Cross talk bit error rate testing of a magnetic head

Details Cross talk bit error rate testing of a magnetic head Cross talk bit error rate testing of a magnetic head

2002-08-27

2004-08-17

Patidar, Jay (Department: 2862)

Electricity: measuring and testing

Magnetic

Magnetic information storage element testing

C360S031000, C369S053380

Reexamination Certificate

active

06777929

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to data storage devices, and more particularly, this invention relates to testing for cross talk instability in magnetic heads and associated hardware.
BACKGROUND OF THE INVENTION
In a disk drive the MR head is mounted on a slider which is connected to a suspension arm, the suspension arm urging the slider toward a magnetic storage disk. When the disk is rotated the slider flies above the surface of the disk on a cushion of air which is generated by the rotating disk. The MR head then plays back recorded magnetic signals (bits) which are arranged in circular tracks on the disk.
The MR sensor is a small stripe of conductive ferromagnetic material, such as Permalloy (NiFe), which changes resistance in response to a magnetic field such as magnetic flux incursions (bits) from a magnetic storage disk. The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which a component of the read element resistance varies as the square of the cosine of the angle between the magnetization in the read element and the direction of sense current flowing through the read element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded medium (the signal field) causes a change in the direction of magnetization in the read element, which in turn causes a change in resistance in the read element and a corresponding change in the sensed current or voltage. Conventional MR sensors based on the AMR effect thus provide an essentially analog signal output, where the resistance and hence signal output is directly related to the strength of the magnetic field being sensed.
FIG. 1
illustrates a cross-sectional view of an MR head
100
, in accordance with the prior art. As shown, the MR read head
100
includes an MR sensor which is sandwiched between a hard bias layer HB which is in turn sandwiched between first and second shield layers S
1
and S
2
, with insulating gap layers G
1
and G
2
separating the sensor and the shield layers. The hard bias layer HB typically includes an upper layer
102
and a seed layer
104
therebeneath. One exemplary material commonly employed for the upper layer
102
is CoPtCr with the seed layer being constructed with Cr.
Lead layers L
1
and L
2
are sandwiched between the hard bias layer HB and shield layer S
2
for providing a sense current to the MR sensor. Magnetic fields from a magnetic disk change the resistance of the sensor procircuital to the strength of the fields. The change in resistance changes the potential across the MR sensor which is processed by channel circuitry as a readback signal.
The MR read head
100
is typically mounted to a slider which, in turn, is attached to a suspension and actuator of a magnetic disk drive. The slider and edges of the MR sensor and other layers of the MR read head
100
form an air bearing surface (ABS). When a magnetic disk is rotated by the drive, the slider and one or more heads are supported against the disk by a cushion of air (an “air bearing”) between the disk and the ABS. The air bearing is generated by the rotating disk. The MR read head
100
then reads magnetic flux signals from the rotating disk.
FIG. 2
illustrates a simplified cross-sectional view of the MR head
100
showing the hard bias layer HB and the MR sensor thereof. It should be noted that such simplified illustration is not drawn to scale, and includes crude blocks to simplistically show the overlap between the MR sensor and the hard bias layer HB, and the associated fields.
As shown
FIG. 2
, the hard bias layer HB includes positive poles
204
and negative poles
206
. In use, the positive poles
204
and negative poles
206
of the hard bias layer HB produce first electromagnetic fields
208
in a first direction, and further produce a second electromagnetic field
210
in a second direction.
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, Co/Cu, or Co/Ru multilayers exhibiting strong antiferromagnetic coupling of the ferromagnetic layers. This GMR effect has also been observed for these types of multilayer structures, but wherein the ferromagnetic layers have a single crystalline structure and thus exhibit uniaxial magnetic anisotropy, as described in U.S. Pat. No. 5,134,533 and by K. Inomata, et al., J. Appl. Phys. 74 (6), Sep. 15, 1993. The physical origin of the GMR effect is that the application of an external magnetic field causes a reorientation of all of the magnetic moments of the ferromagnetic layers. This in turn causes a change in the spin-dependent scattering of conduction electrons and thus a change in the electrical resistance of the multilayered structure. The resistance of the structure thus changes as the relative alignment of the magnetizations of the ferromagnetic layers changes. MR sensors based on the GMR effect also provide an essentially analog signal output.
In high density disk drives bits are closely spaced linearly about each circular track. In order for the MR head to playback the closely spaced bits the MR head has to have high resolution. This is accomplished by close spacing between the first and second shield layers, caused by thin first and second gap layers, so that the MR sensor is magnetically shielded from upstream and downstream bits with respect to the bit being read.
An MR head is typically combined with an inductive write head to form a piggyback MR head or a merged MR head. In either head the write head includes first and second pole pieces which have a gap at a head surface and are magnetically connected at a back gap. The difference between a piggyback MR head and a merged MR head is that the merged MR head employs the second shield layer of the read head as the first pole piece of the write head. A conductive coil induces magnetic flux into the pole pieces, the flux flinging across the gap and recording signals on a rotating disk. The write signals written by the write head are large magnetic fields compared to the read signals shielded by the first and second shield layers. Thus, during the write operation a large magnetic field is applied to one or more of the shield layers causing a dramatic rotation of the magnetic moment of the shield layer.
Magnetic recording data storage technologies, particularly magnetic disk drive technologies, have undergone enormous increases in stored data per unit area of media (areal data density). This has occurred primarily by reducing the size of the magnetic bit through a reduction in the size of the read and write heads and a reduction in the head-disk spacing.
However, sometimes the disk drive exhibits irregularities, such as failed reads and/or writes. Often, a manufacturer will have produced hundreds or thousands of drives before the problem is known. The result is that an entire product line may have to be discarded.
For example, it has been found that some AMR and/or GMR heads exhibit instability such that data cannot be properly read from the disk. One cause of instability is thermal stress caused by transient pulses inside the reader induced by the AC write current inside the writer through inductive and capacitive coupling. This phenomenon is called cross talk instability.
During writing, a write current waveform is applied. As the current changes during writing, a magnetic and electric field is generated, not only at the write head, but also at traces within the suspension interconnect coupling the head to the drive preamp. The change in the magnetic field and line voltage gets coupled to the read circuit, causing crosstalk spikes in the read circuit. These spikes create noise in the read signal, leading to errors. Even worse is that the thermal power of these spikes will cause permanent damage of the reader sensor and result i

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