Abstract:
A magnetic sensor utilizing the spin Hall effect to polarize electrons for use in measuring a magnetic field. The sensor eliminates the need for a pinned layer structure or antiferromagnetic layer (AFM layer), thereby reducing gap thickness for increased data density. The sensor includes a non-magnetic, electrically conductive layer that is configured to accumulate electrons predominantly of one spin at a side thereof when a current flows there-through. A magnetic free layer is located adjacent to the side of the non-magnetic, electrically conductive layer. A change in the direction of magnetization in the free layer relative to the orientation of the spin polarized electrons causes a change in voltage output of the sensor.

Description:
FIELD OF THE INVENTION 
     The present invention relates to magnetic data recording and more particularly to a magnetic sensor that utilizes spin Hall effect to provide electron spin polarization. 
     BACKGROUND OF THE INVENTION 
     At the heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of the rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating, but when the disk rotates air is swirled by the rotating disk. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions. 
     The write head includes at least one coil, a write pole and one or more return poles. When a current flows through the coil, a resulting magnetic field causes a magnetic flux to flow through the write pole, which results in a magnetic write field emitting from the tip of the write pole. This magnetic field is sufficiently strong that it locally magnetizes a portion of the adjacent magnetic disk, thereby recording a bit of data. The write field, then, travels through a magnetically soft under-layer of the magnetic medium to return to the return pole of the write head. 
     A magnetoresistive sensor such as a Giant Magnetoresistance (GMR) sensor or a Tunneling Magnetoresistance (TMR) sensor can be employed to read a magnetic signal from the magnetic media. The magnetoresistive sensor has an electrical resistance that changes in response to an external magnetic field. This change in electrical resistance can be detected by processing circuitry in order to read magnetic data from the adjacent magnetic media. 
     As the need for data density increases there is an ever present need to decrease the bit length in order to increase the linear data density. With regard to the magnetic head, this means reducing the shield to shield spacing of the read head (i.e. the read gap thickness). However, physical limitations as well as manufacturing limitations have constrained the amount by which the gap thickness of the magnetic read head can be reduced. For example current magnetic sensors require a pinned layer structure that includes two anti-parallel coupled magnetic layers with a non-magnetic layer sandwiched between them and an antiferromagnetic (AFM) material layer to pin one of the magnetic layers. This pinned layer structure consumes a large amount of the gap budget and greatly impedes efforts to reduce the gap thickness (and consequently the bit length) of the recording system. Therefore, there remains a need for magnetic sensor design that can provide the reduced gap thickness needed for future magnetic recording requirements. 
     SUMMARY OF THE INVENTION 
     The present invention provides a magnetic sensor that includes a magnetic free layer and a non-magnetic, electrically conductive layer formed adjacent to the magnetic free layer. The non-magnetic, electrically conductive layer is configured to accumulate spin polarized electrons at a side thereof based on a spin Hall effect when an electrical current flows through the non-magnetic, electrically conductive layer. 
     The magnetic free layer can be in direct contact with the non-magnetic, electrically conductive layer, which generates an electric potential at the interface between the magnetic free layer and the non-magnetic, electrically conductive layer. This electrical potential changes in response to changes in the direction of magnetization of the magnetic free layer relative to the spin polarization of electrons in the non-magnetic, electrically conductive layer. 
     Alternatively, a thin-non-magnetic barrier layer may be placed between the magnetic free layer and the non-magnetic, electrically conductive layer. The electrical potential across the non-magnetic barrier layer changes in response to changes in the direction of magnetization of the magnetic free layer. The use of a non-magnetic barrier layer can increase the electrical potential difference across the junction formed by the magnetic free layer and the non-magnetic, electrically conductive layer. 
     In another possible embodiment of the invention, a pair of anti-parallel coupled magnetic free layers can be used, with the non-magnetic, electrically conductive layer being located between the magnetic free layers. The use of two magnetic free layers, essentially doubles the signal output, but also increases the gap thickness somewhat. 
     The sensor using the spin Hall effect to polarize electrons greatly reduces the gap thickness by eliminating the need for a thick pinned layer structure and AFM layer. In addition, the invention eliminates any problems associated with loss of pinning, since the sensor eliminates the need for a pinned layer structure. 
     These and other features and advantages of the invention will be apparent upon reading of the following detailed description of preferred embodiments taken in conjunction with the figures in which like reference numerals indicate like elements throughout. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a fuller understanding of the nature and advantages of this invention, as well as the preferred mode of use, reference should be made to the following detailed description read in conjunction with the accompanying drawings which are not to scale. 
         FIG. 1  is a schematic illustration of a disk drive system in which the invention might be embodied; 
         FIG. 2  is an ABS view of a slider illustrating the location of a magnetic head thereon; 
         FIG. 3  is a schematic air bearing surface view of a magnetic read sensor according to an embodiment of the invention; 
         FIG. 4  is a schematic air bearing surface view of a magnetic read sensor according to an alternate embodiment of the invention; 
         FIG. 5  is a schematic air bearing surface view of a magnetic read sensor according to yet another embodiment of the invention; 
         FIG. 6  is a schematic air bearing surface view of a magnetic read sensor according to still another embodiment of the invention; 
         FIG. 7  is a schematic air bearing surface view of a prior art magnetic read sensor; and 
         FIG. 8  is a graph illustrating spin polarization density as a function of conductor thickness. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description is of the best embodiments presently contemplated for carrying out this invention. This description is made for the purpose of illustrating the general principles of this invention and is not meant to limit the inventive concepts claimed herein. 
     Referring now to  FIG. 1 , there is shown a disk drive  100  embodying this invention. As shown in  FIG. 1 , at least one rotatable magnetic disk  112  is supported on a spindle  114  and rotated by a disk drive motor  118 . The magnetic recording on each disk is in the form of annular patterns of concentric data tracks (not shown) on the magnetic disk  112 . 
     At least one slider  113  is positioned near the magnetic disk  112 , each slider  113  supporting one or more magnetic head assemblies  121 . As the magnetic disk rotates, slider  113  moves radially in and out over the disk surface  122  so that the magnetic head assembly  121  can access different tracks of the magnetic disk where desired data are written. Each slider  113  is attached to an actuator arm  119  by way of a suspension  115 . The suspension  115  provides a slight spring force which biases slider  113  against the disk surface  122 . Each actuator arm  119  is attached to an actuator means  127 . The actuator means  127  as shown in  FIG. 1  may be a voice coil motor (VCM). The VCM comprises a coil movable within a fixed magnetic field, the direction and speed of the coil movements being controlled by the motor current signals supplied by controller  129 . 
     During operation of the disk storage system, the rotation of the magnetic 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. The air bearing thus counter-balances the slight spring force of suspension  115  and supports slider  113  off and slightly above the disk surface by a small, substantially constant spacing during normal operation. 
     The various components of the disk storage system are controlled in operation by control signals generated by 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 slider  113  to the desired data track on disk  112 . Write and read signals are communicated to and from write and read heads  121  by way of recording channel  125 . 
     With reference to  FIG. 2 , the orientation of the magnetic head  121  in a slider  113  can be seen in more detail.  FIG. 2  is an ABS view of the slider  113 , and as can be seen the magnetic head, including an inductive write head and a read sensor, is located at a trailing edge of the slider. The above description of a typical magnetic disk storage system and the accompanying illustration of  FIG. 1  are for representation purposes only. It should be apparent that disk storage systems may contain a large number of disks and actuators, and each actuator may support a number of sliders. 
     As discussed above, in order to increase data density it is necessary to decrease the read gap. The read gap is the spacing between magnetic shields of the read sensor and determines the down-track resolution. Currently used magnetic sensors such as giant magnetoresistance sensors (GMR) and tunneling magnetoresistance sensors (TMR) require a pinned layer structure, a free layer structure and a non-magnetic spacer or barrier layer sandwiched between the pinned and free layer structures. 
     An example of such a GMR or TMR sensor is illustrated with reference to  FIG. 7 .  FIG. 7  shows a read element  700  that includes a sensor stack  702  sandwiched between first and second magnetic shields  704 ,  706 , that also function as leads. The distance between the shields  704 ,  706  defines the gap thickness G. The sensor stack includes a pinned layer structure  708 , a free layer structure  710  and a non-magnetic spacer or barrier layer  712  sandwiched between the free layer structure  710  and pinned layer structure  708 . If the read element  700  is a GMR sensor, then the layer  712  will be a non-magnetic electrically conductive layer such as Cu. If the read element  700  is a TMR sensor, then the layer  712  will be a thin, non-magnetic, electrically insulating barrier layer. 
     The free layer  710  has a magnetization that is biased in a direction parallel with the air bearing surface. Magnetic biasing is provided by hard magnetic bias layers  722  located at either side of the sensor stack. The hard bias layers  722  are separated from the sensor stack  702  and from at least one of the shields by a thin electrically insulating layer  724  that can be constructed of a material such as alumina. A capping layer  726  can be provided at the top of the sensor stack  702  to protect the under-lying layers during manufacture and to magnetically de-couple the free layer  710  from the upper shield  706 . 
     The pinned layer structure  708  includes first and second magnetic layers  714 ,  716  that are anti-parallel coupled across a non-magnetic, anti-parallel coupling layer such as Ru  718 . The first magnetic layer  714  is exchange coupled with a layer of antiferromagnetic material (AFM) such as IrMn or PtMn  720 . In order for the magnetic/AFM coupling to exhibit the necessary pinning strength, the AFM layer  720  must be relatively thick. As can be appreciated, the pinned layer structure  708  and AFM  720  consume a large amount of read gap. In addition, in a current-perpendicular-to-the-plane (CPP) GMR sensor a large fraction of the signal is generated and sensed in the bulk of the magnetic layers. As a consequence, the thickness of the magnetic layers (both in the pinned layer structure and free layer) needs to be long enough compared to the spin diffusion length of the magnetic layers to obtain a sizable signal. This poses limitations on the minimum thickness of these layers. 
     In addition, the lithographic control of the reader width is more difficult with thicker structures. Thus, a thinner reader would also beneficial for achieving narrower track-widths which minimizes side reading and enables higher areal density magnetic recording. 
       FIG. 3  shows a schematic view of a magnetic read head  300  according to a possible embodiment of the invention as viewed from the air bearing surface. The read head  300  operates in a fundamentally different manner than the previously described GMR or TMR sensor, and as can be seen, the read head  300  requires no pinned layer structure. Therefore, the sensor  300  can be made to have a much smaller gap thickness G than has previously been possible using conventional GMR or TMR sensor structures. 
     The sensor  300  includes a sensor stack  302  that is sandwiched between first and second magnetic shields  304 ,  306 . The shields  304 ,  306  can be constructed of a material such as Ni—Fe. The distance between the shields determines the read gap thickness G. The sensor stack  302  includes a non-magnetic, conductive layer  308  having a top side  309  and bottom side  311 , which can be constructed of a material such as platinum (Pt), tantalum (Ta), tungsten (W), or any other non-magnetic metal or alloy that has a large spin Hall angle of at least 0.1 and which exhibits large spin Hall effect. The non-magnetic conductive layer is connected with current source  310  that causes an electrical current i to flow through the electrically conductive, non-magnetic layer  308 . 
     A magnetic free layer  312  is formed adjacent to the non-magnetic electrically conductive layer  308 . The magnetic free layer  312  can be formed of one or more layers of magnetic material, such as Co—Fe, Ni—Fe, and or a Heusler alloy. The magnetic free layer  312  has a magnetization that is biased in a direction that is parallel with the air bearing surface as indicated by arrow  314 , but which is free to move in response to a magnetic field (such as from a nearby magnetic media). The magnetization  314  of the magnetic free layer can be biased by a magnetic bias field provided by first and second hard magnetic bias layers  316 ,  318 , which can be constructed of a high magnetic coercivity material such as CoPt or CoPtCr. The hard magnetic bias layers  316 ,  318  are separated from the non-magnetic, electrically conductive layer  308  by thin insulation layers  320 , which can be constructed of a material such as alumina. A hard bias capping layer  322  such as Rh or some other material can be provided over the top of each of the hard bias layers  316 ,  318 . A capping layer  324  such as Ta can be provided over the top of the magnetic free layer  312 . In addition, optional insulation layers  326  can be provided between the non-magnetic, electrically conductive layer  308  and the adjacent shield  304 . The insulation layers  326  can be constructed of a material such as alumina and can be formed with an opening in the region beneath the free layer  312  so that only laterally extending portions are insulated. These insulation layers  326  can be used to minimize the flow of electrons through the shield in a direction parallel with the non-magnetic, electrically conductive layer  308 , and thus maximize the current density through the active non-magnetic layer  308 . The insulation layers  326  are, however, optional. 
     The sensor  300  operates based on the spin Hall effect. When the electrical current i flows through the non-magnetic conductor  308 , the spin Hall effect causes the spins of electrons in the non-magnetic conductor  308  to become polarized as shown. Electrons predominantly of one spin will accumulate at the first (e.g. top) side  309  of conductive layer  308 , whereas electrons of an opposite spin will accumulate at the second (e.g. bottom) side  311 . This is indicated by arrow tail symbols  328  at the top of the layer  308  and arrow head symbols  330  at the bottom of the layer  308 . These electron spins are oriented perpendicular to the current flow i and perpendicular to the page in  FIG. 3 . 
     It can be seen then, that the magnetization  314  of the magnetic free layer  312  is biased in a direction perpendicular to the direction of the spins  328  of the non-magnetic layer adjacent to the free layer  312 . Since the magnetic free layer  312  is adjacent to the non-magnetic layer  308 , there is an interface  332  between the free layer and the non-magnetic layer  308 . Because of the spin polarization of the electrons  328  and magnetization  314  of the free layer  312 , a spin dependent electrical potential exists across the interface  332 . This electrical potential varies in response to changes in the direction of magnetizations  314  of the free layer  312  relative to the spin polarity of the electrons  328  in the non-magnetic conductive layer  308 . If one of the shields  304  is connected to ground  334  and the other shield  306  is connected with a voltage detector  336 , the change in the voltage across the interface  332  can be read as a signal indicating a change in a nearby magnetic field, (such as from a magnetic media). 
     It can be seen, therefore, that the non-magnetic layer  328  provides spin polarized electrons that would otherwise be provided by a pinned layer structure described above with reference to  FIG. 7 . However, the non-magnetic layer  308  can be made much thinner than the thick pinned layer structure  708  and AFM layer  720  of  FIG. 7 , which are required in prior art GMR and TMR sensors. Therefore, the gap thickness G can be greatly reduced. 
     Spin polarization is generated on the surface of a non-magnetic conductor utilizing spin Hall effect. When electrical current of a density j is applied in an x direction through the conducting layer of a thickness t ( FIG. 3 ) measured in the z direction, the spin orbit coupling of electrons scatters y spins of opposite orientation in opposite directions along the z direction. The same is true for z spins along the y direction, but that is not relevant to the sensor  300 . This effect results in a distribution of y spin polarization density along the z direction, with zero value at the conductor center and maxima of opposite polarization direction at the top and the bottom surfaces as shown in  FIG. 8  for the case when t/2L s =10, where L s  is the spin diffusion length in the non-magnetic conductor. It should be pointed out that the surfaces can have polarization directions opposite to that shown in  FIG. 3 , depending on the sign of the spin-orbit coupling material. The physics of the effect is discussed in detail in M. I. Dyakonov and V. I. Perel, JETP Lett. 13, 467 (1971); M. I. Dyakonov, Phys. Rev. Lett. 99 126601 (2007). The effect has been experimentally verified in both metals and semiconductors. The magnitude of the spin polarization density along the z direction is given as: 
                         S   y     ⁡     (   z   )       =         γ   ⁢           ⁢     L   S       D     ⁢       sinh   ⁡     (     z   /     L   S       )         cosh   ⁡     (       t   /   2     ⁢     L   S       )         ⁢     j   e         ,           (     Equation   ⁢           ⁢   1     )               
where γ is the spin Hall angle, L s  is the spin diffusion length, D is the diffusion coefficient and e is the electron charge.  FIG. 8  shows an example of S y (z) distribution through the thickness of the non-magnetic conductor for the case t/2L s =10. When t is much greater than 2L s  (t&gt;&gt;L s ) the maximum S y  can be obtained at the top and the bottom surfaces:
 
     
       
         
           
             
               
                 S 
                 y 
               
               ⁡ 
               
                 ( 
                 
                   
                     ± 
                     t 
                   
                   / 
                   2 
                 
                 ) 
               
             
             = 
             
               
                 
                   γ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     L 
                     S 
                   
                 
                 D 
               
               ⁢ 
               
                 
                   j 
                   e 
                 
                 . 
               
             
           
         
       
     
     When t=2L S  S y (±t/2) is about ¾ of this maximum value. 
     The spin polarization density generated on the surface of a non-magnetic conductor via the spin Hall effect can be transformed into an effective electrical voltage when adjacent to a magnetic layer, such as the magnetic free layer  312  of  FIG. 3 . More specifically, the spin polarization density induces a spin dependent chemical potential (spin accumulation) that given by:
 
μ s   =S   y   /N ( E   F )
 
     where N(E F ) is the electron density of states at the Fermi level. 
     When the conductive layer (e.g. layer  308 ) is in contact with a ferromagnetic layer (e.g. free layer  312 ) the electric potential
 
φ S =( {right arrow over (P)}·ŷ )μ S   /e  
 
is generated across the interface  332  between the conductor  308  and magnetic layer  312 . In the above equation {right arrow over (P)}=P·{circumflex over (m)} is the interfacial spin polarization vector ({circumflex over (m)}—unit vector in the direction of magnetization of the magnetic layer  318 ; ŷ—unit vector in the direction of the reference surface spin polarization density). The potential difference between parallel and anti-parallel orientations of {right arrow over (P)} and ŷ is
 
     
       
         
           
             
               Δ 
               ⁢ 
               
                   
               
               ⁢ 
               V 
             
             = 
             
               2 
               ⁢ 
               
                   
               
               ⁢ 
               P 
               ⁢ 
               
                   
               
               ⁢ 
               
                 
                   μ 
                   S 
                 
                 e 
               
             
           
         
       
     
     For a non-magnetic conductor  308  of a thickness t and taking into account the Einstein diffusion equation, N(E F )=(e 2 Dρ) −1  where ρ is the resistivity of the non-magnetic conductor  308  the above equation becomes
 
Δ V= 2 PL   S   γρj  tan  h ( t/ 2 L   S )
 
     Therefore, it can be seen that the spin Hall effect results in a voltage being generated across the interface between the conductive layer  308  and the magnetic free layer  314 , and that this voltage depends upon the orientation of the magnetization  314  of the free layer  312  relative to the spin polarization  328  at the interface  332 . This voltage potential can then be measured as a signal in response to a magnetic field, such as from a magnetic media. 
     With reference now to  FIG. 4  a magnetic sensor  400  according to another possible embodiment of the invention is described. The sensor  400  is similar to the sensor  300  of  FIG. 3 , except that a non-magnetic, electrically insulating barrier layer  402  is inserted between the non-magnetic, electrically conductive layer  308  and the magnetic free layer  312 . 
     The presence of the barrier layer  402  may maximize the parameter P in the above equations. The barrier layer also maximizes the current density near the surface of the non-magnetic, electrically conductive layer  308  and thus maximizes reference spin polarization density. This embodiment, having a barrier layer  402 , functions similarly to a tunneling (TMR) sensor, with major difference that electrical current does not flow through the barrier layer  402  and the free layer  312 . This can provide improved reliability and durability over TMR sensors. The voltage across the barrier layer  402  changes depending upon the orientation of the magnetization  314  of the free layer  312  relative to the polarization of spin  328  in the conductive layer. In this embodiment, the spin polarization  328  is provided by the spin Hall effect, rather than by passing a current through a magnetic pinned layer structure as would be the case in a typical TMR sensor. 
       FIG. 6  shows an embodiment of a sensor  600  that is similar to that of  FIG. 4 , except that it requires no hard bias structures at the sides of the free layer  312 . Instead, the shield  306  extends down adjacent to the sides of the free layer  312  to provide a side shielding function. In a sensor where hard bias layers are not needed at the sides of the free layer  312 , the presence of such side shielding helps to better define and reduce the cross-track resolution of the sensor  600 . 
     With reference now to  FIG. 5 , a magnetic sensor  500  according to yet another embodiment of the invention is described. This embodiment includes a pair of anti-parallel coupled magnetic free layers  502 ,  504  located on opposite one another across the non-magnetic, electrically conductive layer  308 . The sensor can also include a capping layer  324  at the top of the one of the magnetic layers  504  and a seed layer  506  at the bottom of the other free layer  502 . The seed layer  506  can help to initiate a desired grain structure in the above deposited free layer  502 . 
     The free layers  502 ,  504  have magnetizations  510 ,  512  that are oriented parallel with the air bearing surface and anti-parallel to one another. The anti-parallel coupling of the layers  502 ,  504  can come from magneto-static coupling or exchange coupling between the magnetic free layers  502 ,  504 . Furthermore, the magnetic layers  502 ,  504  can be constructed such that magnetic anisotropy in the magnetic layers  502 ,  504  causes them to naturally align in a direction parallel with the air bearing surface as shown. While the magnetizations  510 ,  512  tend to align in this anti-parallel manner, they are free to move in response to a magnetic field, such as from a nearby magnetic media. When the magnetizations  510 ,  512  move they do so in a scissor-like fashion. For instance they could both move into the plane of the page or out of the plane of the page in unison in  FIG. 5 . 
     Because of the anti-parallel coupling and shape enhanced magnetic anisotropy a hard bias structure such as that described above may be unnecessary in this embodiment. Therefore, the areas laterally beyond the magnetic free layers  502 ,  504  (between each shield  304 ,  306  and the conductive layer  308 ) can be filled with a non-magnetic, electrically insulating material  514 . 
     Because there are two free layers  502 ,  504 , there are also two interfaces  516 ,  518  between the conductive layer  308  and each of the magnetic free layers  502 ,  504 . In  FIG. 5  it can be seen that, while the magnetizations  510 ,  512  are in opposite directions, the spin polarizations  328 ,  330  at the top and bottom of the conductor  308  are also in opposite directions. Therefore, the signal provided at both of the interfaces  514 ,  516  are additive. In this way, the output of the sensor can be effectively doubled. 
     It should be pointed out, also, that an embodiment (not shown) could be provided that includes features of both the embodiment of illustrated with reference to  FIG. 5  and the embodiment illustrated with reference to  FIG. 4 . That is to say, an embodiment could be constructed with a pair of free layers  502 ,  504  as described with reference to  FIG. 5 , but also having a thin non-magnetic barrier layer (similar to barrier layer  402  of  FIG. 4 ) between each of the magnetic free layers  502 ,  504  and the conductive layer  308 . 
     While various embodiments have been described above, it should be understood that they have been presented by way of example only and not limitation. Other embodiments falling within the scope of the invention may also become apparent to those skilled in the art. Thus, the breadth and scope of the invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.