Abstract:
A current-perpendicular-to-plane (CPP) tunneling magnetoresistance (TMR) or giant magnetoresistance (GMR) read sensor with ferromagnetic amorphous buffer and polycrystalline seed layers is disclosed for reducing a read gap, in order to perform magnetic recording at higher linear densities. The ferromagnetic amorphous buffer and polycrystalline seed layers couples to a ferromagnetic lower shield, thus acting as part of the ferromagnetic lower shield and defining the upper surface of the ferromagnetic polycrystalline seed layer as the lower bound of the read gap. In addition, a CPP TMR or GMR read sensor with nonmagnetic and ferromagnetic cap layers is also disclosed for reducing the read gap, in order to perform magnetic recording at even higher linear densities. The ferromagnetic cap layer couples to a ferromagnetic upper shield, thus acting as part of the ferromagnetic upper shield and defining the lower surface of the ferromagnetic cap layer as the upper bound of the read gap.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention is related to non-volatile magnetic storage devices, and in particular to a hard disk drive including a current-perpendicular-to-plane (CPP) tunneling magnetoresistance (TMR) or giant magnetoresistance (GMR) read sensor with ferromagnetic amorphous buffer and polycrystalline seed layers also acting as ferromagnetic lower shields. 
     2. Statement of the Problem 
     In many non-volatile magnetic storage devices, a hard disk drive is the most extensively used to store data. The hard disk drive includes a hard disk and an assembly of write and read heads. The assembly of write and read heads is supported by a slider that is mounted on a suspension arm. When the hard disk rotates, an actuator swings the suspension arm to place the slider over selected circular data tracks on the hard disk. The suspension arm biases the slider toward the hard disk, and an air flow generated by the rotation of the hard disk causes the slider to fly on a cushion of air at a very low elevation (fly height) over the hard disk. When the slider rides on the air, the actuator moves the suspension arm to position the write and read heads over selected data tracks on the hard disk. The write and read heads write data to and read data from, respectively, data tracks on the hard disk. Processing circuitry connected to the write and read heads then operates according to a computer program to implement writing and reading functions. 
     In a reading process, the read head passes over magnetic transitions of a data track on the rotating hard disk, and magnetic fields emitting from the magnetic transitions modulate the resistance of a read sensor in the read head. Changes in the resistance of the read sensor are detected by a sense current passing through the read sensor, and are then converted into voltage changes that generate read signals. The resulting read signals are used to decode data encoded in the magnetic transitions of the data track. 
     In a typical read head, a current-perpendicular-to-plane (CPP) giant magnetoresistance (GMR) or tunneling magnetoresistance (TMR) read sensor is electrically separated by side oxide layers from longitudinal bias layers in two side regions in order to prevent a sense current from shunting into the two side regions, but is electrically connected with ferromagnetic lower and upper shields, allowing the sense current to flow through the CPP read sensor in a direction perpendicular to the sensor plane. A typical CPP TMR read sensor comprises an electrically insulating barrier layer sandwiched between the lower and upper sensor stacks. The barrier layer is formed by a nonmagnetic MgO x  film having a thickness ranging from 0.4 to 1 nm. When the sense current quantum jumps across the MgO x  barrier layer, changes in the resistance of the CPP TMR read sensor are detected through a TMR effect. A typical CPP GMR read sensor comprises an electrically conducting spacer layer sandwiched between lower and upper sensor stacks. The spacer layer is formed by a nonmagnetic Cu or oxygen-doped Cu (Cu—O) film having a thickness ranging from 1.6 to 4 nm. When the sense current flows across the Cu or Cu—O spacer layer, changes in the resistance of the CPP GMR read sensor are detected through a GMR effect. 
     The lower sensor stack of the CPP TMR read sensor typically comprises a buffer layer formed by a nonmagnetic Ta film, a seed layer formed by a nonmagnetic Ru film, a pinning layer formed by an antiferromagnetic Ir-Mn film, and a flux-closure structure. The flux closure structure comprises a keeper layer formed by a ferromagnetic Co—Fe film, an antiparallel coupling layer formed by a nonmagnetic Ru film, and a reference layer formed by a ferromagnetic Co—Fe—B film. Four fields are induced in the flux-closure structure. First, a unidirectional anisotropy field (H UA ) is induced by exchange coupling between the pinning and keeper layers. Second, an antiparallel-coupling field (H APC ) is induced by antiparallel coupling between the keeper and reference layers and across the antiparallel-coupling layer. Third, a demagnetizing field (H D ) is induced by the net magnetization of the keeper and reference layers. Fourth, a ferromagnetic-coupling field (H F ) is induced by ferromagnetic coupling between the reference and sense layers and across the barrier layer. To ensure proper sensor operation, H UA  and H APC  must be high enough to rigidly pin magnetizations of the keeper and reference layers in opposite transverse directions perpendicular to an air baring surface (ABS), while H D  and H F  must be small and balance with each other to orient the magnetization of a sense layer in a longitudinal direction parallel to the ABS. 
     The upper sensor stack of the CPP TMR read sensor typically comprises a sense layer formed by a ferromagnetic Co—Fe—B film and a cap layer formed by a nonmagnetic Ta film. Both the Co—Fe—B reference and sense layers exhibit a “soft” amorphous phase after deposition, which will be transformed into a polycrystalline phase after annealing. With this crystallization, a Co—Fe—B(001)[110]//MgO x (001)[100]//Co—Fe—B(001)[110] epitaxial relationship is developed, and thus the TMR effect is substantially enhanced. 
     In order for the read head to perform magnetic recording at densities beyond 400 Gb/in 2 , its sensor width has been progressively reduced to below 50 nm for increasing track densities, while its read gap (defined as a distance between the ferromagnetic lower and upper shields) has been progressively reduced to below 30 nm for increasing linear densities. A further reduction in the sensor width poses a stringent photolithography challenge, while a further reduction in the read gap poses an inevitable sensor miniaturization challenge. 
     SUMMARY 
     The invention provides a current-perpendicular-to-plane (CPP) tunneling magnetoresistance (TMR) or giant magnetoresistance (GMR) read sensor with ferromagnetic amorphous buffer and polycrystalline seed layers also acting as ferromagnetic lower shields. 
     The ferromagnetic amorphous buffer layer is preferably formed by a Co—Fe—X film (where X is Hf, Zr or Y). The ferromagnetic amorphous Co—Fe—X buffer layer provides the CPP read sensor with microstructural discontinuity from a ferromagnetic lower shield, thus facilitating the CPP read sensor to grow freely with preferred crystalline textures, and with ferromagnetic continuity to the ferromagnetic lower shield, thus acting as part of the ferromagnetic lower shield. 
     The ferromagnetic polycrystalline seed layer is preferably formed by a Ni—Fe or Ni—Fe—X film (where X is Cu, Cr, Rh, Ru, Ti or W) exhibiting a face-centered-cubic (fcc) structure. The ferromagnetic polycrystalline Ni—Fe or Ni—Fe—X seed layer provides the CPP read sensor with an epitaxial relationship, thus facilitating the CPP read sensor to grow with preferred crystalline textures, thereby exhibiting high pinning fields and good TMR properties, and with ferromagnetic continuity to the ferromagnetic lower shield, thus also acting as part of the ferromagnetic lower shield. 
     These and other features and advantages of the invention will be apparent upon reading the following detailed description of preferred embodiments taken in conjunction with the Figures in which like reference numerals indicate like elements throughout. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The same reference number represents the same element or same type of element on all drawings. 
         FIG. 1  is a schematic diagram illustrating a hard disk drive used as a non-volatile magnetic storage device. 
         FIG. 2  is a schematic diagram illustrating a side view of a portion of a hard disk drive. 
         FIG. 3  is a schematic diagram illustrating an ABS view of a slider. 
         FIG. 4  is a schematic diagram illustrating an ABS view of a read head fabricated on a slider. 
         FIG. 5  is a schematic diagram illustrating an ABS view of a CPP TMR read sensor used in a prior art. 
         FIG. 6  is a schematic diagram illustrating an ABS view of a CPP TMR read sensor in accordance with a preferred embodiment of the invention. 
         FIG. 7  is a schematic diagram illustrating an ABS view of a CPP GMR read sensor in accordance with an alternative embodiment of the invention. 
         FIG. 8  is a schematic diagram illustrating an ABS view of a dual CPP GMR read sensor in accordance with a further alternative embodiment of the invention. 
         FIG. 9  is a chart showing easy-axis magnetic responses of 21.7Ir-78.3Mn(6)/77.5Co-22.5Fe(3.6) films with various buffer/seed layers and Ru(3)/Ta(3)/Ru(4) cap layers after annealing for 2 hours at 280° C. The buffer/seed layers comprise Ta(2)/Ru(2), Ta(2)/91.3Ni-8.7Fe(2) and 66.9Co-9.6Fe-23.5Hf(4)/91.3Ni-8.7Fe(2) films. 
         FIG. 10  is a chart showing J K  versus the seed layer thickness (δ Seed ) for 21.7Ir-78.3Mn(6)/77.5Co-22.5Fe(3.6) films with various buffer/seed layers and Ru(3)/Ta(3)/Ru(4) cap layers after annealing for 2 hours at 280° C. The buffer/seed layers comprise Ta(2)/Ru, Ta(2)/91.3Ni-8.7 Fe and 66.9Co-9.6Fe-23.5Hf(4)/91.3Ni-8.7Fe films. 
         FIG. 11  shows high-field easy-axis magnetic responses of TMR read sensors comprising 21.7Ir-78.3Mn(6)/77.5Co-22.5Fe(2.2)/Ru(0.8)/65.6Co-19.9Fe-14.5B(1.6)/77.5Co-22.5Fe(0.4)/MgO x (0.8)/87.1Co-12.9Fe(0.4)/71.5Co-7.4Fe-21.1B(1.6)/74.0Co-10.8Fe-15.2Hf(1.6)/Ta(2)/Ru(4) films with various buffer/seed layers. The buffer/seed layers comprise Ta(2)/Ru(2), Ta(2)/91.3Ni-8.7Fe(2) and 66.9Co-9.6Fe-23.5Hf(4)/91.3Ni-8.7Fe(2) films. 
         FIG. 12  shows low-field easy-axis magnetic responses of TMR read sensors corresponding to those shown in  FIG. 11 . 
         FIG. 13  shows the ferromagnetic coupling field (H F ) versus the resistance-area product (R J A J ) for the TMR read sensors corresponding to those shown in  FIG. 11 . 
         FIG. 14  shows the TMR coefficient (ΔR T /R J ) versus the resistance-area product (R j A j ) for the TMR read sensors corresponding to those shown in  FIG. 11 . 
     
    
    
     Table 1 is a table summarizing m s , H CE , H UA  and J K  determined from the substantially shifted hysteresis loops of the 21.7Ir-78.3Mn(6)/77.5Co-22.5Fe(3.6) films shown in  FIG. 9 . 
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1-14  and the following description depict specific exemplary embodiments of the invention to teach those skilled in the art how to make and use the invention. For the purpose of teaching inventive principles, some conventional aspects of the invention have been simplified or omitted. Those skilled in the art will appreciate variations from these embodiments that fall within the scope of the invention. Those skilled in the art will appreciate that the features described below can be combined in various ways to form multiple variations of the invention. As a result, the invention is not limited to the specific exemplary embodiments described below, but only by the claims and their equivalents. 
     Referring now to  FIG. 1 , there is shown a hard disk drive  100  embodying the invention. As shown in  FIG. 1 , at least one rotatable hard disk  112  is supported on a spindle  114  and rotated by a disk drive motor  118 . The magnetic recording is performed in the form of annular patterns of concentric data tracks (not shown) on the hard disk  112 . 
     At least one slider  113  is positioned near the hard disk  112 , each slider  113  supporting one or more assemblies of write and read heads  121 . As the hard disk  112  rotates, the slider  113  moves radially in and out over the disk surface  122  so that the assembly of write and read heads  121  may access different tracks of the hard disk  112  where desired data are written. The slider  113  is attached to a suspension  115 , which provides a slight spring force to bias the slider  113  against the disk surface  122 . The suspension  115  is attached to an actuator arm  119 , and the actuator arm  119  is attached to an actuator means  127 . The actuator means  127  may be a voice coil motor (VCM) comprising a coil movable within a fixed magnetic field. The direction and speed of the coil movements in the VCM is controlled by the motor current signals supplied by a control unit  129 . 
     During operation of the hard disk drive  100 , the rotation of the hard disk  112  generates an air bearing between the slider  113  and the disk surface  122  which exerts an upward force or lift on the slider  113 . The air bearing thus counter-balances the slight spring force of suspension  115  and supports the slider  113  off and slightly above the disk surface  122  by a small, substantially constant spacing during sensor operation. 
     The various components of the hard disk drive  100  are controlled in operation by control signals generated by the control unit  129 , such as access control signals and internal clock signals. Typically, the control unit  129  comprises logic control circuits, storage means and a microprocessor. The control unit  129  generates control signals to control various system operations such as drive motor control signals on line  123  and head position and seek control signals on line  128 . The control signals on line  128  provide the desired current profiles to optimally move and position the slider  113  to the desired data track on the hard disk  112 . Write and read signals are communicated to and from write and read heads, respectively, by way of a recording channel  125 . 
       FIG. 2  is a side view of a portion of the hard disk drive  100 . The slider  113  is supported above the hard disk  112 . The slider  113  includes a front end  202  and an opposing trailing end  204 . The slider  113  also includes an air bearing surface (ABS)  206  that faces toward the surface of the hard disk  112 . The assembly of write and read heads (not shown)  121  is formed proximate to the trailing end  204 , which is further illustrated in  FIG. 3 . 
       FIG. 3  is an ABS view of the slider  113 . The ABS  206  of the slider  113  is the surface of the page in  FIG. 3 . The slider  113  may include rails  302  and one or more pads  304  formed on the ABS  206 . The rails and pads  302 ,  304 , which define how the slider  113  flies over the surface of the hard disk  112 , illustrate just one embodiment, and the configuration of the ABS  206  of the slider  113  may take on any desired form. The slider  113  includes a write head  306  and a read head  308  fabricated proximate to the trailing end  204 . 
       FIG. 4  illustrates an ABS view of a read head  308  fabricated on the slider  113 . The read head  308  includes a lower shield  402  and an upper shield  404 . A read sensor  406  having first and second side regions  408 ,  410  is sandwiched between the lower and upper shields  402 ,  404 . In the two side regions  408 ,  410 , side oxide layers  412 ,  414  separate longitudinal bias layers  416 ,  418 , respectively, from the lower shield  402  and the read sensor  406 . 
       FIG. 5  illustrates an ABS view of a CPP TMR read sensor  500  used in a prior art. The CPP TMR read sensor  500  includes an electrically insulating MgO x  barrier layer  530  sandwiched between a lower sensor stack  510  and an upper sensor stack  540 . The lower sensor stack  510  comprises a buffer layer  512  formed by a nonmagnetic Ta film, a seed layer  514  formed by a nonmagnetic Ru or Ni—Fe film, a pinning layer  516  formed by an antiferromagnetic Ir-Mn film, and a flux closure structure  520 . The flux closure structure  520  comprises a keeper layer  522  formed by a ferromagnetic Co—Fe film, an antiparallel coupling layer  524  formed by a nonmagnetic Ru film, and a reference layer  526  formed by a ferromagnetic Co—Fe—B film. The upper sensor stack  540  comprises a sense layer  542  formed by a ferromagnetic Co—Fe—B film and a cap layer  544  formed by a nonmagnetic Ta film. 
     Since the Ta buffer layer  512  is nonmagnetic, it provides a magnetic discontinuity between the Ni—Fe lower shield  510  and the CPP TMR read sensor  500 , and thus its lower surface defines the lower bound of the read gap (RG). Since the Ta buffer layer  512  exhibits a body-centered-cubic (bcc) structure while the Ni—Fe lower shield  510  exhibits a face-centered-cubic (fcc) structure, the Ta buffer layer  512  also provides a microstructural discontinuity between the Ni—Fe lower shield  510  and the TMR read sensor  500 , and may reduce the effect on the development of crystalline textures in the TMR read sensor  500 . Since the Ru seed layer  514  exhibits a hexagonal-closed-packed (hcp) structure (which is formed by two types of closest-packed monolayers) while the Ni—Fe seed layer  514  and the Ir-Mn pinning layer  516  exhibits an fcc structure (which is formed by three types of closest-packed monolayers), the Ru (or Ni—Fe) seed layer  514  provides an epitaxial relationship between the Ru (or Ni—Fe) seed layer  514  and the Ir-Mn pinning layer  516 , thereby facilitating the Ir-Mn pinning layer  516  to grow with a preferred crystalline texture needed for high pinning fields. 
       FIG. 6  illustrates an ABS view of a CPP TMR read sensor  600  in accordance with a preferred embodiment of the invention. The TMR read sensor  600  includes an electrically insulating MgO x  barrier layer  530  sandwiched between a lower sensor stack  610  and an upper sensor stack  640 . 
     The lower sensor stack  610  comprises a buffer layer  612  formed by a 4 nm thick ferromagnetic amorphous 66.9Co-9.6Fe-23.5Hf film (composition in atomic percent), a seed layer  614  formed by a 2 nm thick ferromagnetic polycrystalline 91.3Ni-8.7Fe film, a pinning layer  516  formed by a 6 nm thick antiferromagnetic 21.7Ir-78.3Mn film, and a flux closure structure  620 . The flux closure structure  620  comprises a keeper layer  522  formed by a 2 nm thick ferromagnetic 77.5Co-22.5Fe film, an antiparallel coupling layer  524  formed by a 0.8 nm thick nonmagnetic Ru film, a first reference layer  626  formed by a 1.6 nm thick ferromagnetic 65.6Co-19.9Fe-14.5B film, and a second reference layer  628  formed by a 0.4 nm thick ferromagnetic 77.5Co-22.5Fe film. Thicknesses of the Co—Fe keeper and Co—Fe—B/Co—Fe reference layers are fixed in order to attain designed saturation moments (m s ) of 0.25 and 0.24 memu/cm 2  (equivalent to those of 3.6 and 3.4 nm thick 80Ni-20Fe films sandwiched between two Cu films, respectively), respectively, for ensuring proper sensor operation. 
     More generally, the buffer layer  612  may be formed by a Co—Fe—X film (where X is Hf, Zr or Y) containing Co with a content ranging from 60 to 80 at %, Fe with a content ranging from 0 to 40 at %, and X with a content ranging from 6 to 30 at %, and having a thickness ranging from 0.4 to 20 nm. The seed layer  614  may be formed by a Ni—Fe—X film (where X is Cu, Cr, Rh, Ru, Ti or W) containing Ni with a content ranging from 60 to 100 at %, Fe with a content ranging from 0 to 40 at %, and X with a content ranging from 0 to 10 at %, and having a thickness ranging from 2 to 40 nm. The Co—Fe keeper layer  522  may contain Co with a content ranging from 50 to 90 at % and Fe with a content ranging from 10 to 50 at %, and have a thickness ranging from 1.6 to 3.2 nm. The Co—Fe—B first reference layer  626  may contain Co with a content ranging from 60 to 80 at %, Fe with a content ranging from 0 to 40 at %, and B with a content ranging from 6 to 30 at %, and have a thickness ranging from 1 to 4 nm. The Co—Fe second reference layer  628  may contain Co with a content ranging from 50 to 90 at % and Fe with a content ranging from 10 to 50 at %, and have a thickness ranging from 0.4 to 2 nm. 
     In the preferred embodiment of the invention, since both the Co—Fe—Hf buffer layer  612  and the Ni—Fe seed layer  614  are ferromagnetic, they provide magnetic continuities between the lower shield  510  and the CPP TMR read sensor  600 , and thus the upper surface of the Ni—Fe seed layer  614  defines the lower bound of the read gap. Since the Co—Fe—Hf buffer layer is amorphous, it provides a microstructural discontinuity between the lower shield  510  and the CPP TMR read sensor  600 , and thus establishes a new foundation for the CPP TMR read sensor  600  to grow freely. Since the Ni—Fe seed layer  614  exhibits the fcc structure, it provides an epitaxial relationship between the Ni—Fe seed layer  614  and the Ir-Mn pinning layer  516 , thus facilitating the Ir-Mn pinning layer  516  to grow with a preferred crystalline texture needed for high pinning fields. 
     In the prior art, the Ni—Fe seed layer in contact with the Ta buffer and Ir-Mn pinning layers must be thin enough to ensure high linear resolution, since it acts as part of the read gap, and to prevent the CPP TMR read sensor from exhibiting operational instability, since its residual magnetization is moderately pinned by the Ir-Mn pinning layer. In the invention, by contrast, the Ni—Fe seed layer in contact with the Co—Fe—Hf buffer and Ir-Mn pinning layers can be as thick as desired to reinforce the epitaxial relationship needed for enhancing pinning and TMR properties since it acts as part of the lower shield, while the sensor operation is not interrupted since its residual magnetization is free from the Ir-Mn pinning layer after ferromagnetic coupling to the Co—Fe—Hf buffer layer and the Ni—Fe lower shield. The thick Ni—Fe seed layer, however, requires a plasma treatment to remove unwanted grain boundary grooving on top of columnar polycrystalline grains. As a result, the thick Ni—Fe seed layer may exhibit a smoothened surface with large polycrystalline grains, thus facilitating the TMR read sensor to further enhance pinning and TMR properties. 
     The upper sensor stack  640  comprises a first sense layer  642  formed by a 0.4 nm thick ferromagnetic 87.1Co-12.9Fe film, a second sense layer  644  formed by a 1.6 nm thick ferromagnetic 71.5Co-7.4Fe-21.1B film, a third sense layer  646  formed by a 1.6 nm thick ferromagnetic 74.0Co-10.8Fe-15.2Hf film, a first cap layer  544  formed by a 2 nm thick nonmagnetic Ta film, and a second cap layer  648  formed by a 6 nm thick ferromagnetic 80Ni-20Fe film. The total thickness of the Co—Fe/Co—Fe—B/Co—Fe—Hf sense layers  642 ,  644 ,  646  is fixed in order to attain a designed magnetic moment of 0.32 memu/cm 2  (equivalent to that of a 4.5 nm thick 80Ni-20Fe film sandwiched between two Cu films) for achieving high read sensitivity. 
     More generally, the Co—Fe first sense layer  642  may contain Co with a content ranging from 50 to 90 at % and Fe with a content ranging from 10 to 50 at %, and have a thickness ranging from 0.4 to 2 nm. The Co—Fe—B second sense layer  644  may contain Co with a content ranging from 60 to 80 at %, Fe with a content ranging from 0 to 40 at %, and B with a content ranging from 6 to 30 at %, and have a thickness ranging from 1 to 4 nm. The third sense layer  646  may be formed by a Co—Fe—X film (where X is Hf, Zr or Y) containing Co with a content ranging from 60 to 80 at %, Fe with a content ranging from 0 to 40 at %, and X with a content ranging from 6 to 30 at %, and having a thickness ranging from 1 to 4 nm. The second cap layer  648  can be formed by a Ni—Fe—X film (where X is Cu, Cr, Rh, Ru, Ti or W) containing Ni with a content ranging from 60 to 100 at %, Fe with a content ranging from 0 to 40 at %, and Y with a content ranging from 0 to 10 at %, and having a thickness ranging from 2 to 40 nm. 
     In the prior art, the nonmagnetic cap layer is preferably thin to ensure high linear resolution since it acts as part of the read gap, but also preferably thick enough to eliminate chemical and mechanical processes induced damage since it acts as a protection layer to the sense layers. In the invention, by contrast, the first cap layer can be as thin as desired as long as it is nonmagnetic and causes ferromagnetic decoupling between the sense and second cap layers. On the other hand, the second cap layer can be as thick as desired for protecting the sense layers as long as it is ferromagnetic since it acts as part of the upper shield. The thick Ni—Fe second cap layer, however, requires heavy sputter etching before forming the upper shield, to remove chemical and mechanical damages in processing and to expose a cleaned surface for ferromagnetic coupling to the upper shield. As a result, the lower surface of the second cap layer defines the upper bound of the read gap. 
     In the prior art, the CPP TMR read sensors with sense layers symmetrical and asymmetrical to the lower and upper shields provide 34.8 and 25.2 nm thick read gaps, respectively. In the invention, by contrast, the CPP TMR read sensors with sense layers symmetrical and asymmetrical to the lower and upper shields provide 26.8 and 17.2 nm thick read gaps, respectively. This substantial reduction in the read gap is expected to provide much higher linear resolution in magnetic recording. 
       FIG. 7  illustrates an ABS view of a CPP GMR read sensor  700  in accordance with an alternative embodiment of the invention. The CPP GMR read sensor  700  includes an electrically conducting Cu—O spacer  730  sandwiched between a lower sensor stack  710  and an upper sensor stack  740 . 
     The lower sensor stack  710  comprises a buffer layer  612  formed by a 4 nm thick ferromagnetic amorphous 66.9Co-9.6Fe-23.5Hf film, a seed layer  614  formed by a 2 nm thick ferromagnetic polycrystalline 91.3Ni-8.7Fe film, a pinning layer  516  formed by a 6 nm thick antiferromagnetic 21.7Ir-78.3Mn film, and a flux closure structure  720 . The flux closure structure  720  comprises a keeper layer  522  formed by a 2 nm thick ferromagnetic 77.5Co-22.5Fe film, an antiparallel coupling layer  524  formed by a 0.8 nm thick nonmagnetic Ru film, and a reference layer  726  formed by a 1.6 nm thick ferromagnetic 50Co-50Fe film. Thicknesses of the 77.5Co-22.5Fe keeper and 50Co-50Fe reference layers are fixed in order to attain designed saturation moments (m s ) of 0.25 and 0.24 memu/cm 2  (equivalent to those of a 3.6 and 3.4 nm thick 80Ni-20Fe films sandwiched between two Cu films, respectively), respectively, for ensuring proper sensor operation. An additional reference layer formed by a Co—Fe—X film (where X is Hf, Zr, Y, Al, Ge or Si) with high electrical resistivity may be incorporated into the flux closure structure  720 . 
     The upper sensor stack  740  comprises a sense layer  742  formed by a 2.4 nm thick ferromagnetic 87.1Co-12.9Fe film, a first cap layer  544  by a 2 nm thick nonmagnetic Ta film, and a second cap layer  648  formed by a 6 nm thick ferromagnetic 80Ni-20Fe film. The thickness of the Co—Fe sense layer  742  is fixed in order to attain a designed magnetic moment of 0.32 memu/cm 2  (equivalent to that of a 4.5 nm thick 80Ni-20Fe film sandwiched between two Cu films) for achieving high read sensitivity. An additional sense layer formed by a Ni—Fe—X film (where X is Cu, Cr, Rh, Ru, Ti or W) with high electrical resistivity may be incorporated into the upper sensor stack  740 . 
     More generally, the Co—Fe reference layer  726  may contain Co with a content ranging from 50 to 90 at % and Fe with a content ranging from 10 to 50 at %, and have a thickness ranging from 1.6 to 3.2 nm. The additional Co—Fe—X reference layer (where X is Hf, Zr, Y, Al, Ge or Si) may contain Co with a content ranging from 60 to 80 at %, Fe with a content ranging from 0 to 40 at %, and X with a content ranging from 6 to 30 at %, and having a thickness ranging from 1 to 4 nm. The additional Ni—Fe—X sense layer (where X is Cu, Cr, Rh, Ru, Ti or W) may contain Ni with a content ranging from 60 to 100 at %, Fe with a content ranging from 0 to 40 at %, and X with a content ranging from 0 to 10 at %, and having a thickness ranging from 2 to 4 nm. 
       FIG. 8  illustrates an ABS view of a dual CPP GMR read sensor  800  in accordance with a further alternative embodiment of the invention. The dual CPP GMR read sensor  800  includes a lower sensor stack  710 , a middle sensor stack  830 , and an upper sensor stack  840 . 
     The lower sensor stack  710  comprises a buffer layer  612  formed by a 4 nm thick ferromagnetic amorphous 66.9Co-9.6Fe-23.5Hf film, a seed layer  614  formed by a 2 nm thick ferromagnetic polycrystalline 91.3Ni-8.7Fe film, a first pinning layer  516  formed by a 6 nm thick antiferromagnetic 21.7Ir-78.3Mn film, and a first flux closure structure  720 . The first flux closure structure  720  comprises a first keeper layer  522  formed by a 2 nm thick ferromagnetic 77.5Co-22.5Fe film, a first antiparallel coupling layer  524  formed by a 0.8 nm thick nonmagnetic Ru film, and a first reference layer  726  formed by a 1.6 nm thick ferromagnetic 50Co-50Fefilm. Thicknesses of the 77.5Co-22.5Fe keeper and 50Co-50Fe reference layers are fixed in order to attain designed saturation moments (m s ) of 0.25 and 0.24 memu/cm 2  (equivalent to those of a 3.6 and 3.4 nm thick 80Ni-20Fe films sandwiched between two Cu films, respectively), respectively, for ensuring proper sensor operation. An additional reference layer formed by a Co—Fe—X film (where X is Hf, Zr, Y, Al, Ge or Si) with high electrical resistivity may be incorporated into the first flux closure structure  720 . 
     The middle sensor stack  830  comprises a first electrically conducting Cu—O spacer layer  730 , a sense layer  832  formed by a 2 nm thick ferromagnetic 87.1Co-12.9Fe film, and a second electrically conducting Cu—O spacer layer  834 . The thickness of the Co—Fe sense layer  832  is fixed in order to attain a designed magnetic moment of 0.32 memu/cm 2  (equivalent to that of a 4.5 nm thick 80Ni-20Fe film sandwiched between two Cu films) for achieving high read sensitivity. An additional sense layer formed by a Ni—Fe—X (where X is Cu, Cr, Rh, Ru, Ti or W) film with high electrical resistivity may be incorporated into the middle sensor stack  830 . 
     The upper sensor stack  840  comprises a second flux closure structure  850 , a second pinning layer  842  formed by a 6 nm thick antiferromagnetic 21.7Ir-78.3Mn film, and a cap layer  648  formed by a 6 nm thick ferromagnetic 80Ni-20Fe film. The second flux closure structure  850  comprises a second reference layer  852  formed by a 1.6 nm thick ferromagnetic 50Co-50Fe film, a second antiparallel coupling layer  854  formed by a 0.8 nm thick nonmagnetic Ru film, and a second keeper layer  856  formed by a 2 nm thick ferromagnetic 77.5Co-22.5Fe film. Thicknesses of the 50Co-50Fe second reference and 77.5Co-22.5Fe second keeper layers are fixed in order to attain designed saturation moments (m s ) of 0.24 and 0.25 memu/cm 2  (equivalent to those of a 3.4 and 3.6 nm thick 80Ni-20Fe films sandwiched between two Cu films, respectively), respectively, for ensuring proper sensor operation. An additional reference layer formed by a Co—Fe—X film (where X is Hf, Zr, Y, Al, Ge or Si) with high electrical resistivity may be incorporated into the second flux closure structure  850 . 
       FIG. 9  shows easy-axis magnetic responses of 21.7Ir-78.3Mn(6)/77.5Co-22.5Fe(3.6) films (thickness in nm) with various buffer/seed layers and Ru(3)/Ta(3)/Ru(4) cap layers after annealing for 2 hours at 280° C. The buffer/seed layers comprise Ta(2)/Ru(2), Ta(2)/91.3Ni-8.7Fe(2) and 66.9Co-9.6Fe-23.5Hf(4)/91.3Ni-8.7Fe(2) films. The thickness of the Co—Fe film is selected in order to attain a desired m s  of 0.56 memu/cm 2 , equivalent to that of an 8 nm thick 80Ni-20Fe film sandwiched between two Cu films. Exchange coupling occurring between the Ir-Mn and Co—Fe films can be realized from a hysteresis loop formed when the moment (m) of the Co—Fe film varies as function of a magnetic field (H). The exchange coupling increases the easy-axis coercivity (H CE ) of the Co—Fe film which is determined by the half width of the hysteresis loop, and also induces a unidirectional anisotropy field (H UA ) which is determined by the shift of the hysteresis loop. An exchange coupling energy (J K ), determined by the product of a saturation magnetization (M s ), the Co—Fe film thickness (δ CF ) and H UA , is typically used to characterize an intrinsic pinning strength needed for proper sensor operation. 
     Table 1 summarizes m s , H CE , H UA  and J K  determined from the substantially shifted hysteresis loops of the Ir-Mn/Co—Fe films as shown in  FIG. 7 . The m s  of the 2 nm thick. Ni—Fe seed layer sandwiched between the Ta buffer and Ir-Mn pinning layers cannot be detected at all, indicating a moment loss of as large as ˜0.14 memu/cm 2  caused by mixing at interfaces between the Ta buffer and Ni—Fe seed layers and between the Ni—Fe seed and Ir-Mn pinning layers. On the other hand, the m s  of the Co—Fe—Hf buffer and Ni—Fe seed layers can be determined to be 0.28 memu/cm 2  from a hysteresis loop slightly shifted by 69.7 Oe. It is expected that this shift will be substantially diminished when the Co—Fe—Hf buffer and Ni—Fe seed layers couple to the 1 μm thick Ni—Fe lower shield due to the inverse relationship between m s  and the shift field. Therefore, the Co—Fe—Hf buffer and Ni—Fe seed layers behave as if part of the lower shield. 
     Since the Ru and Ni—Fe seed layers exhibit hcp and fcc structures needed for developing the epitaxial relationship with the Ir-Mn pinning layer, respectively, the Ru and Ni—Fe seed layers facilitate the Ir-Mn pinning and Co—Fe pinned layers to exhibit comparably high H CE , H UA  and J K . On the other hand, since the Co—Fe—Hf buffer layer is amorphous, it may establish a new foundation which may affect the growth of the Ni—Fe seed layer and reduce its effect on the development of the epitaxial relationship. As a result, the use of the Co—Fe—Hf buffer and Ni—Fe seed layers lead to lower H UA  and J K . 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Seed Layers 
                 m S   
                 H CE   
                 H UA   
                 J K   
               
               
                 (nm) 
                 (memu/cm 2 ) 
                 (Oe) 
                 (Oe) 
                 (erg/cm 2 ) 
               
               
                   
               
             
             
               
                 Ta(2)/Ru(2) 
                 0.56 
                 138.5 
                 1196.9 
                 0.67 
               
               
                 Ta(2)/Ni—Fe(2) 
                 0.56 
                 121.1 
                 1200.9 
                 0.67 
               
               
                 Co—Fe—Hf(4)/Ni—Fe(2) 
                 0.56 
                 167.9 
                 1040.7 
                 0.58 
               
               
                   
               
             
          
         
       
     
       FIG. 10  shows J K  versus the seed layer thickness (δ Seed ) for 21.7Ir-78.3Mn(6)/77.5Co-22.5Fe(3.6) films with various buffer/seed layers and Ru(3)/Ta(3)/Ru(4) cap layers after annealing for 2 hours at 280° C. The buffer/seed layers comprise Ta(2)/Ru, Ta(2)/91.3Ni-8.7Fe and 66.9Co-9.6Fe-23.5Hf(4)/91.3Ni-8.7Fe films. In order to maximize J K , the Ru and Ni—Fe seed layers on top of the Ta buffer layers require critical thicknesses of 1.2 and 1.6 nm, respectively, and the Ni—Fe seed layer on top of the Co—Fe—Hf buffer layer 2 nm. 
       FIG. 11  shows high-field easy-axis magnetic responses of TMR read sensors comprising 21.7Ir-78.3 Mn(6)/77.5Co-22.5Fe(2)/Ru(0.8)/65.6Co-19.9Fe-14.5B(1.6)/77.5Co-22.5Fe(0.4)/MgO x (0.8)/87.1Co-12.9Fe(0.4)/71.5Co-7.4Fe-21.1B(1.6)/74.0Co-10.8Fe-15.2Hf(1.6)/Ta(2)/Ru(4) films with various buffer/seed layers. The buffer/seed layers comprise Ta(2)/Ru(2), Ta(2)/91.3Ni-8.7Fe(2) and 66.9Co-9.6Fe-23.5Hf(4)/91.3Ni-8.7Fe(2) films. The TMR read sensors with the Ru and Ni—Fe seed layers on top of the Ta buffer layers exhibit nearly identical hysteresis loops with a spin-fold field (H SF ) of 1,626 Oe, exceeding which the magnetization of the Co—Fe—B/Co—Fe reference layers starts to spin-flop, thus causing instable sensor operation. On the other hand, the TMR read sensor with the Co—Fe—Hf buffer and Ni—Fe seed layers exhibits a hysteresis loop with H SF  of 1,506 Oe. This slight H SF  difference originates from the H UA  difference as described in  FIG. 7 , and indicates a slight degradation in the pinning strength of the Co—Fe—B/Co—Fe reference layers. In spite of this slight degradation, it is still recommended to use the Co—Fe—Hf buffer and Ni—Fe seed layers for reducing the read gap and thus for increasing the linear density. 
       FIG. 12  shows low-field easy-axis magnetic responses of TMR read sensors corresponding to those shown in  FIG. 11 . The TMR read sensors with the Ru and Ni—Fe seed layers on top of the Ta buffer layers exhibit H F  of 98.9 and 104.2 Oe, respectively, while the TMR read sensor with the Co—Fe—Hf buffer and Ni—Fe seed layers 98.2 Oe. The slightly lower H F  may originate from a smoother surface provided by the Co—Fe—Hf buffer layer. It is expected to further decrease H F  after applying the plasma treatment to the Ni—Fe seed layer on top of the Co—Fe—Hf buffer layer. In addition, the m s  of the Co—Fe—Hf buffer and Ni—Fe seed layers can be determined to be 0.28 memu/cm 2  from a hysteresis loop slightly shifted by 68.6 Oe, indicating that the Co—Fe—Hf buffer and Ni—Fe seed layers exchange-couples to the Ir-Mn pinning layer. This exchange coupling will be substantially diminished when the Co—Fe—Hf buffer and Ni—Fe seed layers couple to the 1 μm Ni—Fe lower shield due to the inverse relationship between m s  and the shift field. 
       FIG. 13  shows the ferromagnetic coupling field (H f ) versus the resistance-area product (R J A J ) for the TMR read sensors corresponding to those shown in  FIG. 11 . Although the evidence that the use of the Co—Fe—Hf buffer and Ni—Fe seed layers leads to a lower H F  is not clear, it is still expected that H F  can be further reduced by depositing a thicker Ni—Fe seed layer and applying the plasma treatment to it. It should be noted that while the Ni—Fe seed layer deposited on the Ta seed layer must be thin enough so that its m s  will be diminished due to interface mixing and thus the sensor operation will be stable, the Ni—Fe seed layer deposited on the Co—Fe—Hf seed layer can be as thick as desired since it becomes part of the lower shield. 
       FIG. 14  shows the TMR coefficient (ΔR T /R J ) versus R J A J  for the TMR read sensors corresponding to those shown in  FIG. 11 . The TMR read sensors with the three types of buffer/seed layers basically exhibit similar TMR properties. It is believed that by depositing a thicker Ni—Fe seed layer and applying the plasma treatment to it, TMR properties will be improved due to the epitaxial growth of the TMR read sensor on a smoothened Ni—Fe seed layer.