Abstract:
A method and apparatus for providing amplitude instability data recovery for AMR/GMR heads during amplitude loss using MSE and/or amplitude envelope detection procedures. The method includes (a) initiating a data recovery procedure by selecting a detection mode for detecting when an error condition occurs, (b) performing the selected detection mode, (c) determining a reset action for resetting the magnetic head, (d) performing the reset action to reset the magnetic head and (f) re-reading data in the track after performing the reset action. An embodiment of the invention further includes (g) determining whether the resetting of the magnetic head allowed recovery of the data in the track and (h) terminating the data recovery procedure when the data is recovered. An embodiment of the invention further includes (i) determining whether predetermined limits have been exhausted when the data is not recovered, (j) terminating the data recovery procedure when the predetermined limits have been exhausted and repeating steps (a)-(g) when the predetermined limits have not been exhausted. In an embodiment of the invention, the determining a reset action for resetting the magnetic head further includes applying a maximum write current to the head, toggling a head bias current, toggling the head bias current and the write current, or applying a reset pulse to the head.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention. 
     This invention relates in general to data storage devices, and more particularly to method and apparatus for providing amplitude instability data recovery for AMR/GMR heads. 
     2. Description of Related Art. 
     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, wherein the resistance and hence signal output is directly related to the strength of the magnetic field being sensed. 
     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), Sept. 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, it has been found that some AMR and/or GMR heads exhibit severe amplitude instability such that data cannot be properly read from the disk. In this instance, an error is detected which in turn triggers some corrective action. An error detected while the data is being read form the disk is commonly referred to as a read error, a soft read error is an error that is possible to correct. Many times the correction of the read error is handled without interrupting the computer system which is beyond the rotating disk storage device. The soft read error would also be corrected before the user becomes aware of it. 
     A multistep procedure referred to as a data recovery procedure is (DRP) attempted to recover data when the storage device encounters a soft error. When the steps in the data recovery procedure are unable to correct a read error, then the read error is referred to as a hard error. Hard errors mean that data have been lost. Once data are read with a high error rate or lost from a particular portion of a disk, such as a sector, the area is reallocated to another spare portion on the disk drive. During the reallocation, some errors may be recovered. 
     For example, one problem encountered with MR sensors is Barkhausen noise caused by the irreversible motion of magnetic domains in the presence of an applied filed. It is know that Barkhausen noise is eliminated by creation of a single magnetic domain in the sense current region of the MR element. However, multiple magnetic domains may be formed during fabrication of the MR element. Further, as the dimensions of MR and GMR heads decrease, the MR and GMR heads are increasingly susceptible to low level electrical stress (ES) events that can cause the amplitude of the heads to become unstable and create a high number of soft or hard error events. 
     For example, the head amplitude can become suddenly as low as half of the normal value, and then become normal again, and after an unpredictable period become abnormal once again during write/read operations. 
     It can be seen that there is a need for a method and apparatus for providing amplitude instability data recovery for AMR/GMR heads. 
     SUMMARY OF THE INVENTION 
     To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a method and apparatus for providing amplitude instability data recovery for AMR/GMR heads. 
     The present invention solves the above-described problems by providing a data recovery procedure for head amplitude loss using Mean Square Error (MSE) and/or amplitude envelope detection procedures. 
     A method in accordance with the principles of the present invention includes (a) initiating a data recovery procedure by selecting a detection mode for detecting when an error condition occurs, (b) performing the selected detection mode, (c) determining a reset action for resetting the magnetic head, (d) performing the reset action to reset the magnetic head and (e) re-reading data in the track after performing the reset action. 
     Other embodiments of a system in accordance with the principles of the invention may include alternative or optional additional aspects. One such aspect of the present invention is that method further includes (f) determining whether the resetting of the magnetic head allowed recovery of the data in the track and (g) terminating the data recovery procedure when the data is recovered. 
     Another aspect of the present invention is that the method further includes (h) determining whether predetermined limits have been exhausted when the data is not recovered, (i) terminating the data recovery procedure when the predetermined limits have been exhausted and repeating steps (a)-(f) when the predetermined limits have not been exhausted. 
     Another aspect of the present invention is that the selecting a detection mode further comprises choosing a mean square error procedure, an amplitude envelopeprocedure or a combination procedure. 
     Another aspect of the present invention is that the mean square error procedure further comprises determining whether a mean square error range for data read in the track exceeds a predetermined mean square error limit. 
     Another aspect of the present invention is that the performing the selected detection mode further includes reinitiating the data recovery procedure when the mean square error range for data read in the track does not exceed the predetermined mean square error limit and returning to (c) when the mean square error range for data read in the track exceeds the predetermined mean square error limit. 
     Another aspect of the present invention is that the amplitude envelope procedure further includes determining whether an amplitude of a read signal is below a predetermined amplitude limit. 
     Another aspect of the present invention is that the performing the selected detection mode further includes reinitiating the data recovery procedure when the amplitude of the read signal is not below the predetermined amplitude limit and returning to (c) when the amplitude of the read signal is below the predetermined amplitude limit. 
     Another aspect of the present invention is that the combination procedure further includes determining whether a mean square error range for data read in the track exceeds a predetermined mean square error limit, returning to (c) when the mean square error range for data read in the track exceeds the predetermined mean square error limit, determining when an amplitude of a read signal is below a predetermined amplitude limit when the mean square error range for data read in the track does not exceed the predetermined mean square error limit, reinitiating the data recovery procedure when the amplitude of the read signal is not below the predetermined amplitude limit and returning to (c) when the amplitude of the read signal is below the predetermined amplitude limit. 
     Another aspect of the present invention is that the determining a reset action for resetting the magnetic head further includes applying a maximum write current to the head, toggling a head bias current, toggling the head bias current and the write current, or applying a reset pulse to the head. 
     Another aspect of the present invention is that the performing the reset action further includes applying a maximum write current to the head. 
     Another aspect of the present invention is that the performing the reset action further includes toggling the head bias current. 
     Another aspect of the present invention is that the performing the reset action further includes toggling the head bias current and the write current. 
     Another aspect of the present invention is that the performing the reset action further includes applying a reset pulse to the head. 
     These and various other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and form a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to accompanying descriptive matter, in which there are illustrated and described specific examples of an apparatus in accordance with the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings in which like reference numbers represent corresponding parts throughout: 
     FIG. 1 illustrates a disk drive embodying the present invention; 
     FIG. 2 is a side cross-sectional schematic illustration of the merged MR head; 
     FIG. 3 is a cross-sectional plan view of a spin valve sensor according to the invention; 
     FIG. 4 illustrates a flow chart for providing amplitude instability data recovery for AMR/GMR heads according to the present invention; 
     FIG. 5 is a block diagram that illustrates an exemplary hardware environment for implementing the data recovery procedure of FIG. 4; and 
     FIGS. 6 a-b  illustrate two excitation methods for toggling the MR bias current (Ib) and the write current (Iw) according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description of the exemplary embodiment, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration the specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized as structural changes may be made without departing from the scope of the present invention. 
     The present invention provides a method and apparatus for providing amplitude instability data recovery for AMR/GMR heads. The present invention performs a data recovery procedure for head amplitude loss using MSE and/or amplitude envelope detection procedures. 
     FIG. 1 illustrates a disk drive  100  embodying the present invention. As shown in FIG. 1, at least one rotatable magnetic disk  112  is supported on a spindle  114  and rotated by a disk drive motor  118 . The magnetic recording media on each disk is in the form of an annular pattern of concentric data tracks (not shown) on disk  112 . 
     At least one slider  113  is positioned on the disk  112 , each slider  113  supporting one or more magnetic read/write heads  121  where the head  121  incorporates the MR sensor of the present invention. As the disks rotate, slider  113  is moved radically in and out over disk surface  122  so that heads  121  may access different portions of the disk where desired data is recorded. Each slider  113  is attached to an actuator arm  119  by means of a suspension  115 . The suspension  115  provides a slight spring force which biases slider  113  against the disk surface  122 . Each actuator arm  119  is attached to an actuator means  127 . The actuator means as shown in FIG. 1 may be a voice coil motor (VCM). The VCM comprises a coil movable within a fixed magnetic field, the direction and speed of the coil movements being controlled by the motor current signals supplied by controller  129 . 
     During operation of the disk storage system, the rotation of disk  112  generates an air bearing between slider  113  and disk surface  122  which exerts an upward force or lift on the slider. The air bearing thus counter-balances the slight spring force of suspension  115  and supports slider  113  off and slightly above the disk surface by a small, substantially constant spacing during normal operation. 
     The various components of the disk storage system are controlled in operation by control signals generated by control unit  129 , such as access control signals and internal clock signals. Typically, control unit  129  comprises logic control circuits, storage means and a microprocessor. The control unit  129  generates control signals to control various system operations such as drive motor control signals on line  123  and head position and seek control signals on line  128 . The control signals on line  128  provide the desired current profiles to optimally move and position slider  113   5 o the desired data track on disk  112 . Read and write signals are communicated to and from read/write heads  121  by means of recording channel  125 . 
     The above description of a typical magnetic disk storage system, and the accompanying illustration of FIG. 1 are for representation purposes only. It should be apparent that disk storage systems may contain a large number of disks and actuators, and each actuator may support a number of sliders. 
     FIG. 2 is a side cross-sectional schematic illustration of the merged MR head  200 . The merged MR head  200  includes a read head portion and a write head portion which are lapped to an air beating surface (ABS), the air bearing surface being spaced from the surface of the rotating disk by the air bearing as discussed hereinabove. The read head portion includes an MR sensor which is sandwiched between first and second gaps layers G 1  and G 2  which, in turn, are sandwiched and second shield layers S 1  and S 2 . The write head portion includes a coil layer C and insulation layer I 2  which are sandwiched between insulation layers I 1  and I 3  which in turn are sandwiched between first and second pole pieces P 1  and P 2 . A gap layer G 3  is sandwiched between the first and second pole pieces at their pole tips adjacent the ABS for providing a magnetic gap. When signal current is conducted through the coil layer C, signal flux is induced into the first and second pole layers P 1  and P 2  causing signal fringe flux across the pole tips of the pole pieces at the ABS. This signal fringe flux is induced into circular tracks on the rotating disk  116 , shown in FIG. 1, during a write operation. During a read operation, recorded magnetic flux signals on the rotating disk are induced into the MR sensor of the read head causing a change in the resistance of the MR sensor which can be sensed by a change in potential across the MR sensor responsive to a sense current (not shown) conducted through the MR sensor. These changes in potential are processed by the drive electronics  236  shown in FIG.  2 . The combined head illustrated in FIG. 2 is a merged MR head in which the second shield layer S 2  is employed as a first pole piece P 1  for the combined head. In a piggyback head (not shown) the second shield layer S 2  and the first pole piece P 1  are separate layers. 
     FIG. 2 illustrates the overall physical arrangement of the layers used in forming a merged MR head  200 . However, FIG. 2 does not show the leads to the MR sensor. As mentioned above, the leads required significantly larger area than the area required of the MR sensor. Furthermore, the gap coverage at the edge of the leads is poor and potential for shield-to-lead shorts for high density (thin gap) heads increase significantly. However, since most of the shorts are from the leads to the shields, the leads should be designed to prevent shield-to-lead shorts. FIG. 4 illustrates, with respect to GMR heads, how the leads are attached to the sensors. 
     FIG. 3 depicts an example of a spin valve sensor  300  which the invention may be practiced. The view of FIG. 3 depicts a plan view of the air bearing surface of a substrate  301  containing the spin valve  300 . The substrate&#39;s air bearing surface normally rides upon a cushion of air, which separates it from a magnetic data storage medium such as a disk or tape. 
     The sensor  300  includes a plurality of substantially parallel layers including an antiferromagnetic layer  302 , a ferromagnetic pinned layer  303 , a conductive layer  304 , and a ferromagnetic free layer  305 . The sensor  300  also includes hard bias layers  315 - 316 , the operation of which is discussed in greater detail below. The sensor  300  is deposited upon an insulator  107 , which lies atop the substrate  301 . Adjacent layers preferably lie in direct atomic contact with each other. 
     The antiferromagnetic layer  302  comprises a type and thickness of antiferromagnetic substance suitable for use as a pinned layer in spin valves, e.g., a 400 Å layer of NiO. The ferromagnetic pinned layer  303  comprises a type and thickness of ferromagnetic substance suitable for use in spin valves, e.g., about 10-40 Å of Co. The conductor layer  304  comprises a type and thickness of conductive substance suitable for use in spin valves, e.g., about 20-30 Å of Cu. The ferromagnetic free layer  305  comprises a type and thickness of ferromagnetic substance suitable for use as a free layer in spin valves, e.g., about 30-150 Å of NiFe. The hard bias layers  315 - 316  provide the free layer  305  with a desired quiescent: magnetization. The hard bias layers  315 - 316  preferably comprise a magnetic material with high coercivity, such as CoPtCr. 
     Despite the foregoing detailed description of the sensor  300 , the present invention may be applied using many different sensor arrangements in addition to this example. For example, ordinarily skilled having the benefit of this disclosure will recognize various alternatives to the specific materials and thickness described above. 
     The sensor  300  exhibits a predefined magnetization. Magnetization of the sensor  300 , including the ferromagnetic layers  303 / 305  and the antiferromagnetic layer  302 , is performed in accordance with the invention. The sensor  300  may be magnetized prior to initial operation, such as during the fabrication or assembly processes. Or, the sensor  300  may be magnetized after some period of operating the sensor  300 , where the sensor  300  loses its magnetic orientation due to a traumatic high temperature event such as electrostatic discharge. A process for magnetization of the sensor  100  is discussed in greater detail below. 
     Whether magnetized before or after initial operation of the sensor  300 , the magnetized components of the sensor  300  are ultimately given the same magnetic configuration. In particular, the antiferromagnetic layer  302  has a magnetic orientation in a direction  310 . For ease of explanation, conventional directional shorthand is used herein, where a circled dot indicates a direction coming out of the page (like an arrow&#39;s head), and a circled x indicates a direction going into the page (like an arrow&#39;s tail). The neighboring ferromagnetic pinned layer  303  has a magnetic moment pinned in a parallel direction  311 , due to antiferromagnetic exchange coupling between the layers  302 - 303 . 
     Unlike the pinned layer  303 , the free layer  305  has a magnetic moment that freely responds to external magnetic fields, such as those from a magnetic storage medium. The free layer  305  responds to an external magnetic field by changing its magnetic moment, which in turn changes the resistance of the spin valve  300 . In the absence of any other magnetic fields, the free layer  305  orients itself in a direction  313 , which is oriented 90° to the directions  310 - 311 . This quiescent magnetization direction is due to biasing of the free layer  305  by the hard bias layers  315 - 316 . 
     The sensor  300  may also include various accessories to direct electrical current and magnetic fields through the sensor  300 . A small but constant sense current, for example, is directed through the sensor  300  to provide a source of scattering electrons for operation of the sensor  300  according to the GMR effect. At different times, a relatively large current pulse or waveform is directed through the sensor  300  to establish the magnetization direction of the sensor  300 . FIG. 3 also depicts the sensor  300  in relation to the various features that help direct current through the sensor  300 . 
     The sensor  300  is attached to a pair of complementary leads  308 - 309  to facilitate electrical connection to a sense current source  312 . The leads  308 - 309  also facilitate electrical connection to a pulse current source  323 . The leads  308 - 309  preferably comprise 500 Å of Ta with a 50 Å underlayer of Cr, or another suitable thickness and type of conductive material. The attachment of leads to magnetoresistive sensors and spin valves is a well known technique, familiar to those of ordinary skill in the art. 
     A technique for establishing a predetermined magnetic orientation of spin valve sensor or GMR head has been developed and is disclosed in copending, and commonly owned U.S. patent application Ser. No. 08/855,141, now U.S. Pat. No. 5,748,399, herein incorporated by reference. This technique will be explained with reference to FIG.  3 . 
     Via the leads  308 - 309 , the pulse current source  323  directs an electrical pulse current through the layers  303 - 305 . Chiefly, the pulse current heats the antiferromagnetic layer  302  past its blocking temperature. For an additional measure of magnetization biasing, the pulse current source  323  may be configured to provide pulse current in an appropriate direction to enhance biasing of the antiferromagnetic layer  302  in the direction  310 . The pulse current flows from the lead  309  to the lead  308 . To satisfy the foregoing purposes, the current source  323  comprises a suitable device to provide a current pulse of sufficient amplitude and duration to bring the antiferromagnetic layer  302  past its blocking temperature, thereby freeing the magnetic orientations of this layer as well as the associated ferromagnetic pinned layer  303 . 
     In addition to heat, the current pulse also provides a magnetic field that magnetically orients the antiferromagnetic layer  302  in accordance with the well known right-hand rule of electromagnetics. The pulse current lasts sufficiently long to both remove any magnetic orientation of the antiferromagnetic layer and also to reorient the layers in accordance with the magnetic field created by the flowing current. 
     The magnetic orientation of the antiferromagnetic layer  302  has the effect of pinning the magnetization directions of the ferromagnetic pinned layer  303 . This occurs because of the strong exchange coupling between the antiferromagnet-ferromagnet pair  302 / 303 . More particularly, the antiferromagnetic layer  302  pins the ferromagnetic pinned layer  303  in a direction parallel to its own direction. The pulse current source  323  then applies a bias current to orient the magnetic field of ferromagnet layer  305 . 
     FIG. 4 illustrates a flow chart  400  for providing amplitude instability data recovery for AMR/GMR heads according to the present invention. The method illustrated in the flow chart  400  of FIG. 4 may be performed by the recording channel  125  or control unit  129  of FIG.  1 . First, a specific detection mode is used to activate a head amplitude loss recovery/data recovery procedure  410 . A mean square error (MSE) mode may be implemented  412 . Alternatively, an amplitude envelope mode may be selected  416 . A third alternative involves a combined mode  414 , wherein MSE and an amplitude envelope mode may be used. 
     If the MSE only mode is selected  412 , a determination is made as to whether the MSE range exceeds a predetermined limit  420 . If the MSE range does not exceed a predetermined limit, the amplitude loss recovery method cycles to the beginning  422 . If the MSE range exceeds the predetermined limit  424 , then a determination of the type of head is made and a reset action is selected  450 . 
     If the amplitude envelope only mode is selected  416 , a determination is made as to whether the amplitude is below a predetermined amplitude limit  440 . If the amplitude is not below a predetermined amplitude limit  444 , the amplitude loss recovery method cycles to the beginning  422 . If the amplitude is below a predetermined amplitude limit  442 , then a determination of the type of head is made and a reset action is selected  450 . 
     Thirdly, if the combined mode is selected  414 , a determination is made as to whether the MSE range exceeds a predetermined limit  426 . If the MSE range exceeds the predetermined limit  428 , then a determination of the type of head is made and a reset action is selected  450 . If the MSE range does not exceed the predetermined limit  430 , the amplitude loss recovery method determines whether the amplitude is below a predetermined limit  432 . Then, if the amplitude is not below a predetermined amplitude limit  434 , the amplitude loss recovery method cycles to the beginning  422 . If the amplitude is below a predetermined amplitude limit  442 , then a determination of the type of head is made and a reset action is selected  450 . 
     The head/reset action selection  450  determines the action that is to be performed to determine what data recovery method is to be used. A first method is to apply a maximum write current to the head  460 . A second method involves toggling the MR bias current to the MR element to attempt to reset the head  462 . A third method involves the toggling of the MR bias current (Ib) and the write current (Iw) according to a predetermined sequence  464 , (see, for example, FIG.  6 ). A fourth method, used for GMR heads, involves applying a GMR reset pulse to reset the layers of a GMR head  466 . 
     After the reset action is selected and performed, the problem track is re-read with POR bias settings  470 . Then a determination is made as to whether the data recovery procedure was successful  480 . If the data recovery procedure was successful  482 , the data recovery method is terminated. If the data recovery procedure was not successful  484 , a determination is made as to whether predetermined limits for the data recovery method have been exhausted  490 . If the predetermined limits have been exhausted  496 , the data recovery method is terminated. If the predetermined limits have not been exhausted  492 , the data recovery method recycles to the beginning  410 . 
     FIG. 5 is a block diagram  500  that illustrates an exemplary hardware environment for implementing the data recovery procedure of FIG.  4 . The present invention is typically implemented in a control unit or in the data channel, and comprises a processor  510  including random access memory (RAM), read-only memory (ROM), and other components  512 . The processor  510  operates under the control of an operating system  524 . The processor  510  executes one or more computer programs  526  under the control of the operating system  524 . The present invention comprises a method for providing amplitude instability data recovery for AMR/GMR heads. 
     As described above with reference to FIG. 4, a head/reset action selection determines the action that is to be performed to determine what data recovery method is to be used. The MR bias current (Ib) and the write current (Iw) may be toggled according to a predetermined sequence. FIGS. 6 a-b  illustrate two excitation methods  600 ,  640  for toggling the MR bias current (Ib) and the write current (Iw) according to the present invention. The two excitation methods  600 ,  640  illustrated in FIGS. 6 a-b  may be used to attempt to establish two different states for the MR/GMR heads. For the same head, one of the excitation methods  600 ,  640  is to bring the head into a state with a relatively smaller MSE range, and the other brings the head into a state with a relatively larger MSE range. However, observations have indicated that the latter may actually make the average MSE smaller for some heads. 
     In FIG. 6 a , the write current  610  and the bias current  612  are toggled along the diagonal  620 . The toggling along diagonal  620  involves combining the minimal write current  622  with the minimal bias current  624  and combining the maximal write current  626  with the maximal bias current  628 . In FIG. 6 b , the write current  650  and the bias current  652  are toggled along the diagonal  660 . The toggling along diagonal  660  involves combining the minimal write current  662  with the maximal bias current  664  and combining the maximal write current  666  with the minimal bias current  668 . 
     The foregoing description of the exemplary embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not with this detailed description, but rather by the claims appended hereto.