Abstract:
A magnetic sensor that utilizes Rashba effect to generate spin polarization. The sensor eliminates the need for a pinned layer structure and therefore, greatly reduces the gap thickness of the sensor allowing for greatly improved data density. The sensor includes a two dimensional conductor adjacent to a magnetic free layer, that can also be separated from the free layer by a non-magnetic, electrically insulating barrier layer and that can also be constructed with or without side shields. A current flow through the two-dimensional conductor in a direction parallel with the air bearing surface causes a spin polarization oriented perpendicular to the air bearing surface. The voltage output of the sensor changes with changing magnetization direction of the free layer relative to spin polarization in the two dimensional conductor.

Description:
FIELD OF THE INVENTION 
     The present invention relates to magnetic data recording and more particularly to a magnetic sensor that utilizes Rashba spin orbit interaction in a two dimensional conductor for polarization of electron spins. 
     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 Tunnelling 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 (SS) 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 a relatively thick 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 two-dimensional conductor formed on an electrically conductive substrate, and a magnetic free layer formed adjacent to the two-dimensional conductor. 
     The sensor may also include a non-magnetic, electrically insulating barrier layer sandwiched between the magnetic free layer and the two-dimensional conductor or may be formed so that the magnetic free layer is in direct contact with the two-dimensional conductor. The two dimensional conductor can be formed on an electrically conductive substrate having an electrical conductivity that is lower than that of the two dimensional conductor. 
     When a current flows through the two-dimensional conductor, a spin polarization of electrons is generated in the two dimensional conductor. This spin polarization can be used to induce a voltage across the junction between the magnetic free layer and the two-dimensional conductor. This voltage varies with magnetization orientation of the free layer relative to the spin polarization in the two dimensional conductor. By eliminating the need for a pinned layer and associated AFM layer, the gap thickness of the sensor can be greatly reduced. 
     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 an alternate embodiment of the invention; 
         FIG. 6  is a schematic illustration of the spin polarization generated by the Rashba effect in a two dimensional conductor; 
         FIG. 7  is a graphical illustration of a magnitude of energy splitting of spin sub-band density of states in a two dimensional conductor induced by a Rashba effect; and 
         FIG. 8  is a schematic air bearing surface view of a prior art magnetic read sensor. 
     
    
    
     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. Magnetoresistive sensors such as GMR and TMR sensors 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 prior art GMR or TMR sensor is illustrated with reference to  FIG. 8 .  FIG. 8  shows a read element  800  that includes a sensor stack  802  sandwiched between first and second magnetic shields  804 ,  806 , that also function as leads. The distance between the shields  804 ,  806  defines the gap thickness G. The sensor stack includes a pinned layer structure  808 , a free layer structure  810  and a non-magnetic spacer or barrier layer  812  sandwiched between the free layer structure  810  and pinned layer structure  808 . If the read element  800  is a GMR sensor, then the layer  812  will be a non-magnetic electrically conductive layer such as Cu. If the read element  800  is a TMR sensor, then the layer  812  will be a thin, non-magnetic, electrically insulating barrier layer, such as MgO. 
     The free layer  810  has a magnetization that is biased in a direction parallel with the air bearing surface. Magnetic biasing is provided by hard magnetic bias layers  822  located at either side of the sensor stack. The hard bias layers  822  are separated from the sensor stack  802  and from at least one of the shields by a thin, electrically insulating layer  824  that can be constructed of a material such as alumina. A capping layer  826  can be provided at the top of the sensor stack  802  to protect the under-lying layers during manufacture and to magnetically de-couple the free layer  810  from the upper shield  806 . 
     The pinned layer structure  808  includes first and second magnetic layers  814 ,  816  that are anti-parallel coupled across a non-magnetic, anti-parallel coupling layer such as Ru  818 . The first magnetic layer  814  is exchange coupled with a layer of AFM material such as IrMn or PtMn  820 . In order for the magnetic/AFM coupling to exhibit the necessary pinning strength, the AFM layer  820  must be relatively thick. As can be appreciated, the pinned layer structure  808  and AFM  820  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 to achieve with thicker structures. Thus, thinner readers are also beneficial for achieving narrower track-widths which minimizes side reading and enables higher cross-track resolution in magnetic recording. 
       FIG. 3  is a schematic illustration of a magnetic read sensor  300  according to an embodiment of the invention as viewed from the air bearing surface that utilizes Rashba effect to generate spin polarization. The sensor  300  includes a sensor stack  302  sandwiched between first and second (or upper and lower) electrically conductive, magnetic shields  304 ,  306 . The electrically conductive, magnetic shields  304 ,  306  can be constructed of a material such as Ni-Fe. 
     The sensor stack  302  includes a two-dimensional conductor layer  308  formed on conductive substrate  310 . First and second electrically conductive leads  314  can be connected with opposite ends of the two-dimensional conductor layer  308  and conductive substrate  310 . The leads  314  are connected with an electrical current source for generating an electrical current through the substrate  310  and two-dimensional conductor  308 . An insulation layer  332  can be disposed between the substrate layer  310  and the adjacent shield  304  in order to prevent current shunting through the shield  304 . The insulation layer  332  can, however, be formed with an opening to allow the detection of voltage signal between the shields  304 ,  306  as will be seen below. 
     The substrate  310  has an electrical conductivity that is significantly lower than that of the two-dimensional conductor  308  to prevent shunting of electrons through the substrate and to ensure that a predominant amount of the electrons travel through the two dimensional conductor  308 . Also, to function as a two dimensional conductor, the layer  308  can be constructed very thin, preferably having a thickness no greater than 2 nm, and more preferably less than that. The two dimensional conductor can be constructed of aluminum (Al), copper (Cu), silver (Ag), gold (Au), bismuth (Bi), or lead (Pb) and the substrate  310  can be constructed of tungsten (W), platinum (Pt), silicon (Si) or germanium (Ge). However, any combination of conductive materials that preserves two-dimensional transport through layer  308  and provides strong-enough Rashba spin-orbit interaction of charge carriers in that layer should satisfy. Here, strong-enough means that the energy of spin splitting due to Rashba spin-orbit interaction should be significantly higher than thermal energy at room temperature. The electrical conductivity and thickness of the substrate  310  should be sufficiently low to prevent significant parallel conduction that may shunt current through the two dimensional conductor layer  308 . 
     A magnetic free layer  316  is formed over and in direct contact with the two-dimensional conductor  308 . The magnetic free layer  316  has a magnetization that is biased in a direction that is substantially parallel with the ABS as indicated by arrow  318 , but which is able to move in response to an external magnetic field, such as from a magnetic media. Magnetic biasing of the magnetization  318  of the magnetic free layer  316  can be provided by first and second hard magnetic bias layers  320 , which provide a magnetic bias field in a direction parallel with the ABS. The hard bias layers  320  can be separated from the magnetic free layer  316 , adjacent shield  306  and two dimensional conductor  308  by thin electrically insulating layers  322  to prevent the shunting of electrical sensor current through the hard bias layers  320 . 
     The electrically conductive substrate  310 , two dimensional conductor  308  and magnetic free layer  316  form a quantum well structure that preserves two-dimensional conductivity and Rashba spin orbit interaction in the layer  308 . 
     The magnetic free layer  316  has a width W that determines the reader-width of the magnetic sensor  300  for purposes of magnetic data recording. A non-magnetic capping layer  324  can be formed at the top of the magnetic free layer  324  to protect the magnetic bias layer  316  during manufacturing and to magnetically de-couple the magnetic free layer  316  from the adjacent magnetic shield  306 . 
     The sensor described above can achieve shield to shield spacing significantly narrower than that possible with present magnetoresistive sensor technologies. For example, having a 5 nm thick substrate  310  and a two dimensional conductor  308 , a 3 nm thick magnetic free layer  316  and a 2 nm thick capping layer  324 , the total read gap thickness can be 10 nm. 
     The two-dimensional conductor  308  has a large Rashba energy for generating reference spin polarization density in response to an electrical current density j. The spin polarization is indicated by arrow tail symbols  326 . The spin polarization lies in the plane of the two-dimensional conductor  308  and is directed perpendicular to the current direction and thus perpendicular to the ABS plane (i.e. in the transverse direction, similar to the spin polarization generated by a magnetic pinned layer structure in GMR or TMR magnetic sensor). The spin polarization, however, can be oriented in one of two directions, depending on the type of charge carriers (electrons or holes) and sign of the Rashba spin orbit interaction (positive or negative). 
     The polarization of charge carriers  326  in the two dimensional conductor  308  and the magnetization  318  result in a voltage output that can be measured between the magnetic shields  304 ,  306  (e.g. between lead  328  and ground  330 ), which varies with relative orientation of the magnetization  318  with respect to the direction of spin polarization  326 . In a quiescent state (e.g. magnetization  318  parallel with the ABS) the output voltage is zero. However, movement of the magnetization  318  of the free layer  316  in response to a magnetic field, toward or away from aligning with the transverse spin orientation  308  induces an electrical voltage across the shields  304 ,  306  that will be proportional to sin θ, where θ is the in plane angle between the magnetization direction  318  of the magnetic free layer  316  and the quiescent, longitudinal direction shown in  FIG. 3 . 
       FIG. 4  shows a schematic, ABS view of a magnetic sensor  400  according to an alternate embodiment of the invention. The sensor  400  is similar to the sensor  300  described above with reference to  FIG. 3 , except that the sensor  400  includes a thin, non-magnetic, electrically insulating barrier layer  312  located between the free layer  316  and the two dimensional conductor  308 . The thin, electrically insulating barrier layer  312  is formed on the two-dimensional conductor layer  308  in order to ensure that the layer  308  functions as a two-dimensional conductor and to preserve the Rashba spin-orbit interaction of the two-dimensional conduction states. The layer can also help in providing higher voltage across the junction between the free layer  316  and the two dimensional conductor  308 . 
       FIG. 5  shows a schematic, ABS view of a magnetic sensor  500  according to another embodiment of the invention. The sensor  500  is similar to the sensor  400  of  FIG. 3 , except that it incorporates top side shield and does not require the bias layers  320  of  FIG. 4 . In this embodiment, magnetic biasing of the magnetization  318  is provided by a side shield  306 . The sensor  500  can include insulation layers  502  at either side of the magnetic free layer  316  and capping layer  324 . Although the sensor  500  is shown including the barrier layer  312 , the sensor can be constructed without the barrier layer  312  and with the free layer  316  being in direct contact with the two-dimensional conductor  308  (as with the sensor  300  of  FIG. 3 ). 
     The physics of the Rashba effect and resulting spin polarization in the two dimensional conductor as utilized in the above described sensors  300 ,  400 ,  500  ( FIGS. 3, 4 and 5 ). Rashba effect refers to the lifting of spin degeneracy of the electronic states via spin-orbit interaction in two-dimensional electron (or hole) systems such as quantum wells, surfaces or interfaces. With reference to  FIG. 6 , the spin orbit interaction arises from the intrinsic breaking of the inversion symmetry which results in electric field E perpendicular to a two dimensional electron plane  602 , shown in the x-y direction in  FIG. 5 , and thus transforms into an effective momentum-dependent in-plane magnetic field B R , called a Rashba field, which Zeeman splits the electron density of states near the Fermi level.  FIG. 7  graphically shows the Fermi level energy E F  and related Rashba energy E R  between two spin sub-band densities of states. The magnitude of the effect is quantified by the Rashba energy E R  which refers to the difference between the potential energy of spin-up and spin-down electrons  704 ,  706  induced by the Rashba effect. When electrical current density j ( FIG. 6 ) is applied in the x-direction through a 2 dimensional conductor  602  with Rashba spin-split density of states, the y spin polarization density is generated in the conductor plane. The effect can exist in both metals and semiconductors or their layered structures. The magnitude of the spin polarization density is given as: 
     
       
         
           
             
               
                 
                   
                     S 
                     y 
                   
                   = 
                   
                     
                       
                         E 
                         R 
                       
                       
                         2 
                         ⁢ 
                         
                           v 
                           F 
                         
                         ⁢ 
                         
                           E 
                           
                             F 
                             ⁢ 
                             
                                 
                             
                           
                         
                       
                     
                     ⁢ 
                     
                       j 
                       e 
                     
                   
                 
               
               
                 
                   ( 
                   
                     equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
     where v F  and E F  are the electron Fermi velocity and Fermi energy, respectively, and e is the electron charge. 
     The spin polarization density generated in a two-dimensional conductor via Rashba effect can be transformed into an electrical voltage using a ferromagnetic layer adjacent to the two dimensional conductor. The spin polarization density induces spin dependent chemical potential:
 
μ s   =S   y   /N ( E   F )  (equation 2)
 
     where N(E F ) is the electron density of states at the Fermi level. When the conductor surface is contacted by a ferromagnetic layer FM an electrical potential
 
φ S =( {right arrow over (P)}·ŷ )μ S   /e   (equation 3)
 
is generated across the FM/NM interface. In the above equation {right arrow over (P)}=P·{circumflex over (m)} is the interfacial spin polarization vector, {circumflex over (m)} is the unit vector in the FM magnetization direction; ŷ is the 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               (     equation   ⁢           ⁢   4     )               
Using equations 1, 2 and 4, and the relations N(E F )E F =n/2 and j=nev d  for a two-dimensional conductor (n and v d  are the electron density and drift velocity, respectively) one obtains:
 
     
       
         
           
             
               
                 
                   
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     V 
                   
                   = 
                   
                     2 
                     ⁢ 
                     P 
                     ⁢ 
                     
                       
                         E 
                         R 
                       
                       e 
                     
                     ⁢ 
                     
                       
                         v 
                         d 
                       
                       
                         v 
                         F 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     5 
                   
                   ) 
                 
               
             
           
         
       
     
     In principle, the ratio of V d /V f  can be of the order of 0.1 to 1 in high mobility two-dimensional conductors, while E R  of up to 300meV and P values of up to 0.4 at room temperature have been reported. In the sensors  400 ,  500  of  FIGS. 4 and 5 , the presence of the barrier layer  312  may help to maximize the parameter P in equation 5 above. Thus, the generation of output voltage signals of about 10-100 mV should be possible with sensors as described above with reference to  FIGS. 3, 4 and 5 . 
     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.