mn antiferromagnetic 
material. The initial estimates for the values of I1 and t1 are thus 
selected to satisfy the following: 
EQU (t1*I1.sup.2 *R.sub.SV)/C.sub.SV &gt;(T.sub.b =T.sub.a) (3) 
where R.sub.SV is the resistance and C.sub.SV is the "effective" total heat 
capacity, respectively, of the SV sensor. The effective total heat 
capacity is the total heat capacity of the sensor that is heated by the 
first stage current pulse I1 and includes the heat capacity of the SV 
sensor as well as part of the leads and surrounding material. 
The second stage of the waveform is a current value I2 that provides a 
sufficient magnetic field to hold the pinned layer 39 (FIG. 4) in the 
proper orientation for a sufficient period of time (t2) to allow the SV 
sensor to cool below its blocking temperature. The SV sensor is cooled by 
thermal conduction decay into the magnetic shields (not shown) located on 
the sides of the SV sensor. The duration of the second stage of the 
current pulse I2 should not be less than the time constant associated with 
this thermal decay, which is in the range of 200-1000 ns. The amplitude of 
the holding current I2 should be high enough to generate a magnetic field 
greater than the demagnetizing magnetostatic field H.sub.d of the pinned 
layer 39 in a shielded environment. This can be expressed as follows: 
EQU I2*(2.pi./s)*(f2)&gt;H.sub.d (4) 
However I2 should not be so high that the SV sensor does not get cooled 
below the blocking temperature due to the heat generated by I2. The 
current I2 is the primary source for the field needed to reset the pinned 
layer. However, the passage of current I2 through the SV sensor also 
causes other fields to be induced within the sensor. These include the 
magnetostatic coupling field from the free layer and the coupling field 
H.sub.i. The net of these additional fields also acts to apply a slight 
reset field to the pinned layer. The field due to I2 should also exceed 
any coercivity that the pinned layer may have. The values of I1, I2 and t1 
are experimentally optimized. 
In one specific example for a SV sensor with a free layer thickness of 80 
Angstroms, a stripe height s of 1 micron and a track width of 2 microns, 
R.sub.SV was 35 Ohms, the estimate for C.sub.SV was 3.times.10.sup.-12 
Joules/deg C., the value of I1 was 22 mA and t1 was 20 ns. The value of I2 
was 10 mA and t2 was 300 ns. The use of this reset current waveform 
resulted in recovering approximately 70-95% of the original SV sensor 
signal amplitude that would have been obtained by aligning the pinned 
layer 39 in an oven in the presence of a uniform 5-15 kOe external 
magnetic field. 
The reset pulse generator 132 is shown schematically in FIG. 9. In the 
event of an error-invoked reset head signal 208 from microprocessor 206, 
the SV sensor 30 is first switched out of its operating current source 82 
by switch S5, from position A to position B. Capacitors C1 and C2 are 
charged to the power supply 240 voltage V.sub.DC by putting all switches 
S1, S2, S3, S4 into their A positions. With the capacitors C1 and C2 
charged, all switches S1, S2, S3, S4 are then put into their B positions. 
This puts current through the SV sensor 30 in the opposite polarity as the 
operating bias current I.sub.DC, similar to the waveform shown in FIG. 8. 
Capacitors C1 and C2 are discharged through resistors R1 and R2, 
respectively. Proper selection of resistances and capacitances determines 
the waveform of FIG. 8. The first stage current pulse I1 is given 
approximately by V.sub.DC /(R1+R.sub.SV)+V.sub.DC /(R2+R.sub.SV), where 
R.sub.SV is the resistance of the SV sensor 30. The duration t1 of pulse 
I1 is determined approximately by R1*C1. The second stage current pulse I2 
is given approximately by V.sub.DC /(R2+R.sub.SV) and t2 is determined by 
how long the switches 232, 234, 236, 238 remain in position B before they 
are returned to position A. The value of R2*C2 must therefore be larger 
than t2 to ensure that the holding current I2 is in place while the SV 
sensor is cooled below its blocking temperature. The switches shown in 
FIG. 9 can be implemented by the use of field effect transistors (FETs) or 
bipolar transistors. 
The above explanation is made for the case of a single-ended AE module, 
i.e., where one end of the SV sensor is at ground, and the reset current 
flows in the opposite direction to the bias current. By electronically 
switching the terminals of the SV sensor the same current source that 
provides the bias current can also provide the reset current. 
There are other versions of SV sensors where the coupling field H.sub.i is 
so large or the pinned layer magnetostatic field H.sub.d is so small or of 
opposite sign, that the bias current direction is reversed (from that 
described above) for proper biasing, i.e., for the free layer 
magnetization to be substantially perpendicular to the pinned layer 
magnetization at zero applied field. In this case the field induced by the 
current outside the pinned layer, H.sub.ifs, aids the pinning field. If 
the SV sensor is heated above the blocking temperature and needs to be 
reset, the holding current I2 is in the same direction as the bias current 
I.sub.DC. In this case the reset function can also be implemented by 
merely modifying current source 82 to generate a waveform similar to that 
of FIG. 8 under microprocessor control. 
While the two stage pulse shown in FIG. 8 is preferred, a single pulse with 
a trailing edge decay time comparable to t2 can also be used to recover a 
significant fraction of the SV sensor signal amplitude. This single pulse 
would have an initial amplitude at least as great as I1 so that the SV 
sensor is heated above its blocking temperature and its amplitude during 
the decay would be sufficient to provide the necessary magnetic field to 
reset the mangetization orientation of the pinned layer. 
The above described SV sensor reset techniques (with the reset current 
waveform in either direction relative to the bias current) can also be 
used with SV sensors that use other types of antiferromagnetic materials 
to exchange couple with the pinned layer. 
While the invention has been described as incorporated in a disk drive with 
a PRML readback data channel and a servo control system that detects 
polarity sensitive PES signals, the invention is also applicable to disk 
drives that use other readback data detection, such as peak detection, and 
in disk drives that use other servo detection techniques, such as 
amplitude dependent PES signals. In the case of a disk drive with an 
amplitude dependent PES, the detection of servo errors caused by loss of 
magnetization orientation in the SV sensor comprises verification of the 
polarity of one or several reference marks on the servo or data tracks, 
for instance the first pulse of the AGC field. For low-amplitude caused 
errors in a peak detect channel, verification of the value of the AGC gain 
is done in a similar fashion as that for a PRML channel. 
While the invention has been described in the preferred embodiment where a 
reset head signal is generated in response to the detection of servo 
errors and/or data errors (as shown by the flow chart of FIG. 7) an 
alternate embodiment is a method where the head is reset every time an 
increase in the error rate is detected. This can be done as part of the 
standard error detection and recovery procedure, without verification of 
signal amplitude loss or polarity reversal. Alternatively, the head can be 
reset at periodic intervals, such as at disk drive power on. This could be 
done without any detection of data errors or other verification of signal 
amplitude loss or polarity reversal. 
The reset technique according to the present invention can also be used as 
part of the manufacturing process to reset heads prior to final disk drive 
assembly or test to overcome manufacturing line mild electrostatic 
discharge events. Also, a modified technique can be used in assembly or 
test, wherein the current from the reset pulse generator is used to heat 
the SV sensor above the blocking temperature while an external magnet is 
used to generate the required reset field. 
External Magnetic Field Sensors 
The system for resetting the magnetization of the pinned layer in a SV MR 
read head used in magnetic recording applications can also be applied to 
any SV sensor that has a pinned layer with its magnetization direction 
pinned by an antiferromagnetic layer, such as SV sensors used to sense 
external magnetic fields. IBM's copending application Ser. No. 08/334,659, 
filed Nov. 4, 1994, now U.S. Pat. No. 5,561,368, describes a bridge 
circuit field sensor based on multiple SV elements. These types of SV 
sensors also may experience thermal events that will result in the 
misorientation of the magnetization of the pinned layer. A block diagram 
of a SV field sensor system and the system according to the present 
invention for resetting the pinned layer in such a SV field sensor is 
shown in FIG. 10. During normal operation of the SV field sensor 304, the 
bias current is provided to sensor 304 by a power supply 302. Sensor 304 
converts an oscillating external magnetic field 306 to a voltage output 
308. The oscillating external magnetic field intercepted by the field 
sensor 304 is typically caused by a rotating object. For example, in 
automobile anti-lock braking systems a magnet may be located on the 
rotating disk of the disk brake or the magnet may be fixed and 
ferromagnetic material located on the rotating disk. In either case a 
repetitive oscillating field of essentially a fixed amplitude is presented 
to the sensor as the disk rotates. The rate of oscillation of the field is 
detected as the rate of rotation of the disk. This voltage output 308 is 
amplified by a preamplifier 310 to be detected by a pulse detector 312. 
The pulse detector 312 supplies a digital processor or controller 318 with 
the detected oscillation rate 316 and amplitude 314 of the external 
magnetic field being sensed. The controller 318 compares the detected 
amplitude with a threshold, which may be a previously recorded amplitude. 
Alternatively the controller compares the rate with the expected rate. The 
expected rate is a threshold pre-selected from the known state of the 
apparatus the sensor is located in. If the detected rate and/or amplitude 
is then lower than the predetermined thresholds the controller 318 sends a 
reset sensor signal 320 to reset pulse generater 300. The reset pulse 
generator 300 provides a reset current waveform similar to that shown in 
FIG. 8 to sensor 304. 
While the present invention has been particularly shown and described with 
reference to the preferred embodiments, it will be understood by those 
skilled in the art that various changes in form and detail may be made 
without departing from the spirit and scope of the invention. Accordingly, 
the disclosed invention is to be considered merely as illustrative and 
limited in scope only as specified in the appended claims.