Abstract:
A method and system are provided for implementing discrete step stabilization of AMR read sensors in tape or disk drives. In one implementation, a plurality of discrete step stabilizers and AMR read sensor elements are arranged with a rotational symmetry coupled with a plane inversion for an odd number of steps, and no inversion for an even number of steps. Preferably, the steps are oriented at 45 degrees, or approximately parallel to the desired bias direction. For relatively narrow track widths (e.g., approximately 5 microns), the edges of the sensor element nearest the permanent magnets are especially important to stabilize. Therefore, in one implementation, an edge of a stabilizer preferably intersects the edge of the sensor element at one half of the stripe height. Also, the rising and falling edges of the stabilizers do not always have the same slope. In order to compensate for the different slopes of a stabilizer&#39;s edges, the rising and falling edges of a stabilizer&#39;s pattern are interchanged by a “stabilizer phase” transformation to produce the complement (phase conjugate) of the stabilizer pattern. As such, if a single rising edge of a stabilizer pattern intersects the center of a sensor element, the “stabilizer phase” transformation changes this structure to a single falling edge that intersects the center of the sensor element.

Description:
RELATED APPLICATIONS 
   The present application is related by subject matter to commonly assigned U.S. Pat. No. 7,139,156 entitled “NON-PENETRATION OF PERIODIC STRUCTURE TO PM”, filed on Jun. 20, 2002, and hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to improved data storage technology, and in particular, but not exclusively, to a method and system for implementing discrete step stabilization of narrow-track, Anisotropic Magneto-Resistive (AMR) read sensors in magnetic media storage systems. 
   2. Background of the Invention 
   Tape drives are peripheral mass storage devices often used to archive data on tapes for later access. In certain applications, huge amounts of data are stored directly on magnetic tape for later retrieval and analysis. Tape drives are also used as random access devices in data storage applications where the cost of storage is more important than access time. 
   Data can be stored or written onto a magnetic media (tape or disk) by selectively magnetizing regions of the media with a write head. The magnetized regions of the media produce a magnetic field that can be detected and converted into an electrical signal by a read head. A common type of read head used for carrying out this conversion is an AMR read head. 
   More precisely, the operation of reading data from the magnetic media is performed by sensing the magnetic polarity transitions on the media as it is moved across a read head in a longitudinal direction. The magnetic transitions on the media present a varying magnetic field to a read transducer in the read head. The read transducer converts the magnetic field into an analog read signal that is delivered to a read channel for processing. The read channel converts this analog signal into one or more digital signals that are processed by a computer system. 
   In thin-film heads including a plurality of transducer elements, AMR sensors are often used to read information from the magnetic media, because of the increased sensitivity of the AMR sensors during read operations. During a read operation, an AMR sensor is held very near a disk or in contact with a tape, in order to sense the varying magnetic transitions on a particular track. A constant DC (bias) current is passed through the AMR sensor, and the sensed varying magnetic transitions produce a variable voltage across the sensor due to its varying resistance. This variable voltage signal is the read analog signal, which is then processed and converted to digital form. 
   A common goal in the information storage industry is to magnetically stabilize AMR read elements so that the generated electrical signals are linear. This goal is frequently accomplished by controlling the boundary or end magnetic domains in the read sensors involved. In the past, this boundary magnetic control was provided by permanent magnets or magnetic exchange tabs attached to the ends of the sensors. However, the greater the width of the track containing the stored data, the less effective boundary magnetic control was at the center of the read sensor. 
   One technique used to overcome this limitation has been to form a periodic perturbation (grating structure) which creates a periodic magnetic charge that stabilizes the middle region of the sensor. However, the resulting periodic perturbation can interfere with the magnetic field of the permanent magnet(s). This interference problem is exacerbated as the read heads are made smaller. 
   An improved magnetic stabilization technique for MR read heads is described and claimed in related U.S. Pat. No. 7,139,156 entitled “NON-PENETRATION OF PERIODIC STRUCTURE TO PM”. This technique limits the undulations of the periodic grating structure used for magnetic stabilization of an MR read sensor to regions that are not directly adjacent to the permanent magnets attached at either end of the sensor. This technique allows the permanent magnets to stabilize the magnetization near the ends of the sensor, and also allows the periodic structure to stabilize the magnetization in the middle region of the sensor without perturbing the permanent magnets. 
   Nevertheless, a significant problem that has arisen is that read sensors are being made narrower while the pitch of the periodic structures is constrained. For example, in one of the narrowest track sensors currently being produced, only a single stabilizer centered on the sensor is used. Consequently, it is no longer tenable to regard such a structure as periodic. 
   In actual fact the parts of a stabilizer that influence magnetic behavior are the steps located beneath the sensor (e.g., depressions precisely milled typically in an Aluminum Oxide/Alumina (Al 2 O 3 ) underlayer directly beneath the sensor). Therefore, it is desirable to have an improved method and system for implementing discrete step stabilization of AMR read sensors including, for example, narrow-track, AMR read sensors. 
   SUMMARY OF THE INVENTION 
   The present invention provides a method and system for implementing discrete step stabilization of AMR read sensors including, for example, narrow-track, AMR read sensors in tape drives. In a preferred embodiment of the present invention, a plurality of discrete-step stabilizers and AMR read sensor elements are arranged as shown in  FIG. 3 . Preferably, the steps of the stabilizers are oriented at 45 degrees, or approximately parallel to the desired bias direction. For relatively narrow track widths (e.g., approximately 5 microns), the edges of the sensor element nearest the permanent magnets are especially important to stabilize. Therefore, in one embodiment, an edge of a stabilizer preferably intersects the edge of the sensor element at one half of the stripe height. Also, the rising and falling edges of the stabilizers do not always have the same slope. Therefore, in one embodiment, to compensate for the different slopes of a stabilizer&#39;s edges, the rising and falling edges of a stabilizer&#39;s pattern are interchanged by a “stabilizer phase” transformation to produce the complement (phase conjugate) of the stabilizer pattern, as shown in  FIG. 3 . As such, if a single rising edge of a stabilizer pattern intersects the center of a sensor element, the “stabilizer phase” transformation changes this structure to a single falling edge that intersects the center of the sensor element. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
       FIG. 1  depicts a cutaway view of an AMR read head with a single step stabilizer, which may be used to implement a preferred embodiment of the present invention; 
       FIG. 2  depicts a planar view of an AMR read head structure in accordance with a preferred embodiment of the present invention; 
       FIG. 3  depicts a diagram that illustrates a plurality of discrete step stabilization patterns for an AMR read head, which may be used to implement one or more preferred embodiment(s) of the present invention; 
       FIGS. 4A and 4B  are related diagrams depicting exemplary phase shift (P), reflection (R), and combined reflection and phase shift transformation operations, respectively, that can be performed in accordance with a preferred embodiment of the present invention; and 
       FIGS. 5A-5C  are related diagrams depicting geometric features associated with AMR read sensor elements, which are structured in accordance with a preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   With reference now to the figures and in particular with reference to  FIG. 1 , a cutaway view of an exemplary AMR read head is depicted, which may be used to implement a preferred embodiment of the present invention. Although the following structural description includes particular dimensions and materials, this information is provided for illustrative purposes only and not intended as architectural limitations to be imposed on the present invention. 
   The exemplary cutaway view of AMR read head  100  shows magnetic shields  102  and  118 . Typically, magnetic shields  102 ,  118  are highly permeable magnetic shields that help to focus the magnetic fields emanating from a disk or tape and eliminate stray fields. In a preferred embodiment, each magnetic shield  102 ,  118  is made from a Cobalt-Zirconium Tantalum alloy (CZT) and is approximately 2.5 microns thick. Although not explicitly shown in this cutaway, side view of AMR read head  100 , magnetic shields  102 ,  118  cover substantially the bottom and top surfaces, respectively, of AMR read head  100 . 
   First gap layer  104  is formed directly above bottom magnetic shield  102 . Preferably, for this exemplary embodiment, first gap layer  104  is made of Aluminum Oxide/Alumina (Al 2 O 3 ) and is approximately 1100 Angstroms thick. First gap layer  104  forms a substrate upon which AMR read sensor element  107  can be formed. Also, permanent magnets  106 ,  120  can be placed on first gap layer  104  at each end of AMR read sensor element  107  thus formed. 
   Exemplary AMR read sensor element  107  is shown formed with layers  110 ,  112 ,  114 . Layer  110  is preferably formed from a Cobalt Zirconium Molybdenum (CZM) alloy and is approximately 250 Angstroms thick. An important function of layer  110  is to create a magnetic field in the MR layer (e.g., layer  114  in this embodiment), which allows a quasi-linear magnetic field-(from the storage medium)-to-voltage device operation to occur. 
   Layer  112  is formed directly on layer  110 . Layer  112  is preferably made from Tantalum and is approximately 80 Angstroms thick. An important function of layer  112  is as a spacer layer to prevent direct contact between magnetic layer  110  and (MR) layer  114 . Layer  114  is preferably composed of a Nickel Iron alloy and is approximately 320 Angstroms thick. Layer  114  creates an MR response that converts detected magnetic field changes to resistance and/or voltage changes. 
   Second gap layer  116  is formed on (MR) layer  114 . Second gap layer  116  is preferably composed of Aluminum Oxide/Alumina similar to that of first gap layer  104  and is approximately 1500 Angstroms thick. Second gap layer  116  covers both MR sensor element  107  and permanent magnets  120 . Magnetic shield  118  is formed on second gap layer  116 . As mentioned earlier, magnetic shields  118  and  102  function primarily to block stray magnetic fields. 
   As illustrated by  FIG. 1 , MR sensor element  107  is not entirely planar shaped. For this exemplary embodiment, a single step ( 109 ) is shown. Note that the cutaway view of  FIG. 1  is at the tape-bearing surface, which is half a stripe height below the horizontal midline (as described in more detail below). Therefore, for this example, step  109  occurs to the left-of-center of MR sensor element  107 . Also, note that all structures formed above first gap layer  104  to the left side of step  109  are lower than on the right side of step  109 . As such, the discrete step topography of MR sensor element  107  can be formed by known photolithographic and/or iron milling techniques. 
     FIG. 2  depicts a planar view of an AMR read head structure in accordance with a preferred embodiment of the present invention. AMR read head  200  is shown with the magnetic shields removed for clarity. For this exemplary embodiment, tape-bearing surface  202  is part of MR region  204 . Permanent magnets  206  abut MR region  204  at respective junctions. High conductivity leads  210  provide power to AMR read head  200 . Note, for this illustrative example, that step  207  of stabilizer  208  penetrates into one of permanent magnets  206 . Also, note that only one step ( 209 ) of stabilizer  208  passes through the read sensor element (MR region  204 ) of AMR read head  200 . Step  209  intersects the geometric center of AMR read head  200  in this example. 
     FIG. 3  depicts a diagram that illustrates a plurality of discrete step stabilization patterns for an AMR read head, which may be used to implement one or more preferred embodiment(s) of the present invention. For example, the plurality of stabilizer patterns  300  can be implemented with AMR read head  100  shown in  FIG. 1  and/or AMR read head  200  shown in  FIG. 2 . Notably, as readily seen from the drawings, step stabilizer  208  and MR region  204  in  FIG. 2  are respectively illustrated by the pattern (IA) of discrete step  302  and AMR sensor element  304  in  FIG. 3 . 
   For clarity, the exemplary plurality of stabilizer patterns ( 300 ) illustrated in  FIG. 3  is shown as 16 stabilizer patterns. However, the present invention is not intended to be limited to a specific number of patterns, and the number and type of patterns shown are not to be considered architectural limitations on the present invention. As such, in each stabilizer pattern shown, the shaded rectangle (e.g., element  304 ) represents an AMR sensor element, and the parallelogram (e.g., element  302 ) represents a depression (e.g., precisely milled) under the sensor element, whose edges (e.g., elements  303 ,  305 ) form discrete step structures. 
   Alternatively, instead of a depression with at least one rising edge and one falling edge, each stabilizer pattern may be an elevated area with at least one rising edge and one falling edge. Thus, instead of milling out a “hole” to form a depression in a layer of material under the sensor element, the material around the stabilizer pattern can be milled out to form an elevated area under the sensor element. 
   The operations I, P, R and PR in  FIG. 3  represent transformations that can be applied to the basic stabilizer patterns A, B, C and D. Essentially, for this exemplary embodiment, “I” represents an identity operation that leaves the basic pattern unchanged. Also, as described in detail below, “P” represents a phase shift operation that transforms a rising/falling edge of a step structure to a falling/rising edge of that structure. “R” represents a reflection operation that transforms a step structure of a basic pattern to a mirror image of that step structure about its midpoint. “PR” represents a combination of phase shift and reflection operations that first transforms a basic pattern according to the R operation, and then phase shifts that pattern according to the P operation. 
   Specifically, with reference to  FIGS. 4A and 4B , related diagrams depicting exemplary phase shift (P) and reflection (R) transformation operations, respectively, that can be performed in accordance with a preferred embodiment of the present invention, are shown for illustrative purposes. Referring now to  FIG. 4A , a diagram depicting an exemplary phase shift (P) transformation operation  400   a  is shown. In this case, the P transformation interchanges the location where the falling and rising edges of a stabilizer intersect the horizontal midline of the pattern at the center of the sensor element. In other words, the P transformation is a phase shift of a grating (stabilizer) structure such that the rising and falling edges (discrete steps) of the grating structure are interchanged. 
   For example, the leftmost pattern in  FIG. 4A  (e.g., I or identity pattern) includes stabilizer  402   a  with falling edge  410   a  and rising edge  412   a . In this example, rising edge  412   a  is shown intersecting the midline (e.g., denoted by the dashed line) of the pattern at the geometric center of sensor element  404   a . The P transformation (resulting in the rightmost pattern in  FIG. 4A ) shifts the I pattern to cause falling edge  410   a  to intersect the midline of the pattern at the geometric center of sensor element  404   a.    
   Referring now to  FIG. 4B , a diagram depicting an exemplary reflection (R) transformation operation  400   b  is shown. In this case, the R transformation also interchanges the location where the falling and rising edges of a stabilizer intersect the horizontal midline of the pattern at the center of the sensor element. However, the R transformation rotates the grating (stabilizer) structure 180 degrees about the midpoint of the sensor element such that the rising and falling edges (discrete steps) of the grating structure are interchanged. 
   For example, the leftmost pattern in  FIG. 4B  (e.g., I or identity pattern) includes stabilizer  402   b  with falling edge  410   b  and rising edge  412   b . Again, similar to the I operation in  FIG. 4A , rising edge  412   b  is shown intersecting the midline (e.g., denoted by the dashed line) of the pattern at the geometric center of sensor element  404   b . The R transformation (resulting in the rightmost pattern in  FIG. 4B ) changes the skew of stabilizer  402   b  such that the angle, θ, is transformed to its complementary angle, θ c =180°−θ, as illustrated in the rightmost diagram of  FIG. 4B . Also, the R transformation interchanges the rising and falling edges where they intersect the midline of the pattern at the geometric center of sensor element  404   b . In this example, the I operation in the leftmost pattern in  FIG. 4B  shows rising edge  412   b  intersecting the midline of the pattern at the geometric center of sensor element  404   b . The R transformation operation in the rightmost pattern in  FIG. 4B  also shows rising edge  412   b  intersecting the midline of the pattern at the geometric center of sensor element  404   b , because the R transformation produces the mirror image of the I operation. 
     FIGS. 5A-5C  are related diagrams provided for clarity and definition purposes that depict geometric features associated with AMR read sensor elements, which are structured in accordance with a preferred embodiment of the present invention. Referring to  FIG. 5A , read sensor element  502  is shown with a rectangular shape and its geometric center indicated at  504 . In  FIG. 5B , read sensor element  506  is shown with its horizontal midline indicated at  508  located at half the stripe height. In  FIG. 5C , read sensor element  510  is shown with its horizontal midline indicated at  512  located at half the stripe height. Also,  FIG. 5C  shows w/3 (e.g., ⅓ width) and 2w/3 (e.g., ⅔ width) positions  514  and  516 , respectively, on horizontal midline  512  of read sensor element  510 . 
   Returning now to  FIG. 3 , when reference is made to rising or falling steps, it should be understood that movements with respect to these structural features are viewed with the convention that the midline of a sensor is being traversed from left to right. Also, for the exemplary embodiments shown, the angle, θ, is preferably equal to 45°+/−70°, and the angle, θ c , is preferably equal to 180°−θ. 
   The basic pattern, A (i.e., IA), in  FIG. 3  includes stabilizer  302  with falling step  303  and rising step  305 . As shown, for this exemplary design, rising step  305  is inclined at an angle, θ, and intersects the geometric center of read sensor element  304 . The phase shift transformation (PA) of basic pattern, A, includes stabilizer  308  with falling step  309  inclined at an angle, θ, and intersecting the geometric center of read sensor element  306 . The reflection transformation (RA) of basic pattern, A, includes stabilizer  312  with falling step  311  inclined at an angle, θ c , and intersecting the geometric center of read sensor element  310 . The phase shift/reflection transformation (PRA) of basic pattern, A, includes stabilizer  314  with falling step  315  inclined at an angle, θ c , and intersecting the geometric center of read sensor element  316 . 
   The basic pattern, B (i.e., IB), in  FIG. 3  includes stabilizer  318  with falling step  317  and rising step  319 . As shown, for this exemplary design, falling step  317  and rising step  319  are inclined at an angle, θ, and both steps intersect the left and right edges of read sensor element  320  at its horizontal midline (e.g., half-stripe height). The phase shift transformation (PB) of basic pattern, B, includes two stabilizers ( 322   a ,  322   b ) with both rising step  323   a  of stabilizer  322   a  and falling step  323   b  of stabilizer  322   b  inclined at an angle, θ, and intersecting the left and right edges, respectively, of read sensor element  324  at its horizontal midline (e.g., half-stripe height). The reflection transformation (RB) of basic pattern, B, includes stabilizer  326  with falling step  325  and rising step  327  inclined at an angle, θ c , and intersecting the left and right edges, respectively of read sensor element  328  at its horizontal midline (e.g., half-strip height). The phase shift/reflection transformation (PRB) of basic pattern, B, includes two stabilizers ( 330   a ,  330   b ) with rising step  331   a  and falling step  331   b  inclined at an angle, θ c , and intersecting the left and right edges, respectively, of read sensor element  332  at its horizontal midline (e.g., half-stripe height). 
   The basic pattern, C (i.e., IC), in  FIG. 3  includes two stabilizers  334   a ,  334   b . As shown, for this exemplary design, stabilizer  334   a  includes falling step  335   a  and rising step  335   c , and stabilizer  334   b  includes falling step  335   b , which are all inclined at an angle, θ. Falling steps  335   a  and  335   b  intersect the left and right edges of read sensor element  336  at its horizontal midline (e.g., half-stripe height), and rising step  335   c  intersects the geometric center of read sensor element  336 . The phase shift transformation (PC) of basic pattern, C, includes two stabilizers  338   a ,  338   b  with rising step  339   a  of stabilizer  338   a , and falling step  339   c  and rising step  339   b  of stabilizer  338   b  all inclined at an angle, θ. Rising steps  339   a  and  339   b  intersect the left and right edges, respectively, of read sensor element  340  at its horizontal midline (e.g., half-stripe height), and falling step  339   c  intersects the geometric center of read sensor element  340 . The reflection transformation (RC) of basic pattern, C, includes two stabilizers  342   a ,  342   b  with rising step  343   a  of stabilizer  342   a , and falling step  343   c  and rising step  343   b  of stabilizer  342   b  all inclined at an angle, θ c . Rising steps  343   a  and  343   b  intersect the left and right edges, respectively of read sensor element  344  at its horizontal midline (e.g., half-strip height), and falling step  343   c  intersects the geometric center of read sensor element  344 . The phase shift/reflection transformation (PRC) of basic pattern, C, includes two stabilizers  346   a ,  346   b  with falling step  347   a  and rising step  347   c  of stabilizer  346   a , and falling step  347   b  of stabilizer  346   b  all inclined at an angle, θ c . Falling steps  347   a  and  347   b  intersect the left and right edges, respectively of read sensor element  348  at its horizontal midline (e.g., half-strip height), and rising step  347   c  intersects the geometric center of read sensor element  348 . 
   The basic pattern, D (i.e., ID), in  FIG. 3  includes two stabilizers, stabilizers  350   a  and  350   b . As shown, for this exemplary design, stabilizer  350   a  includes falling step  351   a  and rising step  351   b , and stabilizer  350   b  includes falling step  351   c  and rising step  351   d , which are all inclined at an angle, θ. Falling step  351   a  and rising step  351   d  intersect the left and right edges of read sensor element  352  at its horizontal midline (e.g., half-stripe height). Rising step  351   b  and falling step  351   c  intersect the w/3 and 2w/3 positions, respectively, of read sensor element  352  at its horizontal midline. The phase shift transformation (PD) of basic pattern, D, includes three stabilizers  354   a ,  354   b  and  354   c . Rising step  355   a  of stabilizer  354   a , falling step  355   b  and rising step  355   c  of stabilizer  354   b , and falling step  355   d  of stabilizer  354   c  are all inclined at an angle, θ. Rising step  355   a  and falling step  355   d  intersect the left and right edges, respectively, of read sensor element  356  at its horizontal midline (e.g., half-stripe height). Falling step  355   b  and rising step  355   c  intersect the w/3 and 2w/3 positions, respectively, of read sensor element  356  at its horizontal midline. The reflection transformation (RD) of basic pattern, D, includes two stabilizers  358   a  and  358   b . Falling step  359   a  and rising step  359   b  of stabilizer  358   a , and falling step  359   c  and rising step  359   d  of stabilizer  358   b  are all inclined at an angle, θ c . Falling step  359   a  and rising step  359   d  intersect the left and right edges, respectively of read sensor element  360  at its horizontal midline (e.g., half-strip height). Rising step  359   b  and falling step  359   c  intersect the w/3 and 2w/3 positions, respectively, of read sensor element  360  at its horizontal midline. The phase shift/reflection transformation (PRD) of basic pattern, D, includes three stabilizers  362   a ,  362   b  and  362   c . Rising step  363   a  of stabilizer  362   a , falling step  363   b  and rising step  363   c  of stabilizer  363   b , and falling step  363   d  of stabilizer  362   c  are all inclined at an angle, θ c . Rising step  363   a  and falling step  363   d  intersect the left and right edges, respectively of read sensor element  364  at its horizontal midline (e.g., half-strip height). Falling step  363   b  and rising step  363   c  intersect the w/3 and 2w/3 positions, respectively, of read sensor element  364  at its horizontal midline. 
   It is important to note that while the present invention has been described in the context of a fully functioning magnetic media data storage system, those of ordinary skill in the art will appreciate that the processes and transformations of the present invention are capable of being distributed in the form of a computer readable medium of instructions and a variety of forms and that the present invention applies equally regardless of the particular type of signal bearing media actually used to carry out the distribution. Examples of computer readable media include recordable-type media such a floppy disc, a hard disk drive, a RAM, CD-ROMs, and transmission-type media such as digital and analog communications links. 
   The description of the present invention has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention, the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.