Abstract:
A magnetoresistive read sensor is described. The sensor is a magnetically responsive stack positioned between top and bottom electrodes on an air bearing surface. Current in the sensor is confined to regions close to the air bearing surface by a first multilayer insulator structure between the stack and at least one electrode to enhance reader sensitivity.

Description:
RELATED APPLICATION 
       [0001]    Reference is made to U.S. application Ser. No. 12/727,698 filed Mar. 19, 2010 titled TRILAYER READER WITH CURRENT CONSTRAINT AT THE ABS, which is incorporated by reference. 
       SUMMARY 
       [0002]    A magnetoresistive sensor includes a magnetically responsive stack positioned between top and bottom electrodes on an air bearing surface. A multilayer insulator structure between the stack and at least one electrode such that a current passing through the stack is confined to the vicinity of the air bearing surface to increase sensitivity. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]    According to an embodiment,  FIG. 1  is a schematic cross-sectional view of a magnetic read/write head and magnetic disc taken along a plane normal to an air bearing surface (ABS) of the read/write head. 
           [0004]    According to an embodiment,  FIG. 2  is a schematic ABS view of the magnetic read/write head of  FIG. 1 . 
           [0005]    According to an embodiment,  FIG. 3  shows a schematic ABS view of a typical trilayer current perpendicular to the plane (CPP) sensor stack. 
           [0006]    According to an embodiment,  FIG. 4  is a schematic cross-sectional view of the sensor stack shown in  FIG. 3  taken along section A-A. 
           [0007]    According to an embodiment,  FIG. 5  is a schematic cross-sectional view of a trilayer sensor with a short stripe height taken along section B-B in  FIG. 3 . 
           [0008]      FIG. 5A  is a graph of magnetic field strength in the trilayer sensor of  FIG. 5 . 
           [0009]    According to an embodiment,  FIG. 6  is a schematic cross-sectional view of a trilayer sensor with a long stripe height taken along section B-B in  FIG. 3 . 
           [0010]      FIG. 6A  is a graph of magnetic field strength in the trilayer sensor of  FIG. 6 . 
           [0011]    According to an embodiment,  FIGS. 7-10  are schematic cross-sectional views of four different embodiments of a trilayer sensor. 
           [0012]    According to an embodiment,  FIG. 11  is a schematic cross sectional view of an embodiment of the invention. 
           [0013]    According to an embodiment,  FIG. 12  is a schematic cross sectional view of another embodiment of the invention. 
           [0014]    According to an embodiment,  FIG. 13  is a schematic cross sectional view of another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Reduced shield-to-shield spacing can be achieved through the use of trilayer readers with dual free-layers. In a trilayer structure, two free-layers with magnetization in a scissor orientation are used to detect media magnetic flux. Synthetic antiferromagnetic (SAF) and antiferromagnetic (AFM) layers are not needed and free layer biasing comes from the combination of backend permanent magnet and demagnetization fields when both free layers have ends at the air bearing surface. Since the PM is recessed from the ABS surface, it does not interfere with the ability to achieve smaller shield-to-shield spacing without a sacrifice of PM material properties and bias field. Trilayer readers with a short stripe height and backend magnetic biasing have high readback signal but can be magnetically unstable and are very sensitive to process variations. 
         [0016]      FIG. 1  is a schematic cross-sectional view of an example embodiment of a magnetic read/write head  10  and magnetic disc  12  taken along a plane normal to air bearing surface ABS of read/write head  10 . Air bearing surface ABS of magnetic read/write head  10  faces disc surface  16  of magnetic disc  12 . Magnetic disc  12  travels or rotates in a direction relative to magnetic read/write head  10  as indicated by arrow A. Spacing between air bearing surface ABS and disc surface  16  is preferably minimized while avoiding contact between magnetic read/write head  10  and magnetic disc  12 . 
         [0017]    A writer portion of magnetic read/write head  10  includes top pole  18 , insulator  20 , conductive coils  22 , and bottom pole/top shield  24 . Conductive coils  22  are held in place between top pole  18  and top shield  24  by use of insulator  20 . Conductive coils  22  are shown in  FIG. 1  as two layers of coils but may also be formed of any number of layers of coils as is well known in the field of magnetic read/write head design. 
         [0018]    A reader portion of magnetic read/write head  10  includes bottom pole/top shield  24 , bottom shield  28 , and magnetoresistive (MR) stack  30 . MR stack  30  is positioned between terminating ends of bottom pole  24  and bottom shield  28 . Bottom pole/top shield  24  functions both as a shield and as a shared pole for use in conjunction with top pole  18 . 
         [0019]      FIG. 2  is a schematic view of air bearing surface ABS of the example magnetic read/write head  10  of  FIG. 1 .  FIG. 2  illustrates the location of magnetically significant elements in magnetic read/write head  10  as they appear along air bearing surface ABS of magnetic read/write head  10  of  FIG. 1 . In  FIG. 2  all spacing and insulating layers of magnetic read/write head  10  are omitted for clarity. Bottom shield  28  and bottom pole/top shield  24  are spaced to provide for a location of MR stack  30 . A sense current is caused to flow through MR stack  30  via bottom pole/top shield  24  and bottom shield  28 . While the sense current is injected through the bottom pole/top shield  24  and bottom shield  28  in  FIGS. 1 and 2 , other configurations have MR stack electrically isolated from bottom pole/top shield  24  and bottom shield  28  with additional leads providing the sense current to MR stack  30 . As the sense current is passed through MR stack  30 , the read sensor exhibits a resistive response, which results in a varied output voltage. Because the sense current flows perpendicular to the plane of MR stack  30 , a reader portion of magnetic read/write head  10  is a current perpendicular to plane (CPP) type device. Magnetic read/write head  10  is merely illustrative and other CPP configurations may be used in accordance with various embodiments of the present invention. 
         [0020]      FIG. 3  shows an ABS view of an embodiment of a trilayer CPP MR sensor  50  comprising trilayer MR stack  51 . MR stack  51  includes metal cap layer  52 , first freelayer  54 , nonmagnetic layer  56 , second freelayer  58 , and metal seedlayer  60 . Trilayer MR stack  51  is positioned between bottom pole/top shield  24  and bottom shield  28 . 
         [0021]    In operation, sense current I s  flows perpendicularly to the plane of layers  52 - 60  of trilayer MR stack  51  and experiences a resistance which is proportional to the cosine of an angle formed between the magnetization directions of first freelayer  54  and second free layer  58 . The voltage across trilayer MR stack  51  is then measured to determine the change in resistance and the resulting signal is used to recover encoded information from the magnetic medium. It should be noted that trilayer MR stack  51  configuration is merely illustrative and other layer configurations for trilayer MR stack  51  may be used in accordance with various embodiments of the present invention. 
         [0022]    The magnetization orientations of first freelayer  54  and second freelayer  58  in trilayer MR stack  51  are antiparallel and initially set parallel to the ABS in the absence of other magnetic fields or forces. The alignment of the freelayers in this antiparallel direction is attributed to magnetostatic interactions between the two freelayers and occurs when the reader width (RW) is larger than the stripe height (SH). To increase the sensitivity of the reader, the alignment of the two freelayers is preferably an orthogonal alignment relative to each other and about  45  degrees to the ABS, respectively. This is accomplished by a back bias magnet, (not shown in  FIG. 3 ) behind trilayer MR stack  51  biasing each freelayer.  FIG. 4 , which is a schematic cross-section of the example CPP MR sensor  50  taken along section A-A in  FIG. 3 , shows back bias magnet  62  behind MR stack  51  recessed from the ABS and positioned between bottom pole/top shield  24  and bottom shield  28 . The length of trilayer sensor stack  51  behind the ABS is the stripe height SH and, as will be shown, is an important variable in embodiments to be discussed. 
         [0023]    A schematic cross-section perpendicular to the ABS of trilayer CPP MR sensor  50  along section B-B in  FIG. 3  is shown in  FIG. 5 . Trilayer MR stack  51 A with air bearing surface ABS is shown positioned above recording medium  12 . Back bias magnet  62  is shown positioned above trilayer MR stack  51 A recessed from air bearing surface ABS. 
         [0024]    Trilayer MR stack  51 A has a layer structure identical to trilayer MR stack  51 . Magnetization of back bias magnet  62  is shown by arrow  63  as pointing in a vertical downward direction towards air bearing surface ABS. Magnetizations of first freelayer FL 1  and second freelayer FL 2  of trilayer MR stack  51  are shown schematically by arrows  53 A and  55 A respectively. As noted earlier, in the absence of back bias magnet  62 , magnetizations  53 A and  55 A would be parallel to the ABS and antiparallel to each other. The presence of back bias magnet  62  forces magnetizations  53 A and  55 A into a scissor relationship as shown. 
         [0025]    Curve  57 A in the graph of  FIG. 5A  depicts the magnetic field strength H media  from recording medium  12  in trilayer MR stack  51 A. As shown in  FIG. 5A , the magnetic field strength in the sensor decays exponentially as a function of distance from the ABS. In the sensor geometry shown in  FIG. 5 , the reader width RW is larger than the stripe height SH A  of trilayer stack  51 A. The scissors relation of magnetizations  53 A and  55 A of freelayers FL 1  and FL 2  result in increased sensitivity because both magnetizations freely respond to H media , the media flux. However, minor changes caused by process variability during fabrication can cause unacceptably large variability in sensor output or even magnetically unstable parts that will decrease product yield to unacceptable levels. 
         [0026]    A variation of the sensor geometry shown in  FIG. 5  is shown in  FIG. 6 . Back bias magnet  62  is shown positioned above trilayer MR stack  51 B distal from air bearing surface ABS. Trilayer MR stack  51 B has a layer structure identical to trilayer MR stack  51 . Trilayer MR stack  51 B differs from trilayer MR stack  51 A in that the stripe height SH B  of trilayer MR stack  51 B is longer than the reader width RW of trilayer MR stack  51 B by at least a factor of two. Both sensor stacks  51 A and  51 B have the same reader width RW. Magnetization of back bias magnet  62  is shown by arrow  63  as pointing in a vertical downward direction toward air bearing surface ABS. Magnetizations of first freelayer FL 1  and second freelayer FL 2  are shown schematically by arrows  53 B and  55 B respectively. 
         [0027]    In contrast to the magnetization orientations of trilayer MR stack  51 A, the magnetizations of each freelayer at the backend of trilayer MR stack  51 B are stable and parallel to the magnetization of back bias magnet  62  as indicated by arrow  63 . Due to the long stripe height of trilayer MR stack  51 B, the magnetization of free layers FL 1  and FL 2  naturally relax into the divergent orientations proximate the ABS as shown by arrows  53 B and  55 B due to the magnetostatic interaction between FL 1  and FL 2 . The stability and robustness of trilayer sensor stack  51 B significantly exceeds that of trilayer MR stack  51 A. The increased stability, however, comes with a cost. As a result of the increased stripe height, a majority of the length of trilayer MR stack  51 B does not contribute to the magnetoresistive sensing signal. Rather, the back end of the sensor stack functions as an electrical shunt, thereby decreasing the sensor output. 
         [0028]    Solutions to the problem that provide trilayer reader sensors with robust stability as well as increased sensitivity are shown in  FIGS. 7-10 . 
         [0029]    One embodiment is shown by CPP MR sensor  70  in  FIG. 7 . In CPP MR sensor  70 , trilayer MR stack  71  has a stripe height of at least twice reader width RW as shown in  FIG. 6 . CPP MR sensor  70  is comprised of trilayer MR stack  71  positioned between bottom pole/top shield  24  and bottom shield  28  with back gap magnet  62  behind the trilayer MR stack  51  as in CPP MR sensor  50  shown in  FIG. 4 . The difference is that insulator layer  72  in CPP MR sensor  70  is positioned between trilayer MR stack  71  and bottom shield  28 . Insulator layer  72  extends from the back end of bottom shield  28  to a distance close to the ABS, thereby providing a constriction in the current flow from bottom shield  28  through trilayer MR stack  57  to bottom pole/top shield  24 . By constricting the current flow to the vicinity of the ABS, as shown by the arrows, electrical shunting at the back end of trilayer MR stack  71  is blocked resulting in increased sensor output. 
         [0030]    Another embodiment is shown in  FIG. 8 . CPP MR sensor  80  is comprised of trilayer MR stack  71  with a long stripe height positioned between bottom pole/top shield  24  and bottom shield  28  with back gap magnet  62  behind trilayer MR stack  71 . In this case, insulator layer  73  is positioned between bottom pole/top shield  24  and trilayer MR stack  71 . Insulator layer  73  extends from the back end of bottom shield  28  to a distance close to the ABS, thereby providing a constriction in the current flow from top shield  24  through trilayer MR stack  71  to bottom shield  28  as indicated by the arrows. By constricting the current flow to the vicinity of the ABS, electrical shunting at the back end of trilayer MR stack  71  is blocked resulting in increased sensor output. 
         [0031]    Another embodiment is shown in  FIG. 9 . CPP MR sensor  90  is comprised of trilayer MR stack  71  with a long stripe height positioned between bottom pole/top shield  24  and bottom shield  28  with back gap magnet  62  behind trilayer MR stack  71 . In this case, insulator layer  73  is positioned between bottom pole/top shield  24  and trilayer MR stack  71  and insulator layer  72  is positioned between bottom shield  28  and trilayer MR stack  71 . Insulator layers  72  and  73  extend from the back ends of top and bottom shields  24  and  28  to a distance close to the ABS thereby providing a constriction in the current flow between bottom pole/top shield  24  and bottom shield  28  or between bottom shield  28  and bottom pole/top shield  24  through trilayer MR stack  71 . By constricting the current flow to the vicinity of the ABS, electrical shunting at the back end of trilayer MR stack  71  is blocked, resulting in increased sensor output. 
         [0032]    Another embodiment is shown in  FIG. 10 . CPP MR sensor  100  is comprised of trilayer MR stack  71  with a long stripe height positioned between bottom pole/top shield  24  and bottom shield  28  with back gap magnet  62  behind trilayer MR stack  71 . Insulator layer  72  extends from the back end of bottom shield  28  to the ABS. In this case, a portion of insulator layer  72  proximate the ABS has been treated to transform insulator layer  72  into electrically conducting portion  74 . Electrically conducting portion  74  provides a constriction in the current flow from bottom shield  28  to bottom pole/top shield  24  through trilayer MR stack  71  as indicated by the arrows. By constricting the current flow to the vicinity of the ABS as the current passes through trilayer MR stack  71 , electrical shunting at the back end of trilayer MR stack  71  is blocked, resulting in increased sensor output. 
         [0033]    Insulator layer  72  can be converted to electrically conducting region  74  after the ABS is lapped by a number of processes. Some of these are described here. One approach is to use co-sputtered Fe and SiO 2  as the insulating layer. The resulting Fe/SiO 2  layer is amorphous and electrically resistant. Preferential heat treatment of the ABS to moderate temperatures of about 350° C. to 400° C. by exposing the ABS to a laser beam will cause Fe segregation and the formation of electrically conductive channels close to the ABS. Another approach is to use a TiO x  barrier layer as the insulating layer. Lapping the ABS containing TiO x  insulating layers in an ordinary atmosphere or in hydrogen forms defects in the TiO x  layers that form conductive channels, thereby allowing current flow at the ABS. 
         [0034]    Insulator layers that have been transformed into conducting channels at the ABS to constrict current flow through sensor stack  71  at the ABS can also be positioned between bottom pole/top shield  24  and stack  71  and between bottom shield  28  and stack  71 . It should be noted that the sensor stacks described above are merely illustrative and other configurations may be used in accordance with various embodiments of the present invention. 
         [0035]    It has been found that introduction of insulator layer  72  in bottom shield electrical conductor  28  leads to manufacturing and device performance issues. A key step in the manufacture of CPP MR sensor  70  shown in  FIG. 7  is the planerization of the tops of bottom shield electrical conductor  28  and insulator layer  72 , i.e. surface S, before trilayer MR stack  71  is deposited. Planerization is accomplished by chemical mechanical polishing (CMP) whose techniques are well known to those versed in the art. Difficulties arise because the CMP polishing rates of dissimilar materials are different. This results in discontinuities in surface S. such as peaks and valleys in the surface in the vicinity of the intersection of shield  28  and insulator layer  72 , in dishing in the insulator material, and other problems. The resulting unpredictable nature of surface S after planerization leads to device performance instability, lot to lot variation during processing, and increased manufacturing costs. 
         [0036]    It has been found that introduction of insulator layer  72  in bottom shield electrical conductor  28  leads to manufacturing and device performance issues. A key step in the manufacture of CPP MR sensor  70  shown in  FIG. 7  is the planarization of the tops of bottom shield electrical conductor  28  and insulator layer  72 , i.e. surface S, before trilayer MR stack  71  is deposited. Planarization is accomplished by chemical mechanical polishing (CMP) whose techniques are well known to those versed in the art. Difficulties arise because the CMP polishing rates of dissimilar materials are different. This results in discontinuities in surface S. such as peaks and valleys in the surface in the vicinity of the intersection of shield  28  and insulator layer  72 , in dishing in the insulator material, and other problems. The resulting unpredictable nature of surface S after planarization leads to device performance instability, lot to lot variation during processing, and increased manufacturing costs. 
         [0037]    The problem has been circumvented by the inventive embodiment shown in  FIGS. 11-13 .  FIG. 11  shows CPP MR sensor  110  comprising trilayer MR stack  71  with a long stripe height positioned between bottom pole/top shield  24  and bottom shield  28  with backgap magnet  62  behind trilayer MR stack  71 . Insulator layer  72  has been replaced with multilayer insulator structure  74 . Multilayer insulator structure  74  comprises insulator layer  76  and nonmagnetic metal conducting layer  78 . Nonmagnetic metal conducting layer  78  has CMP polishing rates similar to bottom shield  76 , thereby ensuring planarization of surface S during CMP. Insulating layer  76  contains insulating side wall  77  that ensures there is no conducting path between bottom shield  28  and nonmagnetic conducting layer  78 . The thickness of sidewall  77  is between 3 nm to 5 nm. 
         [0038]    Multilayer insulator structure  74  can also be employed in the embodiment shown in  FIG. 9  as shown in  FIG. 12 .  FIG. 12  shows CPP MR sensor  120  comprising trilayer MR stack  71  with a long stripe height positioned between bottom pole/top shield  24  and bottom shield  28  with back gap magnet  62  behind trilayer MR stack  71 . Insulator layer  72  has been replaced with multilayer insulator structure  74 . Multilayer insulator structure  74  comprises insulator layer  76  and nonmagnetic metal conducting layer  78 . Nonmagnetic metal conducting layer  78  has CMP polishing rates similar to bottom shield  76 , thereby ensuring planarization of surface S during CMP. Insulating layer  76  contains insulating side wall  77  that ensures there is no conducting path between bottom shield  28  and nonmagnetic conducting layer  78 . The thickness of sidewall  77  is between 3 nm to 5 nm. 
         [0039]    Insulating layers  76  and  76 ′ can be, among others, Al 2 O 2 , SiO 2 , and SiON. Nonmagnetic metal conducting layers  78  and  78 ′ can be, among others, Ru, Ta, Cr, and NiCr. 
         [0040]    While the present disclosure has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the claimed embodiments. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the disclosure without departing from the essential scope thereof. Therefore, it is intended that the claimed technology not be limited to the particular embodiment(s) disclosed, but that the disclosure will include all embodiments falling within the scope of the appended claims.