Abstract:
A spin valve sensor is provided with a biasing layer which produces a demagnetizing field which supports a demagnetizing field from a pinned layer structure in counterbalancing a sense current field on the free layer structure. The biasing layer has a high resistance so that a sense current is not excessively shunted therethrough and is a specular reflector so as to reflect conduction electrons to increase a magnetoresistive coefficient dr/R of the sensor. In the preferred embodiment the pinned layer structure is an antiparallel (AP) pinned layer structure.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a spin valve sensor with a biasing layer and, more particularly, to a biasing layer which produces a demagnetizing field which supports a demagnetizing field from a pinned layer structure in opposing a sense current field at a free layer structure in 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 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. 
     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. 
     The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layer structures are 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. 
     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. 
     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 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 1  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. 
     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 H D  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. 
     A net image current field H IM  is obtained by offsetting the spin valve sensor between the first and second read gap layers. For instance, if the spin valve sensor is offset closer to the second shield layer with a thinner second read gap layer the image current field H IM  from the second shield layer will be greater than the net image current field H IM  from the first shield layer which causes the aforementioned net image current field H IM  on the free layer structure. Unfortunately, a thin second read gap layer increases the risk of shorting between the lead layers and the second shield layer. In order to overcome this problem the thickness of the second read gap layer can be increased. Unfortunately, this increases the read gap between the first and second shield layers which reduces the linear bit density of the read head. 
     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 a 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 only a small net demagnetizing field is exerted 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 of the spin valve sensor when subjected to unwanted magnetic fields in the presence of elevated temperatures. 
     Unfortunately, a small demagnetizing field from an AP pinned layer structure makes it difficult to counterbalance the strong sense current field H I  from the sense current. In some spin valve sensors the ferromagnetic coupling field H FC  is very small or zero. This then leaves only the net image current field H IM  from a gap offset in order to provide a field at the free layer structure to support the small net demagnetizing field from the pinned layer structure to oppose the sense current field from the sense current. As stated hereinabove, it is undesirable to employ the gap offset to obtain the net image current field H IM  because of the problem with shorting between the lead layers and one or more of the shield layers. 
     SUMMARY OF THE INVENTION 
     The present spin valve sensor has a biasing layer with a demagnetizing field which supports the net demagnetizing field in opposing the sense current field H I  exerted on the free layer structure. With this arrangement the net demagnetizing field of the pinned layer structure and the biasing layer are parallel with respect to one another. The present invention obviates the necessity of a ferromagnetic coupling field H FC  and/or a net image current field H IM  to counterbalance the sense current field H I  in order to properly bias the free layer structure. An aspect of the invention is that the sense current I S  is fed through the spin valve sensor in a direction which causes the sense current field H I  to orient the magnetic moment of the biasing layer so that the demagnetizing field of the biasing layer supports the net demagnetizing field of the pinned layer structure. 
     In another aspect of the invention the biasing layer is composed of a material which causes specular reflection of conduction electrons back into the mean free path of conduction electrons so as to increase the magnetoresistive coefficient dr/R. A still further aspect of the present invention is that the pinned layer structure is an antiparallel (AP) pinned layer structure which produces a small net demagnetizing field. As stated hereinabove, the AP pinned layer structure promotes high stability for the spin valve sensor. Still another aspect of the invention is that the second AP pinned layer, which interfaces the spacer layer, is magnetically thicker than the first AP pinned layer. With each of the first and second AP pinned layers composed of a cobalt based material, the thicker second AP pinned layer next to the spacer layer will promote an increase in the magnetoresistive coefficient dr/R. Accordingly, the second AP pinned layer controls the orientation of the net demagnetizing field from the AP pinned layer structure on the free layer structure. 
     An object of the present invention is to provide a spin valve sensor wherein a free layer therein can be properly biased without a ferromagnetic coupling field H FC  and/or a net image current field H IM . 
     Another object is to provide a method for making the aforementioned spin valve sensor. 
     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 merged magnetic head as seen in plane  6 — 6  of FIG. 2; 
     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 merged magnetic head; 
     FIG. 8 is a view taken along plane  8 — 8  of FIG. 6 with all material above the coil layer and leads removed; 
     FIG. 9 is an enlarged isometric illustration of the read head with a spin valve sensor; 
     FIG. 10 is an ABS illustration of the present spin valve sensor; and 
     FIG. 11 is a view from the left end of FIG. 10 rotated 90° clockwise. 
    
    
     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 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 . 
     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. 
     FIG. 11 is an enlarged isometric ABS illustration of the read head  40  shown in FIG.  9 . 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 . 
     The Invention 
     FIG. 10 is an ABS illustration of the present spin valve sensor  74  located between the first and second read gap layers  76  and  78 . The spin valve sensor includes a nonmagnetic spacer layer (S)  200  which is located between a ferromagnetic free layer structure  202  and a ferromagnetic pinned layer structure  204 . The pinned layer structure  204  is preferably an antiparallel (AP) pinned layer structure which has an antiparallel coupling (APC) layer  206  which is located between and interfaces first and second AP pinned layers (AP 1 ) and (AP 2 )  208  and  210 . The first AP pinned layer  208  interfaces and is exchange coupled to an antiferromagnetic (AFM) pinning layer  212  which pins a magnetic moment  214  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 strong antiparallel coupling between the first and second AP pinned layers  208  and  210  a magnetic moment  216  of the second AP pinned layer is antiparallel to the magnetic moment  214 . First and second seed layers (SL 1 ) and (SL 2 )  218  and  220  are provided for promoting a desirable microstructure of the layers deposited thereon. 
     The free layer structure  202  preferably has first and second free layers (F 1 ) and (F 2 )  222  and  224 . The first free layer  222  is composed of a cobalt based material, preferably cobalt iron (Co 90 Fe 10 ), and the second free layer  224  is preferably composed of nickel iron (Ni 83 Fe 17 ). The cobalt based first free layer  222  is next to the copper spacer layer  200  for promoting the magnetoresistive coefficient dr/R of the spin valve sensor. The free layer structure  202  has a magnetic moment  226  which is oriented parallel to the ABS and 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 a rotating magnetic disk rotates the magnetic moment  226  of the free layer into the sensor the magnetic moments  226  and  216  become more antiparallel which increases the resistance of the sensor to the sense current I S  and when a signal field rotates the magnetic moment  226  out of the sensor the magnetic moments  226  and  216  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 . 
     FIG. 11, which is a view from the left end of FIG. 10 rotated 90° clockwise, shows the fields exerted on the free layer structure which should completely counterbalance each other so that the magnetic moment  226  of the free layer structure is oriented parallel to the ABS. The ferromagnetic coupling field H FC , shown in phantom, is very small or zero and cannot be employed for counterbalancing the large sense current field H I . As stated hereinabove, the net demagnetizing field H D  from the AP pinned layer structure is not sufficient to counterbalance the sense current field H I . The present invention provides a biasing layer (B)  228  which provides a demagnetizing field H B  which is parallel to and supports the net demagnetizing field H D  so as to completely counterbalance the sense current field H I . Accordingly, the biasing layer  228  has a magnetic moment  230  which is parallel to the magnetic moment  216  of the second AP pinned layer. The processing circuitry  50  in FIG. 3 has means  232  shown in FIG. 10 for feeding the sense current I S  through the spin valve sensor in a direction which causes the sense current field H I  acting on the biasing layer  228  to orient the magnetic moment  230  of the biasing layer parallel to the magnetic moment  216  of the second AP pinned layer. It should be understood that the same result can be achieved by reversing directions of the magnetic moments  214 ,  216  and  230  and the direction of the sense current I S  through the spin valve sensor. With this arrangement the directions of the fields H D , H B  and H I  on the free layer structure  202  will be reversed. A nonmagnetic isolation layer (I)  234 , which is preferably copper (Cu), is located between and interfaces each of the second free layer  224  and the biasing layer  228  for isolating the magnetisms of the second free layer  224  and the biasing layer  228 . A cap layer  236  is located on the biasing layer  228  for protecting the spin valve sensor from subsequent processing steps. 
     In a preferred embodiment each of the first and second AP pinned layers  208  and  210  is composed of a cobalt based material, preferably cobalt iron (Co 90 Fe 10 ), with the second AP pinned layer  210  being thicker than the first AP pinned layer  208 . With this arrangement the cobalt based second AP pinned layer  210  next to the copper spacer layer  200  promotes the magnetoresistive coefficient dr/R of the sensor. Further, the biasing layer  228  is preferably composed of cobalt iron and an additional alloy (CoFeX) wherein X is selected from the group consisting of vanadium (V), chromium (Cr), hafnium (Hf), niobium (Nb) and oxygen (O). X increases the resistance of the biasing layer so that shunting of the sense current by the biasing layer  228  is reduced. Another advantage of the present invention is that the composition of the biasing layer  228  reflects conduction electrons back into the mean free path of conduction electrons for still further increasing the magnetoresistive coefficient dr/R of the spin valve sensor. 
     Exemplary thicknesses and materials of the layers are 30 Å of nickel manganese oxide for the first seed layer  218 , 20 Å of tantalum or nickel iron chromium for the second seed layer  220 , 150 Å of platinum manganese for the pinning layer  212 , 15 Å of cobalt iron for the first AP pinned layer  208 , 8 Å of ruthenium for the antiparallel coupling layer  206 , 20 Å of cobalt iron for the second AP pinned layer  210 , 20 Å of copper for the spacer layer  200 , 15 Å of cobalt iron for the first free layer  222 , 15 Å of nickel iron for the second free layer  224 , 10 Å of copper for the isolation layer  234 , 10 Å of cobalt iron and an additional element X for the biasing layer  228  and 50 Å of tantalum for the cap layer  236 . 
     Discussion 
     It should be understood that the thicknesses and materials of the layers described hereinabove are exemplary. The preferred cobalt iron is Co 90 Fe 10 , the preferred nickel iron is Ni 83 Fe 17  and the preferred platinum manganese is Pt 50 Mn 50 . It should be understood that cobalt (Co) may be substituted for cobalt iron (CoFe) and that other antiferromagnetic materials may be used for the pinning layer such as nickel manganese (NiMn) or iridium manganese (IrMn). It should be understood that the present invention may be employed in a top or bottom spin valve sensor, a spin valve sensor which employs either a simple pinned or antiparallel pinned layer structure and a single or dual spin valve sensor. It should still further be understood that the present invention includes the method of making of the spin valve sensors  200  and  300  shown in FIGS. 10 and 11. 
     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.