Abstract:
Reader asymmetry control has become an important issue as track widths continue to shrink. This has been achieved by providing adaptive adjustment to the asymmetry of individual heads in the form of an additional permanent hard magnet, which may be internal or external to the head. This special ‘tuning’ magnet biases the shields and the sensor to achieve suitable asymmetry and/or amplification. Head bias adjustments may be done individually or in batch. Both the internal and the external magnet versions are described.

Description:
FIELD OF THE INVENTION 
   The invention relates to the general field of magnetic read heads with particular reference to read symmetry/bias point. 
   BACKGROUND OF THE INVENTION 
   Magnetic recording head asymmetry control is very important for head performance and yield. For current and future track widths, reader asymmetry control becomes a progressively more critical issue. 
   In the present invention we disclose a design which allows adaptive adjustment to asymmetry on individual heads by means of an additional special external or internal permanent hard magnet. This special ‘tuning’ magnet biases the shields and the sensor to achieve suitable asymmetry and/or amplification. 
   The biasing magnet of the present invention should not be confused with the permanent magnets used for longitudinal bias. Their purpose is the elimination of Barkhausen noise and they need to abut the sensor (specifically the free layer), which is not the case for the present invention. Furthermore, the tuning magnet of the present invention is set adaptively (its magnetization vector being adjustable) to compensate for head to head (first and second embodiments) or batch to batch (third embodiment) bias and asymmetry variations. The invention applies to all types of magnetic head including all sensor types. 
   A routine search of the prior art was performed with the following reference of interest being found: 
   Jones, Jr., in U.S. Pat. No. 5,742,998, discloses a magnetic head structure including a slider. 
   SUMMARY OF THE INVENTION 
   It has been an object of at least one embodiment of the present invention to provide a process for adjusting the bias point of a magnetic reader head. 
   Another object of at least one embodiment of the present invention has been that said process be applicable to read heads after their manufacture. 
   Still another object of at least one embodiment of the present invention has been to provide a read head whose bias point has been adjusted internally during manufacture. 
   A further object of at least one embodiment of the present invention has been that the application of the invention be independent of sensor type, longitudinal bias type, and current flow type. 
   These objects have been achieved by providing adaptive adjustment to the asymmetry of individual heads by means of an additional external permanent hard magnet. This special ‘tuning’ magnet biases the shields and the sensor to achieve suitable asymmetry and/or amplification. Alternately, batch tuning may be accomplished by use of an internal hard magnet for the same purpose. Both approaches are described. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  Illustrates a first embodiment of the invention wherein an adjustable permanent magnet is located on the top surface of the slider. 
       FIG. 2  Illustrates a second embodiment of the invention wherein an adjustable permanent magnet is located on the front surface of the slider. 
       FIGS. 3 and 4  illustrate a third embodiment of the invention wherein said permanent magnet is internally located as part of the read sensor assembly. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Several embodiments are possible. In embodiments 1 and 2, the tuning (hard) magnet is external to the slider, so its position can be adjusted, as needed, before it is firmly secured in place. For these external magnets, encapsulation of the magnet in epoxy or other materials may be desired to avoid contamination. It is understood that, in all cases, special initialization procedures will be required depending on the tuning magnet and sensor design details, the respective coercivities, the active axis directions, etc. The “initialization” sets the desired magnetization direction and size to achieve the desired asymmetry tuning. In embodiment  3 , the permanent tuning magnet is part of the head assembly itself. 
   We now proceed to a description of three embodiments. Each of these embodiments will be disclosed through a description of a process for its manufacture. Said description will also serve to describe the structure of each embodiment. 
   1 st  Embodiment 
   Referring now to  FIG. 1  the process for manufacturing the first embodiment begins with the provision of slider body  11  having an air bearing surface  12 . A pair of parallel magnetic shields  13  and  14  are formed at one end of the slider body. The shields are positioned to overlap one another and both shields fully overlap magneto-resistive read sensor  15  that is sandwiched between them. The shields and the sensor stack together form the magnetic read head itself. The latter has an air bearing surface which is usually coplanar with that of the slider body. The full read-write head is completed with the fabrication of write head assembly  16 , the latter not being novel. 
   A key novel feature of the invention is the presence on the top surface of slider  11  of adjustable permanent magnet  17 , whose magnetic axis  18  is parallel to the aforementioned top surface. Magnet  17  serves to effect the polarization of shields  13  and  14  at the air bearing surface, which in turn affects the location of the sensor&#39;s bias point. An important advantage of this adjustability of the magnet is that it enables each magnetic read head to be custom tuned with respect to its bias point. Through either of several means, the permanent magnet&#39;s magnetization direction and magnitude are set (or adjusted) in order to tune the bias point of the head. 
   Note also that, during the above-described fabrication process, it is possible to individually shape the two shields. In particular, narrowing and tapering of the shields near the air bearing surface serves to enhance the tuning process discussed above. This will become clearer in the description of the third embodiment. 
   Once the optimum location for the tuning magnet  17  has been determined, it is routine to fix it in place (both to secure it as well as to protect it against possible contamination) by making a permanent bond between it and the slider head (using, for example, epoxy). This does not, however, rule out possible future applications in which the tuning magnet is secured in a manner that makes possible subsequent later adjustments of its position. 
   The exact environment in which the optimum state of the tuning magnet is determined will, of course, vary. An example of such an environment that is of particular importance is worth noting here and that is performing the bias point optimization in combination with perpendicular recording disk media having a magnetically soft underlayer. 
   The application of this embodiment is not dependent on what type of sensor is being used. Thus, it could be used for cases where the magneto-resistive read stack is a Giant Magneto-Resistance (GMR) sensor or a Magnetic Tunnel Junction (MTJ) sensor. 
   Similarly, the application of this first embodiment is not dependent on how the longitudinal bias (discussed in the background section) is applied. Examples of these include (but are not limited to) abutted hard bias, abutted exchange bias, and patterned exchange bias. 
   Finally, the method used to pass current through the sensor does not affect the application of the present invention. Thus, the magneto-resistive read stack could be a Current in Plane (CIP) type or a Current Perpendicular to Plane (CPP) type. 
   2 nd  Embodiment 
   We refer now to  FIG. 2 . The process of the second embodiment begins with the provision of slider body  11  having air bearing surface  12  and, on a surface normal to this, the read-write head is formed of the same elements already enumerated for the first embodiment. 
   The key novel feature (and principal difference from the first embodiment) of the second embodiment is the location where permanent (tuning) magnet  27  is placed. Magnet  27  is placed on end surface  22  of the finished read-write head. Magnetization axis  28  is normal to end surface  22  but other orientations are also possible. 
   It is also possible, in principle, to simultaneously use two magnets such as both  17  and  27  in the same device in order to be able to achieve even finer control of the bias point. 
   As in the first embodiment, the application of the invention is independent of sensor type, longitudinal bias type, and current flow type. 
   3 rd  Embodiment 
   In this embodiment, the permanent (tuning) magnet is internally located within the read-write head assembly. As already noted, this device may be cheaper to manufacture than the first two embodiments. 
   Referring now to  FIG. 3 , the process of the third embodiment begins with the provision of slider body  11  on whose end face magnetic shield  13  is formed. Magneto-resistive read stack  15  is then formed over (as opposed to on) it. Shield  13  must fully overlap stack  15 . 
   This is followed by the deposition, patterning, and magnetization (in direction  38  for example) of permanent magnet  37  which will normally contact shield  13  but must not overlap sensor stack  15 . Formation of the read-head assembly concludes with the deposition and patterning of second magnetic shield  14  (see  FIG. 1  or  2 ) that fully overlaps stack  15 . 
   A variation on the third embodiment is illustrated in  FIG. 4 . The approximate initialization axis here may be as shown by  58  and may be perpendicular to that shown in  FIG. 3 . It is magnetically connected to magnetic shield  13  through magnetic shield extender  43 . This embodiment is intended to show how alternate geometries may allow different (and possibly preferable) initialization directions. 
   As will have been noted,  FIGS. 3 and 4  are cross-sections through  3 — 3  of  FIG. 1  allowing the shapes of shields  13  and  14  to be seen. The narrowing and tapering of (one of) the shields is optional. 
   As in the first and second embodiments, the application of the invention is independent of sensor type, longitudinal bias type, and current flow type.