Abstract:
A differential current-perpendicular-to-the-plane (CPP) giant magnetoresistive (GMR) sensor is provided having nonmagnetic high conductivity leads to achieve low lead resistance. The differential CPP GMR sensor comprises a first spin valve (SV) sensor, a second SV sensor and a metal gap layer disposed between the first and the second SV sensors. Because of the differential operation of the CPP GMR sensor of this invention, there is no need for shield layers to screen the sensor from stray magnetic fields. The shield layers are replaced with thick nonmagnetic lead layers having high conductivity to reduce the lead resistance of the sensor. Suitable materials for forming the leads include tungsten (W), gold (Au), rhodium (Rh), copper (Cu) and tantalum (Ta) because of their conductivity properties and because they are robust with respect to corrosion and smearing.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates in general to magnetic transducers for reading information signals from a magnetic medium and, in particular, to a differential current-perpendicular-to-the-plane giant magnetoresistance sensor with improved non-magnetic high conductivity leads. 
     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 nonmagnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and nonmagnetic layers and within the magnetic layers. 
     GMR sensors using only two layers of ferromagnetic material (e.g., Ni—Fe) separated by a layer of nonmagnetic material (e.g., copper) are generally referred to as spin valve (SV) sensors manifesting the SV effect. 
       FIG. 1  shows an 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 nonmagnetic, electrically conducting spacer layer  115 . Hard bias layers  130  and  135  formed in the end regions  104  and  106 , respectively, provide longitudinal bias for the free layer  110 . Leads  140  and  145  formed on hard bias layers  130  and  135 , respectively, provide electrical connections for sensing the resistance of SV sensor  100 . In the SV sensor  100 , because the sense current flow between the leads  140  and  145  is in the plane of the SV sensor layers, the sensor is known as a current-in-plane (CIP) SV sensor. IBM&#39;s U.S. Pat. No. 5,206,590 granted to Dieny et al. discloses a GMR sensor operating on the basis of the SV effect. 
     Another type of spin valve sensor is an antiparallel pinned (AP) spin valve sensor. The AP-pinned spin valve sensor differs from the simple spin valve sensor in that an AP-pinned structure has multiple thin film layers instead of a single pinned layer. The AP-pinned structure has an antiparallel coupling (APC) layer sandwiched between first and second ferromagnetic pinned layers. The first pinned layer has its magnetization oriented in a first direction by exchange coupling to the antiferromagnetic pinning layer. The second pinned layer is immediately adjacent to the free layer and is antiparallel exchange coupled with the first pinned layer because of the selected thickness (in the order of 8 (E) of the APC layer between the first and second pinned layers. Accordingly, the magnetization of the second pinned layer is oriented in a second direction that is antiparallel to the direction of the magnetization of the first pinned layer. 
     The AP-pinned structure is preferred over the single pinned layer because the magnetizations of the first and second pinned layers of the AP-pinned structure subtractively combine to provide a net magnetization that is less than the magnetization of the single pinned layer. The direction of the net magnetization is determined by the thicker of the first and second pinned layers. A reduced net magnetization equates to a reduced demagnetization field from the AP-pinned structure. Since the antiferromagnetic exchange coupling is inversely proportional to the net pinning magnetization, this increases exchange coupling between the first pinned layer and the antiferromagnetic pinning layer. An AP-pinned spin valve sensor is described in commonly assigned U.S. Pat. No. 5,465,185 to Heim and Parkin. 
     There is a continuing need to increase the MR coefficient and reduce the thickness of GMR sensors. An increase in the spin valve effect and reduced sensor geometry and reduced sensor geometry equates to higher bit density (bits/square inch of the rotating magnetic disk) read by the read head. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to disclose a differential current-perpendicular-to-the-plane (CPP) GMR sensor having nonmagnetic high conductivity leads to achieve low lead resistance. 
     It is another object of the present invention to disclose a differential CPP GMR sensor having an improved delta R/R due to reduced parasitic resistance of the leads. 
     In accordance with the principles of the present invention, there is disclosed a differential CPP GMR sensor comprising a first spin valve (SV) sensor, a second SV sensor and a metal gap layer disposed between the first and the second SV sensors. The differential CPP SV sensor is sandwiched between thick first and second lead layers formed of nonmagnetic high conductivity metals. In a first embodiment, the first SV sensor comprises an antiparallel (AP)-coupled first pinned layer adjacent to a first free layer and the second SV sensor comprises an AP-coupled second pinned layer adjacent to a second free layer. A metal gap layer is sandwiched between the first and second free layers. Because of the differential operation of the CPP GMR sensor of this invention, there is no need for shield layers to screen the sensor from stray magnetic fields. The shield layers are replaced with thick nonmagnetic lead layers having high conductivity to reduce the lead resistance of the sensor. Suitable materials for forming the leads include tungsten (W), gold (Au), rhodium (Rh), copper (Cu) and tantalum (Ta) because of their conductivity properties and because they are robust with respect to corrosion and smearing. 
     The half-bit length of magnetic data recorded on the magnetic media is arranged to be equal to the spacing between the first and second free layers of the differential CPP GMR sensor. With the half-bit length equal to the spacing between the free layers, the signals generated by the first and second spin valve sensors add due to the 180° phase difference of the first and second pinned layers. Because of the differential operation of this CPP sensor, stray magnetic fields do not generate any signal. Therefore, there is no need for ferromagnetic shields on either side of the differential CPP sensor of the present invention. 
     The above as well as additional objects, features, and advantages of the present invention will become apparent in the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a fuller understanding of the nature and advantages of the present invention, as well as of 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 a simplified diagram of a magnetic recording disk drive system using the SV sensor of the present invention; 
         FIG. 3  is a vertical cross-section view, not to scale, of a “piggyback” read/write magnetic head; 
         FIG. 4  is an air bearing surface view, not to scale, of an embodiment of a differential CPP GMR sensor of the present invention; 
         FIG. 5  is an air bearing surface view, not to scale, of a second embodiment of a differential CPP GMR sensor of the present invention; and 
         FIG. 6  is an air bearing surface view, not to scale, of a third embodiment of a differential CPP GMR 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. 2 , there is shown a disk drive  200  embodying the present invention. As shown in  FIG. 2 , at least one rotatable magnetic disk  212  is supported on a spindle  214  and rotated by a disk drive motor  218 . The magnetic recording media on each disk is in the form of an annular pattern of concentric data tracks (not shown) on the disk  212 . 
     At least one slider  213  is positioned on the disk  212 , each slider  213  supporting one or more magnetic read/write heads  221  where the head  221  incorporates the SV sensor of the present invention. As the disks rotate, the slider  213  is moved radially in and out over the disk surface  222  so that the heads  221  may access different portions of the disk where desired data is recorded. Each slider  213  is attached to an actuator arm  219  by means of a suspension  215 . The suspension  215  provides a slight spring force which biases the slider  213  against the disk surface  222 . Each actuator arm  219  is attached to an actuator  227 . The actuator as shown in  FIG. 2  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  229 . 
     During operation of the disk storage system, the rotation of the disk  212  generates an air bearing between the slider  213  (the surface of the slider  213  which includes the head  321  and faces the surface of the disk  212  is referred to as an air bearing surface (ABS)) and the disk surface  222  which exerts an upward force or lift on the slider. The air bearing thus counter-balances the slight spring force of the suspension  215  and supports the slider  213  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  229 , such as access control signals and internal clock signals. Typically, the control unit  229  comprises logic control circuits, storage chips and a microprocessor. The control unit  229  generates control signals to control various system operations such as drive motor control signals on line  223  and head position and seek control signals on line  228 . The control signals on line  228  provide the desired current profiles to optimally move and position the slider  213  to the desired data track on the disk  212 . Read and write signals are communicated to and from the read/write heads  221  by means of the recording channel  225 . Recording channel  225  may be a partial response maximum likelihood (PRML) channel or a peak detect channel. The design and implementation of both channels are well known in the art and to persons skilled in the art. In the preferred embodiment, recording channel  225  is a PRML channel. 
     The above description of a typical magnetic disk storage system, and the accompanying illustration of  FIG. 2  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. 3  is a side cross-sectional elevation view of a “piggyback” magnetic read/write head  300 , which includes a write head portion  302  and a read head portion  304 , the read head portion employing a differential CPP GMR sensor  306  according to the present invention. The sensor  306  is sandwiched between nonmagnetic conductive first and second lead layers  312  and  314 . First and second nonmagnetic insulative layers  308  and  310  separate the first and second lead layers in the region away from the sensor located at the ABS. In response to external magnetic fields, the resistance of the sensor  306  changes. A sense current Is 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  246  shown in FIG.  2 . 
     The write head portion  302  of the magnetic read/write head  300  includes a coil layer  316  sandwiched between first and second insulation layers  318  and  320 . A third insulation layer  322  may be employed for planarizing the head to eliminate ripples in the second insulation layer  320  caused by the coil layer  316 . The first, second and third insulation layers are referred to in the art as an insulation stack. The coil layer  316  and the first, second and third insulation layers  38 ,  320  and  322  are sandwiched between first and second pole piece layers  324  and  326 . The first and second pole piece layers  324  and  326  are magnetically coupled at a back gap  328  and have first and second pole tips  330  and  332  which are separated by a write gap layer  334  at the ABS  340 . An insulation layer  336  is located between the second shield layer  314  and the first pole piece layer  324 . Since the second shield layer  314  and the first pole piece layer  324  are separate layers this read/write head is known as a “piggyback” head. 
     FIRST EXAMPLE 
       FIG. 4  depicts an air bearing surface (ABS) view, not to scale, of a differential CPP GMR sensor  400  according to a first embodiment of the present invention. The sensor  400  comprises end regions  402  and  404  separated from each other by a central region  406 . The active region of the CPP sensor comprises a first SV sensor  410  and a second SV sensor  412  formed in the central region  406 . The first and second SV sensors are separated by a metal gap layer  414 . The first SV sensor  410  is formed on a seed layer  416  deposited on a first lead layer L 1   418  in the central region  406 . The seed layer  416  a nonmagnetic metal layer deposited to modify the crystallographic texture or grain size of subsequent layers. The first lead layer  418  is a layer of nonmagnetic highly conductive metal such as tungsten (W), or alternatively gold (Au), rhodium (Rh), copper (Cu) or tantalum (Ta) deposited on a substrate  408  and extending over the central region  406  and end regions  402  and  404 . Alternatively, the first lead layer  418  may comprise a multilayer of two or more layers, each layer being formed from any of the above listed conductive metals. For example the first lead layer may comprise a bilayer formed of a Ta layer and a Au layer or a bilayer formed of a Ta layer and a Rh layer. The substrate  408  can be any suitable substance including glass, semiconductor material, or a ceramic substance such as alumina (Al 2 O 3 ). 
     The first SV sensor  410  comprises a first pinned layer  422  over the seed layer  416  and a ferromagnetic first free layer  424  deposited over the first pinned layer. The first pinned layer  422  is an AP-coupled layer comprising a first ferromagnetic (FM1) layer  426  adjacent to the seed layer  416 , a second ferromagnetic (FM2) layer  428  and an antiparallel coupling (APC) layer  427  sandwiched between the FM1 and FM2 layers  426  and  428 . The APC layer  427  is formed of a nonmagnetic material, preferably ruthenium (Ru), that allows the FM1 and FM2 layers  426  and  428  to be strongly coupled together antiferromagnetically. 
     The second SV sensor  412  comprises a ferromagnetic second free layer  430  deposited over the metal gap layer  414  and a second pinned layer  424  deposited over the second free layer. The second pinned layer  424  is an AP-coupled layer comprising a third ferromagnetic (FM3) layer  432  adjacent to the second free layer  430 , a fourth ferromagnetic (FM4) layer  434  and an antiparallel coupling (APC) layer  433  sandwiched between the FM3 and FM4 layers  432  and  434 . The APC layer  433  is formed of a nonmagnetic material, preferably ruthenium (Ru), that allows the FM3 and FM4 layers  432  and  434  to be strongly coupled together antiferromagnetically. A cap layer  436  is deposited over the second pinned layer  424 . 
     Insulator layers  440  and  442  of electrically insulating material such as aluminum oxide are formed in the end regions  402  and  404 , respectively, on the first lead layer  418  and in abutting contact with the CPP sensor layers in the central region  406 . A second lead layer L 2   420  of nonmagnetic highly conductive metal such as tungsten (W), or alternatively gold (Au), rhodium (Rh), copper (Cu) or tantalum (Ta), is deposited over the cap layer  436  in the central region  406  and over the insulator layers  440  and  442  in the end regions  402  and  404 . Alternatively, the second lead layer  420  may comprise a multilayer of two or more layers, each layer being formed from any of the above listed conductive metals. For example, the first lead layer may comprise a bilayer formed of a Ta layer and a Au layer or a bilayer formed of a Ta layer and a Rh layer. 
     If longitudinal stabilization of the magnetic domain states of the first and second free layers  424  and  430  is desired, hard bias layers may be provided in the end regions  402  and  404  as is known in the art. IBM&#39;s U.S. Pat. No. 5,720,410 granted to Fontana et al. describes such a longitudinal biasing method. 
     The first and second lead layers  418  and  420  provide electrical connections for the flow of a sensing current I s  from a current source  450  to the CPP sensor  400 . A signal detector  460  which is electrically connected to the first and second lead layers  418  and  420  senses the change in resistance due to changes induced in the first and second free layers  424  and  430 , respectively, 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 first and second free layers relative to the direction of magnetization of the first and second pinned layers  422  and  424 , respectively, which are preferably pinned perpendicular to the ABS. The signal detector  460  preferably comprises a partial response maximum likelihood (PRML) recording channel for processing the signal detected by the MTJ sensor  400 . 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  460  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 sensor  400  may be fabricated in a magnetron sputtering or an ion beam sputtering system to sequentially deposit the multilayer structure shown in FIG.  4 . The first lead layer  418  of tungsten (W), or alternatively gold (Au), rhodium (Rh), copper (Cu), tantalum (Ta) or combinations of these materials, having a thickness in the range of 500-2000 Å is deposited on the substrate  408 . After deposition of the first lead layer a chemical/mechanical polish (CMP) is carried out to provide a smooth surface for deposition of the layer structure of the CPP SV sensor. For the best CMP results, the use of tungsten to form the first lead layer is preferred. The seed layer  416 , the first SV sensor  410 , the metal gap layer  414  and the second SV sensor  412  are sequentially deposited over the first lead layer  418  in the presence of a longitudinal or transverse magnetic field of about 40 Oe to orient the easy axis of all the ferromagnetic layers. The seed layer  416  formed of a nonmagnetic metal, preferably tantalum (Ta), having a thickness of about 30 Å is deposited on the first lead layer  418 . The FM1 layer  426  formed of Ni—Fe having a thickness in the range of 20-50 Å is deposited on the seed layer  416 . The APC layer  427  preferably formed of ruthenium (Ru) having a thickness of about 6 Å is deposited on the FM1 layer  426 . The FM2 layer  428  formed of Ni—Fe having a thickness in the range of 20-50 Å is deposited on the APC layer  427 . The thickness of the FM1 layer  426  is chosen to be greater than the thickness of the FM2 layer  428  so that magnetization  443  (shown as the head of an arrow pointing out of the plane of the paper) of the FM1 layer  426  is greater than the magnetization  444  (shown as the tail of an arrow pointing into the plane of the paper) of the FM2 layer  428 . As a result, the direction of the net magnetization of the AP-coupled first pinned layer  422  has the same direction as the magnetization  443  of the FM1 layer  426 . The first free layer  424  formed of Ni—Fe having a thickness of 20-40 Å is deposited on the FM2 layer  428 . Alternatively, the free layer  428  may be formed of a laminated multilayer comprising a ferromagnetic interface layer formed of cobalt (Co) having a thickness of about 5 Å deposited on the FM1 layer  426  and a ferromagnetic layer formed of Ni—Fe having a thickness of 20-30 Å deposited on the interface layer. 
     The metal gap layer  414  formed of a nonmagnetic metal is deposited over the first free layer  424 . The metal gap layer provides a read gap separating the free layers of the first and second SV sensors  410  and  412  of the differential CPP sensor  400 . With the differential sensor the recorded magnetic half-bit length is arranged to equal the spacing between the first and second free layers  424  and  430 . The magnetization directions  425  and  431  of first and second free layers  424  and  430 , respectively, are arranged to have the same direction, either to the right as shown in  FIG. 4  or, alternatively, to the left. In future high density technology applications the metal gap layer will have a thickness less than 500 Å. 
     The second free layer  430  formed of Ni—Fe having a thickness of about 20-40 Å is deposited on the metal gap layer  414 . Alternatively, the free layer  430  may be formed of a laminated multilayer comprising a ferromagnetic layer formed of Ni—Fe having a thickness of 20-30 Å deposited on the metal gap layer  414  and a ferromagnetic interface layer formed of cobalt (Co) having a thickness of about 5 Å deposited on the ferromagnetic layer of Ni—Fe. The FM3 layer  432  formed of Ni—Fe having a thickness in the range of 20-50 Å is deposited on the second free layer  432 . The APC layer  433  preferably formed of ruthenium (Ru) having a thickness of about 6 Å is deposited on the FM3 layer  432 . The FM4 layer  434  formed of Ni—Fe having a thickness in the range of 20-50 Å is deposited on the APC layer  433 . The thickness of the FM3 layer  432  is chosen to be greater than the thickness of the FM4 layer  434  so that magnetization  445  (shown as the head of an arrow pointing out of the plane of the paper) of the FM3 layer  432  is greater than the magnetization  446  (shown as the tail of an arrow pointing into the plane of the paper) of the FM4 layer  434 . As a result, the direction of the net magnetization of the AP-coupled first pinned layer  422  has the same direction as the magnetization  445  of the FM3 layer  432 . A cap layer  436  of tungsten having a thickness of about 30 Å, formed on the FM4 layer  434  completes the central region  406  of the CPP sensor  400 . The second lead layer  420  of tungsten (W), or alternatively gold (Au), rhodium (Rh), copper (Cu), tantalum (Ta) or combinations of these materials, having a thickness in the range of 500-2000 Å is deposited over the cap layer  436  in the central region  406  and over the insulation layers  440  and  442  in the end regions  402  and  404 . 
     An advantage of the differential CPP GMR sensor  400  of the present invention is that because of the differential operation of the sensor ferromagnetic shields are not required to prevent stray magnetic fields from causing spurious signals. Elimination of the need for shields allows the use of thick high conductivity leads, L 1  and L 2 , to achieve low lead resistance. The low lead resistance provides higher delta R/R for the sensor because parasitic resistance (resistance not contributing to delta R) is lowered. 
     Another advantage of the differential CPP sensor  400  of the present invention is that the first and second pinned layers  422  and  424  of the first and second SV sensors  410  and  412 , respectively, are arranged to be 180° out of phase to provide signal addition for perpendicular or longitudinal transitions where the half-bit length is set equal to the thickness of the metal gap layer  414  (read gap). In order to accomplish this phase relationship of the pinned layers, the thicknesses of ferromagnetic layers FM1, FM2, FM3 and FM4 are selected so that FM2 and FM3 become 180° out of phase during a reset process. This phase relationship may be achieved by choosing the magnetic thickness of FM1 to be greater than the thickness of FM2 and the magnetic thickness of FM3 to be greater than the thickness of FM4. Alternatively, the thickness of FM2 may be chosen to be greater than the thickness of FM1 and the thickness of FM4 may be chosen to be thicker than the thickness of FM3. The magnetic anisotropy differences between FM1, FM2, FM3 and FM4 may also be used to achieve the desired magnetic orientation of these layers. 
     SECOND EXAMPLE 
       FIG. 5  shows an air bearing surface (ABS) view, not to scale, of a differential CPP sensor  500  according to another embodiment of the present invention. The CPP SV sensor  500  differs from the CPP SV sensor  400  shown in  FIG. 4  in having first and second SV sensors  510  and  512  comprising simple pinned layers  518  and  520  with first and second antiferromagnetic (AFM) pinning layers  514  and  516 , respectively, instead of the self-pinned AP-coupled layers  422  and  424  of the SV sensor  400 . The first AFM layer  514  of Pt—Mn or Ir—Mn having a thickness in the range of 50-200 Å is deposited over the seed layer  416 . The first pinned layer  518  of Co—Fe having a thickness in the range of 20-40 Å is deposited over the first AFM layer. The first free layer  424 , metal gap layer  414  and second free layer  430  are sequentially deposited over the first pinned layer  518 . The second pinned layer  520  of Co—Fe having a thickness in the range of 20-40 Å is deposited over the second free layer and the second AFM layer  516  of Pt—Mn or Ir—Mn having a thickness in the range of 50-200 Å is deposited over the second pinned layer  520 . The cap layer  436  is deposited over the second AFM layer  516 . 
     The first AFM layer  514  is set at elevated temperature in the presence of a strong magnetic field, as is known to the art, to pin the direction of the magnetization  519  (shown as the head of an arrow pointing out of the plane of the paper) of the first pinned layer  518  perpendicular to the ABS. The second AFM layer  516  is similarly set to pin the direction of the magnetization  521  (shown as the tail of an arrow pointing into the plane of the paper) of the second pinned layer  520  in an opposite direction to the magnetization  519  of the first pinned layer  518 . Alternatively, the first pinned layer  518  may be pinned so that the magnetization  519  is directed into the plane of the paper and the second pinned layer  520  may be pinned so that the magnetization  521  is directed out of the plane of the paper. With the half-bit length equal to the spacing between the free layers, the signals generated by the first and second spin valve sensors of the differential CPP sensor  500  add due to the 180° phase difference of the magnetizations of the first and second pinned layers. The setting of the first and second AFM layers  514  and  516  180° out of phase may require the use of different AFM materials for each layer and setting procedures known in the art. 
     THIRD EXAMPLE 
       FIG. 6  shows an air bearing surface (ABS) view, not to scale, of a differential CPP sensor  600  according to another embodiment of the present invention. The CPP SV sensor  600  differs from the CPP SV sensor  400  shown in  FIG. 4  in having a first SV sensor  610  comprising a self-pinned AP-coupled first pinned layer  614  and a second SV sensor  612  comprising a simple pinned layer  620  with an antiferromagnetic (AFM) pinning layer  616  instead of the two self-pinned AP-coupled layers  422  and  424  of the SV sensor  400 . The SV sensor  612  having a simple pinned layer and an AFM pinning layer is preferably the top sensor in the stack forming the differential CPP sensor  600  but, alternatively, may be configured as the bottom sensor of the differential CPP sensor. The first SV sensor  610  comprising first pinned layer  614  and first free layer  424  is the same as first SV sensor  410  of CPP sensor  400 . The second pinned layer  620  of the second sensor  612  is formed of Co—Fe having a thickness in the range of 20-40 Å deposited over the second free layer  430 . The AFM layer  616  of Pt—Mn or Ir—Mn having a thickness in the range of 50-200 Å is deposited over the second pinned layer  620 . The cap layer  436  is deposited over the AFM layer  616 . 
     The AFM layer  616  is set at elevated temperature in the presence of a strong magnetic field, as is known to the art, to pin the direction of the magnetization  621  (shown as the head of an arrow pointing out of the plane of the paper) of the second pinned layer  620  perpendicular to the ABS and in an opposite direction to the magnetization  444  of the FM2 layer  428  of the first pinned layer  614 . Alternatively, the FM2 layer  428  may be pinned so that the magnetization  444  is directed into the plane of the paper and the second pinned layer  620  may be pinned so that the magnetization  621  is directed out of the plane of the paper. With the half-bit length equal to the spacing between the free layers, the signals generated by the first and second spin valve sensors of the differential CPP sensor  600  add due to the 180° phase difference of the magnetizations of the FM2 layer  428  and the second pinned layer  620 . 
     While the present invention has been particularly shown and described with reference to the preferred embodiments, it will be understood to 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.