Abstract:
A tunnel junction having reduced free layer coercivity for improved sensitivity. The tunnel valve has a free layer that has been deposited in the presence of a nitrogen gas, which reduces the coercivity of the free layer, thereby improving the sensor&#39;s sensitivity and performance.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a method of making a spin valve sensor with a controlled ferromagnetic coupling field (H F ) wherein a pinned layer structure is sputter deposited in such a manner that a desired ferromagnetic coupling field (H F ) can be obtained within a wider range of deposition times of a copper spacer layer.  
         [0003]     2. Description of the Related Art  
         [0004]     This is a Continuation In Part of U.S. patent application Ser. No. 10/791,106 filed Mar. 01, 2004 which is a Continuation of patent application Ser. No. 09/892,815, now U.S. Pat. No. 6,728,083 filed Jun. 26, 2001 entitled METHOD OF MAKING A SPIN VALVE SENSOR WITH A CONTROLLED FERROMAGNETIC COUPLING FIELD.  
         [0005]     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 signal fields 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.  
         [0006]     An exemplary high performance read head employs a spin valve sensor for sensing the magnetic signal fields from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive first spacer layer sandwiched between a ferromagnetic pinned layer structure and a ferromagnetic free layer structure. An antiferromagnetic pinning layer interfaces the pinned layer structure for pinning a magnetic moment of the pinned layer structure 90° to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the magnetic disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer structure is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or bias point position in response to positive and negative magnetic field signals from a rotating magnetic disk. The quiescent position, which is parallel to the ABS, is the position of the magnetic moment of the free layer structure with the sense current conducted through the sensor in the absence of signal fields.  
         [0007]     The thickness of the spacer layer is chosen so that shunting of the sense current and a ferromagnetic coupling field (H F ) between the free and pinned layer structures is minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons are scattered at the interfaces of the spacer layer with the pinned and free layer structures. When the magnetic moments of the pinned and free layer structures are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. Changes in scattering changes the resistance of the spin valve sensor as a function of cos θ, where θ is the angle between the magnetic moments of the pinned and free layer structures. The sensitivity of the sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in the resistance of the sensor as the magnetic moment of the free layer structure rotates from a position parallel with respect to the magnetic moment of the pinned layer structure to an antiparallel position with respect thereto and R is the resistance of the sensor when the magnetic moments are parallel.  
         [0008]     In addition to the spin valve sensor the read head includes nonconductive nonmagnetic first and second read gap layers and ferromagnetic first and second shield layers. The spin valve sensor is located between the first and second read gap layers and the first and second read gap layers are located between the first and second shield layers. In the construction of the read head the first shield layer is formed first followed by formation of the first read gap layer, the spin valve sensor, the second read gap layer and the second shield layer. Spin valve sensors are classified as a top or a bottom spin valve sensor depending upon whether the pinning layer is located near the bottom of the sensor close to the first read gap layer or near the top of the sensor close to the second read gap layer. Spin valve sensors are further classified as simple pinned or antiparallel pinned depending upon whether the pinned layer structure is one or more ferromagnetic layers with a unidirectional magnetic moment or a pair of ferromagnetic layers that are separated by a coupling layer with magnetic moments of the ferromagnetic layers being antiparallel. Spin valve sensors are still further classified as single or dual wherein a single spin valve sensor employs only one pinned layer and a dual spin valve sensor employs two pinned layers with the free layer structure located therebetween.  
         [0009]     The transfer curve of 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. In a spin valve sensor subjected to positive and negative magnetic signal fields from a moving magnetic disk, which are typically chosen to be equal in magnitude, it is desirable that positive and negative changes in the resistance of the spin valve read head above and below a bias point on the transfer curve of the sensor be equal so that the positive and negative readback signals are equal. When the direction of the magnetic moment of the free layer is substantially parallel to the ABS and the direction of the magnetic moment of the pinned layer is perpendicular to the ABS in a quiescent state (no signal from the magnetic disk) the positive and negative readback signals should be equal when sensing positive and negative fields that are equal from the magnetic disk. Accordingly, 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. When the readback signals are asymmetrical, signal output and dynamic range of the sensor are reduced. Readback asymmetry is defined as  
           V   1     -     V   2         max   ⁡     (       V   1     ⁢             ⁢             ⁢   or   ⁢           ⁢     V   2       )           
 
 For example, +10% readback asymmetry means that the positive readback signal V I  is 10% greater than it should be to obtain readback symmetry. 10% readback asymmetry is acceptable in some applications. +10% readback asymmetry may not be acceptable in applications where the applied field magnetizes the free layer close to saturation. The designer strives to improve asymmetry of the readback signals as much as practical with the goal being symmetry. 
 
         [0010]     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 F ) between the pinned layer and the free layer, a net demagnetizing (demag) field (H D ) from the pinned layer, a sense current field (H I ) from all conductive layers of the spin valve except the free layer and a net image current field (H IM ) from the first and second shield layers if the sensor is offset between the first and second read gap layers. In order to reduce demagnetizing field from the pinned layer on the free layer, the pinned layer may be an antiparallel (AP) pinned layer structure. An AP pinned layer structure has an antiparallel coupling (APC) layer which is located between ferromagnetic first and second AP pinned layers. The first and second AP pinned layers have magnetic moments which are antiparallel with respect to one another because of the strong antiferromagnetic coupling therebetween. The AP pinned layer structure is fully described in commonly assigned U.S. Pat. No. 5,465,185 which is incorporated by reference herein. Because of the partial flux closure between the first and second AP pinned layers of each first and second AP pinned structures, each AP pinned layer exerts only a small demagnetizing field on the free layer. Because of the small demagnetizing field the exchange coupling between the AP pinned layer structure and the pinning layer is increased for promoting high stability when the spin valve sensor is subjected to unwanted magnetic fields in the presence of elevated temperatures.  
         [0011]     The aforementioned dual spin valve sensor includes a ferromagnetic free layer structure between nonmagnetic electrically nonconductive first and second spacer layers which are, in turn, located between ferromagnetic first and second pinned layer structures. The spacer layers are typically copper (Cu) and the pinned layers are typically cobalt iron (CoFe). It has been found that a cobalt iron pinned layer next to a copper spacer layer promotes the magnetoresistive coefficient dr/R of the sensor. The dual spin valve sensor is desirable because its magnetoresistive coefficient dr/R is about 1.5 times greater than the magnetoresistive coefficient dr/R of a single spin valve sensor.  
         [0012]     The dual spin valve sensor is also desirable from the standpoint that there is about an equal amount of conductive material on each side of the free layer structure so that the sense current field (H I ) acting on the free layer structure is essentially zero. It is also desirable that each of the pinned layer structures in the dual spin valve sensor be an antiparallel (AP) pinned layer structure, as discussed hereinabove. Each of the AP pinned layer structures has a low demagnetizing field and the thicknesses of the AP pinned layers of the AP pinned layer structures can be designed so that these demagnetizing fields have nearly total flux closure. Accordingly, the net demagnetizing field from the first and second AP pinned layer structures can be essentially zero. In a preferred embodiment the dual spin valve sensor is not offset between the first and second read gap layers so that adequate insulation can be provided for preventing shorts between the lead layers of the sensor and the first and second shield layers while maintaining a minimum read gap between the first and second shield layers for promoting linear read bit density. Accordingly, virtually the only magnetic field urging the magnetic moment of the free layer structure from a position parallel to the ABS is a net ferromagnetic coupling field (H F ) between each of the first and second AP pinned layer structures and the free layer structure.  
         [0013]     In present methods of sputter depositing the copper spacer layers the ferromagnetic coupling field cannot be controlled within a desirable range from 0 to −10 Oe. As an example, when the deposition time for the first copper spacer layer is 26 seconds the ferromagnetic coupling field (H F ) is −20 Oe and when the deposition time for the first copper spacer layer is 25 seconds the ferromagnetic coupling field (H F ) is +10 Oe. Within a one second deposition time the ferromagnetic coupling field (H F ) has a difference of 30 Oe. Because of process variations, it is very difficult to obtain the aforementioned desirable range of 0 Oe to −10 Oe when the range of the ferromagnetic coupling field (H F ) is 30 Oe for only one second of copper deposition time. The same problem occurs for a single AP pinned layer structure or a single spin valve sensor with a single pinned layer structure. However, in these embodiments a net sense current field and a net demagnetizing field are present for help in properly biasing the free layer structure.  
         [0014]     Another problem that arises, especially with the use of tunnel junction sensor (tunnel valves) is that undesirably high coercivity in the free layer reduces the sensitivity and performance of the sensor. A tunnel valve operates based on the spin dependent tunneling of electrons through a non-magnetic, electrically insulating barrier layer. The tunnel valve is constructed with free and pinned layers similar to GMR senor, however, rather than having an electrically conductive spacer layer, an electrically insulating barrier layer is sandwiched between the free and pinned layers. Unfortunately, the high free layer coercivity in a tunnel valve results in less than desirable tunnel valve sensitivity and performance.  
         [0015]     Therefore, there is a strong felt need for a tunnel valve having reduced magnetic coercivity in its free layer. Such reduced coercivity would greatly improve the performance of such a tunnel valve sensor.  
       SUMMARY OF THE INVENTION  
       [0016]     The present invention overcomes the above described sensitivity problems experienced in prior art tunnel valves by proving a way to reduce the magnetic coercivity of a free layer in a tunnel valve. The present invention achieves this reduced coercivity by depositing the free layer in a chamber in a nitrogen atmosphere. The presence of the Nitrogen modifies the grain structure of the free layer in such a way that a reduced magnetic coercivity results.  
         [0017]     The present invention also overcomes the aforementioned exchange coupling problems by sputter depositing a cobalt iron (CoFe) pinned layer structure reactively in a nitrogen (N 2 ) atmosphere. It is believed that this process alters the microstructure of the cobalt iron pinned layer structure so that it acts as a seed layer for layers deposited thereon, particularly the copper spacer layer, so as to also alter its microstructure. In a dual spin valve sensor the second AP pinned layer of the first AP pinned layer structure is sputter deposited in the nitrogen atmosphere. With this arrangement a net ferromagnetic coupling field (H F ) between 0 Oe to −10 Oe can be obtained with deposition times of the copper spacer layer next to the second AP pinned layer in a range from 26 to 30 seconds. This 4 second range enables present sputter deposition techniques to obtain the desired ferromagnetic coupling field (H F ). In the prior art system the desirable range from 0 Oe to −10 Oe could only be obtained between 25.5 to 26 seconds of copper deposition times which is only one-half of a second. The method also applies to single spin valve sensors with an AP pinned layer structure or a single layer pinned layer structure and also to such embodiments with the altered cobalt iron pinned layer structure fabricated according to the present process.  
         [0018]     An object of the present invention is to provide a method of making a spin valve sensor wherein a desirable ferromagnetic coupling field (H F ) can be obtained with present sputter deposition techniques.  
         [0019]     Another object is to provide a method of making a dual AP pinned spin valve sensor wherein a ferromagnetic coupling field from 0 Oe to −10 Oe can be obtained with a wider range of copper deposition times for a spacer layer.  
         [0020]     A further object is to provide a spin valve sensor wherein a cobalt iron pinned layer structure is altered by the aforementioned methods of making.  
         [0021]     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  
       [0022]      FIG. 1  is a plan view of an exemplary magnetic disk drive;  
         [0023]      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 ;  
         [0024]      FIG. 3  is an elevation view of the magnetic disk drive wherein multiple disks and magnetic heads are employed;  
         [0025]      FIG. 4  is an isometric illustration of an exemplary suspension system for supporting the slider and magnetic head;  
         [0026]      FIG. 5  is an ABS view of the magnetic head taken along plane  5 - 5  of  FIG. 2 ;  
         [0027]      FIG. 6  is a partial view of the slider and a merged magnetic head as seen in plane  6 - 6  of  FIG. 2 ;  
         [0028]      FIG. 7  is a partial ABS view of the slider taken along plane  7 - 7  of  FIG. 6  to show the read and write elements of the magnetic head;  
         [0029]      FIG. 8  is a view taken along plane  8 - 8  of  FIG. 6  with all material above the coil layer and leads removed;  
         [0030]      FIG. 9  is an enlarged isometric illustration of the read head with a spin valve sensor;  
         [0031]      FIG. 10  is an ABS illustration of a first embodiment of the present spin valve sensor;  
         [0032]      FIG. 11  is an ABS illustration of the second embodiment of the present spin valve sensor;  
         [0033]      FIG. 12  is a schematic illustration of an ion beam sputtering system wherein the cobalt iron pinned layer structure is sputter deposited in a nitrogen (N 2 ) atmosphere;  
         [0034]      FIG. 13  is a graph illustrating the ferromagnetic coupling field (H F ) with and without reactive nitrogen (N 2 ) deposition of a cobalt iron (CoFe) pinned layer structure;  
         [0035]      FIG. 14  is an ABS illustration of a tunnel valve magnetoresistive sensor according to an embodiment of the invention; and  
         [0036]      FIG. 15  is a schematic illustration of an ion beam sputtering system wherein the free layer structure is sputter deposited in in a nitrogen (N 2 ) atmosphere. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     Magnetic Disk Drive  
       [0037]     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 .  
         [0038]      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.  
         [0039]      FIG. 6  is a side cross-sectional elevation view of a merged 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. 7  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 .  
         [0040]     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. Since the second shield layer  82  and the first pole piece layer  92  are a common layer this head is known as a merged head. In a piggyback head the second shield layer and the first pole piece layer are separate layers which are separated by a nonmagnetic layer. 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. 8 ) to leads  124  and  126  on the suspension.  
         [0041]      FIG. 9  is an isometric ABS illustration of the read head  40  shown in  FIG. 7 . The read head  40  includes the spin valve sensor  74 . First and second hard bias and lead layers  134  and  136  are connected to first and second side edges  138  and  139  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  74  for stabilizing the magnetic domains therein. The spin valve sensor  74  and the first and second hard bias and lead layers  134  and  136  are located between the nonmagnetic electrically insulative first and second read gap layers  76  and  78 . The first and second read gap layers  76  and  78  are, in turn, located between the ferromagnetic first and second shield layers  80  and  82 .  
         [0042]      FIGS. 10 and 11  are ABS illustrations of spin valve sensors which were tested according to the present invention, as discussed in detail hereinafter. The spin valve sensors can be substituted for the spin valve sensor  74  shown in  FIG. 9 . In  FIG. 10 a  single spin valve sensor  200  is shown between the first and second read gap layers  76  and  78 . The spin valve sensor includes a nonmagnetic electrically nonconductive spacer layer (S)  202  which is located between an antiparallel (AP) pinned layer structure  204  and a free layer structure  206 . The AP pinned layer structure  204  includes an antiparallel coupling (APC) layer  208  which is located between first and second antiparallel (AP) pinned layers (API) and (AP 2 )  210  and  212 . The first AP pinned layer  210  is exchange coupled to an antiferromagnetic (AFM) pinning layer  214  which pins a magnetic moment  216  of the first AP pinned layer perpendicular to the ABS in a direction out of the sensor or into the sensor, as shown in  FIG. 10 . By a strong antiparallel coupling between the first and second AP pinned layers  210  and  212 , a magnetic moment  218  of the second AP pinned layer is antiparallel to the magnetic moment  216 . A seed layer structure  220  may be provided between the first read gap layer  76  and the pinning layer  214  for promoting a desirable microstructure of the layers deposited thereon. The seed layer structure  220  may include first, second and third seed layers (S 1 ), (S 2 ) and (S 3 )  221 ,  222  and  224 , which will be discussed in more detail hereinafter.  
         [0043]     The free layer structure preferably includes first and second free layers (F 1 ) and (F 2 )  226  and  228  and a cap layer  230  may be located on the free layer structure for protecting the spin valve sensor from subsequent processing steps. The free layer structure has a magnetic moment  232  which is oriented parallel to the ABS and to the major planes of the layers in a direction from right to left or from left to right, as shown in  FIG. 10 . When a signal field from the rotating magnetic disk rotates the magnetic moment  232  of the free layer into the sensor the magnetic moments  232  and  218  become more antiparallel which increases the resistance of the sensor to the sense current I S , and when the signal field from the rotating magnetic disk rotates the magnetic moment  232  out of the sensor the magnetic moments  232  and  218  become more parallel which reduces the resistance of the sensor to the sense current I S . These resistance changes are processed as playback signals by the processing circuitry  50  in  FIG. 3 .  
         [0044]     The thicknesses and materials of the layers for the examples of the experiments, which will be discussed hereinafter, are 30 Å of aluminum oxide for the first seed layer  221 , 30 Å of nickel manganese oxide for the second seed layer  222 , 35 Å of tantalum for the third seed layer  224 , 155 Å of platinum manganese for the pinning layer  214 , 17 Å of cobalt iron for the first AP pinned layer  210 , 8 Å of ruthenium for the antiparallel coupling layer  208 , 26 Å of cobalt iron for the second AP pinned layer  212 , 20 Å of copper for the spacer layer  202 , 15 Å of cobalt iron for the first free layer  226 , 25 Å of nickel iron for the second free layer  228  and 40 Å of tantalum for the cap layer  230 .  
         [0045]     As a result of the second AP pinned layer  212  being greater than the thickness of the first AP pinned layer  210 , there is a net demagnetizing field (H D )  234  acting on the free layer structure into the paper, and because of the sense current I S  there is a sense current field (H I )  236  acting on the free layer structure out of the paper. Accordingly, these fields  234  and  236  are antiparallel with respect to each other. Also, there is a ferromagnetic coupling field (H F ) between the second AP pinned layer  212  and the free layer structure  206  which can be a positive ferromagnetic coupling field (H F )  238  or a negative ferromagnetic coupling field (H F )  240 . All of these fields should be balanced so that the orientation of the magnetic moment  232  of the free layer structure is parallel to the ABS and the major planes of the layers.  
         [0046]     A dual AP pinned spin valve sensor  300  is shown in  FIG. 11  and is the same as the spin valve sensor  200 , shown in  FIG. 10 , except for the free layer structure  302 , a second copper spacer layer  304 , a second AP pinned layer structure  306 , a second antiferromagnetic (AFM 2 ) pinning layer  308  and the second AP pinned layer  212  being 20 Å thick instead of 26 Å thick. The free layer structure  302  is located between the first and second spacer layers (S 1 ) and (S 2 )  202  and  304  and the first and second spacer layers  202  and  304  are located between the first and second AP pinned layer structures  204  and  306 . The free layer structure  302  includes first, second and third free layers (F 1 ), (F 2 ) and (F 3 )  310 ,  312  and  314  with the second free layer  312  being located between the first and third free layers  310  and  314 .  
         [0047]     The second AP pinned layer structure  306  includes an antiparallel coupling (APC)  316  which is located between first and second AP pinned layers (AP 1 ) and (AP 2 )  318  and  320 . The first AP pinned layer  318  is exchange coupled to the second pinning layer  308  so that a magnetic moment  322  of the first AP pinned layer is oriented perpendicular to the ABS in a direction into the sensor. By a strong antiparallel coupling between the first and second AP pinned layers  318  and  320 , a magnetic moment  324  of the second AP pinned layer is antiparallel to the magnetic moment  322 .  
         [0048]     The free layer structure has a magnetic moment  326  which is oriented parallel to the ABS and to the major planes of the layers. When a signal field from a rotating magnetic disk rotates the magnetic moment  326  into the sensor the magnetic moment  326  becomes more antiparallel with respect to the magnetic moments  218  and  324 , which increases resistance of the sensor to the sense current I S , and when the signal field from the rotating magnetic disk rotates the magnetic moment  326  out of the sensor the magnetic moment  326  becomes more parallel with respect to the magnetic moments  218  and  324 , which decreases the resistance of the sensor to the sense current I S . Again, these resistance changes are processed by the processing circuitry  50  in  FIG. 3  as playback signals. The magnetoresistive coefficient dr/R of the dual AP spin valve sensor in  FIG. 11  is about 1.5 times as great as the magnetoresistive coefficient dr/R of the single AP pinned layer structure in  FIG. 10 .  
         [0049]     The thicknesses and materials of the layers not already discussed are 15 Å of cobalt iron for the first free layer  310 , 30 Å of nickel iron for the second free layer  312 , 15 Å of cobalt iron for the third free layer  314 , 20 Å of copper for the second spacer layer  304 , 17 Å of cobalt iron for the second AP pinned layer  320 , 8 Å of ruthenium for the antiparallel coupling layer  316 , 20 Å of cobalt iron for the first AP pinned layer  318  and 155 Å of platinum manganese for the second pinning layer  308 . The net demagnetizing field  234 , shown in  FIG. 10 , is not shown in  FIG. 11  because the net demagnetizing field of the first and second AP pinned layer structures  204  and  306  is essentially zero. Further, the sense current field (H I )  236  in  FIG. 10  is not shown in  FIG. 11  since it is essentially zero due to virtually the same amount of conductive material on each side of the free layer structure  302 . The spin valve sensor  300  is preferably located between the first and second read gap layers  76  and  78  without an offset so as to provide adequate material in the first and second read gap layers to prevent shorting between lead layers to the sensor and the first and second shield layers. Accordingly, there is no net image current field (H IM ) acting on the free layer structure. The only remaining field acting on the free layer structure is the ferromagnetic coupling field (H F ) which can be positive, as shown at  328 , or negative, as shown at  330 . It is important that the ferromagnetic coupling field (H F ) be set at a proper level so that the magnetic moment  326  of the free layer structure is essentially parallel to the ABS and the major planes of the layers. A desirable level for the ferromagnetic coupling field (H F ) is between 0 to −10 Oe. Prior art sputtering techniques make it very difficult to obtain a ferromagnetic coupling field within this range.  
       The Invention  
     EXAMPLE 1  
       [0050]     In this example the spin valve sensor  300  in  FIG. 11  was fabricated with the second AP pinned layer  212  being formed by ion beam sputtering in a sputtering chamber which had a nitrogen (N 2 ) atmosphere. A schematic illustration of the sputtering system  400  is shown in  FIG. 12  which has a sputtering chamber  402 . Located within the sputtering chamber is a substrate  404  upon which layers  406  of the spin valve sensor  300  below the second AP pinned layer  212  have been formed. Also located within the chamber is a cobalt iron (CoFe) target  408 . An ion beam (IB) gun  410  projects through the chamber  402  and accelerates xenon (Xe) ions to the target  408  which causes cobalt iron (CoFe) ions to be sputtered from the target  408  to form the second AP pinned layer  212 . Simultaneously, nitrogen (N 2 ) is introduced through an opening  412  in the chamber so that the nitrogen reacts with the cobalt iron at the second AP pinned layer  212  to form the second AP pinned layer  212  with an altered microstructure. Partial pressure of the nitrogen (N 2 ) within the chamber was 2×10 −5  Torr. In this first example, nitrogen (N 2 ) was not introduced into the chamber. The deposition time for each of the first and second spacer layers  202  and  304  was  26  seconds. The rate of deposition was 0.77 Å per second which resulted in each of the spacer layers being 20 Å thick. The ferromagnetic coupling field was −20 Oe. The uniaxial anisotropy field (H K ) was 10 Oe, the easy axis coercivity (H C ) was 10 Oe, the resistance R of the sensor was 13.95 ohms, the magnetoresistive coefficient dr/R was 10.55 and the magnetostriction (lambda) was 0.8×10 6 .  
       EXAMPLE 2  
       [0051]     This example is the same as Example 1 except the deposition time of each of the first and second spacer layers  202  and  304  was 25 seconds instead of 26 seconds. The spacer layers were slightly thinner and the ferromagnetic coupling field (H F ) was +10 Oe. Examples 1 and 2 show that with only a change of 1 second of deposition time that the ferromagnetic coupling field went from −20 Oe to +10 Oe which is a difference of 30 Oe. This demonstrates that it is very difficult to control the ferromagnetic coupling field because of process variations in the ion beam sputtering system shown in  FIG. 12 . The uniaxial anisotropy field (H K ) was 10 Oe, the easy axis coercivity (H C ) was 12 Oe, the resistance R was 13.77 ohms, the magnetoresistive coefficient dr/R was 10.95 and the magnetostriction was 0.2×10 −6 .  
       EXAMPLE 3  
       [0052]     This example is the same as Example I except nitrogen (N 2 ) is introduced into the chamber simultaneously with sputtering cobalt iron (CoFe) from the target  408 .  
         [0053]     The result is that the second AP pinned layer  212  is formed of cobalt iron (CoFe) with an altered microstructure which is very important to the operation of the present invention. In this example, the ferromagnetic coupling field was only −5 Oe which is within the desirable range of 0 Oe to −10 Oe. In the present example, the uniaxial anisotropy (H K ) was 10 Oe, the easy axis coercivity (H C ) was 10 Oe, the magnetostriction (λ) was 0.5×10 −6 , the resistance R was 13.9 ohms and the magnetoresistive coefficient dr/R was 10.5.  
       EXAMPLE 4  
       [0054]     In this example, the spin valve sensor  200  in  FIG. 10  was tested. The second AP pinned layer  212  was ion beam sputter deposited in the chamber  400  in  FIG. 12  without introducing nitrogen (N 2 ). The ferromagnetic coupling field (H F ) was −7 Oe.  
         [0055]     The uniaxial anisotropy (H K ) was 12 Oe, the easy axis coercivity (H C ) was 7.6 Oe, the resistance R was 24 ohms, the magnetoresistive coefficient dr/R was 7.8 and the magnetostriction was 0.6×10 −6 .  
       EXAMPLE 5  
       [0056]     This example is the same as Example 4 except the second AP pinned layer  212  was ion beam sputter deposited in the chamber  402  in  FIG. 12  simultaneously with the introduction of nitrogen (N 2 ) through the opening  412 . The result is that the second AP pinned layer  212  was formed with an altered microstructure which is a basis of the present invention. In both examples, the deposition time for the copper spacer layer  202  was kept constant at 26 seconds. The result in this example was the ferromagnetic coupling field (H F ) was 1 Oe. It can be seen that with the present process that the ferromagnetic coupling field is made more positive by 8 Oe in a similar manner as shown by Example 3 for the dual AP pinned spin valve sensor. In this example the uniaxial anisotropy (H K ) was 8 Oe, the easy axis coercivity (H C ) was 6.3 Oe, the resistance R was 23.6 ohms, the magnetoresistive coefficient dr/R was 7.7 and the magnetostriction was 1.5×10 −6 . It can be seen when compared to the fourth example that the uniaxial anisotropy (H K ) has been reduced by one-third which makes the free layer structure  206  more responsive to field signals from the rotating magnetic disk.  
       OTHER EXAMPLES  
       [0057]     Other examples are shown in the graph in  FIG. 13  which tested a dual AP pinned spin valve sensor as shown in  FIG. 11 . Four different deposition times are shown of the first spacer layer without the introduction of nitrogen (N 2 ) and the same four deposition times are shown with the introduction of nitrogen (N 2 ) when the cobalt iron (CoFe) pinned layer structure is formed. As stated hereinabove, a desirable range for the ferromagnetic coupling field (H F ) is from 0 to −10 Oe. It can be seen that the acceptable deposition times for the copper spacer layer where nitrogen is not employed to fabricate the cobalt iron (CoFe) pinned layer structure is only between 25 seconds to 25.5 seconds. However, when nitrogen (N 2 ) is employed in the sputtering chamber while simultaneously depositing the cobalt iron pinned layer structure an acceptable ferromagnetic coupling field (H F ) can be obtained when the copper deposition time is from about 25.5 second to 30 seconds. This is a range of about 4.5 seconds which is 9 times greater than the range without the nitrogen (N 2 ) atmosphere. Accordingly, the present invention enables the desirable range of ferromagnetic coupling fields (H F ) to be obtained with the ion beam sputtering system shown in  FIG. 12 .  
       Discussion  
       [0058]     It should be understood that cobalt (Co) or any cobalt based alloy may be substituted for cobalt iron (CoFe) without departing from the spirit of the invention.  
         [0059]     Further, it should be understood that the nitrogen (N 2 ) atmosphere changes the microstructure (grain size) of the pinned layer structure by interstitially introducing nitrogen between the atoms of the thin film. The cobalt, cobalt based alloy or nickel iron has a changed crystalline structure which changes the ferromagnetic coupling field (H C ).  
         [0060]     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.  
         [0061]     With reference now to  FIG. 14 , the present invention can be embodied in a tunnel junction sensor (tunnel valve) having improved free layer sensitivity. A tunnel valve operates based upon the spin dependent tunneling of electrons through a thin non-magnetic electrically insulating barrier layer. A tunnel valve  1400  includes a sensor stack  1402  sandwiched between first and second leads  1404 ,  1406 . The leads can be constructed of an electrically conductive, magnetic material such as NiFe so that they may function as magnetic shields as well as leads.  
         [0062]     The sensor stack  1402  includes a pinned layer structure  1408 , a magnetic free layer  1410  and a non-magnetic electrically insulating barrier layer  1412 . The free layer may be constructed one or more magnetic layers which preferably include a layer of CoFe or a layer of Co. The free layer  1410  may include a layer of NiFe located away from the spacer layer  1412  and a layer of Co or CoFe adjacent to the spacer layer. The free layer may also include a layer of CoFe located away from the spacer layer and a layer of Co adjacent to the spacer layer. The free layer  1410  could include multiple layers of CoFe with the layers closest to the spacer layer  1412  having a higher concentration of Co than those located away from the spacer  1412 .  
         [0063]     The barrier layer  1412 , can be, for example aluminum oxide (Al 2 O 3 ) and is relatively thin, preferably having a thickness of 5-15 Angstroms. The pinned layer structure  1408 , may be of several configuration, including, simple pinned, AP pinned, self pinned or AFM pinned. Preferably, the pinned layer structure  1402  is an AFM pinned antiparallel coupled (AP pinned) structure including an API layer  1414 , an AP 2  layer  1416 , and an antiparallel coupling layer  1418  such as Ru sandwiched between the AP 1  and AP 2  layers  1414 ,  1416 . The AP 1  and AP 2  layers  1414 ,  1416  can be for example CoFe or some other magnetic material. The API layer can be exchange coupled with a layer of antiferromagnetic material (AFM layer)  1420  which can be for example PtMn or IrMn. This exchange coupling strongly pins the magnetic moment  1422  of the AP I layer  1414  in a desired direction perpendicular to the ABS. Strong antiparallel coupling between the API and AP 2  layers across the coupling layer  1418  strongly pins the moment  1424  of the AP 2  layer  1416  in a direction antiparallel with the moment  1422  of the AP 1  layer  1414 , and perpendicular to the ABS.  
         [0064]     With continued reference to  FIG. 14 , hard bias layers  1426 ,  1428  formed at either side of the sensor stack  1402  provide a bias field to bias the magnetic moment  1430  of the free layer  1410  in a desired direction parallel with the ABS. The bias layers  1426 ,  1428  can be for example CoPt, CoPtCr or some other high coercivity magnetic material. Since the hard bias layers  1426 ,  1428  are electrically conductive as well as magnetic, an insulation layer  1432  is provided, covering the sides of the sensor stack (as well as the back, stripe height edge, not shown) and also covering at least one of the leads  1404 ,  1406 . The insulation layer  1432  prevents the shunting of sense current through the hard bias layers  1404 ,  1406 . The insulation layer  1432  could be constructed of several non-magnetic, electrically conductive materials and is preferably alumina applied by a conformal deposition process such as atomic layer deposition (ALD). A seed layer  1434  may be provided at the bottom of the sensor stack to promote a desired crystallographic structure in the sensor layers. A capping layer  1436 , such as Ta may be provided at the top of the sensor stack  1402  to protect the senor layers from damage during manufacture.  
         [0065]     As discussed above, the tunnel valve  1400  operates based on the spin dependent tunneling of electrons through the barrier layer  1418 . The electrical resistance through the barrier layer  1412  is lowest when the magnetic moments  1430 ,  1424  of the free layer  1410  and the AP 2  layer  1416  are parallel to one another. When those moments  1430 ,  1424  are antiparallel to one another the resistance through the barrier layer  1412  is at its maximum.  
         [0066]     In order to maintain optimal sensitivity to magnetic fields, the coercivity of the free layer  1410  must be kept sufficiently low. Unfortunately, depositing the free layer material over the barrier layer (such as alumina) results in the free layer having a higher than desired coercivity. However, the present invention provides a way to reduce the coercivity of the free layer, resulting in a tunnel valve  1400  having a desired high sensitivity.  
         [0067]      FIG. 15  describes a method of constructing a tunnel valve sensor according to the present invention that has a low coercivity, highly sensitive free layer. The present invention achieves this by depositing the free layer in a nitrogen atmosphere. With reference to  FIG. 15 , a wafer  1502  including a plurality of sensors is shown in an intermediate stage of manufacture in an ion beam sputtering chamber  1504 . The wafer  1502  includes a substrate  1506  and a portion of the sensor layers  1508 , which may include a seed layer  1432  and layers of the pinned layer structure  1408  ( FIG. 14 ), a barrier layer  1510  is deposited over the other sensor layers  1508 .  
         [0068]     Free layer material  1512  such as Co or CoFe is sputter deposited onto the barrier layer  1510  in the presence of a nitrogen atmosphere. In order achieve this, the deposition chamber  1504  includes an ion beam gun  1514  which projects through the chamber  1404  and accelerates xenon (Xe) ions to a target  1516 . This causes CoFe ions to be sputtered from the target  1516  to form the free layer material  1512 . Simultaneously, nitrogen (N 2 ) is introduced through an opening  1518  in the chamber so that the nitrogen reacts with the CoFe at the free layer  1512  to form a free layer  1512  with an altered microstructure. Partial pressure of the nitrogen within the chamber can be about 2× −5  Torr. The reaction of the nitrogen with the CoFe free layer  1512  alters the microstructure of the free layer  1512  in such a way that the free layer  1512  ( 1410  in  FIG. 14 ) posses a lower magnetic coercivity than would be the case without the introduction of nitrogen into the atmosphere of the chamber  1504 . This reduced free layer coercivity results in a free layer that is more sensitive to magnetic fields and is. As a result, the overall performance of the tunnel valve sensor  1400  ( FIG. 14 ) is greatly improved.  
         [0069]     With continued reference to  FIG. 15 , the other sensor layers may also be deposited in the chamber  1504  in the presence of the nitrogen atmosphere. For example, as discussed with reference to previously described embodiments, one or more layers of the pinned layer structure ( 1408  in  FIG. 14 ) may be deposited in the presence of a nitrogen atmosphere. In addition, the barrier layer  1510  may be deposited in the presence of the nitrogen atmosphere, which may further improve the grain structure of the subsequently deposited free layer  1510 , further reducing free layer coercivity.