Abstract:
A spin valve sensor is disclosed that comprises a first layer of ferromagnetic material and a second layer of ferromagnetic material. A first layer of non-ferromagnetic material is positioned between the first and second layers of ferromagnetic material. A pinning layer is positioned adjacent to the first layer of ferromagnetic material such that the pinning layer is in contact with the first layer of ferromagnetic material. The spin valve includes synthetic antiferromagnetic bias means extending over passive end regions of the second layer of ferromagnetic material for producing a longitudinal bias in the passive end regions of a level sufficient to maintain the passive end regions in a single domain state. A method for forming a spin valve sensor with exchange tabs is also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims the benefit of the filing date of U.S. provisional application Ser. No. 60/131,850 entitled “METHOD OF MAGNETIC SENSOR STABILIZATION USING ANTIFERROMAGNETIC EXCHANGE BIAS WITH A SELF-ALIGNED REMOVAL MECHANISM,” which was filed Apr. 28, 1999, U.S. provisional application Ser. No. 60/131,463 entitled “STRUCTURES TO REDUCE SIDE READING IN SPIN VALVE SENSORS USING ANTIFERROMAGNETIC LONGITUDINAL BIAS,” which was filed Apr. 28, 1999, and U.S. provisional application Ser. No. 60/146,227 entitled “FREEING OF THE FREELAYER IN AN EXCHANGE TAB SPIN-VALVE,” which was filed Jul. 28, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to magnetoresistive read sensors for use in magnetic read heads. In particular, the present invention relates to a spin valve head with antiferromagnetic exchange stabilization and method for forming such a spin valve. 
     A magnetic read head retrieves magnetically-encoded information that is stored on a magnetic medium or disc. The magnetic read head is typically formed of several layers that include a top shield, a bottom shield, and a read sensor positioned between the top and bottom shields. The read sensor is generally a type of magnetoresistive sensor, such as a giant magnetoresistive (GMR) read sensor. The resistance of a GMR read sensor fluctuates in response to a magnetic field emanating from a magnetic medium when the GMR read sensor is used in a magnetic read head and positioned near the magnetic medium. By providing a sense current through the GMR read sensor, the resistance of the GMR read sensor can be measured and used by external circuitry to decipher the information stored on the magnetic medium. 
     A common GMR read sensor configuration is the GMR spin valve configuration in which the GMR read sensor is a multi-layered structure formed of a ferromagnetic free layer, a ferromagnetic pinned layer and a nonmagnetic spacer layer positioned between the free layer and the pinned layer. The magnetization direction of the pinned layer is fixed in a predetermined direction, generally normal to an air bearing surface of the GMR spin valve, while a magnetization direction of the free layer rotates freely in response to an external magnetic field. An easy axis of the free layer is generally set normal to the magnetization direction of the pinned layer. The resistance of the GMR read sensor varies as a function of an angle formed between the magnetization direction of the free layer and the magnetization direction of the pinned layer. This multi-layered spin valve configuration allows for a more pronounced magnetoresistive effect than is possible with anisotropic magnetoresistive (AMR) read sensors, which generally consist of a single ferromagnetic layer. 
     Typically, the magnetization of the pinned layer is fixed in the predetermined direction by exchange coupling an antiferromagnetic layer to the pinned layer. The antiferromagnetic layer is positioned upon the pinned layer such that the antiferromagnetic layer and the free layer form distal edges of the GMR spin valve. The spin valve is then heated to a temperature greater than a Néel temperature of the antiferromagnetic layer. Next, a magnetic field oriented in the predetermined direction is applied to the spin valve, thereby causing the magnetization direction of the pinned layer to orient in the direction of the applied magnetic field. The magnetic field may be applied to the spin valve before the spin valve is heated to the temperature greater than the Néel temperature of the antiferromagnetic layer. While continuing to apply the magnetic field, the spin valve is cooled to a temperature lower than the Neel temperature of the antiferromagnetic layer. Once the magnetic field is removed from the spin valve, the magnetization direction of the pinned layer will remain fixed, as a result of the exchange with the antiferromagnetic layer, so long as the temperature of the spin valve remains lower than the Neel temperature of the antiferromagnetic layer. 
     The free layer of a spin valve sensor must be stabilized against the formation of edge domain walls because domain wall motion results in electrical noise, which makes data recovery impossible. A common way to achieve stabilization is with a permanent magnet abutted junction design. Permanent magnets have a high coercive field (i.e., are hard magnets). The field from the permanent magnets stabilizes the free layer and prevents edge domain formation, and provides proper bias. 
     However, there are several problems with permanent magnet abutted junctions. To properly stabilize the free layer, the permanent magnets must provide more flux than can be closed by the free layer. This undesirable extra flux stiffens the edges of the free layer so that the edges cannot rotate in response to flux from the media, and may also cause shield saturation which adversely affects the ability of the sensor to read high data densities. The extra flux from the permanent magnets may produce multiple domains in the free layer and may also produce dead regions which reduce the sensitivity of the sensor. The junction between the permanent magnet and the various layers must be carefully engineered to minimize the stray flux as well as to minimize the junction resistance. Also, a junction of dissimilar metals can cause unwanted strain in the sensor. The free layer will respond to the strain unless the magnetostriction is exactly zeroed. Another disadvantage of permanent magnet abutted junctions is the nature of hard magnetic materials, which are multi-domained. Variation in grain size and shape leads to a distribution of domain coercivity. Lower coercivity domains may rotate when subjected to external fields. Such a grain near the edge of the free layer could cause domain wall formation and failure. 
     Tabs of antiferromagnetic material or “exchange tabs” have also been used to stabilize the free layer of magnetic sensors. Exchange tabs are deposited upon the outer regions of the free layer and are exchange coupled thereto. Functions of the exchange tabs include pinning the magnetization of the outer regions of the free layer in the proper direction, preventing the formation of edge domains and defining the width of an active area of the free layer by preventing free layer rotation at the outer regions of the free layer. 
     There are several advantages to the use of exchange tabs rather than permanent magnet abutted junctions. There is no junction to produce stray magnetic flux or junction resistance. Also, the lack of a junction of abutted, dissimilar metals makes it less likely that high strain will be produced within the sensor. And control is maintained over the anisotropy of the free layer regardless of how narrow the width of the active area is made. 
     The use of antiferromagnetic exchange tabs in AMR type sensors has been disclosed in U.S. Pat. No. 4,663,685, entitled “MAGNETORESISTIVE READ TRANSDUCER HAVING PATTERNED LONGITUDINAL BIAS”, U.S. Pat. No. 4,713,708, entitled “MAGNETORESISTIVE READ TRANSDUCER”, and U.S. Pat. No. 5,753,131, entitled “MAGNETORESISTIVE DEVICE AND MANUFACTURING METHOD THEREOF”. Longitudinal bias in AMR type sensors is also discussed in U.S. Pat. No. 4,785,366, entitled “MAGNETORESISTIVE READ TRANSDUCER HAVING PATTERNED ORIENTATION OF LONGITUDINAL BIAS”. 
     The use of antiferromagnetic exchange tabs in spin valve type sensors has been disclosed in U.S. Pat. No. 5,206,590, entitled “MAGNETO RESISTIVE SENSOR BASED ON THE SPIN VALVE EFFECT”, and U.S. Pat. No. 5,949,623, entitled “MONOLAYER LONGITUDINAL BIAS AND SENSOR TRACKWIDTH DEFINITION FOR OVERLAID ANISOTROPIC AND GIANT MAGNETORESISTIVE HEADS”. 
     Not all antiferromagnetic materials are suitable for use as exchange tabs in spin valve type sensors. Materials such as MnFe and TbCo are too corrosive for head production. MnFe and similar materials also have relatively low blocking temperatures, which is undesirable because the pinned regions of the free layer may become unpinned if the temperature of the sensor is raised above the blocking temperature of the exchange tabs during operation. MnFe and similar materials also have relatively low coupling constants, which results in weaker exchange coupling and higher side readings. Materials such as Fe 2 O 3  only increase the coercivity of the ferromagnetic layer instead of the pinning field. 
     The prior art does not disclose an exchange tab structure that makes use of a synthetic antiferromagnet, which provides an increased pinning field and reduced side reading. Furthermore, current methods for forming sensors with exchange tabs are inadequate. A major difficulty in manufacturing sensors with exchange tabs using conventional methods is establishing adequate exchange coupling between the exchange tabs and the free layer. Adequate exchange coupling is difficult to obtain because of the presence of a residue layer. It is often necessary to employ some removal process such as a pre-sputter etch or reactive ion etch on the residue layer to expose a clean, unoxidized surface of the free layer and to establish exchange coupling with deposited exchange tab material. A single monolayer of non-magnetic residue material is sufficient to destroy the exchange coupling. The removal step is problematic, though, because a photo mask is typically present on the residue layer during the removal step. It is extremely difficult to remove residue close to the photo mask, and excess residue is often left near the photo mask. The excess residue near the photo mask prevents proper exchange coupling between exchange tab material and the free layer, which results in widening of the effective width of the sensor and generation of an off-track signal. 
     The presence of a photo mask causes another problem during the deposition of the exchange tab material. Near the edge of the photo mask, some amount of shadowing is inevitable, causing reduced thickness of exchange tab material near the photo mask. Thus, even if it were possible using conventional techniques to completely remove the residue near the photo mask, the reduced thickness of the exchange tab material near the photo mask would still cause a problem. Since the pinning strength of exchange tabs is strongly dependent on thickness, it is necessary to compensate for the reduced thickness near the photo mask by increasing the overall layer thickness to provide a margin of safety. As the size of recording heads shrink, thinner layers become more attractive. Collimated deposition techniques can be employed to avoid thinning, but such an approach adds an extra constraint to the deposition process. It would be preferable to optimize for material quality with no geometry constraints. 
     Another issue that arises in fabricating spin valve sensors using conventional techniques is the matching of magnetic flux at the sensor edge. A pre-sputter etch to remove residue will necessarily remove magnetic material from the free layer, thereby reducing the moment of the free layer and creating a mismatch of magnetic flux. The removed material can be replaced by an in situ deposition, but the deposition can never be perfect, especially in the vicinity of a photo mask. 
     Thus, there is a need for a method of forming a spin valve sensor with exchange tabs, which results in better exchange coupling between the exchange tabs and the free layer of the spin valve. 
     BRIEF SUMMARY OF THE INVENTION 
     A spin valve sensor with antiferromagnetic exchange stabilization and method for forming such a sensor are disclosed. The spin valve sensor of the present invention comprises a first layer of ferromagnetic material and a second layer of ferromagnetic material. The second layer of ferromagnetic material has passive end regions separated by a central active region. A first layer of non-ferromagnetic material is positioned between the first and second layers of ferromagnetic material. A pinning layer is positioned adjacent to the first layer of ferromagnetic material such that the pinning layer is in contact with the first layer of ferromagnetic material. The spin valve includes a synthetic antiferromagnetic bias means extending over the passive end regions for producing a longitudinal bias in the passive end regions of a level sufficient to maintain the passive end regions in a single domain state. The synthetic antiferromagnets provide an increased pinning field and reduced side reading. 
     A preferred method for forming the spin valves of the present invention comprises depositing a first layer of antiferromagnetic material. A first layer of ferromagnetic material is deposited upon the first layer of antiferromagnetic material. A spacer layer is deposited upon the first layer of ferromagnetic material. A second layer of ferromagnetic material is deposited upon the spacer layer. A second antiferromagnetic layer is deposited upon the second layer of ferromagnetic material. A photoresist layer is deposited upon a central region of the second layer of antiferromagnetic material. The photoresist layer defines a central region in each of the second layer of antiferromagnetic material and the second layer of ferromagnetic material. The photoresist layer also at least partially defines first and second outer regions in each of the second layer of antiferromagnetic material and the second layer of ferromagnetic material. Contact material is deposited upon the first and second outer regions of the second layer of antiferromagnetic material. The photoresist layer is removed. Antiferromagnetic material is removed from the central region of the second layer of antiferromagnetic material. 
     The method of the present invention provides numerous advantages over the use of conventional techniques. A “perfect” interface is obtained between the exchange tab material and the free layer because there is no photo mask present to cause excess residue, shadowing or other complications. The exchange tabs and the electrical contacts are automatically aligned during the process. The milling of the exchange tab material can be stopped before reaching the interface between the exchange tab material and the free layer without removing any portion of the free layer, as long as the remaining thickness is less than the critical pinning thickness of the exchange tab material. The presence of the contacts allows the transfer curve characteristics of the sensor to be monitored during the mill, indicating if the mill time is adequate. Thus, the moment of the free layer is not reduced and there is not a problem with mismatched flux, stray fields or shield saturation. In addition, the process is easy to perform in high volume. 
     In an alternative embodiment of the method of the present invention, rather than removing antiferromagnetic material from the central region of the second layer of antiferromagnetic material, the central region is exposed to a reactive plasma, which alters the composition of the central region and ruins the exchange coupling. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a magnetic read/write head and magnetic disc taken along a plane normal to an air bearing surface of the read/write head. 
     FIG. 2 is a layer diagram of an air bearing surface of a magnetic read/write head. 
     FIG. 3 is a perspective view of a prior art GMR stack. 
     FIG. 4 is a perspective view of a prior art GMR spin valve stack with permanent magnet abutted junctions. 
     FIG. 5A shows a top view of a GMR spin valve stack with exchange tabs. 
     FIG. 5B shows a side view of a GMR spin valve stack with exchange tabs and with the free layer oriented upwards. 
     FIG. 5C shows a side view of a GMR spin valve stack with exchange tabs and with the free layer oriented downwards. 
     FIG. 6A shows a side view a of GMR spin valve stack with exchange tabs comprising synthetic antiferromagnets. 
     FIG. 6B shows a side view of a GMR spin valve stack with exchange tabs comprising synthetic antiferromagnets with additional ferromagnetic layers. 
     FIGS. 7A-7D illustrate a conventional patterning method which can be used to fabricate a sensor with exchange tabs. 
     FIGS. 8A-8D illustrate a preferred method for fabricating a GMR spin valve stack with exchange tabs. 
     FIGS. 9A-9C show graphs of various sensor characteristics versus time during milling of a GMR stack using the preferred method. 
     FIG. 10 shows a side view of a spin valve stack with exchange tabs formed by an alternative preferred method. 
     FIG. 11A shows a second graph of GMR versus etch time for the method illustrated in FIGS. 8A-8D. 
     FIG. 11B shows a graph of coupling strength versus etch time for the method illustrated in FIGS. 8A-8D. 
     FIG. 12A shows a graph of GMR versus etch time for an alternative preferred method. 
     FIG. 12B shows a graph of coupling strength versus etch time for an alternative preferred method. 
     FIG. 13 shows a graph of sheet resistance versus thickness for an antiferromagnetic exchange tab layer before and after processing using an alternative preferred method. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a cross-sectional view of magnetic read/write head  100  and magnetic disc  102  taken along a plane normal to air bearing surface  104  of read/write head  100 . Air bearing surface  104  of magnetic read/write head  100  faces disc surface  106  of magnetic disc  102 . Magnetic disc  102  travels or rotates in a direction relative to magnetic read/write head  100  as indicated by arrow A. Spacing between air bearing surface  104  and disc surface  106  is preferably minimized while avoiding contact between magnetic read/write head  100  and magnetic disc  102 . 
     A writer portion of magnetic read/write head  100  includes top pole  108 , insulator layer  110 , conductive coils  112  and top shield  114 . Conductive coils  112  are held in place between top pole  108  and top shield  114  by use of insulator  110 . Conductive coils  112  are shown in FIG. 1 as two layers of coils but may also be formed of more layers of coils as is well known in the field of magnetic read/write head design. 
     A reader portion of magnetic read/write head  100  includes top shield  114 , top gap layer  115 , metal contact layer  116 , bottom gap layer  117 , bottom shield  118 , and giant magnetoresistive (GMR) stack  120 . Metal contact layer  116  is positioned between top gap layer  115  and bottom gap layer  117 . GMR stack  120  is positioned between terminating ends of metal contact layer  116  and bottom gap layer  117 . Top gap layer  115  is positioned between top shield  114  and metal contact layer  116 . Bottom gap layer  117  is positioned between metal contact layer  116  and bottom shield  118 . Top shield  114  functions both as a shield and as a shared pole for use in conjunction with top pole  108 . 
     FIG. 2 is a layer diagram of air bearing surface  104  of magnetic read/write head  100 . FIG. 2 illustrates the location of magnetically significant elements in magnetic read/write head  100  as they appear along air bearing surface  104  of magnetic read/write head  100  of FIG.  1 . In FIG. 2, all spacing and insulating layers of magnetic read/write head  100  are omitted for clarity. Bottom shield  118  and top shield  114  are spaced to provide for a location of GMR stack  120 . GMR stack  120  has two passive regions defined as the portions of GMR stack  120  adjacent to metal contact layer  116 . An active region of GMR stack  120  is defined as the portion of GMR stack  120  located between the two passive regions of GMR stack  120 . The active region of GMR stack  120  defines a read sensor width. 
     FIG. 3 is a perspective view of a prior art GMR stack  130 . GMR stack  130  has free layer  132 , spacer layer  134 , pinned layer  136 , and antiferromagnetic layer  138 . Spacer layer  134  is positioned between free layer  132  and pinned layer  136 . A magnetization of pinned layer  136  is fixed in a predetermined direction, generally normal to air bearing surface  140  of GMR stack  130 , while a magnetization of free layer  132  rotates freely in response to an external magnetic field (not shown in FIG.  3 ). Antiferromagnetic layer  138  is positioned on GMR stack  130  such that pinned layer  136  is between spacer layer  134  and antiferromagnetic layer  138 . The magnetization of pinned layer  136  is pinned by exchange coupling pinned layer  136  with antiferromagnetic layer  138 . 
     The resistance of GMR stack  130  varies as a function of an angle that is formed between the magnetization of pinned layer  136  and the magnetization of free layer  132 . The magnetization of pinned layer  136  remains fixed in one direction, while the magnetization of free layer  132  rotates in response to a magnetic field emanating from a magnetic media or disc. The angle formed between the magnetization of free layer  132  and the magnetization of pinned layer  136  is, therefore, directly related to the magnetic field emanating from a magnetic media or disc. Consequently, the resistance of GMR stack  130  is directly related to the magnetic field emanating from the magnetic media or disc. 
     Prior to describing the exchange tabs of the present invention, a specific embodiment of a prior art method of sensor stabilization is described with reference to FIG.  4 . FIG. 4 is a perspective view of prior art GMR spin valve stack  150  with permanent magnet abutted junctions. GMR stack  150  includes permanent magnets  154 A and  154 B, pinning layer  156 , pinned layer  158 , spacer layer  160  and free layer  162 . Pinned layer  158  is positioned over pinning layer  156 . Spacer layer  160  is positioned over pinned layer  158 . Free layer  162  is positioned over spacer layer  160 . Permanent magnets  154 A and  154 B are placed on each side of GMR stack  150 . Junction  152 A is located between permanent magnet  154 A and a first edge of layers  156 - 162 . Junction  152 B is positioned between permanent magnet  154 B and a second edge of layers  156 - 162 . 
     The field from permanent magnets  154 A and  154 B stabilizes free layer  162  and prevents edge domain formation, and provides proper bias. However, there are several:problems with the permanent magnet abutted junction design shown in FIG.  4 . To properly stabilize free layer  162 , permanent magnets  154 A and  154 B must provide more flux than can be closed by free layer  162 . This undesirable extra flux stiffens the edges of free layer  162  and may also cause shield saturation. The junctions  152 A and  152 B must be carefully engineered to minimize this stray flux as well as to minimize the junction resistance. Also, a junction of dissimilar metals can cause unwanted strain in the sensor. Free layer  162  will respond to the strain unless the magnetostriction is exactly zeroed. Another disadvantage of permanent magnetic abutted junctions is the nature of hard magnetic materials, which are multi-domained. Variation in grain size and shape leads to a distribution of domain coercivity. Lower coercivity domains may rotate when subjected to external fields. Such a grain near the edge of free layer  162  could cause domain wall formation and failure. The extra flux from permanent magnets  154  may produce multiple domains in free layer  162  and may also produce dead regions which reduce the sensitivity of the sensor. 
     FIG. 5A shows a top view of GMR spin valve stack  170 . GMR stack  170  includes free layer  174  and exchange tabs  172 A and  172 B (collectively referred to as exchange tabs  172 ). Exchange tabs  172  are placed at the end regions  171 A and  171 B of free layer  174 . As can be seen in FIG. 5A, free layer  174  extends under exchange tabs  172  as represented by the hidden lines. Exchange tabs  172  are antiferromagnetic pinning layers, which are exchange coupled to end regions  171  of free layer  174  and which pin end regions  171  of free layer  174  in the proper direction, which is represented by the arrow on free layer  174 . Exchange tabs  172  also prevent the formation of edge domains in free layer  174 , and define the width of an active area  175  of free layer  174  by preventing free layer rotation in end regions  171 . 
     FIG. 5B shows a side view of GMR spin valve stack  170 . GMR stack  170  includes exchange tabs  172 , free layer  174 , spacer layer  176 , pinned layer  178  and pinning layer  180 . Pinned layer  178  is positioned over pinning layer  180 . Spacer layer  176  is positioned over pinned layer  178 . Free layer  174  is positioned over spacer layer  176 . Exchange tabs  172  are positioned over free layer  174  at each end of free layer  174 . 
     FIG. 5C shows a side view of GMR spin valve stack  190 , which is the same as GMR stack  170  shown in FIG. 5B, but is oriented in an opposite direction. Exchange tabs  172  can be used in either configuration of a GMR stack. 
     Exchange tabs  172  are preferably made from an antiferromagnetic material having a high coupling constant and a high blocking temperature, such as NiMn, PtMn or IrMn. PtMn exhibits good corrosion resistance and low coercivity. IrMn also has a low coercivity. The following table provides coupling constants, blocking temperatures and other characteristics of various antiferromagnetic materials: 
     
       
         
               
               
               
               
               
               
               
               
             
           
               
                   
               
               
                 Materials 
                 MnFe 
                 NiMn 
                 IrMn 
                 PtPdMn 
                 NiO 
                 PtMn 
                 CrMn 
               
               
                   
               
             
             
               
                 Blocking 
                 150° 
                 380° 
                 240° 
                 300° 
                 190° 
                 370° 
                 320° 
               
               
                 temp. 
                 C. 
                 C. 
                 C. 
                 C. 
                 C. 
                 C. 
                 C. 
               
               
                 Coupling 
                 0.1 
                 ˜0.3 
                 0.2 
                 0.12 
                 0.09 
                 0.2 
                 0.2 
               
               
                 Constant 
               
               
                 (Erg/cm 2 ) 
               
               
                 Bulk 
                 150 
                 200 
                 200 
                 150 
                 insulator 
                 170 
                 300 
               
               
                 Resistivity 
               
               
                 (micro- 
               
               
                 Ohms 
               
               
                 cm) 
               
               
                 Annealing 
                 No 
                 270° 
                 No 
                 250° 
                 No 
                 270° 
                 230° 
               
               
                 Require- 
                   
                 C. 
                   
                 C. 
                   
                 C. 
                 C. 
               
               
                 ment 
               
               
                   
               
             
          
         
       
     
     Exchange tabs  172  increase the effective Hk of end regions  171  of free layer  174 , thereby reducing the permeability of the end regions  171 . The effective Hk of the end regions  171  of free layer  174  is the sum of H p  (pinning field) and H c  (coercivity). Materials with high coupling constants are desirable for exchange tabs  172  because these materials provide for a greater increase in the effective Hk of end regions  171  and therefore provide for a greater reduction in the side reading. A high blocking temperature is desirable because the pinned regions of the free layer may become unpinned if the temperature of the sensor is raised above the blocking temperature of the exchange tabs during operation. 
     FIG. 6A shows a side view of GMR spin valve stack  200 . GMR stack  200  includes antiferromagnetic layers  202 A and  202 B, CoFe layers  204 A and  204 B, Ru layers  206 A and  206 B, free layer  208 , spacer layer  210 , pinned layer  212  and pinning layer  214 . The combination of antiferromagnetic layer  202 A, CoFe layer  204 A, Ru layer  206 A and the portion of free layer  208  located under Ru layer  206 A (i.e., end region  207 A) is referred to as a synthetic antiferromagnet (SAF). Similarly, on the other side of free layer  208 , the combination of antiferromagnetic layer  202 B, CoFe layer  204 B, Ru layer  206 B and the portion of free layer  208  located under Ru layer  206 B (i.e., end region  207 B) is also referred to as a SAF. When two ferromagnetic layers, such as CoFe layers  204  and free layer  208  are separated by an Ru spacer of an appropriate thickness, the two ferromagnetic layers couple strongly with magnetic moments anti-parallel as shown by the arrows on these layers in FIG.  6 A. The coupling between CoFe layers  204  and free layer  208  results in a very large effective Hk in the end regions  207  of free layer  208 . If a SAF is set along the air bearing surface direction of a sensor, an applied field perpendicular to the air bearing surface direction cannot easily rotate the two ferromagnetic layers  204  and  208 . Therefore, the stiffness of free layer  208  in the end regions  207  is greatly enhanced (by up to a factor of  10 ). In a preferred embodiment, CoFe layers  204  are about  20  angstroms thick, Ru spacer layers  206  are about 10 angstroms thick, and free layer  208  is about 30 angstroms thick. For these preferred dimensions, a field of 5000 Oe is needed to rotate the pinned regions  207 . Pinned layer  212  may also be a SAF structure to provide enhanced stiffness. 
     FIG. 6B shows a side view of GMR spin valve stack  220 . GMR stack  220  includes antiferromagnetic layers  222 A and  222 B, CoFe layers  224 A and  224 B, Ru layers  226 A and  226 B, CoFe layers  228 A and  228 B, Ru layers  230 A and  230 B, free layer  232 , spacer layer  234 , pinned layer  236  and pinning layer  238 . GMR stack  220  is the same as GMR stack  200  shown in FIG. 6A, but includes additional CoFe layers  224  and Ru layers  226 . The addition of CoFe layers  224  and Ru layers  226  balances extra flux along the air bearing surface and thereby eliminates the stray field from unbalanced exchange tabs like those shown in FIG.  6 A. The extra flux is caused by the generation of poles in CoFe layers  204 . Pinned layer  236  may also be a SAF structure to provide enhanced stiffness. The magnetizations of various layers are shown by arrows in FIG.  6 B. 
     Prior to describing a preferred method for fabricating spin valve sensors with exchange tabs in accordance with the present invention, conventional fabrication techniques will be discussed with reference to FIGS. 7A-7D. FIGS. 7A-7D illustrate a conventional patterning method which can be used to fabricate a sensor  250  with exchange tabs. A major difficulty in manufacturing sensors with exchange tabs using conventional methods is establishing adequate exchange coupling between the exchange tabs and the free layer. As shown in FIG. 7A, a photoresist lift-off mask  252  is typically present during deposition of the exchange tab material on free layer  256 . Unless a cap layer is used, producing the lift-off mask  252  necessarily involves exposing free layer  256  to atmosphere, photoresist and developer. At a minimum, a layer of oxide forms on free layer  256 . For this reason, a cap layer is frequently used to prevent oxidation or other chemical reactions. In the discussion that follows, layer  254  is referred to as a “residue”layer, and represents any material such as oxide or a cap layer that would adversely affect the necessary exchange coupling. Whether a cap layer is used or not, it is necessary to employ some removal process such as a pre-sputter etch or reactive ion etch on residue layer  254  to expose a clean, unoxidized surface of free layer  256  to establish exchange coupling with deposited exchange tab material. A single monolayer of non-magnetic material is sufficient to destroy the exchange coupling. The removal step is problematic because of the presence of photo mask  252 . 
     An in situ pre-sputter etch is typically performed to remove residue layer  254 , but the presence of photo mask  252  makes it extremely difficult to remove residue close to photo mask  252 . (See FIG.  7 B). The excess residue  258 A and  258 B near photo mask  252  prevents proper exchange coupling between exchange tab material  262 A and  262 B (which is deposited on top of free layer  256 ) and free layer  256 . (See FIG.  7 C). Without this exchange coupling, the portions of free layer  256  under excess residue  258 A and  258 B rotate in response to field from the media, producing an off-track signal. FIG. 7D illustrates the effective width of the sensor in the presence of a feathered tail of excess residue  258 . A final step in the conventional method is to deposit contact material  260 A and  260 B over exchange tab material  262 . 
     Photo mask  252  causes another problem during the deposition of the exchange tab material  262 . Near the edge of the photo mask  252 , some amount of shadowing is inevitable, causing reduced thickness of exchange tab material  262  near photo mask  252 . Thus, even if it were possible using conventional techniques to completely remove the residue  254  near photo mask  252 , the reduced thickness of the exchange tab material  262  near photo mask  252  would still cause a problem. Since the pinning strength of exchange tabs  262  is strongly dependent on thickness, it is necessary to compensate for the reduced thickness near the photo mask  252  by increasing the overall layer thickness to provide a margin of safety. As the size of recording heads shrink, thinner layers become more attractive. Collimated deposition techniques can be employed to avoid thinning, but such an approach adds an extra constraint to the deposition process. It would be preferable to optimize for material quality with no geometry constraints. 
     Another issue that arises with conventional techniques is the matching of magnetic flux at the sensor edge. A pre-sputter etch to remove residue  254  will necessarily remove magnetic material from free layer  256 , thereby reducing the moment of free layer  256  and creating a mismatch of magnetic flux. This material can be replaced by an in situ deposition, but the deposition can never be perfect, especially in the vicinity of photo mask  252 . 
     The method illustrated in FIGS. 8A-8D alleviates the problems associated with the previously described conventional techniques. FIG. 8A shows photo mask  272 , exchange tab layer  274  and free layer  276 . Rather than starting with a photo mask  272 , the exchange tab layer  274  is deposited on free layer  276  before patterning. (See FIG.  7 A). Since the deposition takes place in a vacuum, free layer  276  is not exposed to air prior to the addition of exchange tab layer  274 . This ensures that the interface between free layer  276  and exchange tab layer  274  is clean and uncomplicated by the presence of features. At this stage of the process, the free layer  276  is not free, but pinned. Photo mask  272  is deposited on a central region of exchange tab layer  274 . Electrical contacts  278 A and  278 B are then deposited using conventional procedures (See FIG.  8 B). Photo mask  272  is removed. (See FIG.  8 C). The final step is a mill of exchange tab material  274  between contacts  278 . (See FIG.  8 D). The electrical contacts  278 A and  278 B act as a mask during the milling step. Once the exchange tab material  274  between contacts  278  is removed, the portion of free layer  276  between contacts  278  is once again free to rotate. The portions of free layer  276  located under contacts  278  remain pinned. The active area or reader width is shown in FIG.  8 D. As can be seen in FIG. 8D, there is no widening of the reader width caused by excess residue. To highlight novel aspects of the present method, FIGS. 8A-8D only show the top few layers of spin valve  270 . A spacer layer, pinned layer and pinning layer are also part of spin valve  270 , and are deposited using conventional techniques. 
     It will be recognized by those of ordinary skill in the art that the method described above with respect to FIGS. 8A-8D may also be used to fabricate spin valve sensors having exchange tabs comprising synthetic antiferromagnets. For such sensors, rather than being a single layer of antiferromagnetic material, exchange tab layer  274  represents a multi-layer stack comprising an antiferromagnetic layer, one or more CoFe layers and one or more Ru layers. 
     FIGS. 9A-9C show graphs of various sensor characteristics versus time as the mill step (discussed above with respect to FIG. 8D) progresses. FIG. 9A shows M s  (left vertical axis) of free layer  276  in normalized units and ΔS (right vertical axis) versus time. M s  represents the moment of free layer  276  and ΔS represents change in conductance. M s  is proportional to the thickness of free layer  276 . The graph shows that M s  is constant until approximately  18  minutes into the etch, where it begins to drop linearly. Where M s  is constant, the exchange tab material  274  is being etched away. At 18 minutes, the exchange tab material  274  has been completely removed and free layer  276  is being etched. This is consistent with the ΔS data, which also shows a sharp drop around 18 minutes. The slope of the ΔS versus time curve is expected to equal rσ, where r is the mill rate, and σ is the conductivity. A change in slope indicates a change in material. 
     FIG. 9B shows the GMR ratio (left vertical axis) and ΔR (right vertical axis) versus etch time. The GMR ratio represents the fractional change in resistance when free layer  276  is rotated, and ΔR represents the absolute change in resistance when free layer  276  is rotated. Both characteristics rise steadily as the exchange tab material  274  is milled away and exhibit a plateau before free layer  276  is reached, at which point the characteristics begin to drop precipitously. 
     FIG. 9C shows H 1  (left vertical axis) and H c1  (right vertical axis) versus etch time. H 1  represents the strength of the coupling between exchange tab material  274  and free layer  276 . H c1  represents the coercivity of free layer  276 . The values of GMR, ΔR, H 1  and H 1 c1  from a sensor fabricated using the method illustrated in FIGS. 8A-8D are all indistinguishable from the values of an as-grown spin valve sensor. There is little or no degradation in the GMR characteristics due to the milling step of the preferred method. 
     The method illustrated in FIGS. 8A-8D provides numerous advantages over the use of conventional methods. A “perfect” interface is obtained between exchange tab material  274  and free layer  276  because there is no photo mask  272  present to cause excess residue, shadowing or other complications. The exchange tabs and the electrical contacts  278 A and  278 B are automatically aligned during the process. The milling of exchange tab material  274  can be stopped before removing any portion of the free layer  276 , because the presence of contacts  278  allows the transfer curve characteristics of the sensor to be monitored during the mill. Examination of the transfer curve would show if the mill time were too short. A wafer could be milled for an additional period until the desired results are obtained. Thus, the moment of free layer  276  is not reduced and there is not a problem with mis-matched flux, stray fields or shield saturation. In addition, the process is easy to perform in high volume. 
     In an alternative preferred method for forming a spin valve sensor with exchange tabs, rather than freeing free layer  276  by milling the exchange tab material  274  between contacts  278 A and  278 B as described above and shown in FIG. 8D, the exchange tab material  274  between contacts  278 A and  278 B is exposed to a reactive plasma that reacts with at least some of the materials in the exchange tab material  274 . In a preferred embodiment, exchange tab material  274  is exposed to oxygen, CF 4 , CHF 3  or similar gas. Contacts  278  act as a mask during the process. The process is referred to as a two step reactive ion etch (RIE), wherein the first step is removal of a tantalum capping layer that is typically present over the exchange tab material  274 , and the second step is exposure to the reactive plasma. Even though the process is referred to as an “etch”, in a preferred embodiment, the exchange tab material  274  is actually left in place after exposure to the reactive plasma, and is not etched away. The reaction changes the chemical composition of the exchange tab material  274  in the region between contacts  278 , and results in a new material, which will be referred to as “AFM*”. The reaction ruins the exchange coupling between free layer  276  and exchange tab material  274  in the region between contacts  278 , and thereby frees free layer  276  in this region. 
     FIG. 10 shows a side view of spin valve  300 , which is formed by the alternative preferred method. Spin valve  300  includes contacts  302 A and  302 B exchange tabs  304 A and  304 B, AFM* material  304 C and GMR stack  306 . As with the GMR stacks discussed above, GMR stack  306  includes a pinned layer, a pinning layer, a spacer layer and a free layer. The free layer of GMR stack  306  is positioned at the top of GMR stack  306 , and is adjacent to AFM* material  304 C and exchange tabs  304 A and  304 B. As can be seen in FIG. 10, AFM* material  304 C remains after processing is complete. After the reactive ion etch, the exchange coupling between AFM* material  304 C and the portion of the free layer located under AFM* material  304 C is destroyed, while exchange tabs  304 A and  304 B remain exchange coupled to the free layer. 
     The alternative preferred method provides additional advantages over the method illustrated in FIGS. 8A-8D. The additional advantages will be discussed with respect to the graphs shown in FIGS. 11-13. 
     FIG. 11A shows a graph of GMR versus etch time for the method illustrated in FIGS. 8A-8D. GMR remains essentially constant until approximately 120 seconds into the etch. At about 120 seconds, exchange tab material  274  has been milled away and free layer  276  is attacked. As free layer  276  is milled, GMR decreases sharply. 
     FIG. 11B shows a graph of the strength of coupling between free layer  276  and pinned layer  178  (which is represented by H1) versus etch time in seconds for the method illustrated in FIGS. 8A-8D. As shown in FIG. 11B, the coupling strength starts out fairly constant and then decreases as exchange tab material  274  is milled. At approximately  120  seconds into the etch, free layer  276  is attacked and the coupling strength sharply increases. 
     FIG. 12A shows a graph of GMR versus etch time for the alternative preferred method. As shown in FIG. 12A, the GMR rises during the first couple of minutes of the etch, and then levels off and remains substantially constant. The GMR does not decrease even after an extended exposure to the reactive plasma. 
     FIG. 12B shows a graph of the coupling strength between free layer  276  and pinned layer  178  versus etch time for the alternative preferred method. The coupling strength decreases during the first couple of minutes into the process, and then remains substantially constant. The coupling strength does not increase even after extended exposure to the reactive plasma. 
     In comparing FIG. 11A with FIG.  12 A and FIG. 11B with FIG. 12B, it is evident that the alternative preferred method has a greater process latitude than the method illustrated in FIGS. 8A-8D. For the method illustrated in FIGS. 8A-8D, if the exchange tab material  274  is milled too long, free layer  276  will be attacked, resulting in a decrease in the GMR effect (see FIG. 11A) and an increase in the coupling between free layer  276  and pinned layer  178  (see FIG.  11 B). The increased coupling between free layer  276  and pinned layer  178  is undesirable because it results in poor symmetry in the final device. The magnetization of free layer  276  should be perpendicular to pinned layer  178 , but as the coupling between the two layers gets stronger, the magnetization of free layer  276  will tend to move parallel to pinned layer  178 . 
     There is approximately a 20-30 second ion mill time window for a given wafer using the method illustrated in FIGS. 8A-8D. For example, assuming that the coupling strength H 1  must be less than about 18 Oe, the milling of exchange tab material  274  must stop within about 10-15 seconds before or after the time at which the minimum coupling strength occurs (See FIG.  11 B). If the exchange tab material  274  is not milled long enough, there will be hysteresis in the R versus H loop, which will show up as noise in the final device. In addition, the excess exchange tab material  274  may result in shunting. If there is any mill rate variation or wafer-to-wafer variation in the thickness or uniformity of exchange tab material  274 , the process could end up outside of the  20 - 30  second window and all devices could be ruined. 
     The alternative preferred method provides a larger process latitude by allowing the use of long process times to ensure that the exchange coupling between free layer  276  and exchange tab material  274  is destroyed on all of the devices without harming the free layer on any of the devices. The alternative preferred method ensures that all devices are processed sufficiently and none are processed too long. The alternative preferred method is not as sensitive to wafer-to-wafer thickness variations and poor uniformity as the method illustrated in FIGS. 8A-8D. Furthermore, the alternative preferred method does not use ion milling, which may cause ESD or EOS failures. 
     FIG. 13 shows a graph of sheet resistance of AFM* material  304 C versus thickness before and after the reactive ion etch. For the graph, IrMn was used for AFM* material  304 C. As shown in FIG. 13, AFM* material  304 C has a very high resistivity after the reactive ion etch. The high resistivity ensures that AFM* material  304 C will not shunt any of the read back current away from the spin valve sensor. The high resistivity also contributes to an increase in GMR of spin valve sensors. After the reactive ion etch, the remaining high resistivity AFM* material  304 C may also help prevent reader-to-shield shorting. The GMR stack cannot contact a shield or the device will be ruined. Because of the high resistivity of the AFM* layer  304 C, AFM* layer  304 C may act as an insulator between the shield and the GMR stack and prevent shorting. 
     Patterned devices have been formed on top of AFM* layer  304 C and then exposed to various processing conditions such as photoresist stripping, plasma etches, and spin rinse dryers, and no delamination of the films was observed. The AFM* material  304 C, therefore, appears to be chemically stable and should not corrode in later steps of processing. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.