Abstract:
A magnetic tunnel junction (MTJ) device for use as a magnetic field sensor in a magnetic disk drive or as a memory cell in a magnetic random access (MRAM) array has an antiferromagnetic (AFM) layer formed of electrically insulating antiferromagnetic material. The magnetic tunnel junction in the sensor is formed on a first shield, which also serves as an electrical lead, and is made up of a stack of layers forming an MTJ sensor stripe. The layers in the stack are an AFM layer, a pinned ferromagnetic layer exchange biased with the AFM layer so that its magnetic moment cannot rotate in the presence of an applied magnetic field, a free ferromagnetic layer whose magnetic moment is free to rotate in the presence of an applied magnetic field, and an insulating tunnel barrier layer disposed between the pinned layer and the free layer. The MTJ sensor stripe is generally rectangularly shaped with parallel side edges and a back edge and a front edge at the air bearing surface (ABS). The pinned layer extends away from the ABS beyond the back edge of the AFM layer to contact the first shield providing a path for sensing current to bypass the electrically insulating AFM layer and flow to the tunnel junction layer. A layer of electrically insulating material isolates the pinned layer and the first shield from the second shield which also serves as an electrical lead for the MTJ sensor.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates in general to magnetic tunnel junction transducers for reading information signals from a magnetic medium and, in particular, to a magnetic tunnel junction sensor with an electrically insulating antiferromagnetic layer, and to magnetic storage systems which incorporate such sensors. 
     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. 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 . 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 . 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. 
     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 change in the sensed current or voltage. IBM&#39;s U.S. Pat. No. 5,650,958 granted to Gallagher et al., incorporated in its entirety herein by reference, discloses an MTJ sensor operating on the basis of the magnetic tunnel junction effect. 
     FIG. 2 a  shows a prior art MTJ sensor  200  comprising a first electrode  204 , a second electrode  202 , and a tunnel barrier layer  215 . The first electrode  204  comprises a pinned layer (pinned ferromagnetic layer)  220 , an antiferromagnetic (AFM) layer  230 , and a seed layer  240 . The magnetization of the pinned layer  220  is fixed through exchange coupling with the AFM layer  230 . The second electrode  202  comprises a free layer (free ferromagnetic layer)  210  and a cap layer  205 . The free layer  210  is separated from the pinned layer  220  by a non-magnetic, electrically insulating tunnel barrier layer  215 . In the absence of an external magnetic field, the free layer  210  has its magnetization oriented in the direction shown by arrow  212 , that is, generally perpendicular to the magnetization direction of the pinned layer  220  shown by arrow  222  (tail of an arrow pointing into the plane of the paper). A first lead  260  and a second lead  265  formed in contact with first electrode  204  and second electrode  202 , respectively, provide electrical connections for the flow of sensing current I S  from a current source  270  to the MTJ sensor A signal detector  280 , typically including a recording channel such as a partial-response maximum-likelihood (PRML) channel, connected to the first and second leads  260  and  265  senses the change in resistance due to changes induced in the free layer  210  by the external magnetic field. 
     FIG. 2 b  is a cross-sectional view perpendicular to the air bearing surface of the prior art MTJ sensor  200 . The MTJ sensor  200  comprises a sensor stripe  290  having a front edge  291  at the ABS and extending away from the ABS to a back edge  292  defined by the back edge of the tunnel barrier layer  215 . The leads  260 ,  265  provide electrical connections for the flow of the sensing current I S  in a direction perpendicular to the tunnel barrier layer  215 . An electrical insulating layer  250  prevents shunting of the sensing current around the tunnel barrier layer at the back edge  292  of the sensor stripe  290 . 
     Since, in an MTJ sensor, the sensing current flows in a direction perpendicular to the tunnel barrier layer, a reasonably high electrical conductivity is needed for all the layers disposed between the lead layers except for the tunnel barrier layer. One of these layers is the AFM layer used to fix (pin) the magnetization direction of the ferromagnetic pinned layer. Mn-Fe is an antiferromagnet with good electrical conductivity that has been used in previous MTJ sensors. However, Mn-Fe has poor corrosion resistance which is a concern during the fabrication process and undesirable for long term stability of the MTJ sensor in a disk drive environment. Alternate AFM materials that have high corrosion resistance are NiO and α-Fe 2 O 3 /NiO bilayer, however these AFM materials are electrically insulating and therefore do not provide a path for the sensing current to flow between the leads in a direction perpendicular to the tunnel barrier layer with the usual MTJ sensor structure. 
     What is needed is a structure for an MTJ sensor which allows the use of electrically insulating AFM materials, such as NiO and α-Fe 2 O 3 /NiO with their high corrosion resistance, for the pinning layer used to fix the magnetization of the pinned layer and a process for fabrication of an MTJ sensor with this structure. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to disclose a magnetoresistive tunnel junction (MTJ) sensor having an electrically insulating AFM layer. 
     It is another object of the present invention to disclose an MTJ sensor having an AFM layer made of NiO. 
     It is yet another object of the present invention to disclose an MTJ sensor using an electrically insulating AFM layer and having a pinned layer structure in electrical contact to a magnetic shield, the shield also serving as an electrical lead. 
     In accordance with the principles of the present invention, there is disclosed an MTJ sensor comprising an MTJ sensor stripe having a generally rectangular shape and two opposite side edges, a back edge and a front edge at the ABS surface. The sensor stripe includes a stack of layers including an AFM layer of NiO, an insulating antiferromagnetic material. A ferromagnetic pinned layer deposited on the AFM layer makes electrical contact with a ferromagnetic first shield behind the back edge of the MTJ sensor stripe (the edge opposite to the front edge of the stripe at the air bearing surface) to provide a path for the sensing current to bypass the electrically insulating AFM layer. The sensing current then flows from the pinning layer transversely through the tunnel barrier layer and the free layer of the MTJ sensor stripe to the ferromagnetic second shield which serves as a second electrical lead for the MTJ sensor. 
     The MTJ sensor comprises a seed layer, an AFM layer, a pinned ferromagnetic layer, a tunnel barrier layer, a free ferromagnetic layer and a cap layer sequentially deposited on the first shield. After deposition of the seed layer and the AFM layer, the AFM layer is defined by photolithography to have an AFM back edge extending beyond the back edge of the sensor stripe. The pinned ferromagnetic layer is deposited on the AFM layer, over the AFM back edge and on the first shield making electrical contact with the first shield. The tunnel barrier layer, free layer and cap layer are then deposited sequentially and patterned by photolithography to form the MTJ sensor stripe. An electrically insulating layer is then deposited over the entire MTJ sensor area. The photoresist covering the MTJ sensor stripe is then removed and the second shield of ferromagnetic material is deposited over the MTJ sensor making direct electrical contact with the second electrode of the MTJ sensor stripe. 
     In the MTJ sensor structure of the present invention, the ferromagnetic first and second shield layers provide magnetic shielding from stray magnetic fields as is known to the art and also provide the electrical leads to supply sensing current to the first and second electrodes, respectively, of the MTJ stack. Because the AFM layer used in this embodiment is electrically insulating, direct contact between the first shield and the pinned layer beyond the back edge of the sensor stripe provides the sensing current path to the first electrode of the MTJ sensor. The electrically insulating layer of material at the end regions of the MTJ sensor and at the back edge of the sensor stripe prevent sensing current flow from being shunted around the tunnel barrier layer between the first and second shields. 
     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 a  is an air bearing surface view, not to scale, of a prior art magnetic tunnel junction sensor; 
     FIG. 2 b  is a cross-section, not to scale, perpendicular to the air bearing surface of a prior art magnetic tunnel junction sensor; 
     FIG. 3 a  is a simplified drawing of a magnetic recording disk drive system; 
     FIG. 3 b  is a vertical cross-section view, not to scale, of an inductive write/MTJ read head with the MTJ read head located between the shields and adjacent to the inductive write head; 
     FIG. 4 a  is an air bearing surface view, not to scale, of an embodiment of the magnetic tunnel junction sensor of the present invention; 
     FIG. 4 b  is a cross-section, not to scale, perpendicular to the air bearing surface of the magnetic tunnel junction sensor of the present invention; 
     FIG. 5 a  is an air bearing surface view, not to scale, of another embodiment of the magnetic tunnel junction sensor of the present invention; and 
     FIG. 5 b  is a cross-section, not to scale, perpendicular to the air bearing surface of yet another embodiment of the magnetic tunnel junction 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. 3, there is shown a disk drive  300  embodying the present invention. As shown in FIG. 3, at least one rotatable magnetic disk  312  is supported on a spindle  314  and rotated by a disk drive motor  318 . The magnetic recording media on each disk is in the form of an annular pattern of concentric data tracks (not shown) on the disk  312 . 
     At least one slider  313  is positioned on the disk  312 , each slider  313  supporting one or more magnetic read/write heads  321  where the head  321  incorporates the MTJ sensor of the present invention. As the disks rotate, the slider  313  is moved radially in and out over the disk surface  322  so that the heads  321  may access different portions of the disk where desired data is recorded. Each slider  313  is attached to an actuator arm  319  by means of a suspension  315 . The suspension  315  provides a slight spring force which biases the slider  313  against the disk surface  322 . Each actuator arm  319  is attached to an actuator  327 . The actuator as shown in FIG. 3 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  329 . 
     During operation of the disk storage system, the rotation of the disk  312  generates an air bearing between the slider  313  (the surface of the slider  313  which includes the head  321  and faces the surface of the disk  312  is referred to as an air bearing surface (ABS)) and the disk surface  322  which exerts an upward force or lift on the slider. The air bearing thus counterbalances the slight spring force of the suspension  315  and supports the slider  313  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  329 , such as access control signals and internal clock signals. Typically, the control unit  329  comprises logic control circuits, storage chips and a microprocessor. The control unit  329  generates control signals to control various system operations such as drive motor control signals on line  323  and head position and seek control signals on line  328 . The control signals on line  328  provide the desired current profiles to optimally move and position the slider  313  to the desired data track on the disk  312 . Read and write signals are communicated to and from the read/write heads  321  by means of the recording channel  325 . 
     The above description of a typical magnetic disk storage system, and the accompanying illustration of FIG. 3 are for representation purposes only. It should be apparent that disk storage systems may contain a large number of disks and actuators, and each actuator may support a number of sliders. 
     FIG. 3 b  shows a cross-sectional schematic view of the read/write head  321  embodying the present invention which includes an MTJ read head portion and an inductive write head portion. The head  321  is lapped to form an ABS. The read head includes an MTJ sensor  340  disposed between first and second shield layers Si and S 2 . An insulating gap layer G 1  is disposed between the first and second shield layers S 1  and S 2  in the region away from the MTJ sensor. The write head includes a coil layer C and an insulation layer IN 2  which are disposed between insulation layers IN 1  and IN 3  which are, in turn, disposed between first and second pole pieces P 1  and P 2 . A gap layer G 2  is disposed between the first and second pole pieces P 1 , P 2  for providing a magnetic write gap at their pole tips adjacent to the ABS. The combined read/write head  321  shown in FIG. 3 b  is a “merged” head in which the second shield layer S 2  of the read head is employed as a first pole piece P 1  for the write head. 
     FIG. 4 a  shows an air bearing surface (ABS) view of an MTJ sensor  400  according to the preferred embodiment of the present invention. The MTJ sensor  400  comprises end regions  464  and  466  separated from each other by a central region  462 . The active region of the MTJ sensor  400  is an MTJ sensor stripe  403  formed in the central region  462 . The MTJ sensor stripe  403  has a generally rectangular shape with two opposite side edges  407 ,  408  and a back edge (not shown) opposite to a front edge  491  at the ABS. The MTJ sensor stripe  403  comprises a first electrode  404 , a second electrode  402  and a tunnel barrier layer  415  disposed between the first electrode  404  and the second electrode  402 . The first electrode  404  comprises a pinned layer  420 , an AFM layer  430 , and a seed layer  440 , where the pinned layer  420  is disposed between the tunnel barrier layer  415  and the AFM layer  430  and the AFM layer  430  is disposed between the pinned layer  420  and the seed layer  440 . The second electrode  402  comprises a free layer  410  and a cap layer  405 , where the free layer  410  is disposed between the tunnel barrier layer  415  and the cap layer  405 . 
     The AFM layer  430  is exchange coupled to the pinned layer  420  providing an exchange field to pin the magnetization direction of the pinned layer  420  perpendicular to the ABS. The magnetization of the free layer  410  is oriented parallel to the ABS and is free to rotate in the presence of a signal magnetic field. 
     In the preferred embodiment of the present invention, the MTJ sensor stripe  403  is formed in the central region  462  over a first shield (S 1 )  460 . The first shield  460  is a layer of soft ferromagnetic material such as Ni—Fe (permalloy), or alternatively Al—Fe—Si (Sendust), deposited on a substrate  401  and extending over the central region  462  and the end regions  464  and  466  to provide magnetic shielding of the MTJ sensor from stray magnetic fields. An insulator layer  450  of electrically insulating material such as Al 2 O 3  is formed in the end regions  464  and  466  and behind the back edge of the MTJ sensor stripe  403 . A second shield (S 2 )  461  of soft ferromagnetic material such as Ni—Fe, or alternatively Al—Fe—Si, is formed on the insulator layer  450  in the end regions  464  and  466  and over the MTJ sensor stripe  403  in the central region  462 . 
     FIG. 4 b  shows the cross-section of the MTJ sensor  400  perpendicular to the ABS. The MTJ sensor stripe  403  has a front edge  491  at the ABS and extending away from the ABS to a back edge  492  defined by the back edges of the tunnel barrier layer  415 . Because the AFM layer in the MTJ sensor of the present invention is formed of electrically insulating material, it is necessary to provide a path for the sensing current to bypass the AFM layer  430  and to flow perpendicular to the tunnel barrier layer  415 . The path for sensing current flow is formed by patterning the AFM layer  430  to define an AFM back edge  494  significantly further away from the ABS than the MTJ sensor stripe back edge  492  and then depositing the pinned ferromagnetic layer  420  over the AFM layer  430  and over the exposed region of the first shield  460  further away from the ABS than the AFM back edge  494 . The AFM back edge  494  may be patterned to be in the range of 10-50 micrometers away from the ABS while the MTJ sensor stripe back edge  492  is only about 0.5 micrometers away from the ABS. This structure provides a path for the flow of the sensing current I S  from the first shield  460 , into and along the plane of the pinned layer  420 , and transversely through the tunnel barrier layer  415  and the free layer  410  to the second shield  461 . The insulating layer  450  deposited over the pinned layer  420  beyond the MTJ sensor stripe back edge  492  provides electrical isolation between the first and second shields  460  and  461  and prevents the sensing current from shunting around the MTJ sensor stripe  403 . Since the sensing current flows in the plane of the pinned layer  420 , its magnetic field may be used to achieve a stable magnetic state in the free layer  410 . 
     Referring again to FIG. 4 a , the first and second shields  460  and  461  provide electrical connections for the flow of the sensing current I S  from a current source  470  to the MTJ sensor stripe  403 . A signal detector  480  which is electrically connected to shields  460  and  461  senses the change in the resistance due to changes induced in the free layer  410  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  410  relative to the direction of magnetization of the pinned layer  420  which is preferably pinned perpendicular to the ABS. The signal detector  480  preferably includes a digital recording channel such as a partial response maximum-likelihood (PRML) channel as is known to those skilled in the art or other types of well known recording channels such as peak detect or maximum likelihood channels. The signal detector  480  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 MTJ sensor  400  may be fabricated in a magnetron sputtering or an ion beam sputtering system to deposit sequentially the multilayer structure shown in FIGS. 4 a  and  4   b . The first shield (S 1 )  460  of Ni—Fe (permalloy) having a thickness in the range of about 5000-10000 Å is deposited on the substrate  401 . The seed layer  440 , the AFM layer  430 , the pinned layer  420 , the tunnel barrier layer  415 , the free layer  410 , and the cap layer  405  are sequentially deposited over the first shield  460  in the presence of a longitudinal or transverse magnetic field of about 40 Oe to orient the easy axis of all of the ferromagnetic layers. The seed layer  440  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  having a thickness of about 30-50 Å. The AFM layer  430  formed of NiO having a thickness of about 100-400 Å is deposited on the seed layer  440  by sputtering a nickel target in the presence of a reactive gas that includes oxygen. The AFM layer  430  is patterned by photolithography to define the AFM back edge  494 . The ferromagnetic pinned layer  420  is deposited on the AFM layer  430  and on the area of the first shield  460  exposed by the patterning of the AFM back edge  494 . The pinned layer  420  may be formed of Ni—Fe having a thickness in the range of about 20-50 Å, or alternatively, may be formed of a sub-layer of Ni—Fe having a thickness in the range of 20-50 Å and an interface layer of Co having a thickness of about 5 Å deposited on the Ni—Fe sublayer. The tunnel barrier layer  415  is formed of Al 2 O 3  by depositing and then plasma oxidizing an 8-20 Å aluminum (Al) layer on the pinned layer  420 . The ferromagnetic free layer  410  may be formed of Ni—Fe having a thickness in the range of about 20-50 Å, or alternatively, may be formed of an interface layer of Co having a thickness of about 5 Å deposited on the tunnel barrier layer  415  and a sub-layer of Ni—Fe having a thickness of about 20-50 Å deposited on the Co interface layer. The cap layer  405  formed of Ta having a thickness of about 50 Å is deposited on the free layer  410 . A photoresist layer is deposited on the cap layer  405  and photolithography and ion milling processes well known in the art may be used to define the back edge  492  and the central region  462  of the MTJ sensor stripe  403 . 
     The insulator layer  450  can now be deposited on the exposed portion of the pinned layer  420  in the area behind the MTJ stripe back edge  492  and on the first shield (S 1 )  460  in the end regions  464 ,  466 . The insulator layer  450  is formed of Al 2 O 3  by depositing and then plasma oxidizing an aluminum (Al) layer having a thickness approximately equal to the total thickness of the MTJ sensor active layers in the central region  462 . The photoresist protecting the MTJ sensor stripe  403  is then removed and the second shield  461  of Ni—Fe (permalloy) having a thickness in the range of about 5000-10000 Å is deposited on the exposed MTJ sensor stripe  403  and on the insulator layer  450 . 
     The second shield  461  makes electrical contact to the second electrode  402 . The free ferromagnetic layer  410  is separated from the second shield  461  by the thin cap layer  405  to break magnetic coupling between the free layer  410  and the second shield  461 . 
     FIG. 5 a  shows an ABS view of an MTJ sensor  500  according to another embodiment of the present invention. This embodiment only differs from the embodiment shown in FIGS. 4 a  and  4   b  in having the seed layer  440  and the AFM layer  430  extend over the first shield (S 1 )  460  in the end regions  464  and  466  as well as in the central region  462 . Since the AFM layer  430  is made of an electrically insulating AFM material, such as NiO, the AFM layer  430  in the end regions  464 ,  466  provides electrical insulation between the first shield (S 1 )  460  and the second shield (S 2 )  461  which together with the insulating layer  450  prevents electrical shorting between S 1  and S 2 . The structure at the back edge of the MTJ sensor and the method of making electrical contact of the pinned layer  420  to the first shield (S 1 )  460  to provide a sensing current path may be the same as shown in FIG. 4 b  for the preferred embodiment or, alternatively, may have the structure and method of making electrical contact shown in FIG. 5 b.    
     FIG. 5 b  shows the cross-section of an MTJ sensor  510  perpendicular to the ABS according to yet another embodiment of the present invention. In this embodiment, the seed layer  440  and the AFM layer  430  deposited on the first shield (S 1 ) extend away from the ABS over the first shield (S 1 ). Since the AFM layer  430  is formed of electrically insulating material, it is necessary to provide a path for the sensing current I S  to bypass the AFM layer  430  and to flow in a direction perpendicular to the tunnel barrier layer  415 . The path for the sensing current flow is made by forming an opening (via)  496  through the AFM layer  430  prior to the deposition of the pinned layer  420  over the AFM layer  430 . The via  496  is formed in the region of the AFM layer  430  further away from the ABS than the MTJ sensor stripe back edge  492  using methods well known to the art. The pinned layer  420  is deposited on the AFM layer  430  and on the area of the first shield (S 1 )  460  exposed by the via  496  through the AFM layer  430 . The pinned layer  420  makes electrical contact to the first shield (S 1 )  460  through the via  496  providing a path for the flow of the sensing current I S  from the first shield (S 1 )  460 , into and along the plane of the pinned layer, and transversely through the tunnel barrier layer  415  and the free layer  410  to the second shield (S 2 )  461 . The insulating layer  450  deposited over the pinned layer  420  beyond the MTJ stripe back edge  492  provides electrical isolation between the first and second shields  460  and  461  and prevents the sensing current from shunting around the MTJ sensor stripe  403 . 
     Alternatively, AFM layer  430  may be made of an α-Fe 2 O 3 /NiO bilayer to fabricate the MTJ sensor  400  according to the present invention. 
     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 in scope only as specified in the appended claims.