Abstract:
A shield for a read element of a magnetic recording head includes a first domain with boundaries remote from the read element and stabilized with a patterned bias element. The patterned bias element comprises a topographical pattern of grooves formed on the shield substrate.

Description:
BACKGROUND 
       [0001]    A magnetic read head retrieves magnetically encoded information that is stored on a magnetic medium or disc. The magnetic read head is typically formed of several layers that include a top shield, a bottom shield, and a read element or sensor positioned between the top and bottom shield. The read element may be a magnetoresistive device, such as a magnetoresistive (MR) sensor, a giant magnetoresistive (GMR) sensor, or a tunneling magnetoresistive (TMR) sensor. The read element includes at least one layer of a ferromagnetic material, which is magnetized along an easy axis of low coercivity. The read element may be oriented such that the easy axis is transverse to the direction of disc rotation and parallel to the plane of the disc. Magnetic flux from the disc surface causes rotation of the magnetization vector of the ferromagnetic layer of the read element, which in turn, causes a change in electrical resistivity of the read element. The change in resistivity of the read element can be detected by passing a sense current through the read element and measuring a voltage across the read element. External circuitry then converts the voltage information into an appropriate format and manipulates that information as necessary. 
         [0002]    During a read operation, the top and bottom shields ensure that the read element reads only the information stored directly beneath it on a specific track of the magnetic medium or disc by absorbing any stray magnetic fields emanating from adjacent tracks and transitions. A plurality of magnetic domains exist within the bottom shield. The magnetic domains are separated by one of a plurality of magnetic domain walls. Each domain has a magnetization that is oriented in a direction different from the magnetization of all adjacent domains. The application of an external magnetic field either during manufacture or from an adjacent track or transition of the magnetic storage medium during operation to a shield can cause the magnetization of each of the domains within that shield to rotate, thereby causing the domains to move, grow, or shrink. Thus, the domain walls are relocated due to the external magnetic field. The movement of a domain wall through a portion of the shield that is directly adjacent the read element results in Barkhausen noise, which is a local perturbation of the magnetic structure within the read element, producing an unwanted change in the resistance of the read element. Until recently, Barkhausen noise induced by domain wall movement has been negligible. However, as storage densities on magnetic media and discs have increased, the read element has necessarily become smaller, more sensitive, and more susceptible to Barkhausen noise created by domain wall movement. 
       SUMMARY 
       [0003]    The present invention is a magnetic shield that reduces Barkhausen noise by ensuring domain wall stability within a shield associated with a magnetic reader. The shield includes a topographically patterned bias in the vicinity of the reader that results in a domain structure with domain walls sufficiently removed from the reader so that they do not interfere with the reader. The patterned bias can be achieved, for example, by creating a topographical pattern on the substrate beneath a seed layer in the form of closely spaced parallel grooves. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIG. 1  is a simplified diagram of a disc drive storage system. 
           [0005]      FIG. 2  is a cross-sectional view of a magnetic read/write head and a magnetic disc taken along a plane normal to an air bearing surface of the read/write head. 
           [0006]      FIG. 3  is a layer diagram of a magnetic read/write head. 
           [0007]      FIG. 4  is a Kerr micrograph of a lower shield showing a domain configuration. 
           [0008]      FIG. 5  is a diagram of a lower shield showing the domain configuration. 
           [0009]      FIG. 6A  is a diagram of a lower shield showing topographical features (grooves) that stabilize the domain configuration. 
           [0010]      FIG. 6B  is a cross-section through AA of  FIG. 6A  showing the grooves and magnetic layers. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]      FIG. 1  is a top view of disc drive system  10 , which includes magnetic disc  12  mounted for rotational movement about an axis defined by spindle  14  within housing  16 . Disc drive  10  also includes actuator  18  mounted to base plate  20  of housing  16  and pivotally movable relative to disc  14  about axis  22 . Cover  24  covers a portion of actuator  18 . Drive controller  26  is coupled to actuator  18 . Drive controller  26  is either mountable within disc drive system  10  or is located outside of disc drive system  10  with suitable connection to actuator  18 . Actuator  18  includes actuator arm assembly  28 , rigid support member  30 , and head gimbal assembly  32 . Head gimbal assembly  32  includes flexure arm  34  coupled to rigid member  30  and air bearing slider  36  coupled to flexure arm  34  by a gimbal. Slider  36  supports a magnetic read/write transducer or head for reading information from disc  12  and writing information to disc  12 . 
         [0012]    During operation, drive controller  26  receives position information indicating a portion of disc  12  to be accessed. Drive controller  26  receives the position information from either an operator, a host computer, or from another suitable controller. Based on the position information, drive controller  26  provides a position signal to actuator  18 . The position signal causes actuator  18  to pivot about axis  22 . This causes slider  36  to move radially over the surface of disc  12  in a generally arc-like path indicated by arrow  38 . Drive controller  26  and actuator  18  operate in a known closed loop, negative feedback manner so that the transducer carried by slider  36  is positioned over the desired portion of disc  12 . Once the transducer is appropriately positioned, drive controller  26  then executes a desired read or write operation. 
         [0013]      FIG. 2  is a cross-sectional view of magnetic read/write head  50  and magnetic disc  12  taken along a plane normal to air bearing surface  54  of read/write head  50 .  FIG. 2  illustrates magnetic read/write head  50  and its placement relative to magnetic disc  12 . Air bearing surface ABS of magnetic read/write head  50  faces disc surface  56  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. The spacing between air bearing surface  54  and disc surface  56  is preferably minimized while avoiding contact between magnetic read head  50  and magnetic disc  12 . Contact between magnetic read head  50  and magnetic disc  12  can potentially destroy both magnetic read head  50  and magnetic disc  12 . 
         [0014]    A reader portion of read/write head  50  includes bottom gap layer  58 , top gap layer  60 , metal contact layer  62 , bottom shield  64 , top shield  66 , and read element  68 . Read gap  70  is defined on air bearing surface  54  between bottom gap layer  58  and metal contact layer  62 . Metal contact layer  62  is positioned between bottom gap layer  58  and top gap layer  60 . Read element  68  is positioned between terminating ends of bottom gap layer  58  and metal contact layer  62 . 
         [0015]    A writer portion of magnetic read/write head  50  includes top shield  66 , write gap layer  72 , top pole  74 , conductive coil  76 , and insulator layer  78 . Write gap  80  is defined on air bearing surface ABS by write gap layer  72  between terminating ends of top pole  74  and top shield  66 . Electrically conductive coils  76  are provided to generate magnetic fields across write gap  80  and are positioned in insulator layer  78  between top pole  74  and write gap layer  72 . While  FIG. 2  shows a single layer of conductive coils  76 , it should be understood that several layers of conductive coils, separated by insulating layers, may be used. 
         [0016]      FIG. 3  is a layer diagram of magnetic read/write head  50 .  FIG. 3  illustrates the location of a plurality of magnetically significant elements of magnetic read/write head  50  as they appear along air bearing surface  54  of magnetic read/write head  50  shown in  FIG. 2 . In  FIG. 3 , all spacing and insulating layers are omitted for clarity. Bottom shield  64  and top shield  66  are spaced to provide for a location of read element  68 . Read element  68  has two passive regions defined as the portions of read element  68  positioned adjacent to metal contacts  62 A and  62 B. An active region of read element  68  is defined as the portion of read element  68  located between the two passive regions of read element  68 . The active region of read element  68  defines a read sensor width. 
         [0017]    Read element  68  is preferably a magnetic sensor such as a magnetoresistive (MR) element, a giant magnetoresistive (GMR) sensor stack, or a tunneling magnetoresistive (TMR) sensor stack. A magnetoresistive element is generally formed of a ferromagnetic material whose resistance fluctuates in response to an external magnetic field, preferably from a magnetic medium or disc. By providing a sense current through the magnetoresistive element, a change in resistance of the magnetoresistive element can be measured and used by external circuitry to decipher the information stored on the magnetic medium or disc. A giant magnetoresistive stack operates similarly, but allows for a more pronounced magnetoresistive effect. GMR and TMR sensor stacks generally include: a ferromagnetic free layer, a ferromagnetic pinned or reference layer; and a non-magnetic spacer layer positioned between the free layer and the pinned layer. A pinned magnetization of the pinned layer is held constant while a free magnetization of the free layer is free to rotate in response to an external magnetic field, i.e. a transition from a magnetic disc. The resistivity of the stack varies as a function of an angle between the direction of the free magnetization and the pinned magnetization. In a GMR sensor stack, the spacer layer is an electrical conductor; in a TMR sensor stack, the spacer layer is an electrical insulator or barrier layer. 
         [0018]    Read element  68  of magnetic read/write head  50  shown in  FIG. 3  is configured as a current in plane (CIP) sensor with the sense current between electrodes  62 A and  62 B directed parallel to the ABS and transverse to the direction of motion relative to magnetic read/write head  10  as indicated by arrow A. It is to be understood that current-perpendicular-to-plane (CPP) sensor geometries can also be employed. 
         [0019]    One problem that can be encountered with magnetic sensors (such as MR, GMR, and TMR sensors) is Barkhausen noise. Barkhausen noise results from domain wall instability in magnetic shields in the vicinity of the magnetic sensor. The domain wall instability and resulting domain wall migration produces magnetic fluctuations that are sensed by the magnetic sensor and result in noise in the sensor signal. To minimize or eliminate Barkhausen noise, it is essential to (1) minimize the number of domain boundaries in the vicinity of the sensor, and (2) to restrict or eliminate domain wall motion under the influence of an external magnetic field. 
         [0020]    Top and bottom shields of a read element have been formed in a number of shapes including circular, square, rectangular, trapezoidal, and combinations thereof. The domain pattern in any shield is a function of a number of parameters including shape, magnetocrystalline anisotropy, internal stress and other effects. External magnetic fields such as those applied to a magnetic shield of a read element during manufacture and use tend to randomize the magnetic domain patterns of the magnetic shields. For example, when magnetic fields of sufficient magnitude are applied to the magnetic shield in its easy axis direction, the magnetic shield can saturate in the easy axis direction. When the external field is reduced to zero, square and rectangular geometries used in the prior art tend to develop unpredictable domain structures. 
         [0021]      FIG. 4  is a Kerr micrograph showing the domain pattern in a magnetic shield with a modified trapezoidal shape. This particular shield geometry has proved to be relatively stable and suitable for top and bottom shield applications for a magnetic reader. The modified trapezoidal shield geometry results in domain boundaries that are distal from the reader and a primary domain that surrounds the reader and has an easy axis of magnetization parallel to air bearing surface ABS. The domain boundary configuration in the Kerr micrograph of  FIG. 4  is shown following magnetization or “setting” of the reader using bias magnets parallel to air bearing surface ABS. 
         [0022]    A schematic of the domain boundary configuration in  FIG. 4  is shown in  FIG. 5  and described herein.  FIG. 5  shows reader shield  100  with trapezoidal-like boundaries  102 ,  104 ,  106  and  108  with corners  110 ,  112 ,  114  and  116 . Reader  300  is surrounded by primary domain  120 . Shorter domain boundaries  142  and  144  are attached to obtuse angles  112  and  114 . 
         [0023]    Primary domain  120  has magnetization vector  200  parallel to air bearing surface ABS and domain boundaries  125  and  155  are spaced away from reader  300 . The shape of shield  100  encourages domains  120 ,  130 ,  140  and  150  to form following magnetization of the setting field. Following saturation magnetization, as the setting field is reduced, domain boundaries initially nucleate at the acute angles of corners  110  and  116  to minimize domain boundary energy. This results in the formation of large domains  120 ,  130  and  150  sharing domain boundaries  125  and  155  attached to corners  110  and  116 . Shorter domain boundaries  142  and  144  are attached to obtuse angles  112  and  114 . The remanent magnetization of major domain  120  is parallel to air bearing surface ABS as shown. Magnetization vectors  200 ,  210 ,  220  and  230  indicate a closed magnetic circuit. The ideal domain configuration shown in  FIG. 4  does not always result following initial magnetization. 
         [0024]    The use of a patterned bias for domain control reinforces the ideal domain configuration shown in  FIGS. 4 and 5 . This patterned bias results in repeateable production of shields having the domain configurations and magnetic orientations shown in  FIGS. 4 and 5 . 
         [0025]    The domain control involves forming a pattern of parallel grooves on the substrate inside the outline of a shield to control the domain pattern and magnetization directions in the shield.  FIG. 6A  shows a diagram of shield  400  having a groove pattern designed to force the domain pattern shown in  FIG. 5 . Longitudinal grooves  410 ,  420 , and  430  are substantially parallel to ABS and to the perimeter boundaries of the shield outline. As discussed below, the final magnetization directions will follow the grooves after magnetization. Dotted lines  440 ,  450 ,  460  and  470  are on the diagram to indicate positions of the domain boundaries following magnetization. The domain boundaries will follow a line connecting the points where the grooves and resulting magnetization directions in the final film abruptly change direction. 
         [0026]    Cross section  500  of shield  400  along section AA of  FIG. 6A  is shown in  FIG. 6B . Seedlayer  520  is formed on substrate  510  and ferromagnetic layer  530  is formed on seedlayer  520 . Although grooves  410  and  420  are shown with rectangular cross sections, other cross sections including but not limited to semi-circular or trapezoidal shapes can be used. 
         [0027]    Topographical patterns of grooves such as those discussed herein can be formed on the substrate by ion beam etching, chemical etching, reactive ion etching, plasma etching, liftoff and other techniques known in the art. 
         [0028]    The grooves on the substrate force a longitudinal geometry for the magnetization directions in the magnetic film following magnetization. The longitudinal pattern is dictated by shape anisotropy. As a result of shape anisotropy, magnetization favors high length-to-width aspect ratios. A demagnetizing field and resulting magnetostatic energy are lowest when the magnetization follows the grooves, and the ideal domain pattern shown in  FIG. 5  is strongly reinforced when the substrate contains the groove patterns shown in  FIG. 6A . 
         [0029]    As described above, the disclosure includes a method to create a patterned bias on lower and shared shields that ensure the easy axes of the primary domains in the vicinity of the reader are parallel to the air bearing surface. In addition, the domain boundaries of the primary domain are sufficiently removed from the reader so that the domain boundaries do not interfere with operation of the reader. 
         [0030]    Although the use of grooved substrate templates to create domain configurations has been described with respect to a thin film magnetic shield geometry pertaining to a modified trapezoidal shape, it should be understood that the method can be applied to any shield geometry wherein the replication of particular equilibrium domain boundary configurations is desired. Furthermore, the technique is generally applicable to any magnetic thin film structure comprising a domain structure. 
         [0031]    The implementations described above and other implementations are within the scope of the following claims.