Abstract:
A magnetoresistive sensor is generally disclosed. Various embodiments of a sensor can have at least a trilayer sensor stack biased with a back biasing magnet adjacent a back of the trilayer sensor. The back biasing magnet, the trilayer sensor stack, or both have substantially trapezoidal shapes to enhance the biasing field and to minimize noise.

Description:
RELATED APPLICATION 
     This application is a divisional application of copending U.S. patent application Ser. No. 12/502,104 filed on Jul. 13, 2009. 
    
    
     SUMMARY 
     A magnetoresistive sensor includes at least a trilayer sensor stack with a front width proximate an ABS, and a back width distal from an ABS and a back biasing magnet with a trapezoidal shape with a front width and a back width. The trapezoidal shape concentrates the magnetic field at the front of the biasing magnet in the vicinity of the sensor stack. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic diagram showing micromagnetic magnetization patterns in a rectangular sample. 
         FIG. 1B  is a schematic diagram showing a “C” type micromagnetic magnetization pattern in the sample of  FIG. 1A . 
         FIG. 1C  is a schematic diagram showing an “S” type micromagnetic magnetization pattern in the sample of  FIG. 1A . 
         FIG. 1D  is a schematic showing a “C” type micromagnetic magnetization pattern in a trapezoidal sample. 
         FIG. 2  is a top view of a first example of a read head in accord with the present invention. 
         FIG. 3  is an ABS view of the read head in  FIG. 2  in accord with the present invention. 
         FIG. 4A  is a schematic top view of the trilayer sensor in  FIG. 2  showing biasing in the absence of external bit flux. 
         FIG. 4B  is a schematic top view of the trilayer sensor in  FIG. 4A  under the influence of a first state of data. 
         FIG. 4C  is a schematic top view of the trilayer sensor in  FIG. 4A  under the influence of a second state of data. 
         FIG. 5  is a top view of a second example of a read head in accord with the present invention. 
         FIG. 6  is an ABS view of the read head in  FIG. 5  in accord with the present invention. 
         FIG. 7A  is a schematic top view of the trilayer sensor in  FIG. 5  showing biasing in the absence of external bit flux. 
         FIG. 7B  is a schematic top view of the trilayer sensor in  FIG. 7A  under the influence of a first state of data. 
         FIG. 7C  is a schematic top view of the trilayer sensor in  FIG. 7A  under the influence of a second state of data. 
         FIGS. 8A-8K  illustrate the fabrication steps to produce the read head illustrated in  FIGS. 2 and 3 . 
         FIGS. 9A-9K  illustrate the fabrication steps to produce the read head illustrated in  FIGS. 5 and 6 . 
     
    
    
     DETAILED DESCRIPTION 
     The various embodiments of shapes disclosed herein increase the performance of a reader by increasing the bias field at the front of a back bias magnet and by decreasing signal noise. The origin of these effects is shown in  FIGS. 1A-1C .  FIG. 1A  illustrates possible micromagnetic magnetization patterns in a rectangular magnetic sample under a magnetization oriented generally from the left to right. Magnetization vectors  12 ′ and  14 ′ originate at the corners of the sample and are directed to the center where they converge at magnetization vector  10 ′. Magnetization vector  10 ′ diverges into vectors  16 ′ and  18 ′ as it approaches the right side of the sample.  FIG. 1  shows all possible micromagnetic magnetization patterns. Two patterns are energetically favored.  FIG. 1B  illustrates a “C” pattern comprised of vectors  12 ′,  10 ′ and  16 ′. An alternative “C” pattern comprises vectors  14 ′,  10 ′ and  18 ′.  FIG. 1C  illustrates an “S” pattern comprised of vectors  12 ′,  10 ′ and  18 ′ or alternatively vectors  14 ′,  10 ′ and  16 ′. The energy difference between the “C” state and the “S” state is very small and during magnetic switching, thermally activated transitions between both patterns contribute to measurable sensor noise. 
     By changing the geometry of a magnetic element, one or the other of the “C” and “S” states can be energetically favored.  FIG. 1D  illustrates how the “C” state can be favored by a trapezoidal shape of the micromagnetic element. This shape will be used in what follows to tailor magnetization in the back bias permanent magnet of a trilayer reader as well as in the freelayers of the reader itself. Although trapezoidal geometries are discussed herein to favor “C” shape micromagnetic magnetization patterns, it should be noted that other geometries such as half moon shapes can be used to obtain similar beneficial results. 
       FIGS. 2 and 3  illustrate one non-limiting aspect of an example trilayer reader.  FIG. 2  is a top view of an embodiment of the trilayer read head  10 , and  FIG. 3  is an ABS view of read head  10 . Read head  10  comprises rectangular trilayer reader stack  20  (comprising ferromagnetic freelayers  22  and  24  and spacer layer  26 ) in front of trapezoidal back bias magnet  30 . Magnetic side shields  40  and  42  abut both sides of bias magnet  30  and trilayer reader stack  20 . Trilayer reader stack  20 , bias magnet  30 , and side shields  40  and  42  are separated from each other by insulating layer  50 . Side shields  40  and  42  may also be replaced by an insulator preferably an oxide of aluminum. 
     The ABS view of trilayer read head  10  in  FIG. 3  shows top shield  60 , bottom shield  70  and side shields  40  and  42  adjacent trilayer reader stack  20  and insulator layer  50 . Ferromagnetic freelayers  22  and  24  of trilayer reader stack  20  are separated by spacer layer  26 . If spacer layer  26  is a nonmagnetic electrical conductor, read head  10  is a GMR head. If spacer layer  26  is a nonmagnetic electrical insulator, read head  10  is a TGMR head. Read head  10  can be a current perpendicular to plane (CPP) head wherein electrical contact is made to trilayer reader stack  20  through top shield  60  and bottom shield  70 . 
     If spacer layer  26  is nonmagnetic, and electrically conducting, it may be fabricated from, for example, copper. If spacer layer  26  is nonconducting, it may be fabricated from, for example, aluminum oxide (Al 2 O 3  or Al x O where x may or may not be an integer) or magnesium oxide. Ferromagnetic layers  22  and  24  may be fabricated from magnetic material such as, for example, nickel-iron-cobalt (Ni—Fe—Co) compositions. The shield layers may be fabricated from, for example, a soft magnetic material such as nickel-iron (Ni—Fe). Back bias magnet  30  may be fabricated from a permanent magnet material such as, for example, a cobalt-platinum (Co—Pt) alloy. 
     The operation of read head  10 , according to one aspect of the invention is described in conjunction with  FIGS. 4A-4C .  FIGS. 4A ,  4 B and  4 C show top views of read head  10  with magnetization vector  30 ′ of back bias layer  30  oriented with respect to magnetization vectors  22 ′ and  24 ′ of freelayers  22  and  24  to achieve optimum response of freelayers  22  and  24  to external magnetic fields. In the absence of back bias magnetization, freelayer magnetization vectors  22 ′ and  24 ′ would be antiparallel and commonly parallel to the ABS. Under the bias of magnetization vector  30 ′, they arrange in a scissor orientation for optimum sensitivity. One benefit of the trapezoidal shape of back bias magnet  30  is that the smaller base near the back of trilayer reader stack  20  results in magnetic flux concentration in that region resulting in deeper penetration of the biasing field into reader stack  20  in the direction of the ABS. 
       FIGS. 4A-4C  illustrate an example effect of varying bit magnetization on recorded media on the magnetization directions  22 ′ and  24 ′ of first freelayer  22  and second freelayer  24  respectively.  FIG. 4A  shows an example trilayer reader stack  10  in a quiescent magnetic state when it is not under the influence of magnetic flux emanating from recording media. The angle of magnetization between first ferromagnetic freelayer  22  and second ferromagnetic freelayer  24  at the ABS is in a scissors relation for optimum sensor response.  FIG. 4B  is a top view of an embodiment of the read head  10  showing trilayer reader stack  20  under the influence of a first state of data D 1  corresponding to a positive bit. This first state of data causes the angle of magnetization between first freelayer  22  and second freelayer  24  to increase at the ABS. When this occurs, the resistance across trilayer reader stack  20  changes and is detected when a sense current is passed through trilayer reader stack  20 .  FIG. 4C  is a top view of an example read head  10  showing trilayer reader stack  20  under the influence of a second state of data D 2  corresponding to a negative bit. This second state of data causes the angle of magnetization between first freelayer  22  and second freelayer  24  to decrease at the ABS. As with the first state of data, the second state of data causes a change in resistance across trilayer reader stack  20  and is detected when a sense current is passed through trilayer reader stack  20 . 
       FIGS. 5 and 6  illustrate another non-limiting embodiment.  FIG. 5  is a top view of trilayer reader head  110 , and  FIG. 6  is an ABS view of read head  110 . Read head  110  comprises trapezoidal trilayer reader stack  120  comprising ferromagnetic freelayers  122  and  124  and spacer layer  126  in front of trapezoidal back bias magnet  130 . Magnetic side shields  140  and  142  are adjacent both sides of back bias magnet  130  and freelayer stack  120 . Trilayer reader stack  120 , back bias magnet  130 , and side shields  140  and  142  are separated from each other by insulating layer  150 . Side shields  140  and  142  may also be replaced by an insulator, preferably an oxide of aluminum. In this aspect of the invention, trilayer reader stack  120  has a trapezoidal shape. A benefit of the trapezoidal shape is that a “C” pattern of micromagnetic magnetization in reader stack  120  is preferred. The ABS view of trilayer read head  110  in  FIG. 6  shows top shield  160 , bottom shield  170  and side shields  140  and  142  adjacent trilayer reader stack  120  and insulator layer  150 . Ferromagnetic freelayers  122  and  124  of trilayer reader stack  120  are separated by spacer layer  126 . If spacer layer  126  is nonmagnetic, read head  110  is a GMR head. If spacer layer  126  is an insulator, read head  110  is a TGMR head. Read head  110  can be a current perpendicular to plane (CPP) head wherein electrical contact is made to trilayer reader stack  120  through top shield  160  and bottom shield  170 . 
     If spacer layer  126  is nonmagnetic and electrically conducting, it may be fabricated from, for example, copper. If spacer layer  126  is nonconducting, it may be fabricated from, for example, aluminum oxide (Al 2 O 3  or Al x O where x may be not be an integer) or magnesium oxide. Ferromagnetic layers  122  and  124  may be fabricated from magnetic materials, such as, for example, nickel-iron-cobalt (Ni—Fe—Co) compositions. The shield layers may be fabricated from, for example, a soft magnetic material such as nickel-iron (Ni—Fe). Back bias magnet  130  may be fabricated from a permanent magnet material such as, for example, a cobalt-platinum (Co—Pt) alloy. 
     The operation of read head  110  according to one embodiment is described in conjunction with  FIGS. 7A-7C .  FIGS. 7A ,  7 B and  7 C show top views of read head  110  with magnetization vector  130 ′ of back bias layer  130  oriented with respect to magnetization vectors  122 ′ and  124 ′ of freelayers  122  and  124  to achieve optimum response of freelayers  122  and  124  to external magnetic fields. In the absence of back bias magnetization  130 ′, freelayer magnetization vectors  122 ′ and  124 ′ would be antiparallel and parallel to ABS  160 . Under the back bias of magnetization  130 ′, they arrange in a scissor orientation for optimum sensitivity. A benefit of the trapezoidal shape of back bias magnet  130  is that the smaller base at trilayer reader stack  120  results in magnetic flux concentration in that region resulting in deeper penetration of the biasing field into reader stack  120  in the direction of the ABS. 
       FIGS. 7A-7C  illustrate the effect of varying bit magnetizations on recorded media on the magnetization directions  122 ′ and  124 ′ of first freelayer  122  and second freelayer  124  respectively.  FIG. 7A  shows trilayer reader stack  120  in a quiescent magnetic state when it is not under the influence of magnetic flux emanating from recording media. The angle of magnetization between first ferromagnetic freelayer  122  and second ferromagnetic freelayer  124  at the ABS is in a scissors relation for optimum sensor response.  FIG. 7B  is a front view of read head  110  showing trilayer reader stack  120  under the influence of a first state of data D 1  corresponding to a positive bit. This first state of data causes the angle of magnetization between first freelayer  122 ′ and second freelayer  124 ′ to increase at the ABS. When this occurs, the resistance across trilayer reader stack  120  changes and is detected when a sense current is passed through trilayer reader stack  120 .  FIG. 7C  is a top view of read head  110  showing trilayer reader stack  120  under the influence of a second state of data D 2  corresponding to a negative bit. This second state of data causes the angle of magnetization between first freelayer  122 ′ and second freelayer  124 ′ to decrease at the ABS. As with the first state of data, the second state of data causes a change in resistance across trilayer reader stack  120  and is detected when a sense current is passed through trilayer reader stack  120 . 
     The operation of read head  110  is similar to that discussed for read head  10  and schematically illustrated in  FIG. 4A-4C , with one exception. The trapezoidal shape of trilayer reader stack  120  encourages a “C” type of micromagnetic magnetization in freelayers  124  and  126 . This forces the magnetization vectors into orientations parallel to the ABS and discourages the formation of “S” type micromagnetic magnetization patterns in the freelayers, thereby minimizing noise resulting from “C” type to “S” type switching behavior during operation. 
     The formation of reader  10  with trapezoidal back bias magnet  30  shown in  FIGS. 2 and 3  is schematically illustrated in  FIGS. 8A-8K .  FIG. 8A  shows a substrate coated with reader stack  220 . The reader stack can be a GMR or a TGMR stack. In the next step, photoresist (PR) layer  260 , covering the center portion of reader stack  220 , is deposited as shown in  FIG. 8B . In the next step, shown in  FIG. 8C , exposed reader stack  220  has been removed by ion beam machining or etching or by other means known in the art. Following removal of exposed reader stack  220 , insulating layer  250  is deposited on each side of reader stack  220  and PR layer  260  as shown in  FIG. 8D . Insulating layer  250 , as mentioned earlier, is preferably aluminum oxide and is preferably deposited by atomic layer deposition (ALD). In the next step permanent bias magnet  230  is then deposited as shown in  FIG. 8E  comprising reader stack  220  with bias magnets  230  above and below reader stack  220  separated from reader stack  220  by insulating layers  250 . The structure in  FIG. 8E  is then covered with PR layer  260   b  with a narrow center width and wider ends as shown in  FIG. 8F . The exposed structure not covered with PR layer  260   b  is then removed by ion beam machining or etching or other means known in the art as shown in  FIG. 8G . Insulator layer  250  is then deposited on each side of the structure covered with PR layer  260   b  as shown in  FIG. 8H . Side shields  240  and  242  are deposited to form the structure shown in  FIG. 8I . Side shields  240  and  242  could be replaced with insulator layer  250  if needed. Removing PR layer  260   b  in  FIG. 8I  reveals the structure shown in  FIG. 8J  comprising rectangular reader stack  220  separated from side shields  240  and  242  and trapezoidal bias magnets  230  by insulating layer  250 . Masking the top half of the structure shown in  FIG. 8J  and removing the remainder creates reader structure  10  shown in  FIG. 8K  comprising rectangular reader stack  220 , side shields  240  and  242  and trapezoidal back bias magnet  230  separated from each other by insulating layer  250 . Air bearing surface ABS is indicated in  FIG. 8K . 
     The formation of reader  110  with trapezoidal back bias magnet  130  and trapezoidal reader stack  120  shown in  FIGS. 5 and 6  is schematically illustrated in  FIGS. 9A-9K .  FIG. 9A  shows a substrate coated with reader stack  320 . The reader stack can be a GMR or a TGMR stack. Photoresist (PR) layer  360 , covering the center portion of reader stack  320 , is deposited as shown in  FIG. 9B . In the next step, shown in  FIG. 9C , exposed reader stack  320  has been removed by ion beam machining or etching or by other means known in the art. Following removal of exposed reader stack  320 , insulating layer  350  is deposited on each side of reader stack  320  and PR layer  360  as shown in  FIG. 9D . Insulating layer  350 , as mentioned earlier, is preferably aluminum oxide and is preferably deposited by atomic layer deposition (ALD). In the next step, permanent bias magnet  330  is then deposited as shown in  FIG. 9E  comprising reader stack  320  with bias magnets  330  above and below reader stack  320  separated from reader stack  320  by insulating layer  350 . The structure in  FIG. 9E  is then covered with PR layer  360   b  with a narrow center width and asymmetrically wider ends as shown in  FIG. 9H . The exposed structure not covered with PR layer  360   b  is then removed by ion beam machining or etching or other means known to produce the structure shown in  FIG. 9G . Insulator layer  350  is then deposited on each side of the structure in  FIG. 9G  to produce the structure shown in  FIG. 9H . Side shields  340  and  342  are deposited on each side to form the structure shown in  FIG. 9I . Side shields  340  and  342  could be replaced with insulator layer  350  if needed. Removing PR layer  360   b  in  FIG. 9K  reveals the structure shown in  FIG. 9J  comprising trapezoidal reader stack  320 , side shields  340  and  342  and trapezoidal bias magnet  330 . All are separated by insulating layer  350 . Masking the top half of the structure shown in  FIG. 9J  and removing the remainder creates reader structure  110  shown in  FIG. 9K  comprising trapezoidal trilayer reader stack  320 , side shields  340  and  342 , and trapezoidal back bias magnet  330  separated from each other by insulating layer  350 . Air bearing surface ABS is indicated in  FIG. 9K . 
     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 technology. 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 provided technology not be limited to the particular embodiment(s) disclosed, but will include all embodiments falling within the scope of the appended claims.