Abstract:
A Lorentz magnetoresistive sensor that employs a gating voltage to control the momentum of charge carriers in a quantum well structure. A gate electrode can be formed at the top of the sensor structure to apply a gate voltage. The application of the gate voltage reduces the momentum of the charge carriers, which makes their movement more easily altered by the presence of a magnetic field, thereby increasing the sensitivity of the sensor.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to magnetoresistive sensors employing Lorentz forces for magnetoresistive effect, and more particularly to the use of gating in such a sensor to increase magnetoresistive sensitivity and spatial resolution. 
     BACKGROUND OF THE INVENTION 
     The heart of a computer&#39;s long term memory 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 a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head and the pole piece layers are connected at a back gap. Current conducted to the coil layer includes a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write sap at the ABS for the purpose of within, the aforementioned magnetic impressions in tracks on the involving media, such as in circular tracks on the aforementioned rotating disk. 
     For some time a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a free layer. First and second leads are connected to the spin valve sensor for conduction a sense current therethrough. The magnetization of the pinned layer is oriented generally perpendicular to the air bearing surface (ABS) and the magnetic moment of the free layer is oriented generally parallel to the ABS, but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer. 
     The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos ⊖, where ⊖ is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals. Another sensor employing the spin valve structure and using tunneling (i.e. A MAGNETIC TUNNEL JUNCTION) has also been recently employed or sensing magnetic fields from the rotating magnetic disk. 
     The drive for ever increased data rate and data capacity has, however, lead researchers to search for new types of magnetoresistive sensors, capable of increased sensitivity and high signal to noise ratio at decreased track widths. One type of magnetoresistive sensor that has been proposed is a magnetoresistive sensor that employs Lorentz forces to alter the path of an electrical current in a magnetoresistive sensor. Such sensors have been referred to as Lorentz magnetoresistive sensors. An advantage of Lorentz magnetoresistive sensors is that the active region of the sensor is constructed of non-magnetic semiconductor materials, and does not suffer from the problem of magnetic noise that exists in giant magnetoresistive sensors (GMR) and tunnel valves, both of which use magnetic films in their active regions. 
     A Lorentz magnetoresistive sensor can include a pair of voltage leads and a pair of current leads in contact with one side of the active region and an electrically conductive shunt in contact with the other side of the active region. In the absence of an applied magnetic field, sense current conducted through the current leads passes into the semiconductor active region and is shunted through the shunt. When an applied magnetic field is present, current is deflected from the shunt and passes primarily through the semiconductor active region. The change in electrical resistance due to the applied magnetic field is detected across the voltage leads. An EMR sensor is described by T. Zhou et al., “Extraordinary magnetoresistance in externally shunted van der Pauw plates”, Appl. Phys. Lett., Vol. 78, No. 5, 29 January 2001, pp. 667-669. 
     SUMMARY OF THE INVENTION 
     The present invention provides a Lorentz magnetoresistive sensor that includes a quantum well structure formed between first and second semiconductor layers. An insulation layer is formed over a portion of the second semiconductor layer at a side opposite the quantum well structure, and a gate electrode is formed over the insulation layer such that the insulation layer is sandwiched between the second semiconductor layer and the gate electrode. 
     A voltage source can be connected with the gate electrode by an electrically conductive lead. The application of a gale voltage to the gate electrode can be used to form an electrical field in the second semiconductor. This can be used to reduce and control the momentum of charge carriers in the quantum well structure. This reduction and control of charge carrier momentum advantageously increases the sensitivity of the Lorentz magnetoresistive sensor, requiring less magnetic field to deflect the path of charge carriers in the quantum well 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; 
         FIG. 3  is schematic isometric view of an EMR device according to an embodiment of the invention, shown with voltage and current leads removed for clarity; and 
         FIG. 4  is a top-down, view of the sensor of  FIG. 3 , shown with voltage and current leads attached. 
     
    
    
     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  which could embody 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  may 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  113 . 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 . The read portion of the head  121  call be an Extraordinary Magnetoresistive (EMR) sensor such as will be described below. 
       FIG. 2  shows an enlarged view of the slider  113  having a magnetic head  113  formed thereon. The slider  113  is shown as viewed from the air bearing surface (ABS). The magnetic head  121  is formed on the trailing edge  202  of the slider  113 , and includes a write head and a Lorentz magnetoresistive sensors which will be described in greater detail below. 
     With reference now to  FIG. 3 , a Lorentz magnetoresistive sensor  300  for use in a magnetic head  121  ( FIG. 2 ) is shown according to a possible embodiment of the invention. The Lorentz magnetoresistive sensor  300  includes a mesa structure  301  formed on a substrate  304  such as a wafer. The mesa structure  301  includes a heterostructure  302  that includes a quantum well  308  such as a 2 Dimensional Electron Gas (2-DEG) or hole gas or other conducting layer sandwiched between first and second layers of higher resistance semiconductor layers  306 ,  310 . The mesa structure  301  can also include a buffer layer  312  underlying the structure  302 . A capping layer  314  may also be provided at the top of the mesa structure to protect the underlying layers  306 - 310  of the mesa structure  301 . The Lorentz magnetoresistive sensor  301  also includes an electrically conductive shunt structure  316 , the top end of which is shown, but which extends downward into the mesa structure  301  as will be described in greater detail herein below. 
     The Lorentz magnetoresistive sensor  300  may include a structure  302  that is a III-V heterostructure formed on a semiconductor substrate  304  such as GaAs. However, the Lorentz magnetoresistive sensor described in this invention is not restricted to II-V semiconductor materials. For example, it may also be formed on the basis of silicon or germanium. The heterostructure  302  can include a first layer  306  of semi-conducting material having a first band-gap, a second layer  308  of semi-conducting material formed on the first layer  306  and having a second bandgap that is smaller than that of the first layer  306 , and a third semi-conducting layer  310  of semi-conducting material formed on top of the second layer  308  and having a third band gap that is greater than the second band gap. The materials in the first and third layers  306 ,  310  may be similar or identical. An energetic potential well (quantum well) is created by the first, second and third semi-conducting material layers  306 ,  308 ,  310  due to the different band-gaps of the different materials. Thus, carriers can be confined inside layer  308 , which is considered the Lorentz magnetoresistive active film in the sensor  300 . Because the layer  308  is extremely thin, and because electrons travel very fast and at very long distances without scattering, this layer  308 , forms what has been referred to as a 2 Dimensional Electron Gas (2DEG) or 2 Dimension Hole Gas. 
     The first layer  306  is typically formed on top of a buffer layer  312  that may be one or more layers. The buffer layer  312  comprises several periods of a superlattice structure that functions to prevent impurities present in the substrate from migrating into the functional layers  306 ,  308 ,  310 . In addition, the buffer layer  312  is chosen to accommodate the typically different lattice constants of the substrate  304  and the functional layers of the heterostructure  302  to thus act as a strain relief layer between the substrate and the functional layers. 
     One or more doped layers can be incorporated into the semiconducting material in the first layer  306 , the third layer  310 , or both layers  306  and  310 , and spaced apart from the boundary of the second and third semiconducting materials. Dopants are also sometimes incorporated in layer  312  or  314  at locations near layers  306  or  310 . The doped layers provide electrons (if n-doped) or holes (if p doped) to the quantum well. The electrons or holes are concentrated in the quantum well in the form of a two dimensional electron-gas or hole-gas, respectively. 
     The layers  306 ,  308 ,  310  may be a Al 0.09 In 0.91 Sb/InSb/Al 0.09 In 0.91 Sb or AlSb/InAs/AlSb heterostructure grown onto a semi-insulating GaAs substrate  304  with a buffer layer  312  in between. InSb and InAs can be narrow band-gap semiconductor. Narrow band-gap semiconductors typically have a high electron mobility, since the effective electron mass is greatly reduced. Typical narrow band-gap materials are InSb and InAs. For example, the room temperature electron mobility of InSb and InAs are 70,000 cm 2 /Vs and 35,000 cm 2 /Vs, respectively. 
     The bottom Al 0.09 In 0.91 Sb or GaAlSb layer  306  formed on the buffer layer  312  has a thickness in the range of approximately 1-3 microns and the top Al 0.09 In 0.91 Sb or AlSb layer  310  has a thickness in the range of approximately 2 to 1000 nm. The doping layers incorporated into layers  306 ,  310  have a thickness from one monolayer (delta-doped layer) up to 10 nm. The doping layer is spaced from the InSb/Al 0.09 In 0.91 Sb boundaries of first and second or second and third semi-conducting materials by a distance of 10-300 Angstrom. N-doping is preferred, since electrons typically have higher mobility than holes. The typical n-dopant is silicon with a concentration of about 1×10 19 /cm 3 . The deposition process for the heterostructure  302  is preferably molecular-beam-epitaxy, but other epitaxial growth methods can be used. 
     A capping layer  314  is formed over the heterostructure  302  to protect the device from corrosion. The capping layer  314  is formed of an insulating material such as oxides or nitrides of aluminum or silicon (e.g., Si 3 N 4 , Al 2 O 3 ) or a non-corrosive semi-insulating semiconductor. The layers  312 ,  306 ,  308 ,  310 ,  314  together form a structure that can be referred to as a mesa structure  301 . 
     As can be seen, in  FIG. 3 , the mesa structure  301  can be configured with cutout notches  326  formed in a side of the mesa structure. The notches provide a contact region for electrical leads that are not shown in  FIG. 3 , but which will be shown and described in subsequent figures. The leads and also an optional fill layer have been removed from  FIG. 3 , in order to more clearly show the mesa structure  300  and associated notches  326 . As can be seen, the notches  326  extend from the top of the mesa structure  301  to a point beneath the quantum well layer  308 , also referred to as the magnetically active region or 2-DEG  308 , and preferably extend beyond the entire heterostructure  302 . As will be seen below, the notches  326  are optional (e.g. they can be configured with a depth from the side that can vary down to zero). However, the presence of the notches increases the surface area over which the leads (not shown in  FIG. 3 ) can make contact. 
       FIG. 4 , shows top down view of the sensor of  FIG. 3 .  FIG. 4  therefore, shows the mesa structure  301  as well as the shunt  316  passing there-through. A set of electrically conductive leads  402 ,  404 ,  406 ,  408  extend into the notches formed in the mesa structure. The leads  402 ,  404 ,  406 ,  408  can be constructed of an electrically conductive material such as, for example, Au or AuGe, and can be constructed of the same material as the shunt structure  316 . 
     With continued reference to  FIG. 4 , two of the leads, such as  404  and  408 , are current leads for supplying a sense current to the sensor  300  and, more specifically, to the 2-DEG layer  308 . Therefore, lead layer  408  can be a first current lead II, and lead layer  404  can be a second current lead layer  12 . Lead layers  406  and  402  call provide voltage leads for measuring a change in resistance associated with the presence of a magnetic field, as will be described below. Therefore, lead layer  406  can provide a first voltage lead V 1  and lead layer  402  can provide a second voltage lead V 2 . 
     As mentioned above, the current leads  408 ,  404  provide a sense current through the sensor  300 . In the absence of a magnetic field, a majority of this current (indicated by dashed line  410 ) passes from the first current lead  408 , through a portion of the active layer  308  ( FIG. 3 ) to the shunt structure  316 . This current then passes through the shunts structure  316  with a relative low resistance before passing back through the active layer  308  back to the second current lead  404 . However, in the presence of magnetic field H oriented generally perpendicular to the plane of the active layer  308 , a relatively larger portion of the current is deflected from the shunt  316  to travel through the 2-DEG layer  308  as indicated by clashed line  412 . This detection of carriers is due to Lorentz forces of the magnetic field on the charge carriers. The active layer  308  form is a quantum well, which traps carriers in this thin layer  308 , between the layers  306 ,  308 . This active layer  308 , therefore, has a much higher resistance than the shunt layer. Therefore, when more of the carriers are forced into the active layer  308  as indicated by line  412 , the electrical resistance across the voltage leads  402 ,  406  increases. This increased the electrical resistance can be detected by measuring a voltage across the voltage leads  406 ,  402 . 
     As can be seen in  FIG. 4 , the leads  402 - 408  can extend into the notches  326 . Therefore, the perimeter of contact between a lead and the notch  326  is essentially twice the depth of the notch plus the width of the lead. This increases the electrical contact area, advantageously reducing resistance between the leads  402 - 408  and the active layer  308 . Perhaps more importantly, the leads  402 - 408  can be self aligned with the notches  326  and also with the shunt, by a single photolithographic step, as will be explained more fully below. 
     One factor that greatly affects the performance of a Lorentz magnetoresistive sensor such as the sensor  300  described above is the momentum of charge carriers in the active layer  308  and shunt  316 . In a Lorentz magnetoresistive sensor the trajectories of carriers is affected by the magnetic field through the Lorentz force: F=qv×B, where v is the velocity vector and B is the magnetic field vector. The change in trajectories leads to different trajectories that are detected either through a voltage induced by the charge on the carriers or by counting their current down separate paths directly. The Lorentz force leads to an acceleration “a” of the carrier perpendicular to both the velocity and magnetic field, and is of magnitude a=F/m*=qvB/m* where m* is the carrier effective mass. Thus to optimize the Lorentz magnetoresistance signal one must consider both the carrier velocity av and its mass m* (or equivalently velocity v and momentum p=m*v). 
     The relationship between Lorentz force and deflection in a spatially localized magnetic field can be defines as F=qvB=ma. The time in the device is t=L/v, where L is the device length over which the magnetic field is applied and v is velocity in the x-direction. The distance traveled in a y direction during that portion of the trajectory in the magnetic field is δy=at 2 /2=(qvB/m) L/v) 2 =qBL 2 /2m*v. The angle is approximately ω=δy/L=qBL/2m*v. For a device in a 0.05 T field this is ω=1.6E-19©5E-2[T] 20E-9[m]/2.6E02 9E02[kg] 1E6m/s]2=3.4E-3 radians=3.0 degrees. Thus a reduction in p=m*v of a factor of 3-10 would greatly increase the deflection of carriers to angles that could be easily distinguished in practical devices. In the quantum regime, the relationship ω=δy/L=qBL/2m*v still holds where the momentum p=m*v is replaced with P=h/2πλ, where λ is the electron wavelength. 
     Therefore, if the momentum of the charge carriers is too high, then a higher magnetic field H is needed to deflect the charge carriers out of the shunt  316  and into the active area  302 . Also, the higher velocity associated with a higher momentum means that the charge carriers spend less time in the presence of (and under the influence of) the magnetic field H. On the other hand, a charge carrier momentum that is to low means that fewer charge carriers are available to provide a magnetoresistive effect. It would be very advantageous, therefore, to be able to control the momentum of charge carriers, whether electrons or holes, to maximize the magnetoresistive sensitivity of the sensor  300 . 
     To this end, with reference to  FIGS. 3 and 4 , a gate electrode  328  is provided at the top of the mesa structure  301 . The gate electrode is preferably constructed of an electrically conductive metal and is separated from the mesa structure by an insulation layer  330 . The gate electrode can be constructed of a conducting material such as Au, Al, Pd or titanium nitride, and the gate insulator can be constricted of an electrically insulating material such as aluminum oxide, silicon oxide, hafnium oxide or some other suitable material. 
     A gate voltage can be applied to the gate electrode by a voltage lead represented schematically as voltage lead line  332 . Applying a voltage to the gate electrode results in an electric field between the gate electrode  328  and the active layer  308 , which decreases the momentum of the charge carriers flowing through the sensor  302  (e.g. as represented by lines  410 ,  412 . The greater the voltage applied the greater the reduction in carrier momentum. Therefore, in order to actively control the momentum of charge carriers in the active layer  308 , the lead  332  can be electrically connected with a gate voltage source  334  that is preferably controllable to adjust the amount of gate voltage applied to the gate electrode  328 . 
     In order to localize the sensitivity of the sensor, the gate electrode preferably covers an area just adjacent to the voltage electrodes as shown in  FIGS. 3 and 4 . This advantageously prevents the side reading from nearby magnetic sources, for example in nearby magnetic recording bits. This greatly increases the spatial resolution of the sensor  300 , reducing the size of bits that can be accurately read. This of course allows an advantageous increase in data density. It is desirable to add the gate electrode where magnetic field most affects the trajectories of electrons, or where one wants the slowing of electrons to induce a lengthening of the time spent in the magnetic field, thereby increasing the magnetic field effect. It is also possible for the gate to be located below the device, for example in an additional conducting layer or 2DEG, or for the entire device to be back gated. 
     There are a variety of carrier velocities that might be appropriate for an active layer  308  constructed of, for example, InAs, InSb or graphene. They can range from 5E7 cm/s to 1E8 cm/s. Some values that have been measured for un-gated materials are: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 InAs, thermal velocity 
                 7E7 cm/s 
               
               
                   
                 In As, measured saturation velocity 
                 5E7 cm/s 
               
               
                   
                 InAs, theoretical saturation velocity 
                 1E8 cm/s 
               
               
                   
                 Graphene, theoretical saturation velocity 
                 1E8 cm/s. 
               
               
                   
                   
               
             
          
         
       
     
     The application of a gate voltage  332  allows the Fermi level of the semiconductor layer  314  to be raised or lowered as desired. The velocity and momentum of the carriers can be obtained by evaluating the band structure of the material. The velocity of the charge carriers is given by v(kfermi)=[(1/h)dE/dk] at k=kfermi and the effective mass is given by m*(kfermi)=h/[dE 2 /dk 2 ] at k=kfermi. Thus, one can look at the band structure for guidance regarding how a gate voltage could affect the carrier momentum. For example, the velocity of a parabolic band ill be largest near the band edge. 
     Therefore, as can be seen, the application and control of a gate voltage greatly increases the performance of a Lorentz magnetoresistor. 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. For example, although the invention has been described as providing an EMR sensor for use in a magnetic data recording system such as a disk drive, the present invention could also be used in the constructional of an EMR sensor to be used in another device such as a scanning magnetometer or in any other application where a magnetic signal can be read. 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.