Abstract:
A current-perpendicular-to-plane (CPP) magnetoresistance sensor and a method for forming a current-perpendicular-to-plane (CPP) magnetoresistance sensor. The method includes providing a ferromagnetic shield layer and disposing one or more seed layers on the ferromagnetic shield layer. The method also includes disposing a pinning layer on the one or more seed layers, wherein the pinning layer excludes PtMn, and disposing a pinned layer on the pinning layer. The shield layer, each of the one or more seed layers, the pinning layer, and the pinned layer are comprised of compounds having face-centered-cubic structures.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a current-perpendicular-to-plane (CPP) tunneling magnetoresistance (TMR) or giant magnetoresistance (GMR) sensor, and more particularly to a CPP TMR or GMR sensor epitaxially grown on a bottom shield. 
     2. Description of the Related Art 
     The heart of a computer is a non-volatile storage device that is referred to as a magnetic disk drive. The magnetic disk drive includes a magnetic disk as well as write and read heads. The write and read heads are supported by a slider that is mounted on a suspension arm. When the magnetic disk rotates, an actuator swings the suspension arm to place the write and read heads over selected circular tracks on the surface of the rotating magnetic disk. An air flow generated by the rotation of the magnetic disk causes an air bearing surface (ABS) of the slider to fly at a very low elevation (referred to as the fly height) over the surface of the rotating magnetic disk. The write and read heads write magnetic transitions to and read magnetic transitions from the rotating magnetic disk, respectively. Processing circuitry connected to the write and read and heads then operates according to a computer program to implement writing and reading functions. 
     The write head includes a coil embedded in an insulation stack that is sandwiched between main and auxiliary poles. The main and auxiliary poles are magnetically coupled at a back gap and are coated with an overcoat. A write current conducted to the coil induces a magnetic flux in the main pole that causes a magnetic field to write the aforementioned magnetic impressions to the rotating magnetic disk. 
     The read head includes a sensor electrically connected with top and bottom electrodes, but electrically insulated by insulating films from bias stacks at two side regions. A sense current conducted through the top electrode, sensor, and bottom electrode causes changes of resistance in response to external magnetic fields of magnetic transitions from the rotating magnetic disk. 
     In some cases, the performance of the sensor may be degraded due to temperature increases during sensor processing or manufacturing. To prevent the sensor from this performance degradation, there is a desire to improve the thermal stability of the sensor. Accordingly, what is needed are a sensor and method for providing the sensor with improved thermal stability. 
     SUMMARY OF THE INVENTION 
     Embodiments of the invention generally provide a current-perpendicular-to-plane (CPP) magnetoresistance sensor and a method for forming a current-perpendicular-to-plane (CPP) magnetoresistance sensor. In one embodiment, the method includes providing a ferromagnetic shield layer and disposing one or more seed layers on the ferromagnetic shield layer. The method also includes disposing a pinning layer on the one or more seed layers, wherein the pinning layer excludes PtMn, and disposing a pinned layer on the pinning layer. The shield layer, each of the one or more seed layers, the pinning layer, and the pinned layer are comprised of compounds having face-centered-cubic structures. 
     One embodiment of the invention provides a current-perpendicular-to-plane (CPP) magnetoresistance sensor including a ferromagnetic shield layer and one or more seed layers disposed on the ferromagnetic shield layer. The sensor also includes a pinning layer disposed on the one or more seed layers, wherein the pinning layer excludes PtMn, and a pinned layer disposed on the pinning layer. The shield layer, each of the one or more seed layers, the pinning layer, and the pinned layer are comprised of compounds having face-centered-cubic structures. 
     One embodiment of the invention also provides a current-perpendicular-to-plane (CPP) giant magnetoresistance (GMR) sensor includes a ferromagnetic shield layer, one or more seed layers disposed on the ferromagnetic shield layer, and a pinning layer disposed on the seed layers, wherein the pinning layer excludes PtMn. The sensor also includes a pinned layer disposed on the pinning layer, a spacer layer disposed on the pinned layer, a free layer disposed on the spacer layer, and a cap layer disposed on the free layer. The shield layer, each of the one or more seed layers, the pinning layer, and the pinned layer comprise compounds having face-centered-cubic structures. 
     One embodiment of the invention provides a current-perpendicular-to-plane (CPP) tunneling magnetoresistance (TMR) sensor. The sensor includes a ferromagnetic shield layer, one or more seed layers disposed on the ferromagnetic shield layer, and a pinning layer disposed on the seed layers, wherein the pinning layer excludes PtMn. The sensor also includes a pinned layer disposed on the pinning layer, a barrier layer disposed on the pinned layer, a free layer disposed on the barrier layer, and a cap layer disposed on the free layer. The shield layer, each of the one or more seed layers, the pinning layer, and the pinned layer comprise compounds having face-centered-cubic structures. 
     Another embodiment of the invention provides a magnetic storage system including a magnetic storage medium having a plurality of tracks for recording data and a current-perpendicular-to-plane (CPP) magnetoresistance sensor. The sensor includes a ferromagnetic shield layer, one or more seed layers disposed on the ferromagnetic shield layer, and a pinning layer disposed on the one or more seed layers, wherein the pinning layer excludes PtMn. The sensor also includes a pinned layer disposed on the pinning layer. The shield layer, each of the one or more seed layers, the pinning layer, and the pinned layer comprise compounds having face-centered-cubic structures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above-recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  is a block diagram depicting a hard drive according to one embodiment of the invention. 
         FIG. 2  is a block diagram depicting the structure of a current perpendicular-to-plane (CPP) tunneling magnetoresistive (TMR) sensor according to one embodiment of the invention. 
         FIG. 3  is a flow diagram depicting a method for forming a CPP TMR sensor according to one embodiment of the invention. 
         FIG. 4  is a block diagram depicting the structure of a CPP giant magnetoresistive (GMR) sensor according to one embodiment of the invention. 
         FIG. 5  is a flow diagram depicting a method for forming a CPP GMR sensor according to one embodiment of the invention. 
         FIG. 6A  is a graph of the TMR coefficient over the junction resistance-area product for CPP TMR sensors according to one embodiment of the invention. 
         FIG. 6B  is a graph of the TMR coefficient over the pinned magnetic field for CPP TMR sensors according to one embodiment of the invention. 
         FIG. 6C  is a graph of the ferromagnetic coupling field over junction resistance-area product for CPP TMR sensors according to one embodiment of the invention. 
         FIG. 6D  is a graph of the TMR coefficient over junction resistance-area product for CPP TMR sensors according to one embodiment of the invention. 
         FIG. 6E  is a graph of the ferromagnetic coupling field over junction resistance-area product for CPP TMR sensors according to one embodiment of the invention. 
         FIG. 6F  is a graph of the TMR coefficient measured at an operating voltage versus the operating voltage for TMR sensors according to one embodiment of the invention. 
         FIG. 6G  is a graph of the TMR coefficient measured at 40 mV versus a stressing voltage for TMR sensors according to one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the following, reference is made to embodiments of the invention. However, it should be understood that the invention is not limited to specific described embodiments. Instead, any combination of the following features and elements, whether related to different embodiments or not, is contemplated to implement and practice the invention. Furthermore, in various embodiments the invention provides numerous advantages over the prior art. However, although embodiments of the invention may achieve advantages over other possible solutions and/or over the prior art, whether or not a particular advantage is achieved by a given embodiment is not limiting of the invention. Thus, the following aspects, features, embodiments and advantages are merely illustrative and, unless explicitly present, are not considered elements or limitations of the appended claims. 
     An Exemplary Hard Drive 
       FIG. 1  is a block diagram depicting a hard drive  100  according to one embodiment of the invention. The hard disk drive  100  includes a magnetic media hard disk  112  mounted upon a motorized spindle  114 . An actuator arm  116  is pivotally mounted within the hard disk drive  100  with a slider  120  disposed upon a distal end  122  of the actuator arm  116 . During operation of the hard disk drive  100 , the hard disk  112  rotates upon the spindle  114  and the slider  120  acts as an air-bearing surface (ABS) adapted for flying above the surface of the disk  112 . The slider  120  includes a substrate base upon which various layers and structures that form a magnetoresistive sensor, described in greater detail below, are fabricated. 
     Layers of a Tunneling Magnetoresistance (TMR) Sensor 
       FIG. 2  is a block diagram depicting exemplary layers of a typical current-perpendicular-to-plane (CPP) tunneling magnetoresistance (TMR) sensor  200  according to one embodiment of the invention. As depicted, the TMR sensor  200  may comprise a bottom shield  202 , one or more seed layers  240 , a pinning layer  208 , a flux-closure stack  250 , a barrier layer  216 , a sense layer  218 , and a cap layer  220 . The TMR sensor  200  may include a bottom side  232 , a top side  234 , a front side  236  facing an air-bearing surface (ABS), and a back side  238 . As described below, the seed layers  240  may be used to provide a microstructural base upon which other layers of the TMR sensor  200  are deposited. Also, as described below, the composition of the seed layers  240  may affect magnetic and TMR properties of the other layers deposited on the seed layers  240 . 
     In one embodiment, the flux-closure stack  250  may comprise keeper and reference layers ( 210  and  214 ) separated by an antiparallel (AP) coupling layer  212 . The keeper layer  210  may have a magnetization  258  in a given direction defined by the pinning layer  208 . The reference layer  214  may have a magnetization  254  in a direction antiparallel to the given direction. Both the magnetizations  258  and  254  are rigidly pinned due to strong antiferromagnetic coupling energy provided by the pinning layer  208  and strong antiparallel coupling energy provided by the AP coupling layer  212 . This rigid pinning facilitates proper operation of the TMR sensor  200 . 
     The TMR sensor  200  is typically annealed in a magnetic field in the predetermined direction  258  to thermally set the antiferromagnetism of the pinning layer  208 , thereby rigidly pinning the magnetization  258  of the keeper layer  210  through exchange coupling between the pinning layer  208  and the keeper layer  210 , and the magnetization  254  of the reference layer  214  through antiparallel coupling across the AP coupling layer  212 . To ensure the rigid pinning for proper sensor operation, a high pinning field induced by strong exchange and antiparallel coupling is typically desired in the TMR sensor  200 . 
     In one embodiment, to achieve strong exchange and antiparallel coupling, the pinning layer  208  may be made of an antiferromagnetic Ir—Mn-based film, the keeper and reference layers  210  and  214  may be made of Co—Fe-based films, and the AP coupling layer  212  may be made of a Ru film. In one embodiment, the pinning layer  208  has an Ir content of 22 atomic percent (at %), a Mn content of 75 at %, and a Cr content of 3 at %. The keeper layers  210  may have a Co content of 75 at % and an Fe content of 25 at %, and the reference layer  214  may have a Co content of 51 at %, an Fe content of 34 at %, and a B content of 15 at %. While described above with respect to layers of specific thicknesses and compositions, it should be apparent to those of ordinary skill in the art that embodiments of the invention may be used with any ordering of layers, with layers of any composition, and with layers of any desired thickness. 
     The sense layer  218  of the TMR sensor  200  may have a magnetization  256  which can be rotated by external magnetic fields. In one embodiment, the sense layer  218  may be made of a ferromagnetic Co—Fe-based film. For example, the sense layer  218  may comprise lower and upper portions  262 ,  264 . The lower portion  262  of the sense layer  218  may be formed of a Co—Fe—B film having a Co content of 60 at %, an Fe content of 20 at %, and a B content of 20 at % which may provide improved TMR properties. The upper portion  264  of the sense layer  218  may be formed of a Co—Fe—B film, having a Co content of 72 at %, an Fe content of 8 at %, and a B content of 20 at % which may provide improved ferromagnetic properties. The TMR sensor  200  may be coated with a cap layer  220 , formed of a Ru or Ta film for protecting the TMR sensor  200  during subsequent processing. 
     During sensor operation in magnetic fields representing data stored on a magnetic disk  112 , a sense current I tunneling through the barrier layer  216  of the TMR sensor  200  in a direction perpendicular to the planes of the TMR sensor  200  may be used to detect a resistance change and sense a TMR effect. When the magnetic fields cause the magnetization  256  of the sense layer  218  to rotate from a direction parallel to the magnetization  254  of the reference layer  214  to a direction antiparallel to the magnetization  254  of the reference layer  214 , the junction resistance may increase from a first resistance value R J  to a second resistance value R J +ΔR T . The TMR effect is characterized by a TMR coefficient, defined as ΔR T /R J . In one embodiment, the barrier layer  216  may be formed of a magnesium oxide (Mg—O) film. In other embodiments, the barrier layer  216  may be formed of an aluminum oxide (Al—O) or titanium oxide (Ti—O) film. 
     In one embodiment, the seed layer  240  may provide a microstructural base upon which other layers of the TMR sensor  200  may be epitaxially grown with desired magnetic and TMR properties, as well as high thermal stability. For example, the seed layer  240  may provide substantial grain coarsening and a crystalline texture, thereby inducing a large TMR effect. The seed layer  240  may be made of Ni—Cr—Fe, Ni—Fe, Ru, Cu or Pt film. In one embodiment, the seed layer  240  may comprise a lower seed layer  204  and an upper seed layer  206 . The lower seed layer  204  may be formed of a Ni—Cr—Fe film, having a Ni content of 48 at %, a Cr content of 40 at %, and an Fe content of 12 at %. The upper seed layer  206  may be formed of a Ni—Fe film, having a Ni content of 91.5 at % and an Fe content of 8.5 at %. 
     In one embodiment, to provide a solid microstructural base, the seed layer  240  may have the same structure as the pinning layer  208 , for example, a face-centered-cubic (FCC) structure. Thus, in one embodiment, the lower seed layer  204  may be made of a Ni—Cr—Fe film and the upper seed layer  204  may be made of a Ni—Fe film, which both provide the FCC structure. As a result, epitaxial growth of subsequent layers in the sensor may provide large polycrystalline grains in the subsequent layers of the TMR sensor  200 . With few or no grain boundaries after patterning into a small dimension of the layers, the TMR sensor  200  may exhibit good magnetic and TMR properties, as well as high thermal stability. 
     In one embodiment, the bottom shield  202  may be made of a Ni—Fe film which also have the FCC structure and which may be as thick as 1,000 nm. Because the bottom shield  202  may have the FCC structure, the bottom shield  202  may be considered a thick seed layer upon which other layers are deposited. In one embodiment, by depositing subsequent layers which have only an FCC structure, the benefits described above may be obtained. Furthermore, in one embodiment, subsequently deposit layers between the bottom shield  202  and the barrier layer  216  may not include tantalum (Ta) which has a body-centered-cubic (BCC) structure and may reduce the benefits obtained from only depositing layers with the FCC structure. 
     In one embodiment, the TMR sensor  200  may include a lower seed layer  204  made of a 2 nm thick 48Ni-40Cr-12Fe film (which is directly deposited on top of the bottom shield  202  made of a 1,000 nm thick Ni—Fe film), an upper seed layer  206  made of a 0.8 nm thick 91.5Ni-8.5Fe film, a pinning layer  208  made of a 6 nm thick 22Ir-75Mn-3Cr film, a keeper layer  210  made of a 2.1 nm thick 75Co-25Fe film, an AP coupling layer  212  made of a 0.43 nm thick Ru film, a reference layer  214  made of a 1.9 nm thick 51Co-34Fe-15B film, a barrier layer  216  made of a 0.85 nm thick Mg—O film, a lower sense layer  262  made of a 60Co-20Fe-20B film, an upper sense layer  264  made of a 72Co-8Fe-20B film, and a cap  220  made of a 6 nm thick Ru film. While described above with respect to layers of specific thicknesses and compositions, it should be apparent to those of ordinary skill in the art that embodiments of the invention may be used with any ordering of layers, with layers of any composition, and with layers of any desired thickness. 
       FIG. 3  is a block diagram depicting a process  300  for manufacturing a TMR sensor  200  according to one embodiment of the invention. As depicted, the process  300  may begin at step  302  where the bottom shield  202  made of a 2,000 nm thick Ni—Fe film is formed and chemically-mechanically polished (CMP). In one embodiment, during the formation process of the Ni—Fe bottom shield  202 , a seed layer made of an 80 nm thick Ni—Fe film is sputtered on a wafer. A bottom shield frame is formed with photolithographic patterning, and a 2,400 nm thick Ni—Fe film is then electroplated into the bottom shield frame. In one embodiment, during the CMP process, the photoresist and the Ni—Fe seed layer outside of the bottom shield frame are removed, a 2,400 nm thick Al 2 O 3  film is deposited, and CMP is then applied until a smooth 2,000 nm thick lower shield  202  is formed. 
     At step  304 , seed layers  204  and  206  may be deposited on the bottom shield  202 . At step  306 , a pinning layer  208  may be deposited on the upper seed layer  206 . Then, at step  308 , the flux-closure stack  250  may be deposited on the pinning layer  208 . At step  310 , the barrier layer  216  may be deposited on the flux-closure stack  250 . At step  312 , the sense layer  218  may be deposited on the barrier layer  216 . Then, at step  314 , the cap layer  220  may be deposited on the sense layer  218 . At step  316 , the TMR sensor  200  may be annealed. In one embodiment, annealing may be performed for 5 hours at 285 degrees C in a field of 50,000 Oe in a vacuum oven. With respect to each of the deposition steps described above, any deposition method known to those of skill in the art may be used, including DC magnetron sputtering, ion-beam sputtering, etc. 
     In one embodiment, during the process  300  for manufacturing the TMR sensor  200 , the Ni—Fe seed layer  206  may provide a high-conductance path for electroplating, instead of controlling the grain growth in the electroplated Ni—Fe bottom shield  202 . The Ni—Fe bottom shield  202  may thus exhibit its own equal-axial grains with various sizes independent of the Ni—Fe seed layer  206  and the film thickness. In some cases, the TMR sensor  200  deposited on these grains with various sizes may exhibit TMR properties tightly distributed over the 5″ diameter wafer. Optionally, in some cases, the grain sizes may be controlled during the process  300  to further ensure uniform distribution of the TMR properties. 
     In one embodiment, to ensure uniform grain-size distribution, a 3 nm thick Ta block layer can be sputtered to block the electroplated Ni—Fe film after the CMP process, and a Ni—Fe film, as thick as up to 400 nm, can be sputtered on top of the Ta block layer. This sputtered Ni—Fe film may exhibit desired columnar grains which nucleate on the Ta block layer and grow to a uniform size, and may also exhibit soft anisotropic ferromagnetic properties. In some cases, if the sputtering process continues to attain a Ni—Fe film thickness exceeding 400 nm, grains may grow to various sizes due to heat-induced grain boundary coalescence, and thus the ferromagnetic properties may deteriorate. Therefore, in one embodiment, the single electroplated Ni—Fe film may be replaced with laminated Ta/Ni—Fe films which may be deposited via sputtering, where each Ni—Fe laminate has a thickness of no more than 400 nm. In one embodiment, to improve the smoothness of the sputtered Ni—Fe film, nitrogen can be incorporated into the sputtering process for the surface smoothness, or the CMP process can be applied again to remove unwanted grain boundary grooving. 
     Layers of a CPP Giant Magnetoresistive (GMR) Sensor 
       FIG. 4  is a block diagram depicting exemplary layers of a typical current-perpendicular-to-plane (CPP) giant magnetoresistance (GMR) sensor  400  according to one embodiment of the invention. As depicted, the CPP GMR sensor  400  may comprise a bottom shield  402 , one or more seed layers  440 , a pinning layer  408 , a flux-closure stack  450 , a spacer layer  416 , a sense layer  418 , and a cap layer  420 . The CPP GMR sensor  400  may include a bottom side  432 , a top side  434 , a front side  436  facing an air-bearing surface (ABS), and a back side  438 . As described below, the seed layers  440  may be used to provide a microstructural base upon which other layers of the TMR sensor  400  are deposited. Also, as described below, the composition of the seed layers  440  may affect magnetic and CPP GMR properties of the other layers deposited on the seed layers  440 . 
     During sensor operation in magnetic fields representing data stored on a magnetic disk  112 , a sense current I flowing through the spacer layer  416  of the CPP GMR sensor  400  in a direction perpendicular to the planes of the CPP GMR sensor  400  may be used to detect a resistance change and sense a CPP GMR effect in the sensor  400 . For example, when the magnetic fields cause the magnetization  456  of the sense layer  418  to rotate from a direction parallel to the magnetization  454  of the reference layer  414  to a direction antiparallel to the magnetization  454  of the reference layer  214 , the junction resistance may increase from R J  to R J +ΔR G . The CPP GMR effect is characterized by a CPP GMR coefficient, defined as ΔR G /R J . In one embodiment, the spacer layer  416  may be formed of a copper oxide (Cu—O) film. Other aspects of the CPP GMR sensor  400  such as the compositions of rest of the CPP GMR sensor  400  may be similar or identical to those described above with respect to the TMR sensor  200 . 
       FIG. 5  is a block diagram depicting a process  500  for manufacturing a CPP GMR sensor  400  according to one embodiment of the invention. As depicted, the process  500  may begin at step  502  where the lower shield  402  made of a 2,000 nm thick Ni—Fe film is formed and chemically-mechanically polished (CMP). In one embodiment, during the formation process of the Ni—Fe bottom shield, a seed layer made of an 80 nm thick Ni—Fe film is sputtered on a wafer. A bottom shield frame is formed with photolithographic patterning, and a 2,400 nm thick Ni—Fe film is then electroplated into the bottom shield frame. In one embodiment, in the CMP process, the photoresist and the Ni—Fe seed layer outside of the bottom shield frame are removed, a 2,400 nm thick Al 2 O 3  film is deposited, and CMP then applied until a smooth 2,000 nm thick lower shield  402  is formed. At step  504 , seed layers  404  and  406  may be deposited on the bottom shield  402 . At step  506 , a pinning layer  408  may be deposited on the upper seed layer  406 . Then, at step  508 , the flux-closure stack  450  may be deposited on the pinning layer  408 . At step  510 , the barrier layer  416  may be deposited on the flux-closure stack  450 . At step  512 , the sense layer  418  may be deposited on the barrier layer  416 . Then, at step  514 , the cap layer  420  may be deposited on the sense layer  418 . At step  516 , the CPP GMR sensor  400  may be annealed. In one embodiment, annealing may be performed for 5 hours at 285 degrees C. in a field of 50,000 Oe in a vacuum oven. With respect to each of the deposition steps described above, any deposition method known to those of skill in the art may be used, including DC magnetron sputtering, ion-beam sputtering, etc. The process  500  can be further performed in the same ways as described with respect to the process  300 . 
     Exemplary Sensor Characteristics 
       FIGS. 6A-G  depict experimental results of a TMR sensor fabricated according to one embodiment of the invention in contrast with a conventional TMR sensor. The TMR sensors used in one embodiment include Ni—Cr—Fe(2)/Ni—Fe(0.8)/Ir—Mn—Cr(6)/Co—Fe(2.1)/Ru(0.43)/Co—Fe—B(1.9)/Mg—O/Co—Fe—B(4)/Ru(6) films (thickness in nm). Two exemplary types of the TMR sensors are depicted in the graphs, one with and the other without a 3 nm thick Ta block layer. Each of the sensors depicted are deposited on a chemically-mechanically polished 2,000 nm thick Ni—Fe bottom shield, and then annealed for 5 hours at 285° C. in 50,000 Oe in a vacuum oven. Without performing photolithographic patterning, the junction resistance-area product (R J A J ) and the TMR coefficient (ΔR T /R J ) of the TMR sensors are measured with a current-in-plane tunneling technique. 
       FIG. 6A  shows the TMR coefficient (ΔR T /R J ) versus the junction resistance-area product (R J A J ) for (a) a TMR sensor  200  according to one embodiment of the invention and (b) a conventional TMR sensor. ΔR T /R J  and R J A J  are measured with a current-in-plane tunneling technique. As depicted in  FIG. 6A , as the thickness of the Mg—O barrier layer  216  increases from 0.85 to 1.05 nm, the junction resistance-area product (R J A J ) increases from 1.0 to 4.3 Ω-μm 2  and the TMR coefficient (ΔR T /R J ) increases from 11.9 to 79.4% for the conventional TMR stack, while R J A J  increases from 1.1 to 4.0 Ω-μm 2  and ΔR T /R J  increases from 26.1 to 90.7% for the TMR stack according to one embodiment of the invention. With the Mg—O barrier layer  216  of 1 nm in thickness for a conservatively designed R J A J  of 3.4 Ω-μm 2 , the epitaxially growth exhibited by the multiple layers with an identical FCC structure an increase in ΔR T /R J  67.0 to 83.6%. 
     The TMR sensors are then patterned into 700 nm diameter round TMR sensors. The ferromagnetic coupling field (H F , the shift of a sense-layer hysteresis loop), pinning field (H 90 , where ΔR T /R J  decreases to 90% of its original value), R J A J  and ΔR T /R J  are measured with a four-point probe. 
       FIG. 6B  shows magnetic responses of (a) a TMR sensor  200  according to one embodiment of the invention, and (b) a conventional TMR sensor. The field (H) is applied in the easy-axis direction of the Co—Fe keeper layer  210  and the Co—Fe—B reference layer  214 . The ferromagnetic coupling field (H F , the shift of a sense-layer hysteresis loop), pinning field (H 90 , where the TMR coefficient ΔR T /R J  decreases to 90% of its original value), the junction resistance-area product (R J A J ), and the TMR coefficient (ΔR T /R J ) are measured with a four-point probe. The epitaxially growth exhibited by the multiple layers with an identical FCC structure causes a decrease in H F  from 65.0 to 34.4 Oe, a decrease in H 90  from 1619.4 to 1418.8 Oe, and an increase in ΔR T /R J  from 65.2 to 87.3%. In spite of the lower H 90 , the TMR sensors according to one embodiment of the invention in fact still exhibits ΔR T /R J  at H 90  of as high as 78.6%, indicating strong pinning desired for proper sensor operation. 
       FIG. 6C  shows a graph of the ferromagnetic coupling field (H F ) versus the junction resistance-area product (R J A J ) for (a) a TMR sensor according to one embodiment of the invention, and (b) a conventional TMR sensor.  FIG. 6D  shows a graph of the TMR coefficient (ΔR T /R J ) versus the junction resistance-area product (R J A J ) for (a) a TMR sensor according to one embodiment of the invention, and (b) a conventional TMR sensor.  FIG. 6E  is a graph of the ferromagnetic coupling field (H F ) over junction resistance-area product (R J A J ) provided for (a) a TMR sensor according to one embodiment of the invention, and (b) a conventional TMR sensor. As the thickness of the Mg—O barrier layer  216  increases from 0.85 to 1.05 nm, R J A J  increases from 1.0 to 4.4 Ω-μm 2 , H F  decreases from 150.9 to 38.1 Oe, and ΔR T /R J  increases from 18.4 to 76.3% for the conventional TMR sensor, while R J A J  increases from 1.1 to 3.7 Ω-μm 2 , H F  decreases from 95.6 to 33.4 Oe, and ΔR T /R J  increases from 27.9 to 87.2% for the TMR sensor formed according to one embodiment of the invention, as depicted in  FIGS. 6C-E . With the Mg—O barrier layer  216  of 0.85 nm in thickness for an aggressively designed R J A J  of 1 Ω-μm 2 , the epitaxial growth provided by the multiple layers with an identical FCC structure causes a decrease in H F  from 150.9 to 95.6 Oe and an increase in ΔR T /R J  from 18.4 to 27.9%. The substantial decrease in H F  and the substantial increase in ΔR T /R J  indicate that the TMR sensor formed according to one embodiment of the invention will be still viable after a sensor miniaturization process, and thus will play an important role in magnetic recording at ultrahigh densities. 
       FIG. 6F  is a graph of the TMR coefficient (ΔR T /R J ) versus operation voltage (V O ) for (a) a TMR sensor according to one embodiment of the invention, and (b) a conventional TMR sensor. With respect to  FIG. 6F , the TMR sensors are tested by applying V O  increasing by every 20 mV in alternative directions. Their thermal stability is characterized by a critical voltage (V 90 , where ΔR T /R J  decreases to 90% of its original value). As depicted, the epitaxial growth exhibited by the multiple layers with an identical FCC structure causes an increase in ΔR T /R J  at V O =20 mV from 71.4% to 96.7%, and a decrease in V 75  from 227.3 to 183.2 mV. In spite of the lower V 75 , the TMR sensor formed according to one embodiment of the invention in fact still exhibits ΔR T /R J  at V 75  of as high as 72.5%, indicating higher thermal stability desired for magnetic recording at higher densities. 
       FIG. 6G  is a graph of the TMR coefficient (ΔR T /R J ) measured at 40 mV versus the stressing voltage (V S ) for (a) a TMR sensor according to one embodiment of the invention, and (b) a conventional TMR sensor. With respect to  FIG. 6G , the TMR sensors are stressed by applying V S  increasing by every 20 mV and staying for 100 ms at each V S , and tested by applying V O  of 40 mV after each stressing. Their thermal stability is characterized by a breakdown voltage (V B ), where the TMR effects are destroyed and thus ΔR T /R J  becomes zero. As depicted, the epitaxial growth provided by the multiple layers with an identical FCC structure causes an increase in V B  from 338 to 384 mV, thus providing higher thermal stability desired for magnetic recording at higher densities. 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.