Patent Publication Number: US-2012040089-A1

Title: Current-perpendicular-to-plane (cpp) read sensor with multiple reference layers

Description:
RELATED APPLICATIONS 
     The patent application is a divisional of U.S. patent application having the Ser. No. 11/964,673, and filed on Dec. 26, 2007, which is incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention is related to the field of magnetic storage systems, and in particular, to a disk drive including a current-perpendicular-to-plane (CPP) read sensor with multiple reference layers. 
     2. Statement of the Problem 
     In many magnetic storage systems, a hard disk drive is the most extensively used to store data. The hard disk drive typically includes a hard disk along with 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 surface of the rotating hard disk. An air flow generated by the rotation of the hard disk causes the slider with an air bearing surface (ABS) to fly on a cushion of air at a particular height over the rotating hard disk. The height depends on the shape of the ABS. As the slider flies on the air bearing, the actuator moves the suspension arm to position the write and read heads over selected data tracks on the surface of the hard disk. The write and read heads thus write data to and read data from, respectively, a recording medium on the rotating 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 transitions of a data track in the magnetic medium, and magnetic fields emitting from the 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 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 for preventing a sense current from shunting into the two side regions, but is electrically connected with lower and upper shields for the sense current to flow in a direction perpendicular to the sensor plane. A typical CPP GMR read sensor comprises an electrically conducting spacer layer sandwiched between lower and upper sensor stacks. The spacer layer is typically formed by a nonmagnetic Cu or oxygen-doped Cu (Cu—O) film having a thickness ranging from 1.6 to 4 nanometers. When the sense current flows across the Cu or Cu—O spacer layer, changes in the resistance of the CPP GMR read sensor is detected through a GMR effect. A typical CPP TMR read sensor comprises an electrically insulating barrier layer sandwiched between the lower and upper sensor stacks. The barrier layer is typically formed by a nonmagnetic oxygen-doped Mg (Mg—O) or Mg oxide (MgO X ) film having a thickness ranging from 0.4 to 1 nanometers. When the sense current “quantum-jumps” across the Mg—O or MgO X  barrier layer, changes in the resistance of the CPP GMR read sensor is detected through a TMR effect. 
     The lower sensor stack comprises nonmagnetic seed layers, an antiferromagnetic pinning layer, a ferromagnetic keeper layer, a nonmagnetic antiparallel-coupling layer, and a ferromagnetic reference layer. The upper sensor stack comprises ferromagnetic sense (free) layers and a nonmagnetic cap layer. In the lower sensor stack, the keeper layer, the antiparallel-coupling layer, and the reference layer form a flux-closure structure where four fields are induced. First, a unidirectional anisotropy field (H UA ) is induced by exchange coupling between the antiferromagnetic pinning layer and the keeper layer. Second, an antiparallel-coupling field (H APC ) is induced by antiparallel coupling between the keeper layer and the reference layer across the antiparallel-coupling layer. Third, a demagnetizing field (H D ) is induced by the net magnetization of the keeper layer and the reference layer. Fourth, a ferromagnetic-coupling field (H F ) is induced by ferromagnetic coupling between the reference layer and the sense layer across the spacer or barrier layer. To ensure proper sensor operation, H UA  and H APC  should be high enough to rigidly pin magnetizations of the keeper layer and the reference layer in opposite transverse directions perpendicular to the ABS, while H D  and H F  should be small and balance with each other to orient the magnetization of the sense layers in a longitudinal direction parallel to the ABS. 
     In the flux-closure structure of the CPP TMR read sensor, the Co—Fe keeper layer is selected to ensure high exchange and antiparallel coupling. Its composition is optimized and its magnetic moment is small, so that high H UA  and H APC  can be attained. The Co—Fe—B reference layer is selected to ensure a strong TMR effect and mild ferromagnetic coupling. Its B content is high enough for B atoms, which are much smaller than Co and Fe atoms, to occupy interstitial sites of a crystalline structure and thus interfere with the ability of the Co and Fe atoms to crystallize. As a result, an interstitial-type amorphous film with a flat surface is formed, which facilitates the Mg—O or MgO X  barrier layer to grow with a preferred &lt;001&gt; crystalline texture on the flat surface, thus increasing a TMR coefficient (ΔR T /R J ) and decreasing H F . Its Co and Fe contents are optimized and its magnetic moment is small, so that a high H APC  can be attained. 
     The use of the Co—Fe—B reference layer in the prior art generally meets the requirements of high H APC , low H F , and high ΔR T /R J . However, it is still desirable to further improve the reference layer for the CPP TMR sensor to operate more robustly. 
     SUMMARY 
     Embodiments of the invention include a CPP read sensor with multiple reference layers. According to one embodiment, the multiple reference layers of a CPP TMR read sensor include a first reference layer formed by a ferromagnetic polycrystalline Co—Fe film, a second reference layer formed by a ferromagnetic substitute-type amorphous Co—Fe—X film where X is Zr, Hf or Y, and a third reference layer formed by a ferromagnetic interstitial-type amorphous Co—Fe—B film. The first reference layer facilitates the TMR sensor to exhibit high exchange and antiparallel-coupling fields. The second reference layer provides a thermally stable flat surface, thus facilitating the CPP TMR sensor to exhibit a low ferromagnetic-coupling field. The multiple reference layers induce spin-dependent scattering, thus facilitating the CPP TMR sensor to exhibit a high TMR coefficient. 
     The invention may include other exemplary embodiments as described below. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The same reference number represents the same element or same type of element on all drawings. 
         FIG. 1  illustrates a hard disk drive used as a magnetic storage system. 
         FIG. 2  is a side view of a hard disk drive. 
         FIG. 3  is an ABS view of a slider. 
         FIG. 4  illustrates an ABS view of a typical read head fabricated on a slider. 
         FIG. 5  illustrates an ABS view of a TMR read sensor in an exemplary embodiment of the invention. 
         FIG. 6  illustrates multiple reference layers in an exemplary embodiment of the invention. 
         FIG. 7  is a flow chart illustrating a method of fabricating a TMR read sensor in an exemplary embodiment of the invention. 
         FIG. 8  illustrates an ABS view of the detailed structure of a TMR read sensor in an exemplary embodiment of the invention. 
         FIG. 9  illustrates magnetic responses of Ta(3)/Ru(3)/77.5Co-22.5Fe(2.1)/Ru(0.8)/X/Ta(3) films (thickness in nanometer), where X is 77.5Co-22.5Fe (2.1), 64.6Co-19.7Fe-15.7Zr (2.4), or 51.9Co-34.6Fe-13.5B (2.4) in an exemplary embodiment of the invention. 
         FIG. 10  illustrates the ferromagnetic coupling field (H F ) versus the resistance-area product (R J A J ) for TMR read sensors in an exemplary embodiment of the invention. 
         FIG. 11  illustrates the TMR coefficient (ΔR T /R J ) versus the resistance-area product (R J A J ) for the TMR read sensors in an exemplary embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1-11  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 embodiments described below, but only by the claims and their equivalents. 
       FIG. 1  illustrates a hard disk drive  100  used as a magnetic storage system. The hard disk drive  100  includes a spindle  102 , a hard disk  104 , a control system  106 , an actuator  108 , a suspension arm  110 , and a slider  114  having an assembly of write and read heads. The spindle  102  supports and rotates the hard disk  104  in a direction indicated by the arrow. A spindle motor (not shown) rotates the spindle  102  according to control signals from the control system  106 . The slider  114  is supported by the suspension arm  110 , and the actuator  108  is configured to rotate the suspension arm  110  in order to position the assembly of write and read heads over a desired data track in a magnetic medium on the hard disk  104 . The hard disk drive  100  may include other components not shown in  FIG. 1 , such as a plurality of hard disks, actuators, suspension arms, and sliders. 
     When the hard disk  104  rotates, an air flow generated by the rotation of the hard disk  104  causes the slider  114  with an air bearing surface (ABS) to fly on a cushion of air at a particular height over the rotating hard disk  104 . The height depends on the shape of the ABS. As the slider  114  flies on the air, the actuator  108  moves the suspension arm  110  to position a write head (not shown) and a read head (not shown) over selected data tracks on the surface of the hard disk  104 . The write and read heads write data to and read data from, respectively, a recording medium on the rotating hard disk  104 . Processing circuitry connected to the write and read heads then operates according to a computer program to implement writing and reading functions. 
       FIG. 2  is a side view of hard disk drive  100 . The slider  114  is supported above the surface of the hard disk  104 . The slider  114  includes a front end  202  and an opposing trailing end  204 . The slider  114  also includes an air bearing surface  206  that faces toward the surface of the hard disk  104 . A write head (not shown) and a read head (not shown) are formed proximate to the trailing end  204 , which is further illustrated in  FIG. 3 . 
       FIG. 3  is an ABS view of slider  114 . The ABS  206  of slider  114  is the surface of the page in  FIG. 3 . The slider  114  has a cross rail  303 , two side rails  304 ,  305 , and a center rail  306  on the ABS  206 . The rails, which define how the slider  114  flies over the surface of the hard disk  104 , illustrate just one embodiment, and the configuration of the ABS  206  of the slider  114  may take on any desired form. The slider  114  includes a write head  210  and a read head  112  fabricated proximate to the trailing end  204 . 
       FIG. 4  illustrates an ABS view of a typical read head  212  fabricated on the slider  116 . The read head  212  includes a first (lower) shield  401  and a second (upper) shield  402  that sandwich a TMR read sensor  404  and two side regions at edges of the TMR read sensor  404 . In the two side regions, side oxide layers  405 - 406  separate longitudinal bias layers  407 - 408 , respectively, from the first shield  401  and the TMR read sensor  404 . 
       FIG. 5  illustrates an ABS view of a TMR read sensor  404  in an exemplary embodiment of the invention. The layers  511 - 517  shown for the TMR read sensor  404  illustrate just one embodiment, and the TMR read sensor  404  may include additional layers or different layers in other embodiments. The TMR read sensor  404  includes an electrically insulating barrier layer  515  sandwiched between a lower sensor stack and an upper sensor stack. The lower sensor stack includes one or more nonmagnetic seed layers  510 , an antiferromagnetic pinning layer  511 , a ferromagnetic keeper layer  512 , an antiparallel-coupling layer  513 , and multiple reference layers  514 . The upper sensor stack includes one or more ferromagnetic sense layers  516 , and a nonmagnetic cap layer  517 . 
       FIG. 6  illustrates multiple reference layers  514  in an exemplary embodiment of the invention. The multiple reference layers  514  include a first reference layer  601  formed by a ferromagnetic polycrystalline Co—Fe film. The first reference layer  601  may be formed by a Co—Fe film including Co with a content ranging from 50 to 90 at % and Fe with a content ranging from 10 to 50 at %, and having a thickness ranging from 0.2 to 1 nanometers. The multiple reference layers  514  further include the second reference layer  602  formed by a ferromagnetic substitute-type amorphous Co—Fe—X (where X is Zr, Hf or Y) film. The second reference layer  602  may be formed by a Co—Fe—X film including 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 %, where X is Hf, Zr, or Y, and having a thickness ranging from 0.6 to 2 nanometers. The substitute-type amorphous film is formed by adding Zr, Hf or Y atoms, which are much larger than Co and Fe atoms, to occupy substitute sites of a crystalline structure and thus to interfere with the ability of the Co and Fe atoms to crystallize. The multiple reference layers  514  further include a third reference layer  603  formed by a ferromagnetic interstitial-type amorphous Co—Fe—B film. The third reference layer  603  may be formed by a Co—Fe—B film including 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 having a thickness ranging from 1 to 2 nanometers. The interstitial-type amorphous film is formed by adding B atoms, which are much smaller than Co and Fe atoms, to occupy interstitial sites of a crystalline structure and thus to interfere with the ability of the Co and Fe atoms to crystallize. 
     Although  FIG. 5  illustrates a TMR read sensor, those skilled in the art will appreciate that that the concept of multiple reference layers as described in  FIG. 6  may also apply to other CPP read sensors, such as a CPP GMR read sensor. In a CPP GMR read sensor, a nonmagnetic spacer layer replaces the barrier layer  515  in the CPP GMR lower and upper sensor stacks. 
       FIG. 7  is a flow chart illustrating a method  700  of fabricating the TMR read sensor  404  in an exemplary embodiment of the invention. The steps of the flow chart in  FIG. 7  are not all inclusive and may include other steps not shown. Step  702  comprises forming a first or lower ferromagnetic shield  401  on a wafer. The wafer is then smoothened with chemical mechanical polishing (CMP) in order for the lower shield  401  to provide a smooth surface for the TMR read sensor to grow. Step  703  comprises depositing one or more nonmagnetic seed layers  510  on the lower shield  401 . Step  704  comprises depositing an antiferromagnetic pinning layer  511  on the seed layers  510 . The term “on” as used herein may refer to being deposited directly on top of a previously deposited film. Step  706  comprises depositing a ferromagnetic keeper layer  512  on the antiferromagnetic pinning layer  511 . Step  708  comprises depositing a nonmagnetic antiparallel coupling layer  513  on the keeper layer  512 . Step  710  comprises depositing the first reference layer  601  on the antiparallel coupling layer  513 . Step  712  comprises depositing the second reference layer  602  on the first reference layer  601 . Step  714  comprises depositing the third reference layer  603  on the second reference layer  602 . Steps  710 - 714  may be performed in situ in the same module of a sputtering system for improving processing efficiency and TMR properties. Step  716  comprises forming the barrier layer  515  on the third reference layer  603 . Step  718  comprises depositing one or more sense layers  516  on the barrier layer  515 . Step  720  comprises depositing a cap layer  517  on the sense layers  516 . After patterning the TMR read sensor  404  and depositing side oxide and longitudinal bias layers into side regions of the TMR read sensor  404 , step  722  is performed to form the second shield  402  on the cap layer  517 . 
       FIG. 8  illustrates an ABS view of the detailed structure of a TMR read head  800  in an exemplary embodiment of the invention. The TMR read head  800  is a detailed embodiment that is in no way intended to limit the scope of the invention, as exemplary layers of the TMR read head  800  are shown. Thus, those skilled in the art understand that the TMR read head  800  may include other layers in other exemplary embodiments. In  FIG. 8 , the TMR read head  800  includes a TMR read sensor  810  sandwiched between a first or lower shield  831  formed by a 1 μm thick Ni—Fe film and a second or upper shield  832  formed by another 1 μm thick Ni—Fe film. 
     The TMR read sensor  810  includes an electrically insulating barrier layer  819  sandwiched between a lower sensor stack and an upper sensor stack. The barrier layer  819  may be formed by a nonmagnetic oxygen-doped Mg (Mg—O) film in-situ formed in only one module of a sputtering system, as described below. After heavily cleaning a Mg target for 60 seconds with a target power of 600 W, a 0.2 nanometer thick Mg film is DC sputtered in an argon gas of 3×10 −4  torr with a target power of 40 W. A first oxygen treatment in an oxygen gas of 5×10 −7  torr is then applied to the Mg film, resulting in oxygen doping into the Mg film. A 0.4 nanometer thick Mg—O film is then DC sputtered in a mixture of argon and oxygen gases of 3 and 0.4×10 −4  torr, respectively, with a target power of 100 W. A second oxygen treatment in an oxygen gas of 5×10 −7  ton is then applied to the Mg—O film. A 0.2 nanometer thick Mg—O film is then DC sputtered in a mixture of argon and oxygen gases of 3 and 0.1×10 −4  ton, respectively, with a target power of 100 W. A third oxygen treatment in an oxygen gas of 5×10 −5  ton is then applied to the Mg—O film. 
     The lower sensor stack comprises a first seed layer  811  formed by a 3 nanometer thick nonmagnetic Ta film, a second seed layer  812  formed by a 3 nanometer thick nonmagnetic Ru film, a pinning layer  813  formed by a 6 nanometer thick antiferromagnetic 21.7Ir-78.3Mn film (composition in atomic percent), a keeper layer  814  formed by a 2.1 nanometer thick ferromagnetic 77.5Co-22.5 Fe film, and an antiparallel coupling layer  815  formed by a 0.8 nanometer thick nonmagnetic Ru film. The lower sensor stack further comprises a first reference layer  816  formed by a 0.4 nanometer thick ferromagnetic 77.5Co-22.5 Fe film, a second reference layer  817  formed by a 0.6 nanometer thick ferromagnetic 64.6Co-19.7 Fe-15.7Zr film, and a third reference layer  818  formed by a 1 nanometer thick ferromagnetic 51.9Co-34.6Fe-13.5B film. 
     The upper sensor stack comprises a first sense layer  820  formed by a 0.4 nanometer thick ferromagnetic 87.1Co-12.9Fe film, a second sense layer  821  formed by a 2.6 nanometer thick ferromagnetic 71.5Co-7.4Fe-21.1B film, and a cap layer  822  formed by a 6 nanometer thick nonmagnetic Ru film. 
     The first reference layer  816  facilitates the flux-closure structure to exhibit a high antiparallel-coupling field (H APC ), which is defined as a critical field aligning 95% of the saturation magnetization of the flux-closure structure in its direction.  FIG. 9  illustrates magnetic responses of Ta(3)/Ru(3)/77.5Co-22.5Fe(2.1)/Ru(0.8)/X/Ta(3) films (thickness in nanometer), where X is 77.5Co-22.5Fe(2.1), 64.6Co-19.7Fe-15.7Zr (2.4) or 51.9Co-34.6Fe-13.5B (2.4). H APC  values reach 6,299, 4,136 and 6,450 Oe for the Co—Fe(2.1), Co—Fe—Zr(2.4) and Co—Fe—B(2.4), respectively. The Co—Fe first reference layer  816  acts as a buffer layer to achieve a high H APC . While the Co—Fe—B can also act as a buffer layer to achieve a litter higher H APC , it is still suggested to use the Co—Fe first reference layer  816  but with a higher Fe content to achieve an even higher H APC . 
     The second reference layer  817  provides a thermally stable flat surface, thus facilitating the TMR read sensor  800  to exhibit a low ferromagnetic-coupling field (H F ).  FIG. 10  illustrates the ferromagnetic coupling field (H F ) versus the resistance-area product (R J A J ) for the TMR read sensors in an exemplary embodiment of the invention. After annealing for 5 hours at 240 and 280° C., the TMR read sensors with Co—Fe—B and Co—Fe/Co—Fe—Zr/Co—Fe—B reference layers exhibit comparable H F , indicating that the Co—Fe—Zr second reference layer  817  plays the same role as the Co—Fe—B third reference layer  818  in facilitating the Mg—O film to grow as a flat barrier layer. In addition, it is found from cross-sectional transmission electron microscopy that, after annealing for 5 hours at 280° C., the substitute-type amorphous Co—Fe—Zr second reference layer  817  still remains as amorphous, while the interstitial-type amorphous Co—Fe—B third reference layer  818  becomes polycrystalline. It is thus expected that the substitute-type amorphous Co—Fe—Zr reference layer  817  might play a better role than the Co—Fe—B third reference layer  818  in facilitating the Mg—O film to grow as a flat barrier layer in the TMR read sensor annealed at even higher temperatures. 
     The multiple reference layers might induce spin-dependent scattering, thus facilitating the TMR read sensor  800  to exhibit a high TMR coefficient (ΔR T /R J ).  FIG. 11  shows ΔR T /R J  versus the resistance-area product (R J A J ) for the TMR read sensors in an exemplary embodiment of the invention. After annealing for 5 hour at 240° C., the TMR read sensor with the Co—Fe/Co—Fe—Zr/Co—Fe—B reference layers exhibits ΔR T /R J  higher than that with the Co—Fe—B reference layer. In addition, after annealing for 5 hour at 280° C., while ΔR T /R J  for the TMR read sensor with the Co—Fe—B reference layer slightly increases, ΔR T /R J  for the TMR read sensor with the Co—Fe/Co—Fe—Zr/Co—Fe—B reference layers substantially increases. It should be noted that the thickness of the Co—Fe—B reference layer in the prior art must be large enough to exhibit a high ΔR T /R J , while that of the Co—Fe—B third reference layer in the exemplary embodiment of the invention is reduced by 50%, but an even higher ΔR T /R J  is attained. The Co—Fe first reference layer and the Co—Fe—Zr second reference layer, as well as two newly created interfaces in the multiple reference layers, thus also play crucial roles in increasing ΔR T /R J . 
     Although specific embodiments were described herein, the scope of the invention is not limited to those specific embodiments. The scope of the invention is defined by the following claims and any equivalents thereof.