Patent Publication Number: US-6907655-B2

Title: Method for manufacturing a spin valve having an enhanced free layer

Description:
REFERENCE TO RELATED APPLICATION 
     This is a divisional of application Ser. No. 09/753,968 filed Jan. 2, 2001, now U.S. Pat. No. 6,700,757. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an enhanced free layer for a spin valve sensor and, more particularly, to such a free layer and a method of making wherein a desirable negative ferromagnetic coupling field is maintained when a copper layer is located between the free layer and a capping layer for the purpose of increasing a magnetoresistive coefficient dr/R of the spin valve sensor. 
     2. Description of the Related Art 
     The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic field signals from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions. 
     An exemplary high performance read head employs a spin valve sensor for sensing the magnetic field signals from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90° to the air bearing surface (ABS). First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic signal fields from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is preferably parallel to the ABS, is when the sense current is conducted through the sensor without magnetic field signals from the rotating magnetic disk. If the quiescent position of the magnetic moment is not parallel to the ABS the positive and negative responses of the free layer will not be equal which results in read signal asymmetry, which is discussed in more detail hereinbelow. 
     The sensitivity of the spin valve sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in resistance of the spin valve sensor from minimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the spin valve sensor at minimum resistance. Because of the high magnetoresistance of a spin valve sensor it is sometimes referred to as a giant magnetoresistive (GMR) sensor. Changes in resistance of the spin valve sensor are a function of cos θ, where θ is the angle between the magnetic moments of the pinned and free layers. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals from the rotating magnetic disk. 
     The transfer curve for a spin valve sensor is defined by the aforementioned cos θ where θ is the angle between the directions of the magnetic moments of the free and pinned layers. The bias point should be located midway between the top and bottom of the transfer curve. When the bias point is located below the midway point the spin valve sensor is negatively biased and has positive asymmetry and when the bias point is above the midway point the spin valve sensor is positively biased and has negative asymmetry. The location of the transfer curve relative to the bias point is influenced by four major forces on the free layer of a spin valve sensor, namely a ferromagnetic coupling field H FC  between the pinned layer and the free layer, a net demagnetizing (demag) field HD from the pinned layer, a sense current field H I  from all conductive layers of the spin valve except the free layer, a net image current field H IM  from the first and second shield layers. The strongest magnetic force on the free layer structure is the sense current field H I . 
     SUMMARY OF THE INVENTION 
     In the present invention a negative ferromagnetic coupling field −H FC  is obtained for the purpose of counterbalancing other magnetic fields acting on the free layer so as to more adequately position the bias point on the transfer curve of the spin valve sensor. In a preferred embodiment this is accomplished by providing a pinning layer which is composed of platinum manganese (PtMn) and providing a first seed layer composed of nickel manganese oxide (NiMnO) and a second seed layer composed of tantalum (Ta) wherein the first seed layer interfaces the first read gap layer, which is composed of aluminum oxide (Al 2 O 3 ), and the second seed layer is located between the first seed layer and the pinning layer. The invention further includes a copper (Cu) layer which is located between the free layer and a capping layer wherein the capping layer is preferably tantalum (Ta). The purpose of the copper (Cu) layer, which is also referred to as a spin filter layer, is to increase the magnetoresistive coefficient dr/R. Unfortunately, the spin filter layer reduces the magnitude of the negative ferromagnetic coupling field which is being sought for proper balancing of the free layer. Further, the spin filter layer can result in a decrease of the magnetoresistive coefficient dr/R instead of an increase. 
     The present invention obviates reduction of the negative ferromagnetic coupling field by oxidizing a top of the free layer before formation of the capping layer. This may be accomplished by first sputter depositing the top of the free layer, which may be nickel iron (NiFe) or cobalt iron (CoFe) or cobalt (Co), and then introducing oxygen into a sputtering chamber for oxidizing the top of the deposited layer. Accordingly, the free layer has an oxidized film portion and an unoxidized film portion wherein the oxidized film portion is located between the unoxidized film portion and the capping layer. In my experiments I have shown that without the spin filter layer the negative ferromagnetic coupling field −H FC  is about −16 Oe, that when the spin filter layer is added the negative ferromagnetic coupling field −H FC  is degraded to about −8 Oe, and that when the top of a nickel iron (NiFe) free layer is oxidized before forming the capping layer that the negative ferromagnetic coupling field −H FC  is restored to −16 Oe. Further studies optimized the magnetoresistive coefficient dr/R of the present invention by appropriately sizing the thickness of the copper layer. The magnetoresistive coefficient dr/R was maximized when the thickness of the copper layer was about 6 Å. The invention also includes oxidizing fully or a top portion of the copper layer and/or oxidizing top portions of multiple films of the free layer and capping layers. 
     Another aspect of the invention is that when the copper spacer layer of the spin valve sensor is made thinner the dr/R is increased. However, when the thickness of the spacer layer is decreased the ferromagnetic coupling field increases which may adversely affect the biasing of the free layer. The present invention enables the spin filter layer to be employed for increasing the dr/R in combination with a thinner spacer layer for further increasing the dr/R. When a negative ferromagnetic coupling field −H FC  of −16 Oe is obtained by the present invention the more positive ferromagnetic coupling field due to a thinner spacer layer is offset by the −16 Oe. A resultant −8 Oe or lower can still be used effectively for properly biasing the free layer. 
     An object of the present invention is to provide a spin valve sensor wherein a negative ferromagnetic coupling field H FC  is not degraded when a copper layer is employed between a free layer and a capping layer for the purpose of increasing the magnetoresistive coefficient dr/R of the spin valve sensor. 
     Another object is to accomplish the foregoing object as well as appropriately sizing the copper layer so as to optimize the magnetoresistive coefficient dr/R. 
     Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of an exemplary magnetic disk drive; 
         FIG. 2  is an end view of a slider with a magnetic head of the disk drive as seen in plane  2 — 2  of  FIG. 1 ; 
         FIG. 3  is an elevation view of the magnetic disk drive wherein multiple disks and magnetic heads are employed; 
         FIG. 4  is an isometric illustration of an exemplary suspension system for supporting the slider and magnetic head; 
         FIG. 5  is an ABS view of the magnetic head taken along plane  5 — 5  of  FIG. 2 ; 
         FIG. 6  is a partial view of the slider and a piggyback magnetic head as seen in plane  6 — 6  of  FIG. 2 ; 
         FIG. 7  is a partial view of the slider and a merged magnetic head as seen in plane  7 — 7  of  FIG. 2 ; 
         FIG. 8  is a partial ABS view of the slider taken along plane  8 — 8  of  FIG. 6  to show the read and write elements of the piggyback magnetic head; 
         FIG. 9  is a partial ABS view of the slider taken along plane  9 — 9  of  FIG. 7  to show the read and write elements of the merged magnetic head; 
         FIG. 10  is a view taken along plane  10 — 10  of  FIG. 6  or  7  with all material above the coil layer and leads removed; 
         FIG. 11  is an enlarged isometric illustration of a read head which has a spin valve sensor; 
         FIG. 12  is an ABS illustration of a spin valve sensor wherein a negative ferromagnetic coupling field −H FC  is obtained; 
         FIG. 13  is the same as  FIG. 12  except a copper layer is located between the free layer and a capping layer; 
         FIG. 14  is the same as  FIG. 13  except a top portion of the free layer has been oxidized; 
         FIG. 15  is the same as  FIG. 14  except a top portion of the copper layer is also oxidized; 
         FIG. 16  is the same as  FIG. 15  except the copper spacer layer is 6 Å thick instead of 10 Å thick; 
         FIG. 17  is a chart showing the change in a negative ferromagnetic coupling field −H FC  in various Examples 1-4; 
         FIG. 18  is a chart showing the change in resistance R of the spin valve sensor with various thicknesses of a copper layer in Example 3; 
         FIG. 19  is a chart showing the change in resistance dr of the spin valve sensor with various thicknesses of the copper layer in Example 3; 
         FIG. 20  is a chart showing the change in a magnetoresistive coefficient dr/R of the spin valve sensor with various thicknesses of the copper layer in Example 3; 
         FIG. 21  is a chart showing the change in uniaxial anisotropy field H K with  various thicknesses of the copper layer in Example 3; 
         FIG. 22  is a change in easy axis coercivity H C  of a spin valve sensor with various thicknesses of the copper layer in Example 3; 
         FIG. 23  is an ABS illustration of another embodiment of the invention wherein the free layer has a top film composed of cobalt iron (CoFe) which has a top oxidized portion; 
         FIG. 24  is the same as  FIG. 23  except additional films composed of nickel iron (NiFe) and cobalt iron (CoFe) of the free layer have oxidized portions; and 
         FIG. 25  is the same as  FIG. 24  except the copper layer has a top oxidized portion. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Magnetic Disk Drive 
     Referring now to the drawings wherein like reference numerals designate like or similar parts throughout the several views,  FIGS. 1-3  illustrate a magnetic disk drive  30 . The drive  30  includes a spindle  32  that supports and rotates a magnetic disk  34 . The spindle  32  is rotated by a spindle motor  36  that is controlled by a motor controller  38 . A slider  42  has a combined read and write magnetic head  40  and is supported by a suspension  44  and actuator arm  46  that is rotatably positioned by an actuator  47 . A plurality of disks, sliders and suspensions may be employed in a large capacity direct access storage device (DASD) as shown in FIG.  3 . The suspension  44  and actuator arm  46  are moved by the actuator  47  to position the slider  42  so that the magnetic head  40  is in a transducing relationship with a surface of the magnetic disk  34 . When the disk  34  is rotated by the spindle motor  36  the slider is supported on a thin (typically, 0.05 μm) cushion of air (air bearing) between the surface of the disk  34  and the air bearing surface (ABS)  48 . The magnetic head  40  may then be employed for writing information to multiple circular tracks on the surface of the disk  34 , as well as for reading information therefrom. Processing circuitry  50  exchanges signals, representing such information, with the head  40 , provides spindle motor drive signals for rotating the magnetic disk  34 , and provides control signals to the actuator for moving the slider to various tracks. In  FIG. 4  the slider  42  is shown mounted to a suspension  44 . The components described hereinabove may be mounted on a frame  54  of a housing, as shown in FIG.  3 . 
       FIG. 5  is an ABS view of the slider  42  and the magnetic head  40 . The slider has a center rail  56  that supports the magnetic head  40 , and side rails  58  and  60 . The rails  56 ,  58  and  60  extend from a cross rail  62 . With respect to rotation of the magnetic disk  34 , the cross rail  62  is at a leading edge  64  of the slider and the magnetic head  40  is at a trailing edge  66  of the slider. 
       FIG. 6  is a side cross-sectional elevation view of a piggyback magnetic head  40 , which includes a write head portion  70  and a read head portion  72 , the read head portion employing a dual spin valve sensor  74  of the present invention.  FIG. 8  is an ABS view of FIG.  6 . The spin valve sensor  74  is sandwiched between nonmagnetic electrically insulative first and second read gap layers  76  and  78 , and the read gap layers are sandwiched between ferromagnetic first and second shield layers  80  and  82 . In response to external magnetic fields, the resistance of the spin valve sensor  74  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  50  shown in FIG.  3 . 
     The write head portion  70  of the magnetic head  40  includes a coil layer  84  sandwiched between first and second insulation layers  86  and  88 . A third insulation layer  90  may be employed for planarizing the head to eliminate ripples in the second insulation layer caused by the coil layer  84 . The first, second and third insulation layers are referred to in the art as an “insulation stack”. The coil layer  84  and the first, second and third insulation layers  86 ,  88  and  90  are sandwiched between first and second pole piece layers  92  and  94 . The first and second pole piece layers  92  and  94  are magnetically coupled at a back gap  96  and have first and second pole tips  98  and  100  which are separated by a write gap layer  102  at the ABS. An insulation layer  103  is located between the second shield layer  82  and the first pole piece layer  92 . Since the second shield layer  82  and the first pole piece layer  92  are separate layers this head is known as a piggyback head. As shown in  FIGS. 2 and 4 , first and second solder connections  104  and  106  connect leads from the spin valve sensor  74  to leads  112  and  114  on the suspension  44 , and third and fourth solder connections  116  and  118  connect leads  120  and  122  from the coil  84  (see  FIG. 10 ) to leads  124  and  126  on the suspension. 
       FIGS. 7 and 9  are the same as  FIGS. 6 and 8  except the second shield layer  82  and the first pole piece layer  92  are a common layer. This type of head is known as a merged magnetic head. The insulation layer  103  of the piggyback head in  FIGS. 6 and 8  is omitted. 
       FIG. 11  is an isometric ABS illustration of the read head  72  shown in  FIG. 6  or  8 . The read head  72  includes the present dual spin valve sensor  74  which is located on an antiferromagnetic (AFM) pinning layer  132 . First and second hard bias and lead layers  134  and  136  are connected to first and second side edges  138  and  140  of the spin valve sensor. This connection is known in the art as a contiguous junction and is fully described in commonly assigned U.S. Pat. No. 5,018,037 which is incorporated by reference herein. The first hard bias and lead layers  134  include a first hard bias layer  140  and a first lead layer  142  and the second hard bias and lead layers  136  include a second hard bias layer  144  and a second lead layer  146 . The hard bias layers  140  and  144  cause magnetic fields to extend longitudinally through the spin valve sensor  130  for stabilizing the magnetic domains therein. The spin valve sensor  130  and the first and second hard bias and lead layers  134  and  136  are located between nonmagnetic electrically insulative first and second read gap layers  148  and  150 . The first and second read gap layers  148  and  150  are, in turn, located between ferromagnetic first and second shield layers  152  and  154 . 
     EXAMPLE 1 
       FIG. 12  shows a spin valve sensor  200  which is located on the first read gap (G1)  148  wherein the first read gap is composed of aluminum oxide (Al 2 O 3 ). The spin valve sensor  200  includes a pinned layer  202  which has a magnetic moment  204  which is pinned by an antiferromagnetic (AFM) pinning layer  206 . The magnetic moment  204  is pinned perpendicular to the ABS in a direction toward the ABS or away from the ABS, as shown in FIG.  12 . In this example the pinned layer was an antiparallel (AP) pinned layer which included an antiparallel coupling layer (APC)  208  which was located between ferromagnetic first and second antiparallel layers (AP 1 ) and (AP 2 )  210  and  212 . A spacer layer (S)  214  is located between the second AP pinned layer  212  and a free layer  216  which has a magnetic moment  218  which is oriented parallel to the ABS and parallel to the major thin film surfaces of the layers when the bias point of the spin valve sensor is located midway on its transfer curve. The free layer  216  includes first and second free films (F 1 ) and (F 2 )  220  and  222 . On top of the second free film  222  is a cap layer  224 . First and second seed layers (SL 1 ) and (SL 2 )  226  and  228  are provided between the first read gap layer  148  and the pinning layer  206  with the second seed layer  228  being located between the first seed layer  226  and the pinning layer  206 . 
     Exemplary thicknesses of the Al 2 O 3  first read gap layer  146  are 200Å-700 Å. The thicknesses and materials of the other layers are 30 Å of nickel manganese oxide (NiMnO) for the first seed layer  226 , 35 Å of tantalum (Ta) for the second seed layer  228 , 175 Å of platinum manganese (PtMn) for the pinning layer  206 , 17 Å of cobalt iron (CoFe) for the first AP pinned layer  210 , 8 Å of ruthenium (Ru) for the antiparallel coupling layer  208 , 26 Å of cobalt iron (CoFe) for the second AP pinned layer  212 , 20 Å of copper (Cu) for the spacer layer  214 , 15 Å of cobalt iron (CoFe) for the first free film  220 , 25 Å of nickel iron (NiFe) for the second free film  222  and 50 Å of tantalum (Ta) for the cap layer  224 . 
     A sense current I S  may be directed from right to left or from left to right as shown in FIG.  12 . When a field signal from a rotating magnetic disk rotates the magnetic moment  218  upwardly into the sensor the magnetic moments  204  and  218  become more parallel which reduces the resistance of the sensor to the sense current I S  and when a field signal from the rotating magnetic disk rotates the magnetic moment  218  downwardly out of the sensor the magnetic moments  204  and  218  become more antiparallel which increases the resistance of the sensor to the sense current I S . These increases and decreases in the resistance of the spin valve sensor are processed as playback signals by the processing circuitry  50  in FIG.  3 . 
     As discussed in the Summary of the Invention various magnetic forces affect the positioning of the magnetic moment  218  of the free layer. The free layer is properly biased by these magnetic forces when the magnetic moment  218  is parallel to the ABS, as shown in  FIG. 12. A  negative ferromagnetic coupling field −H FC  is desirable for counterbalancing one or more of these magnetic forces for properly biasing the free layer  216 . This negative ferromagnetic coupling field has been achieved by employing the pinning layer  206  composed of platinum manganese (PtMn) and the first and second seed layers  226  and  228  which are composed of nickel manganese oxide (NiMnO) and tantalum (Ta) respectively. Further, the first seed layer  226  must interface an aluminum oxide (Al 2 O 3 ) first read gap layer  148  or be located on an aluminum oxide (Al 2 O 3 ) seed layer which is approximately 30 Å thick. The spin valve sensor  200  in  FIG. 12  was tested for its ferromagnetic coupling field −H FC  and it was found to be −16 Oe, which is shown at Example 1 in FIG.  17 . 
     It should be noted that when the spacer layer  214  is decreased in thickness that the magnetoresistive coefficient dr/R is increased. However, when the spacer layer  214  is decreased in thickness the ferromagnetic coupling field −H FC  increases in a positive direction, which may adversely affect biasing of the free layer  216 . Accordingly, the negative ferromagnetic coupling field −H FC  of −16 Oe in Example 1 is desirable because the thickness of the spacer layer  214  can now be reduced to increase the dr/R and the increase in the ferromagnetic coupling field due to the thinner spacer layer can be offset by a part of the −16 Oe. 
     EXAMPLE 2 
       FIG. 13  shows a spin valve sensor  300  which is the same as the spin valve sensor  200  in  FIG. 12  except a spin filter layer (SF)  302  is located between the second free film  222  and the cap layer  224 . Upon testing the spin valve sensor  300  it was found that the negative ferromagnetic coupling field −H F  was −8 Oe as shown by Example 2 in FIG.  17 . It can be seen that the negative magnetic coupling field of Example 2 had dropped by 50% as compared to the negative ferromagnetic coupling field −H F  for Example 1 in FIG.  12 . The thickness and material of the spin filter layer  302  was 10 Å of copper (Cu). While a negative ferromagnetic coupling field −H FC  of −8 Oe may be desirable for biasing the free layer  216 , it is too low to provide any offset when the thickness of the spacer layer  214  is decreased for the purpose of further increasing the dr/R. 
     EXAMPLE 3 
     The spin valve sensor  400  in  FIG. 14  is the same as the spin valve sensor  300  in  FIG. 13  except a top portion  402  of the second free film  222  has been oxidized. Accordingly, the second free film has an unoxidized portion  222  of nickel iron (NiFe) and an oxidized portion  402  which is composed of nickel iron oxide (NiFeO). The oxidized film portion  402  is located directly between the unoxidized film portion  222  and the spin filter layer  302 . After sputter depositing the second free film  222  oxygen was exposed into the sputtering chamber and the second film was exposed to the oxygen for 30 seconds at approximately 2×10 −5  Torr. This exposure caused the oxidized portion  402  to develop. The spin valve sensor  400  was tested for its negative ferromagnetic coupling field −H FC  and it was found to be −16 Oe which is shown at Example 3 in FIG.  17 . Accordingly, the present invention, shown in  FIG. 14 , completely restored the negative ferromagnetic coupling field to a value obtained in Example 1 so that the spin filter layer  302  can be employed for obtaining the advantages of the spin filter layer as explained hereinbelow. It is speculated that the increase in the negative ferromagnetic coupling is due to the fact that the oxidization caused a smoother interface between the layers  222  and  302 . 
     The thickness of the copper layer  302  in  FIG. 14  was then varied in order to determine the effect of this thickness on resistance R of the spin valve sensor, the effect on the change in resistance dr of the spin valve sensor, the change on the magnetoresistive coefficient dr/R of the spin valve sensor, the change in uniaxial anisotropy field H K  of the spin valve sensor and the change in easy axis coercivity H C  of the spin valve sensor, as shown in  FIGS. 17-20 , respectively. The thickness of the spin filter layer  302  was tested without the spin filter layer and then with thicknesses of the spin filter layer of 5 Å, 10 Å, 15 Å and 20 Å. Without the spin filter it can be seen from  FIG. 17  that the resistance R was 24, that from  FIG. 18  the change in resistance dr was 1.8, from  FIG. 19  that the magnetoresistive coefficient dr/R was 8, from  FIG. 20  that the uniaxial anisotropy field H K  was 12.5 and from  FIG. 21  that the easy axis coercivity H C  was 7. When the spin filter layer was 5 Å thick the resistance R was 23, the change in resistance dr was 1.9, the magnetoresistive coefficient dr/R was 8.3, the uniaxial anisotropy field H K  was 10 Oe and the coercivity H C  was 6.7 Oe, as shown in  FIGS. 17-20 , respectively. When the thickness of the spin filter layer was increased to 10 Å the resistance R was 22, the change in resistance dr was 1.75, the magnetoresistive coefficient dr/R was 8.15, the uniaxial anisotropy field H K  was 9 Oe and the coercivity H C  was 6.2 Oe, as shown in  FIGS. 17-20 , respectively. When the thickness of the spin filter layer was increased to 15 Å the resistance R was 20.5, the change in resistance dr was 1.6, the magnetoresistive coefficient dr/R was 7.95, the uniaxial anisotropy field H K  was 9.5 Oe and the coercivity H C  was 15 Oe, as shown in  FIGS. 17-20 , respectively. When the thickness of the spin filter layer was further increased to 20 Å the resistance R was 19, the change in resistance dr was 1.45, the magnetoresistive coefficient dr/R was 7.65, the uniaxial anisotropy field H K  was 10 Oe and the coercivity H C  was 6.4 Oe, as shown in  FIGS. 17-20 . It is desirable that the thickness of the spin filter layer be optimized for maximizing the magnetoresistive coefficient dr/R as shown in FIG.  19 . Accordingly, optimum thickness for the spin filter layer is approximately 6 Å which will achieve a magnetoresistive coefficient dr/R of 8.45. It can also be seen from  FIG. 20  that when the thickness of the spin filter layer is 6 Å that the uniaxial anisotropy field H K  is at a minimum at 7.5 Oe. This is desirable so that the free layer has soft magnetic characteristics for responding freely to field signals from the rotating magnetic disk. Further, the coercivity H C  in  FIG. 21  is nearer its low point when the thickness of the spin filter layer is about 6 Å. This further indicates that the free layer has soft magnetic properties which are desirable. A desirable range for the thickness of the spin filter layer would be between 5-7 Å, as shown from FIG.  19 . When the thickness of the spin filter layer was 6 Å the resistance R was 22, the change in resistance dr was 1.85, the magnetoresistive coefficient dr/R was 8.45, the uniaxial anisotropy field H K  was 7.5 Oe and the coercivity H C  was 6.3 Oe, as shown in  FIGS. 17-20 , respectively. 
     EXAMPLE 4 
     The spin valve sensor  500  in  FIG. 15  is the same as the spin valve sensor  400  in  FIG. 14  except a top portion  502  of the spin filter layer  302  has been oxidized. Accordingly, the spin filter layer has an unoxidized film portion of copper (Cu) and an oxidized film portion of copper oxide (CuO)  502 . The oxidized film portion  502  is located between the unoxidized film portion  302  and the cap layer  224 . Upon testing the spin valve sensor  500  it was found that the negative ferromagnetic coupling field −H F  was −15 Oe as shown in Example 4 in FIG.  17 . Accordingly, the negative ferromagnetic coupling field of Example 4 is substantially the same as the negative ferromagnetic coupling field of Example 3. 
     EXAMPLE 5 
     The spin valve sensor  550  in  FIG. 16  is the same as the spin valve sensor  500  in  FIG. 15  except the spin filter layer  552  in  FIG. 16  is only 6 Å thick and is shown oxidized throughout its thickness. As stated in Example 3 this thickness is the optimized and preferred thickness for the spin filter layer. This is shown by  FIGS. 17-23 . 
     EXAMPLE 6 
     The spin valve sensor  600  in  FIG. 23  is the same as the spin valve sensor  550  in  FIG. 13  except a free layer  602  is provided which has a third free film (F3)  604  which was deposited as 15 Å of cobalt iron (CoFe). The third free film  604  has an unoxidized film portion of cobalt iron  604  and an oxidized film portion  606  which is cobalt iron oxide (CoFeO). 
     EXAMPLE 7 
     The spin valve sensor  700  in  FIG. 24  is the same as the spin valve sensor  600  in  FIG. 23  except each of the first and second free films  220  and  222  have an unoxidized portion and an oxidized portion. The first free film  220  has an unoxidized film portion of cobalt iron (CoFe) and an oxidized film portion  702  of cobalt iron oxide (CoFeO). The second film  222  has an unoxidized film portion  222  and an oxidized film portion  704  of cobalt iron oxide (CoFeO). 
     EXAMPLE 8 
     The spin valve sensor  800  in  FIG. 25  is the same as the spin valve sensor  700  in  FIG. 24  except the spin filter layer  552  of  FIG. 16  is employed. 
     DISCUSSION 
     All of the oxide films may be formed in the same manner as the oxide film  402  in FIG.  14 . It should be understood that the forming of the various layers may be accomplished in any type of sputtering system, such as RF or DC sputtering, ion beam sputtering or magnetron sputtering. It should be understood that the tantalum (Ta)  224  in all embodiments may be fully or partially oxidized. 
     Clearly, other embodiments and modifications of this invention will occur readily to those of ordinary skill in the art in view of these teachings. Therefore, this invention is to be limited only by the following claims, which include all such embodiments and modifications when viewed in conjunction with the above specification and accompanying drawings.