Abstract:
A current perpendicular to plane (CPP) magnetoresistive sensor having a current path defined by first and second overlying insulation layers between which an electrically conductive lead makes content with a surface of the sensor stack. The current path being narrower than the width of the sensor stack allows the outer edges of the sensor stack to be moved outside of the active area of the sensor. This results in a sensor that is unaffected by damage at outer edges of the sensor layers. The sensor stack includes a free layer that is biased by direct exchange coupling with a layer of antiferromagnetic material (AFM layer). The strength of the exchange field can be controlled by adding Cr to the AFM material to ensure that the exchange field is sufficiently weak to avoid pinning the free layer.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to magnetoresistive sensors and more particularly to a current perpendicular to plane (CPP) magnetoresistive sensor having a track width defined by a lead contact area. 
       BACKGROUND OF THE INVENTION 
       [0002]    The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of the rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). 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. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions 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. 
         [0003]    The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head and the pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write gap at the ABS for the purpose of writing the aforementioned magnetic impressions in tracks on the moving media, such as in circular tracks on the aforementioned rotating disk. 
         [0004]    In current read head designs a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to the air bearing surface (ABS) and the magnetic moment of the free layer is located parallel to the ABS, but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer. 
         [0005]    The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos θ, where θ is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals. 
         [0006]    When a spin valve sensor employs a single pinned layer it is referred to as a simple spin valve. When a spin valve employs an antiparallel (AP) pinned layer it is referred to as an AP pinned spin valve. An AP spin valve includes first and second magnetic layers separated by a thin non-magnetic coupling layer such as Ru. The thickness of the spacer layer is chosen so as to antiparallel couple the magnetizations of the ferromagnetic layers of the pinned layer. A spin valve is also known as a top or bottom spin valve depending upon whether the pinning layer is at the top (formed after the free layer) or at the bottom (before the free layer). 
         [0007]    The spin valve sensor is located between first and second nonmagnetic electrically insulating read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. In a merged magnetic head a single ferromagnetic layer functions as the second shield layer of the read head and as the first pole piece layer of the write head. In a piggyback head the second shield layer and the first pole piece layer are separate layers. 
         [0008]    Magnetization of the pinned layer is usually fixed by exchange coupling one of the ferromagnetic layers (AP 1 ) with a layer of antiferromagnetic material such as PtMn. While an antiferromagnetic (AFM) material such as PtMn does not in and of itself have a magnetization, when exchange coupled with a magnetic material, it can strongly pin the magnetization of the ferromagnetic layer. 
         [0009]    The ever increasing demand for increased data rate and data capacity has lead a relentless push to develop magnetoresistive sensors having improved signal amplitude and reduced track width. Sensors that show promise in achieving higher signal amplitude include current perpendicular to plane (CPP) sensors. Such sensors conduct sense current from top to bottom, perpendicular to the planes of the sensor layers. Examples of CPP sensors include CPP GMR sensors and tunnel valve sensors. A CPP GMR sensor operates based on the spin dependent scattering of electrons through the sensor, similar to a more traditional CIP GMR sensor except that, as mentioned above, the sense current flows perpendicular to the plane of the layers. A tunnel valve operates based on the spin dependent tunneling of electrons through a non-magnetic, electrically insulating barrier layer. 
         [0010]    Traditionally, the magnetization of the free layer has been accomplished by hard magnetic bias layers formed at either side of the sensor stack. However, the push for ever increased areal density has lead a continual push for sensors of ever decreasing size. As the size of the sensor shrinks, the free layer becomes less stable. The need for increased free layer stability is even more pronounced at higher signal amplitudes. 
         [0011]    Another problem that arises as a result of the push for smaller sensors, is that as track width decreases, the magnetic performance of the sensor layers suffers. Manufacturing processes used to define the sensor (such as ion milling) cause a certain amount of damage to the sensor layers at the outside edges of the sensor. Areas nearer to the center are, however, unaffected by the manufacturing processes that damage the outside of the sensor. When sensors have a large track width, the damaged portion of the sensor layers at the outside of the sensor stack are a sufficiently small percentage of the sensor as a whole that the overall performance is not significantly affected. However, as sensor track width decreases, the damaged portion of the sensor makes up an unacceptably large percentage of the sensor, and performance suffers. 
         [0012]    Therefore, there is a need for a sensor design that can provide increased free layer stability in CPP magnetoresistive sensor. There is also a need for a sensor design that can overcome the challenges presented by decreased track widths. Such a sensor would preferably overcome the performance problems caused by sensor damage at outside edges of the sensor stack. 
       SUMMARY OF THE INVENTION 
       [0013]    The present invention provides a current perpendicular to plane (CPP) magnetoresistive sensor having a current path defined by first and second overlying insulation layers between which an electrically conductive lead makes contact with a surface of the sensor stack. The sensor stack includes a free layer that is biased by direct exchange coupling with a layer of antiferromagnetic material (AFM layer). 
         [0014]    The strength of the exchange field can be controlled by adding Cr to the AFM material to ensure that the exchange field is sufficiently weak to avoid strongly pinning the free layer. Furthermore, the amount of Cr can be adjusted to provide a desired exchange field for biasing the free layer without making the free layer too stiff. 
         [0015]    As discussed above in the Background of the Invention, the outer edges of a sensor are often damaged by processes such as ion milling used to define the sensor stack. However, according to the present invention, the sensor has a lead that contacts the sensor stack in central area removed from the outer edges of the sensor. This advantageously allows the sensor to have an active area that is remove from the outer edges of the sensor, resulting in a sensor that is unaffected by damage at outer edges of the sensor layers. 
         [0016]    These and other features and advantages of the invention will be apparent upon reading of the following detailed description of preferred embodiments taken in conjunction with the Figures in which like reference numerals indicate like elements throughout. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    For a fuller understanding of the nature and advantages of this invention, as well as the preferred mode of use, reference should be made to the following detailed description read in conjunction with the accompanying drawings which are not to scale. 
           [0018]      FIG. 1  is a schematic illustration of a disk drive system in which the invention might be embodied; 
           [0019]      FIG. 2  is an ABS view of a slider illustrating the location of a magnetic head thereon; 
           [0020]      FIG. 3  is an enlarged ABS view taken from circle  3  of  FIG. 2  rotated 90 degrees counterclockwise; and 
           [0021]      FIG. 4  is a view similar to that of  FIG. 3  of showing an alternate embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]    The following description is of the best embodiments presently contemplated for carrying out this invention. This description is made for the purpose of illustrating the general principles of this invention and is not meant to limit the inventive concepts claimed herein. 
         [0023]    Referring now to  FIG. 1 , there is shown a disk drive  100  embodying this invention. As shown in  FIG. 1 , at least one rotatable magnetic disk  112  is supported on a spindle  114  and rotated by a disk drive motor  118 . The magnetic recording on each disk is in the form of annular patterns of concentric data tracks (not shown) on the magnetic disk  112 . 
         [0024]    At least one slider  113  is positioned near the magnetic disk  112 , each slider  113  supporting one or more magnetic head assemblies  121 . As the magnetic disk rotates, slider  113  moves radially in and out over the disk surface  122  so that the magnetic head assembly  121  may access different tracks of the magnetic disk where desired data are written. Each slider  113  is attached to an actuator arm  119  by way of a suspension  115 . The suspension  115  provides a slight spring force which biases slider  113  against the disk surface  122 . Each actuator arm  119  is attached to an actuator means  127 . The actuator means  127  as shown in  FIG. 1  may be a voice coil motor (VCM). The VCM comprises a coil movable within a fixed magnetic field, the direction and speed of the coil movements being controlled by the motor current signals supplied by controller  129 . 
         [0025]    During operation of the disk storage system, the rotation of the magnetic disk  112  generates an air bearing between the slider  113  and the disk surface  122  which exerts an upward force or lift on the slider. The air bearing thus counter-balances the slight spring force of suspension  115  and supports slider  113  off and slightly above the disk surface by a small, substantially constant spacing during normal operation. 
         [0026]    The various components of the disk storage system are controlled in operation by control signals generated by control unit  129 , such as access control signals and internal clock signals. Typically, the control unit  129  comprises logic control circuits, storage means and a microprocessor. The control unit  129  generates control signals to control various system operations such as drive motor control signals on line  123  and head position and seek control signals on line  128 . The control signals on line  128  provide the desired current profiles to optimally move and position slider  113  to the desired data track on disk  112 . Write and read signals are communicated to and from write and read heads  121  by way of recording channel  125 . 
         [0027]    With reference to  FIG. 2 , the orientation of the magnetic head  121  in a slider  113  can be seen in more detail.  FIG. 2  is an ABS view of the slider  113 , and as can be seen the magnetic head including an inductive write head and a read sensor, is located at a trailing edge of the slider. The above description of a typical magnetic disk storage system, and the accompanying illustration of  FIG. 1  are for representation purposes only. It should be apparent that disk storage systems may contain a large number of disks and actuators, and each actuator may support a number of sliders. 
         [0028]    With reference now to  FIG. 3 , a CPP magnetoresistive sensor  300  according to an embodiment of the invention is described, the sensor being shown as viewed from the air bearing surface (ABS). The invention will be described in terms of a tunnel valve, but it should be understood that the invention would also apply to a CPP GMR sensor as well. 
         [0029]    The sensor  300  includes a sensor stack  302  which is sandwiched between first and second leads  304 ,  306 . The leads  304 ,  306  are constructed of an electrically conductive material and may be constructed of a material that is magnetic as well (such as NiFe) so that the leads  304 ,  306  can function as magnetic shields as well as leads. 
         [0030]    First and second side insulation layers  308 ,  310  are provided at either lateral side of the sensor  302 , filling most of the space between the leads  304 ,  306 . The side insulation layers  304 ,  306  may be constructed of, for example, alumina (Al 2 O 3 ). First and second overlay insulation layers  312 ,  314  are also provided. The overlay insulation layers  312 ,  314  extend over outer portions of the sensor stack  302 , and may extend over the side insulation layers  308 ,  310  as well. The overlay insulation layers  312 ,  314  terminate at inner ends, which are separated by a width W 2 . As can be seen, the second lead  306  contacts the top of the sensor stack  302  in a location between the first and second overlay insulation layers  312 ,  314 , having a contact width W 2 . The sensor stack  302  has a physical width W 1  that is significantly larger than the width W 2  between the inner ends of the overlay insulation layers  312 ,  314 . 
         [0031]    Because the current flowing from the second lead  306  is limited to an area having a width W 2  between the overlay insulation layers  312 ,  314 , the effective sensor track width is smaller than the actual width W 1  of the sensor. However, due to the spreading of current as it passes through the sensor stack  302 , the effective track width will be larger than the width W 2  between the overlay insulation layers  312 ,  314 . Therefore, the effective track width of the sensor  300  will be somewhere between W 2  and W 1 . 
         [0032]    As can be seen, the effective track width will be removed inward from the outer edges  316 ,  314  of the sensor stack  302 . As mentioned above, the outer edges  316 ,  318  of the layers of the sensor stack can become damaged during manufacture so that the magnetic qualities of the sensor layers at the outer edges is much poorer than at the inner portions of the sensor stack  302 . Because the effective track width of the sensor  300  is removed inward from the outer edges  316 ,  318  of the sensor stack  302 , the sensor  300  can enjoy a narrow track width while also being unaffected by any damage to the outer edges of the sensor stack  302 . 
         [0033]    With continued reference to  FIG. 3 , the sensor stack includes a free layer  320 , a pinned layer structure  322  and a thin, non-magnetic, electrically insulating barrier layer  324  such as alumina sandwiched between the free and pinned layers  320 ,  322 . If the sensor  300  is embodied in a CPP GMR sensor, the layer  324  would be a non-magnetic, electrically conductive spacer layer, such as Cu. 
         [0034]    The pinned layer structure  322 , can be of various configurations, and is preferably an antiparallel coupled (AP coupled), AFM pinned structure, that includes first and second magnetic layers AP 1   326 , AP 2   328  which are antiparallel coupled across an AP coupling layer  330  such as Ru. The AP 1  and AP 2  layers  326 ,  328  can be, for example CoFe or some other magnetic material. The AP 1  layer  326  is exchange coupled with a layer of antiferromagnetic material (first AFM layer)  332  such as PtMn, IrMn, etc. The exchange coupling with the AFM layer strongly pins the magnetization  334  of the AP 1  layer  326  in a desired direction perpendicular to the ABS. Antiparallel coupling between the AP 1  layer and the AP 2  layer pins the magnetization  336  of the AP 2  layer  328  in a direction perpendicular to the ABS and antiparallel with the magnetization  334  of the AP 1  layer  326 . A seed layer  338  may be provided at the bottom of the sensor stack to initiate a desired crystalline growth in the layers deposited there over. 
         [0035]    With reference still to  FIG. 3 , the free layer  320  has a magnetization  338  that is biased in a desired direction parallel with the ABS. This biasing is provided by a second AFM layer  340  that is deposited directly on top of and in contact with the free layer  320 . The second AFM layer  340  is configured to have a weaker exchange field than that of the first AFM layer  332  so that the second AFM layer  340  does not pin the magnetization  338  of the free layer  320  or make it too stiff. Accordingly, the second AFM layer  340  should have an exchange field of 50 to 300 Oe or more preferably 50 to 100 Oe. The exchange field strength can be adjusted, for example by adding Cr to the AFM layer  340 . For example, the AFM layer  340  could be constructed of PtMnCr or IrMnCr, and increasing the Cr content will decrease the exchange field. A capping layer  342 , such as Ta can be formed over the top of the sensor stack  302  to protect the sensor layers during manufacture. 
         [0036]    Because the sensor  300  is a CPP structure, sense current will not be shunted through the second AFM layer  340  as would be the case in a CIP structure. With reference to  FIG. 4 , in the event that even greater free layer stability is needed, a free layer  402  having an AP coupled structure can be provided. In such a design, the free layer  402  can be constructed with first and second magnetic layers F 1  and F 2   404 ,  406  which are antiparallel coupled across a coupling layer  408 , which can be, for example Ru. The second AFM layer  340  biases the magnetization  410  of the F 2  layer  406  in a desired direction parallel with the ABS, and the antiparallel coupling biases the magnetization  412  of the F 1  layer  404  in an opposite direction. Again, since the sensor  300  is a CPP sensor, current shunting will not be a problem as a result of the thicker free layer structure  402 . 
         [0037]    While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Other embodiments falling within the scope of the invention may also become apparent to those skilled in the art. Thus, the breadth and scope of the invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.