BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates in general to spin valve magnetoresistive sensors for reading information signals from a magnetic medium and, in particular, to a spin valve sensor with multiple antiparallel coupling layers in the pinned layer for improved bias properties. 
     2. Description of Related Art 
     Computers often include auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces. 
     In high capacity disk drives, magnetoresistive (MR) read sensors, commonly referred to as MR sensors, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater track and linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an “MR element”) as a function of the strength and direction of the magnetic flux being sensed by the MR layer. 
     The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization in the MR element and the direction of sense current flowing through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage. 
     Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers. 
     GMR sensors using only two layers of ferromagnetic material (e.g., Ni—Fe) separated by a layer of non-magnetic material (e.g., copper) are generally referred to as spin valve (SV) sensors manifesting the SV effect. FIG. 1 shows a prior art SV sensor  100  comprising end regions  104  and  106  separated by a central region  102 . A first ferromagnetic layer, referred to as a pinned layer  120 , has its magnetization typically fixed (pinned) by exchange coupling with an antiferromagnetic (AFM) layer  125 . The magnetization of a second ferromagnetic layer, referred to as a free layer  110 , is not fixed and is free to rotate in response to the magnetic field from the recorded magnetic medium (the signal field). The free layer  110  is separated from the pinned layer  120  by a non-magnetic, electrically conducting spacer layer  115 . Leads  140  and  145  formed in the end regions  104  and  106 , respectively, provide electrical connections for sensing the resistance of SV sensor  100 . IBM&#39;s U.S. Pat. No. 5,206,590 granted to Dieny et al., incorporated herein by reference, discloses a SV sensor operating on the basis of the GMR effect. 
     Another type of SV sensor is an antiparallel (AP)-pinned SV sensor. In AP-pinned SV sensors, the pinned layer is a laminated structure of two ferromagnetic layers separated by a non-magnetic coupling layer such that the magnetizations of the two ferromagnetic layers are strongly coupled together antiferromagnetically in an antiparallel orientation. The AP-pinned SV sensor provides improved exchange coupling of the antiferromagnetic (AFM) layer to the laminated pinned layer structure than is achieved with the pinned layer structure of the SV sensor of FIG.  1 . This improved exchange coupling increases the stability of the AP-pinned SV sensor at high temperatures which allows the use of corrosion resistant and electrically insulating antiferromagnetic materials such as NiO for the AFM layer. 
     Referring to FIG. 2, an AP-pinned SV sensor  200  comprises a free layer  210  separated from a laminated AP-pinned layer structure  220  by a nonmagnetic, electrically-conducting spacer layer  215 . The magnetization of the laminated AP-pinned layer structure  220  is fixed by an AFM layer  230 . The laminated AP-pinned layer structure  220  comprises a first ferromagnetic layer  226  and a second ferromagnetic layer  222  separated by an antiparallel coupling (APC) layer  224  of nonmagnetic material (usually ruthenium (Ru)). The two ferromagnetic layers  226 ,  222  (FM 1  and FM 2 ) in the laminated AP-pinned layer structure  220  have their magnetization directions oriented antiparallel, as indicated by the arrows  227 ,  223  (arrows pointing out of and into the plane of the paper respectively). 
     The transfer curve (readback signal of the spin valve head versus applied signal from the magnetic disk) for a spin valve is linear and is defined by sin θ where θ is the angle between the directions of the magnetic moments of the free and pinned layers. FIG. 3 a  is an exemplary transfer curve for a spin valve sensor having a bias point (operating point)  300  at the midpoint of the transfer curve, at which point the positive and negative readback signals V 1  and V 2  (positive and negative changes in the GMR of the spin valve above and below the bias point) are equal (symmetrical) when sensing positive and negative fields having the same magnitude from the magnetic disk. FIGS. 3 b  and  3   c  illustrate transfer curves having bias points  302  and  304  shifted in the positive and negative directions, respectively, so that the readback signals V 1  and V 2  are asymmetrical with respect to the bias point. 
     The desirable symmetric bias transfer curve of FIG. 3 a  is obtained when the SV sensor is in its quiescent state (no magnetic signal from the disk) and the direction of the magnetic moment of the free layer is perpendicular to the magnetic moment of the pinned layer which is fixed substantially perpendicular to the disk surface. The bias point may be shifted from the midpoint of the transfer curve by various influences on the free layer which in the quiescent state can act to rotate its magnetic moment relative to the magnetic moment of the pinned layer. 
     The bias point is influenced by four major forces on the free layer, namely a ferromagnetic coupling field H FC  between the pinned layer and the free layer, a demagnetization field H demag  on the free layer from the pinned layer, a sense current field Hsc from all conductive layers of the spin valve except the free layer, and the AMR effect from the free layer which is also present in a spin valve sensor. The influence of the AMR on the bias point is the same as a magnetic influence thereon and can be defined in terms of magnitude and direction referred to herein as the AMR EFFECT. IBM&#39;s U.S. Pat. No. 5,828,529 to Gill, incorporated herein by reference, discloses an AP-pinned spin valve with bias point symmetry obtained by counterbalancing the combined influence of H FC , H demag  and H SC  by the influence of the AMR EFFECT on the free layer. 
     A problem with the prior art sensors arises as the size of spin valve sensors is decreased in order to address the need for higher storage density disk files. The AMR effect in the thinner free layer decreases and therefore the AMR EFFECT is no longer sufficient to counterbalance the influences of H FC , H demag  and H SC  resulting in a shift of the bias point toward a positive asymmetry. The asymmetric bias results in asymmetric readback signal response for positive and negative magnetic signals and to reduced signal output and dynamic range of the SV sensor. 
     Therefore there is a need for an SV sensor that provides a symmetric bias point on the transfer curve and improved signal output without sacrificing other desirable characteristics such as the strength of pinning of the pinned layer and in-file resettability of the antiferromagnetic layer. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to disclose a spin valve sensor which provides high amplitude with near zero signal asymmetry while maintaining bias polarity for pinning and in-file resettability of the antiferromagnet. 
     It is another object of the present invention to disclose a spin valve sensor having an effectively thicker ruthenium layer while maintaining stronger antiparallel coupling. 
     It is a further object of the present invention to disclose a spin valve sensor having an effectively thicker ruthenium layer to increase the signal amplitude. 
     It is yet another object of the present invention to disclose a spin valve sensor having an AP-pinned structure with multiple ruthenium antiparallel coupling (APC) layers. 
     In accordance with the principles of the present invention, there is disclosed a preferred embodiment of the present invention wherein a spin valve sensor has a plurality of APC layers (e.g., ruthenium) interleaved between ferromagnetic pinned layers, in order to effectively increase the ruthenium thickness while avoiding a decrease in the antiferromagnetic coupling between ferromagnetic layers which would normally accompany a substantial increase in the thickness of a single ruthenium layer. In the preferred embodiment, the spin valve sensor has a laminated AP-pinned layer comprising two APC layers, preferably made of ruthenium, separating three ferromagnetic pinned layers. With this AP-pinned layer structure, the forces on the free layer that influence the bias point on the sensor transfer curve are oriented so that the combined effects of the demagnetization field H demag  and the sense current field H SC  are counterbalanced by the combined effects of the AMR EFFECT and the ferromagnetic coupling field H FC  resulting in near zero asymmetry of the read signal. 
     When the SV sensor of the present invention is positioned asymmetrically between first and second shield layers, a net image field H image  due to images of the free layer current in the first and second shields is present at the free layer and has an influence on the bias point on the transfer curve. With the center of the free layer positioned a greater distance from the nearest surface of the first shield than the distance of the center of the free layer from the nearest surface of the second shield, H image  is in the same direction as H FC  and the AMR EFFECT. The combined influences of H image , H FC  and the AMR EFFECT counterbalance the combined influences of H demag  and H SC  resulting in near zero asymmetry of the read signal. 
     The above, as well as additional objects, features and advantages of the present invention will become apparent in the following detailed written description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a fuller understanding of the nature and advantages of the present invention, as well as the preferred mode of use, reference should be made to the following detailed description read in conjunction with the accompanying drawings. In the following drawings, like reference numerals designate like or similar parts throughout the drawings. 
     FIG. 1 is an air bearing surface view, not to scale, of a prior art SV sensor; 
     FIG. 2 is an air bearing surface view , not to scale, of a prior art AP-pinned SV sensor; 
     FIG. 3 a  is a transfer curve for a spin valve sensor having a bias point at the midpoint of the transfer curve so that positive and negative readback signals are symmetrical about a zero bias point; 
     FIG. 3 b  is a transfer curve for a spin valve sensor having a bias point shifted in the positive direction of the transfer curve so that positive and negative readback signals are asymmetrical about the bias point; 
     FIG. 3 c  is a transfer curve for a spin valve sensor having a bias point shifted in the negative direction of the transfer curve so that positive and negative readback signals are asymmetrical about the bias point; 
     FIG. 4 is a block diagram of a magnetic recording disk drive system; 
     FIG. 5 is a vertical cross-section view (not to scale) of a “piggyback” read/write magnetic head; 
     FIG. 6 is a vertical cross-section view (not to scale) of a “merged” read/write magnetic head; 
     FIG. 7 is an air bearing surface view (not to scale) of the improved spin valve sensor of the present invention; 
     FIG. 8 is a side cross-section view (not to scale) of the improved spin valve sensor of the present invention; and 
     FIG. 9 is a vertical cross-section view (not to scale) of a read head portion of a read/write magnetic head with the improved spin valve sensor of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following description is the best embodiment presently contemplated for carrying out the present invention. This description is made for the purpose of illustrating the general principles of the present invention and is not meant to limit the inventive concepts claimed herein. 
     Referring now to FIG. 4, there is shown a disk drive  400  embodying the present invention. As shown in FIG. 4, at least one rotatable magnetic disk  412  is supported on a spindle  414  and rotated by a disk drive motor  418 . The magnetic recording media on each disk is in the form of an annular pattern of concentric data tracks (not shown) on the disk  412 . 
     At least one slider  413  is positioned on the disk  412 , each slider  413  supporting one or more magnetic read/write heads  421  where the head  421  incorporates the SV sensor of the present invention. As the disks rotate, the slider  413  is moved radially in and out over the disk surface  422  so that the heads  421  may access different portions of the disk where desired data is recorded. Each slider  413  is attached to an actuator arm  419  by means of a suspension  415 . The suspension  415  provides a slight spring force which biases the slider  413  against the disk surface  422 . Each actuator arm  419  is attached to an actuator  427 . The actuator as shown in FIG. 4 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 a controller  429 . 
     During operation of the disk storage system, the rotation of the disk  412  generates an air bearing between the slider  413  (the surface of the slider  413  which includes the head  421  and faces the surface of the disk  412  is referred to as an air bearing surface (ABS)) and the disk surface  422  which exerts an upward force or lift on the slider. The air bearing thus counter-balances the slight spring force of the suspension  415  and supports the slider  413  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 the control unit  429 , such as access control signals and internal clock signals. Typically, the control unit  429  comprises logic control circuits, storage chips and a microprocessor. The control unit  429  generates control signals to control various system operations such as drive motor control signals on line  423  and head position and seek control signals on line  428 . The control signals on line  428  provide the desired current profiles to optimally move and position the slider  413  to the desired data track on the disk  412 . Read and write signals are communicated to and from the read/write heads  421  by means of the data recording channel  425 . 
     The above description of a typical magnetic disk storage system, and the accompanying illustration of FIG. 4 are for representation purposes only. It should be apparent that disk storage systems may contain a large number of disks and actuator arms, and each actuator arm may support a number of sliders. 
     FIG. 5 is a side cross-sectional elevation view of a “piggyback” magnetic read/write head  500 , which includes a write head portion  502  and a read head portion  504 , the read head portion employing a spin valve sensor  506  according to the present invention. The spin valve sensor  506  is sandwiched between nonmagnetic insulative first and second read gap layers  508  and  510 , and the read gap layers are sandwiched between ferromagnetic first and second shield layers  512  and  514 . In response to external magnetic fields, the resistance of the spin valve sensor  506  changes. A sense current I S  conducted through the sensor causes these resistance changes to be manifested as potential changes. These potential changes are then processed as readback signals by the processing circuitry of the data recording channel  425  shown in FIG.  4 . 
     The write head portion  502  of the magnetic read/write head  500  includes a coil layer  516  sandwiched between first and second insulation layers  518  and  520 . A third insulation layer  522  may be employed for planarizing the head to eliminate ripples in the second insulation layer  520  caused by the coil layer  516 . The first, second and third insulation layers are referred to in the art as an insulation stack. The coil layer  516  and the first, second and third insulation layers  518 ,  520  and  522  are sandwiched between first and second pole piece layers  524  and  526 . The first and second pole piece layers  524  and  526  are magnetically coupled at a back gap  528  and have first and second pole tips  530  and  532  which are separated by a write gap layer  534  at the ABS  540 . An insulation layer  536  is located between the second shield layer  514  and the first pole piece layer  524 . Since the second shield layer  514  and the first pole piece layer  524  are separate layers this read/write head is known as a “piggyback” head. 
     FIG. 6 is the same as FIG. 5 except the second shield layer  514  and the first pole piece layer  524  are a common layer. This type of read/write head is known as a “merged” head  600 . The insulation layer  536  of the piggyback head in FIG. 5 is omitted in the merged head  600  of FIG.  6 . 
     FIG. 7 shows an air bearing surface (ABS) view of an antiparallel (AP)-pinned spin valve (SV) sensor  700  according to the preferred embodiment of the present invention. The SV sensor  700  comprises end regions  712  and  714  separated from each other by a central region  716 . The substrate  725  can be any suitable substance, including glass, semiconductor material, or a ceramic material, such as alumina (Al 2 O 3 ). The seed layer  723  is a layer deposited to modify the crystallographic texture or grain size of the subsequent layers, and may not be needed depending on the material of the subsequent layer. If used the seed layer may be formed of tantalum (Ta), zirconium (Zr), nickel-iron (Ni—Fe), or Al 2 O 3 . An antiferromagnetic (AFM) layer  724  is deposited over seed layer  723  to the thickness at which the desired exchange properties are achieved, typically 200-500 Å. A laminated AP-pinned layer  720  is formed on the AFM layer  724  in the central region  716 . A free layer (free ferromagnetic layer)  718  is separated from the pinned layer  720  by a nonmagnetic, electrically conducting spacer layer  722 . The magnetization of the free layer  718  is preferably parallel to the ABS in the absence of an external field as indicated by the arrow  740 . A cap layer  742  formed on the free layer  718 , completes the central region  716  of the SV sensor  700 . In the present embodiment, the cap layer  742  is formed of tantalum (Ta). 
     As can be seen in the view of FIG. 7, the AP-pinned layer  720  comprises a first ferromagnetic pinned layer (FM 1 )  758 , a second ferromagnetic pinned layer (EM 2 )  754 , and a third ferromagnetic pinned layer (FM 3 )  750 . The FM 1  layer  758  and the M 2  layer  754  are separated by a first antiparallel coupling (APC 1 ) layer  756 . Similarly, the FM 2  layer  754  and the FM 3  layer  750  are separated by a second antiparallel coupling (APC 2 ) layer  752 . The APC 1  layer  756  and the APC 2  layer  752  are formed of a nonmagnetic material, preferably ruthenium (Ru), that allows the FM 1  layer  758 , FM 2  layer  754  and the FM 3  layer  750  to be strongly coupled together antiferromagnetically. 
     The SV sensor  700  further comprises layers  726  and  728  formed on the end regions  712  and  714 , respectively, for providing a longitudinal bias field to the free layer  740  to ensure a single magnetic domain state in the free layer. Lead layers  731  and  732  are also deposited on the end regions  712  and  714 , respectively, to provide electrical connections for the flow of a sensing current I s  from a current source  750  to the SV sensor  700 . A signal detector  760  which is electrically connected to leads  731  and  732  senses the change in resistance due to changes induced in the free layer  718  by the external magnetic field (e.g., field generated by a data bit stored on a disk). The external magnetic field acts to rotate the direction of magnetization of the free layer  718  relative to the direction of magnetization of the pinned layer  720  which is preferably pinned perpendicular to the ABS. The signal detector  760  preferably comprises a partial response maximum likelihood (PRML) recording channel for processing the signal detected by SV sensor  700 . Alternatively, a peak detect channel or a maximum likelihood channel (e.g., 1,7 ML) may be used. The design and implementation of the aforementioned channels are known to those skilled in the art. The signal detector  760  also includes other supporting circuitries such as a preamplifier (electrically placed between the sensor and the channel) for conditioning the sensed resistance changes as is known to those skilled in the art. 
     The SV sensor  700  is fabricated in a magnetron sputtering or an ion beam sputtering system to sequentially deposit the multilayer structure shown in FIG.  7 . The sputter deposition process is carried out in the presence of a transverse magnetic field of about 40 Oe. The AFM layer  724  formed of NiO, generally having a thickness in the range of 200-500 Å and preferably having a thickness of about 400 Å, is directly deposited on an Al 2 O 3  substrate layer  725  by sputtering a nickel target in the presence of a reactive gas that includes oxygen. 
     The FM 1  layer  758  is formed of Ni—Fe (permalloy) having a thickness in the range of 5-30 Å deposited on the AFM layer  724 . The APC 1  layer  756  is formed of Ru having a thickness of about 6 Å deposited on the EM 1  layer  758 . The FM 2  layer  754  is formed of NiFe having a thickness in the range of 5-30 Å deposited on the APC 1  layer  756 . The APC 2  layer  752  is formed of Ru having a thickness of about 6 Å deposited on the FM 2  layer  754 . The FM 3  layer  750  is formed of cobalt (Co) having a thickness in the range of 10-30 Å deposited on the APC 2  layer  752 . Alternatively, the FM 1  layer  758  and the FM 2  layer  754  may be formed of FeCo. The thicknesses of the FM 1 , FM 2  and FM 3  layers  758 ,  754  and  750  are selected to achieve a net magnetic thickness of the pinned layer  720  equivalent to about 10 Å of NiFe. 
     The nonmagnetic, conducting spacer layer  722  is formed of copper (Cu) having a thickness of about 20 Å deposited on the FM 3  layer  750 . The free layer  718  is formed of NiFe having a thickness in the range of 20-50 Å deposited on the spacer layer  722 . The cap layer  742  is formed of Ta having a thickness in the range of 20-50 Å deposited on the free layer  718 . 
     After the deposition of the central portion  716  is completed, the sensor is annealed in the presence of a magnetic field of about 800 Oe oriented in the transverse direction to the ABS and is then cooled while still in the magnetic field to set the exchange coupling of the AFM layer  724  with the laminated pinned layer  720  transverse to the ABS. The FM 1  layer  758  has a surface which interfaces with a surface of the AFM layer  724  so that the AFM layer pins the magnetic moment  748  (represented in FIG. 7 by the tail of an arrow pointing into the plane of the paper) of the FM 1  layer  758  in a direction perpendicular to and away from the ABS. The moment of the FM 1  layer  758  is pinned in this direction by exchange coupling with the AFM layer  724 . The APC 1  layer  756  is very thin (about 6 Å) which allows an antiferromagnetic exchange coupling between the FM 1  layer  758  and the FM 2  layer  754 . Accordingly, the magnetic moment  746  (represented by the head of an arrow pointing out of the plane of the paper) of the FM 2  layer  754  is directed in an opposite direction to the magnetic moment  748  of the FM 1  layer  758 , namely perpendicular to and towards the ABS. Similarly, the APC 2  layer  752  allows an antiferromagnetic exchange coupling between the FM 2  layer  754  and the FM 3  layer  750 . The magnetic moment  744  of the FM 3  layer  750  is directed in an opposite direction to the magnetic moment  746  of the FM 2  layer  754 , namely perpendicular to and away from the ABS. 
     FIG. 8 is a side cross-sectional view perpendicular to the ABS  540  of the SV sensor  700  of the present invention. The thicknesses of the FM 1 , FM 2  and FM 3  layers  758 ,  754  and  750  determine the net magnetic moment of the AP-pinned layer  720  and are chosen so that the net magnetic moment of the AP-pinned layer  720  is approximately equivalent to 10 Å of NiFe directed perpendicular to and away from the ABS  540 . The small magnitude of the net magnetic moment promotes strong antiferromagnetic coupling of the pinned layer  720  to the AFM layer  724  leading to good thermal stability of the SV sensor  700 . The direction of the net magnetic moment of the pinned layer  720  is important in achieving the desired symmetric bias point for operation of the SV sensor  700  of the present invention to be described in detail hereafter. The FM 1  layer  758  has a preferred thickness in the range of 15-30 Å, the FM 2  layer  754  has a preferred thickness in the range of 15-30 Å, and the FM 1  layer  750  has a preferred equivalent thickness of NiFe in the range of 15-30 Å in the present embodiment of the invention. 
     Various influences on the free layer  718  and consequently various influences on the bias point of the transfer curve for the SV sensor  700  are shown in FIG.  8 . The influences on the magnetic moment  740  of the free layer  718  are H demag    810 , H FC    816 , the AMR EFFECT  814 , and H SC    812 . H demag    810  is due to the net moment of the AP-pinned layer  720 , H FC    816  is due to a ferromagnetic coupling between the free layer  718  and the FM 3  layer  750 , the AMR EFFECT  814  is due to an AMR effect which is proportional to the thickness of the free layer  718 , and H SC    812  is the net sense current field on the free layer due to conduction of the sense current through the layers  758 ,  756 ,  754 ,  752 ,  750  and  722 . 
     An advantage of the SV sensor  700  is that the influences on the free layer  718  of the AMR EFFECT  814  and H FC    816  are in the same direction and opposite in direction to the influences of H demag    810  and H SC    812 . Accordingly, the influences on the free layer  718  of the AMR EFFECT  814  and H FC    816  act to counterbalance the influence of H demag    810  and H SC    812 . Referring to FIGS. 3 a ,  3   b  and  3   c , the AMR EFFECT  814  and H FC    816  act on the free layer  718  so as to cause the bias point on the transfer curve to be shifted in the negative direction (as indicated by bias point  304  in FIG. 3 c ) causing an asymmetry so that positive readback signals V 1  will be greater than negative readback signals V 2 . Conversely, H demag    810  and H SC    812  act on the free layer  718  so as to cause the bias point on the transfer curve to be shifted in the positive direction (as indicated by bias point  302  in FIG. 3 b ) causing an asymmetry so that negative readback signals V 2  will be greater than positive readback signals V 1 . Due to the counterbalancing of the influences of the AMR EFFECT  814  and H FC    816  by the influences of H derng    810  and H SC    812 , the resulting bias point on the transfer curve of the SV sensor  700  will be more nearly at the midpoint of the curve (as indicated by bias point  300  in FIG. 3 a ) resulting in a symmetric or nearly symmetric response to positive and negative readback signals. The net influence on the free layer of H FC , H demag  the AMR effect and H SC  is considered to be substantially zero when the asymmetry of the readback signal response is less than ±10% (asymmetry is defined by (V 1 −V 2 )/V 1 ×100% for V 1 &gt;V 2  or by (V 1 −V 2 )/V 2 ×100% for V 2 &gt;V 1 ). 
     It should be noted that having a second Ru layer (APC 2   752 ) and an additional ferromagnetic pinned layer (FM 3   750 ) in the laminated AP-pinned layer  720  of the SV sensor  700  allows the direction of the ferromagnetic coupling field H FC    816  to be directed opposite to the two other fields H SC    812  and H demag    810 . As a result, H FC    816  adds to the AMR EFFECT  814  to counterbalance the net effect of H SC    812  and H demag    810  to achieve near zero asymmetry. In the prior art AP-pinned SV sensor having a single Ru layer in the AP-pinned layer, H FC  is in the same direction as H SC  and H demag  resulting in bias point asymmetry. 
     Referring back to FIG. 5, in the foregoing description, the free layer of SV sensor  506  has been assumed to be symmetrically located between the first shield  512  and the second shield  514 . However, if the free layer of the SV sensor  506  is not equidistant from the first and second shields  512  and  514 , a net image field H image  from the first and second shields due to the free layer sense current acts on the magnetic moment of the free layer and may become a significant factor affecting the bias point of the transfer curve. 
     FIG. 9 shows a read head  900  having an SV sensor  910  located asymmetrically within the gap between the first shield  512  and the second shield  514 . The SV sensor  910  is positioned so that the center of the free layer  718  is a distance G 1  from the nearest surface of the first shield  512  and a distance G 2  from the nearest surface of the second shield  514 . When G 1 =G 2 , the image field from the first shield  512  due to the current current flowing in the free layer  718  is cancelled by the image field from the second shield  514  due to the same current flowing in the free layer  718 . However, when G 1  is significantly larger than G 2 , a net image field H image    920  directed perpendicular to and away from the ABS  540  is present at the free layer  718 . Accordingly, the influences on the free layer  718  of H image    920 , H FC    816  and the AMR EFFECT  814  are in the same direction and opposite in direction to the influences of H demag    810  and H SC    812 . The influences on the free layer  718  of H image    920 , H FC    816  and the AMR EFFECT  814  act to counterbalance the influence of H demag    810  and H SC    812 . The resulting bias point on the transfer curve of the SV sensor  910  will be nearly at the midpoint of the curve (as indicated by the bias point  300  in FIG. 3 a ) resulting in a nearly symmetric response to positive and negative readback signals. The influence of H image    920  in obtaining an exact or nearly exact counterbalance of the influences on the free layer  718  is particularly important when the AMR EFFECT  814  is small or negligible and G 1  is approximately twice G 2  so that the combined influences of Hinge  920  and H FC    816  are sufficient to counterbalance the combined influences of H demag    810  and H SC    812 . 
     Another advantage of an AP-pinned layer having multiple Ru layers (antiparallel coupling layers) is that the resultant SV valve structure has greater total Ru layer thickness. It has been experimentally observed that as the Ru layer thickness increases, for example from 6 Å to 10 Å, read head amplitude increases by about 40%. However, with the usual AP-pinned SV sensor having a single APC layer formed of Ru, a thicker Ru layer results in a decrease of the antiferromagnetic coupling between the ferromagnetic pinned layers resulting in weaker pinning of the pinned layer magnetization. With the multiple Ru layer structure of the AP-pinned SV sensor  700  of the present invention, the effective Ru thickness is increased while maintaining strong antiferromagnetic coupling by limiting the thickness of individual APC layers formed of Ru to about 6 Å. 
     A further advantage of the SV sensor  700  of the present invention is that the sense current through the free layer  718  will cause a sense current field which is imposed on and increases the magnetic strength of the pinning moment  748  of the FM 1  layer  758 . This will promote thermal stability of the sensor from the standpoint that high temperature incursions due to contact with asperities on the rotating disk or electrostatic discharge from an object will not disorient the direction of the magnetic moment  748  until a higher temperature is reached. However, should this higher temperature be reached, which is referred to as the blocking temperature of the antiferromagnetic layer  724 , there is provided a unipolar amplifier  770  for resetting the orientation of the antiferromagnetic layer  724  by conducting a resetting current I reset  through the SV sensor  700 . This current is of a higher magnitude than the sense current Is and typically would be three times Is for a very short period of time, such as 30 nanoseconds, to avoid overheating the antiferromagnetic layer  724 . It should be noted that the direction of the reset current I reset  is in the same direction as the sense current I s . In-file resettability of the AFM layer  724  is an advantage of SV sensor  700  made possible by having the magnetic moment  748  of the FM 1  layer  758  oriented in the same direction as the sense current field at the FM 1  layer  758 . 
     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, scope and teaching of the invention. Accordingly, the disclosed invention is to be considered merely as illustrative and limited only as specified in the appended claims.
An antiparallel (AP)-pinned spin valve (SV) sensor is provided which has positive and negative read signal symmetry about a zero bias point of a transfer curve upon sensing positive and negative magnetic incursions of equal magnitude from a moving magnetic medium. The SV sensor includes a ferromagnetic free layer which has a magnetic moment which is free to rotate in first and second directions from a position which corresponds to the zero bias point upon sensing positive and negative magnetic incursions, respectively, an AP-pinned layer, an antiferromagnetic layer which pins the magnetic moment of the AP-pinned layer along a pinned direction, and a spacer layer sandwiched between the AP-pinned layer and the free layer. The AP-pinned layer includes at least two antiparallel coupling (APC) layers made of ruthenium interleaved between ferromagnetic pinned layers in order to effectively increase the ruthenium thickness while avoiding a decrease in the antiferromagnetic coupling between the ferromagnetic pinned layers. With this AP-pinned layer structure, the forces on the free layer that influence the bias point on the sensor transfer curve are oriented so that the combined effects of a demagnetization field and a sense current field are counterbalanced by the combined effects of an anisotropic magnetoresistive effect and a ferromagnetic coupling field resulting in near zero asymmetry of the read signal.