Patent Publication Number: US-2003235016-A1

Title: Stabilization structures for CPP sensor

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] This invention relates in general to magnetic transducers for reading information signals from a magnetic medium and, in particular, to a current perpendicular to the plane sensor with an improved stabilization structure which allows addition of ferromagnetic coupling and magnetostatic bias at the free layer.  
       [0003] 2. Description of the Related Art  
       [0004] 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.  
       [0005] 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.  
       [0006] 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.  
       [0007] 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.  
       [0008] 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.  
       [0009]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 . 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., incorporated herein by reference, discloses a GMR sensor operating on the basis of the SV effect.  
       [0010] Another type of spin valve sensor is an antiparallel pinned (AP) spin valve sensor. The AP-pinned spin valve sensor differs from the simple 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 Å) 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.  
       [0011] 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. The AP-pinned spin valve sensor is described in commonly assigned U.S. Pat. No. 5,465,185 to Heim and Parkin which is incorporated by reference herein.  
       [0012] Another type of magnetic device currently under development is a magnetic tunnel junction (MTJ) device. The MTJ device has potential applications as a memory cell and as a magnetic field sensor. The MTJ device comprises two ferromagnetic layers separated by a thin, electrically insulating, tunnel barrier layer. The tunnel barrier layer is sufficiently thin that quantum-mechanical tunneling of charge carriers occurs between the ferromagnetic layers. The tunneling process is electron spin dependent, which means that the tunneling current across the junction depends on the spin-dependent electronic properties of the ferromagnetic materials and is a function of the relative orientation of the magnetic moments, or magnetization directions, of the two ferromagnetic layers. In the MTJ sensor, one ferromagnetic layer has its magnetic moment fixed, or pinned, and the other ferromagnetic layer has its magnetic moment free to rotate in response to an external magnetic field from the recording medium (the signal field). When an electric potential is applied between the two ferromagnetic layers, the sensor resistance is a function of the tunneling current across the insulating layer between the ferromagnetic layers. Since the tunneling current that flows perpendicularly through the tunnel barrier layer depends on the relative magnetization directions of the two ferromagnetic layers, recorded data can be read from a magnetic medium because the signal field causes a change of direction of magnetization of the free layer, which in turn causes a change in resistance of the MTJ sensor and a corresponding change in the sensed current or voltage. Because the sensing current is perpendicular to the plane of the sensor layers, the MTJ sensor is known as a current-perpendicular-to-plane (CPP) sensor. IBM&#39;s U.S. Pat. No. 5,650,958 granted to Gallagher et al a MTJ sensor operating on the basis of the magnetic tunnel junction effect.  
       [0013] Two types of current-perpendicular-to-plane (CPP) sensors have been extensively explored for magnetic recording at ultrahigh densities (≦20 Gb/in 2 ). One is a GMR spin valve sensor and the other is a MTJ sensor. When the CPP sensor is used, magnetic stabilization of the free (sense) layer can be difficult due to the use of insulating layers to avoid current shorting around the active region of the sensor. Therefore, theres is a continuing need to improve the magnetic stabilization of CPP type magnetoresistive sensors to improve sensor stability.  
       SUMMARY OF THE INVENTION  
       [0014] It is an object of the present invention to disclose current-perpendicular-to-plane (CPP) spin valve (SV) and magnetic tunnel junction (MTJ) sensors having an antiparallel (AP)-pinned longitudinal bias stack for instack biasing to stabilize the free layer.  
       [0015] It is another object of the present invention to disclose CPP SV and MTJ sensors having an AP-pinned longitudinal bias stack in which the bias field from the bias layer stack adds to the coupling field between the free layer and the bias stack.  
       [0016] It is a further object of the present invention to disclose CPP SV and MTJ sensors having a longitudinal bias stack adjacent to the free layer comprising a spacer layer, a first ferromagnetic (FM 1 ) layer, an antiparallel coupling (APC) layer, a second ferromagnetic (FM 2 ) layer and an antiferromagnetic (AFM) layer.  
       [0017] It is yet another object of the present invention to disclose CPP SV and MTJ sensors having a longitudinal bias stack adjacent to the free layer in which the FM 1  layer is made thicker than the FM 2  layer if the ferromagnetic coupling between the free layer and the FM 1  layer is negative (antiparallel).  
       [0018] It is still another object of the present invention to disclose CPP SV and MTJ sensors having a longitudinal bias stack adjacent to the free layer in which the FM 2  layer is made thicker than the FM 1  layer if the ferromagnetic coupling between the free layer and the FM 1  layer is positive (parallel).  
       [0019] In accordance with the principles of the present invention, there is disclosed a first embodiment of the present invention wherein a CPP SV sensor comprises a SV stack and a longitudinal bias stack adjacent to and in contact with a free (sense) layer of the SV stack. The bias stack comprises an antiparallel (AP)-pinned layer including FM 1  and FM 2  layers separated by an APC layer. The FM 1  layer is separated from the free layer of the SV stack by a nonmagnetic spacer layer. Depending on material and thickness of the spacer layer, ferromagnetic coupling between the FM 1  layer and the free layer may be either positive or negative. By choosing the relative magnetic thicknesses of the FM 1  layer and the FM 2  layer, the bias field H B  from the AP-pinned layer and the ferromagnetic coupling field H FC  across the spacer layer can be made additive at the free layer for either positive or negative coupling. If the coupling across the spacer layer is positive (ferromagnetic), the thickness of the FM 2  layer is chosen to be greater than the thickness of the FM 1  layer. If the coupling across the spacer layer is negative (antiferromagnetic), the thickness of the FM 1  layer is chosen to be greater than the thickness of the FM 2  layer. By ensuring that the bias field adds to the coupling field, the stability of the free layer by in-stack biasing is improved.  
       [0020] In accordance with the principles of the present invention, there is disclosed a second embodiment of the present invention wherein a CPP MTJ sensor comprises an MTJ stack and a longitudinal bias stack adjacent to and in contact with a free (sense) layer of the MTJ stack. The bias stack comprises an antiparallel (AP)-pinned layer including FM 1  and FM 2  layers separated by an APC layer. The FM 1  layer is separated from the free layer of the SV stack by a nonmagnetic spacer layer. Depending on material and thickness of the spacer layer, ferromagnetic coupling between the FM 1  layer and the free layer may be either positive or negative. By choosing the relative magnetic thicknesses of the FM 1  layer and the FM 2  layer, the bias field H B  from the AP-pinned layer and the ferromagnetic coupling field H FC  across the spacer layer can be made additive at the free layer for either positive or negative coupling. If the coupling across the spacer layer is positive (ferromagnetic), the thickness of the FM 2  layer is chosen to be greater than the thickness of the FM 1  layer. If the coupling across the spacer layer is negative (antiferromagnetic), the thickness of the FM 1  layer is chosen to be greater than the thickness of the FM 2  layer. By ensuring that the bias field adds to the coupling field, the stability of the free layer by in-stack longitudinal biasing is improved. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0021] 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.  
     [0022]FIG. 1 is an air bearing surface view, not to scale, of a prior art SV sensor;  
     [0023]FIG. 2 is a simplified diagram of a magnetic recording disk drive system using the MTJ sensor of the present invention;  
     [0024]FIG. 3 is a vertical cross-section view, not to scale, of a “piggyback” read/write magnetic head;  
     [0025]FIG. 4 is a vertical cross-section view, not to scale, of a “V merged” read/write magnetic head;  
     [0026]FIG. 5 is an air bearing surface view, not to scale, of a CPP spin valve embodiment of the present invention; and  
     [0027]FIG. 6 is an air bearing surface view, not to scale, of a CPP magnetic tunnel junction embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
     [0028] 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.  
     [0029] 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 a coating on the surfaces of the disk  212  on which the data is recorded as an annular pattern of concentric data tracks (not shown).  
     [0030] 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 that is 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 .  
     [0031] 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.  
     [0032] 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 (PMRL) 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 PMRL channel.  
     [0033] 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.  
     [0034]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 CPP magnetoresistive sensor  306  according to the present invention. The sensor  306  is sandwiched between nonmagnetic insulative first and second read gap layers  308  and  310 , and the read gap layers are sandwiched between ferromagnetic first and second shield layers  312  and  314 . In response to external magnetic fields, the resistance of the sensor  306  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  246  shown in FIG. 2.  
     [0035] 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.  
     [0036]FIG. 4 is the same as FIG. 3 except the second shield layer  414  and the first pole piece layer  424  are a common layer. This type of read/write head is known as a “merged” head  400 . The insulation layer  336  of the piggyback head in FIG. 3 is omitted in the merged head  400  of FIG. 4.  
     FIRST EXAMPLE  
     [0037]FIG. 5 shows an air bearing surface (ABS) view, not to scale, of a CPP spin valve (SV) sensor  500  according to a first embodiment of the present invention. The SV sensor  500  comprises end regions  504  and  506  separated from each other by a central region  502 . The active region of the SV sensor comprises a CPP spin valve (SV) stack  508  and a longitudinal bias stack  510  formed in the central region  502 . The seed layer  512  is a layer deposited to modify the crystallographic texture or grain size of the subsequent layers, and may not be needed depending on the subsequent layer. The SV stack  508  sequentially deposited over the seed layer  512  comprises a first antiferromagnetic (AFM 1 ) layer  514 , a ferromagnetic pinned layer  516 , a conductive spacer layer  518  and a ferromagnetic free (sense) layer  520 . The AFM 1  layer  514  has a thickness, typically 50-500 Å, at which the desired exchange properties are achieved with the pinned layer  516 .  
     [0038] The longitudinal bias stack  510  sequentially deposited over the SV stack  508  comprises a nonmagnetic spacer layer  522 , a first ferromagnetic (FM 1 ) layer  524 , an antiparallel coupling (APC) layer  526 , a second ferromagnetic (FM 2 ) layer  528 , and a second antiferromagnetic (AFM) layer  530 . The APC layer  526  is formed of a nonmagnetic material, preferably ruthenium (Ru), that allows the FM 1  and FM 2  layers  524  and  528  to be strongly coupled together antiferromagnetically forming an AP-pinned layer structure whose magnetization is pinned by the second AFM layer  530 . The AFM 2  layer  530  has a thickness, typically 50-500 Å, at which the desired exchange properties are achieved with the FM 2  layer  528 . A cap layer  532 , formed on the AFM 2  layer  530 , completes the central region  502  of the SV sensor  500 .  
     [0039] The AFM 1  layer  514  is exchange coupled to the pinned layer  516  to provide a pinning magnetic field to pin the magnetization of the pinned layer perpendicular to the ABS as indicated by the arrow head  517  pointing out of the plane of the paper. The free layer  520  has a magnetization  521  that is free to rotate in the presence of an external (signal) magnetic field. The magnetization  521  of the free layer  520  is preferably oriented parallel to the ABS in the absence of an external magnetic field, and may, alternatively, have an orientation opposite in direction to the magnetization  521 .  
     [0040] The AFM 2  layer  530  is exchange coupled to the AP-pinned layer comprising the FM 1  and FM 2  layers  524  and  528  to provide a pinning magnetic field to pin the magnetizations of the two ferromagnetic layers parallel to the ABS as indicated by the arrows  525  and  529 , respectively. The net magnetization of the AP-pinned layer provides a longitudinal bias field which forms a flux closure with the free layer  520  to provide longitudinal stabilization of the magnetic domain states of the free layer.  
     [0041] First and second shield layers  552  and  554  adjacent to the seed layer  512  and the cap layer  632  provide electrical connections for the flow of a sensing current Is from a current source  560  to the SV sensor  500 . A signal detector  570  which is electrically connected to first and second shields  552  and  554  senses the change in resistance due to changes induced in the sense layer  520  by the external magnetic field (e.g., field generated by a data bit stored on a disk). The external field acts to rotate the direction of magnetization of the sense layer  520  relative to the direction of magnetization of the pinned layer  516  which is preferably pinned perpendicular to the ABS. The signal detector  570  preferably comprises a partial response maximum likelihood (PRML) recording channel for processing the signal detected by MTJ sensor  500 . 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  570  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.  
     [0042] The SV sensor  500  is fabricated in a magnetron sputtering or an ion beam sputtering system to sequentially deposit the multilayer structure shown in FIG. 5. The sputter deposition process is carried out 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 first shield layer  552  formed of Ni—Fe having a thickness in the range of 5000-10000 Å is deposited on a substrate  501 . The seed layer  512  formed of a nonmagnetic metal, preferably tantalum (Ta), having a thickness of about 30 Å is deposited on the first shield  512 . The SV stack  508  is formed on the seed layer by sequentially depositing the AFM 1  layer  514  of Pt—Mn having a thickness of 100-200 Å, the pinned layer  516  of Ni—Fe, or alternatively of Co—Fe, having a thickness in the range of 20-50 Å, the conductive spacer layer  518  formed of copper having a thickness of about 20 Å, and the free layer  520  formed of Ni—Fe, or alternatively of Co—Fe, having a thickness in the range of 10-40 Å.  
     [0043] The longitudinal bias stack  510  is formed on the SV stack  508  by sequentially depositing the spacer layer  522  formed of copper (Cu), or of alternatively ruthenium (Ru), rhodium (Rh), tantalum (Ta) or some combination of these materials, having a thickness in the range of 5-30 Å, the FM 1  layer  524  formed of Co—Fe, or alternatively of Co, Ni—Fe or Co—Fe—Ni, having a thickness in the range of 10-30 Å, the APC layer  526  formed of ruthenium (Ru) having a thickness of about 8 Å, the FM 2  layer  528  formed of Co—Fe, or alternatively of Co, Ni—Fe or Co—Fe—Ni, having a thickness in the range of 10-30 Å, and the AFM 2  layer  530  formed of PtMn having a thickness in the range of 100-200 Å. Alternatively, the AFM 2  layer may be formed of an antiferromagnetic material having a blocking temperature different from the material of the AFM 1  layer. The cap layer  532  formed of tantalum (Ta) having a thickness of about 50 Å is deposited on the AFM 2  layer  530 .  
     [0044] The second shield layer  554  formed of Ni—Fe having a thickness in the range of 5000-10000 Å is deposited over the cap layer  532 . An insulating layer  556  formed of Al 2 O 3  deposited between the first shield layer  552  and the second shield layer  554  in the end regions  504  and  506  provides electrical insulation between the shields/leads and prevents shunting of the sense current around the active region  502  of the sensor.  
     [0045] After the deposition of the central portion  502  is completed, the AFM 1  layer  514  is set transverse to the ABS and the AFM 2  layer  530  is set longitudinal to the ABS using procedures well known to the art.  
     [0046] According to the invention, the longitudinal bias field H B  at the free layer provided by the longitudinal bias stack  510  is always additive with the ferromagnetic coupling field H FC  between the FM 1  layer  524  and the free layer  520 . With prior art in-stack longitudinal bias structures using a simple pinned layer, addition of H B  and H FC  can only be achieved for the case of negative ferromagnetic coupling across the spacer layer disposed between the longitudinal bias layer stack and the free layer. Since the sign and strength of coupling across a spacer layer is strongly dependent on both thickness and material, this restriction can be a problem for achieving a good in-stack bias design. With the AP-pinned layer structure in the bias stack  510  of the present invention, additive fields H B  and H FC  can be achieved for both positive (ferromagnetic) and negative (antiferromagnetic) coupling across the spacer layer  522  by proper choice of the relative magnetic thicknesses of the FM 1  and FM 2  layers  524  and  528 .  
     [0047] For the embodiment shown in FIG. 5, positive coupling between the bias (FM 1 ) layer and the free layer has been assumed. For positive coupling, H FC  at the free layer is a field having the same direction as the magnetization of the FM 1  layer as indicated by the arrow  525 . In order for closure of the instack bias field H B  from the bias strack  510  to have the same direction as H FC  at the free layer, the net magnetization of the AP-pinned layer must have the same direction as the magnetization of the FM 2  layer  528  as indicated by the arrow  529 . This requirement is met by choosing the thickness of the FM 2  layer  528  to be greater than the thickness of the FM 1  layer  524  (FM 2 &gt;FM 1 ). With this choice of the relative thicknesses of FM 1  and FM 2 , the bias field H B  and the ferromagnetic coupling field H FC  are additive at the free layer and have the direction indicated by the magnetization  521  of the free layer.  
     [0048] Alternatively, if the coupling between the bias (FM 1 ) layer and the free layer is negative (antiparallel), H FC  at the free layer is a field having the opposite direction to the magnetization of the FM 1  layer as indicated by the arrow  525 . In order for closure of the instack bias field H B  from the bias stack  510  to have the same direction as H FC  at the free layer, the net magnetization of the AP-pinned layer must have the same direction as the magnetization of the FM 1  layer  524  as indicated by the arrow  525 . This requirement is met by choosing the thickness of the FM 1  layer  524  to be greater than the thickness of the FM 2  layer  528  (FM 1 &gt;FM 2 ). With this choice of the relative thicknesses of FM 1  and FM 2 , the bias field H B  and the ferromagnetic coupling field H FC  are additive at the free layer and have the opposite direction to that indicated by the arrow  521  in FIG. 5.  
     SECOND EXAMPLE  
     [0049]FIG. 6 shows an air bearing surface (ABS) view, not to scale, of a CPP magnetic tunnel junction (MTJ) sensor  600  according to a second embodiment of the present invention. The MTJ sensor  600  differs from the SV sensor  500  in having an MTJ stack  608  in place of the SV stack  508 . The active region of the MTJ sensor comprises the MTJ stack  608  and the longitudinal bias stack  510  formed in the central region  502 . The MTJ stack  608  sequentially deposited over the seed layer  512  comprises a first antiferromagnetic (AFM 1 ) layer  514 , a ferromagnetic pinned layer  516 , an insulating tunnel barrier layer  618  and a ferromagnetic free (sense) layer  520 . The insulating tunnel barrier layer  618 , preferably formed of Al 2 O 3 , replaces the conductive spacer layer  518  of the CPP SV sensor  500  of the first example.  
     [0050] The longitudinal bias stack  510  sequentially deposited over the MTJ stack  608  has the same structure as the bias stack of the first example including a spacer layer  522 , an AP-pinned layer comprising FM 1  and FM 2  layers  524  and  528 , respectively, separated by an APC layer  526  and an AFM 2  layer  530 . The net magnetization of the AP-pinned layer provides a longitudinal bias field which forms a flux closure with the free layer  520  to provide longitudinal stabilization of the magnetic domain states of the free layer  521  of the MTJ stack  608 .  
     [0051] The MTJ sensor  600  is fabricated in a magnetron sputtering or an ion beam sputtering system to sequentially deposit the multilayer structure shown in FIG. 6. The sputter deposition process is the same as that used to fabricate the CPP SV sensor  500  except for deposition of the tunnel barrier layer  618  in place of the conductive spacer layer  518 . The tunnel barrier layer  618  of Al 2 O 3  is deposited on the pinned layer  516  by depositing and then plasma oxidizing an 8-20 Å aluminum (Al) layer. The free layer  520  is then deposited on the tunnel barrier layer  618 .  
     [0052] The process of choosing the relative thicknesses of the FM 1  layer  524  and the FM 2  layer  528  so that the bias field H B  and the ferromagnetic coupling field H FC  are additive at the free layer  520  for either positive or negative coupling of spacer layer  522  is the same as discussed above with respect to the first example. If the coupling across the spacer layer  522  is positive (ferromagnetic), the thickness of the FM 2  layer  528  is chosen to be greater than the thickness of the FM 1  layer  524 . If the coupling across the spacer layer  522  is negative (antiferromagnetic), the thickness of the FM 1  layer  524  is chosen to be greater than the thickness of the FM 2  layer  528 .  
     [0053] 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.