Abstract:
The present invention provides systems and method utilizing magnetoelectric materials such as Cr 2 O 3  to construct tunneling magnetoresistence and/or giant magnetoresistence structures for memory and/or logical circuitry. An applied voltage differential induces a magnetic moment in the magnetoelectric material, which in turn tunes an exchange field between it and one or more adjacent ferromagnetic layers. The resulting magnetoresistence of the device may be measured. Devices in accordance with the present invention may be utilized for MRAM read heads, memory storage cells and/or logical circuitry such as XOR or NXOR devices.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Patent Application No. 60/686,356, filed Jun. 1, 2005 and entitled “Magnetic Spin Valve with a Magnetoelectric Element” which is hereby incorporated by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not applicable.  
       BACKGROUND  
       [0003]     The circuitry used for computing systems has advanced at a rapid rate. Circuit elements for both logical circuitry and memory circuitry, such as magnetic random access memories (MRAM), have steadily shrunk and, in many cases, required ever decreasing amounts of power to operate. However, the ever increasing demands for yet faster processing, greater storage capacity, and lower power consumption continues to require new developments for the circuitry and circuit elements used in memory and processing circuitry.  
         [0004]     One example of the past years&#39; advancements in computer circuitry are read heads or field sensors that take advantage of the tunneling magnetoresistance (TMR) effect. In a typical TMR-type structure, two magnetic layers are separate by an ultra thin insulating barrier material, such as Al 2 O 3 . When driving an electric current through such a tri-layer system, the resistance of the system depends on the relative orientation of the magnetization in the two magnetic layers. The magnetic orientation of the sensor layer may rotate in accordance with the stray field created by a magnetic bit of MRAM, while the pinned magnetization of the second layer remains constant in magnitude and direction. The combination of these two layers, in turn, provide the read head functionality of the device, as the varying electrical resistance across the device will indicate whether the magnetic fields of the two layers are aligned or unaligned, which depends upon the magnetic properties of the magnetic bit being read. Similarly, the TMR effect in multi-layer structures may be used in the development and fabrication of MRAM itself. In MRAM, one bit of information may be encoded in the relative orientation of a top magnetic layer relative to a pinned bottom layer. The magnetization state of the top layer may be set via the magnetic stray-field of a writing current. The information written in this manner may subsequently be read by driving a current through the structure and, based upon the observed resistance, determining whether the magnetic orientations of the two layers are aligned or unaligned, which may correspond to either a one or a zero.  
       SUMMARY  
       [0005]     The present invention improves upon structures previously using conventional TMR or giant magnetoresistance (GMR) type structures, as well as creates novel logic devices, by replacing the passive insulating barrier material, such as Al 2 O 3 , with an active magnetoelectric material, such as Cr 2 O 3 . In a magnetoelectric material, an applied electric field induces a magnetic moment. An anti-ferromagnetic magnetoelectric thin film used in this fashion may serve as a dielectric tunnel junction between two ferromagnetic metallic layers and replace the conventional passive tunneling barrier previously employed. A tunnel barrier is an ideal system for sustaining very high electric fields, such as those reaching up to one volt per nanometer. The electrically induced magnetization in the magnetoelectric barrier interacts via magnetic exchange with the two adjacent ferromagnetic layers. This creates a shift of the magnetization curves of both ferromagnetic layers proportional to the magnetization in the magnetoelectric layer, which is dependent upon the applied voltage in the device. An anti-ferromagnetic magnetoelectric thin film may also be used to pin a magnetic layer to form a device useful as a memory cell, or even as a logic device. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0006]     Illustrative embodiments of the present invention are described in detail below with reference to the attached drawing figures, which are incorporated by reference herein and wherein:  
         [0007]      FIG. 1  illustrates an example of a device in accordance with the present invention;  
         [0008]      FIG. 2  further illustrates an example of a device in accordance with the present invention;  
         [0009]      FIG. 3  illustrates the magnetoresistance properties of a device in accordance with the present invention;  
         [0010]      FIG. 4  illustrates a further example of a device in accordance with the present invention;  
         [0011]      FIG. 5  further illustrates a further example of a device in accordance with the present invention;  
         [0012]      FIG. 6  illustrates the magnetoresistance properties of a further device in accordance with the present invention;  
         [0013]      FIG. 7  illustrates a method in accordance with the present invention;  
         [0014]      FIG. 8  illustrates a method in accordance with the present invention; and  
         [0015]      FIG. 9  illustrates a method in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0016]     Various embodiments of the present invention utilize an anti-ferromagnetic magnetoelectric thin film material in conjunction with ferromagnetic metallic layers. An applied voltage creates an electric field in the anti-ferromagnetic magnetoelectric thin film, which induces a magnetic moment in the film. The electrically induced magnetization in the anti-ferromagnetic magnetoelectric thin film interacts via magnetic exchange with one or more adjacent ferromagnetic films, which creates a shift in the magnetization curve of adjacent ferromagnetic layers proportional to the magnetization in the anti-ferromagnetic magnetoelectric film, which depends upon the voltage applied to the device.  
         [0017]     Referring now to  FIG. 1 , a device.  100  in accordance with the present invention is illustrated. Device  100  may comprise a first ferromagnetic metallic layer  110  and a second ferromagnetic metallic layer  120  separated by an anti-ferromagnetic magnetoelectric thin film junction  130 . First ferromagnetic metallic layer  110  and second ferromagnetic metallic layer  120  may comprise CrO 2 , while anti-ferromagnetic magnetoelectric thin film junction  130  may comprise Cr 2 O 3 . One skilled in the art will realize, however, that other materials with similar electrical properties may be used in conjunction with the present invention. First ferromagnetic metallic layer  110  may be a hard magnetic layer with a fixed magnetic field. Alternatively, the magnetic field in first ferromagnetic metallic layer  110  may be fixed using a pinning layer (not illustrated) adjacent to it. A voltage differential may be applied by voltage source  140  between electrical contact  142  on the first ferromagnetic metallic layer  110  and electrical contact  144  on second ferromagnetic metallic layer  120 . One skilled in the art will realize that electrical contacts  142 ,  144  need not be positioned as illustrated and may be indirect. The magnetic orientation of second ferromagnetic metallic layer  120  may be soft, while the magnetic orientation of the anti-ferromagnetic magnetoelectric thin film junction  130  will vary depending upon the voltage applied by voltage source  140 . Anti-ferromagnetic magnetoelectric thin film junction may sustain a very high electric field, reaching up to 1 V/nm for Cr 2 O 3  and possibly higher for other materials. An exchange field may be generated having a value of the order of the saturation field of the soft magnetic second ferromagnetic metallic layer  120 . This exchange field may be several mT in magnitude and provides control of the magnetization direction of the second ferromagnetic metallic layer  120 .  FIG. 1  illustrates the magnetic orientation of anti-ferromagnetic magnetoelectric thin film junction when a first voltage is applied by voltage source  140 .  
         [0018]      FIG. 2  illustrates device  100  when a second voltage has been applied by voltage source  140 . As can be seen in  FIG. 2 , the orientation of magnetic field within anti-ferromagnetic magnetoelectric thin film junction  130  has changed, resulting in a changed magnetoresistance of device  100  from first contact  142  to second contact  144 .  
         [0019]     The resistance value of device  100  as a function of electric field resulting from the applied voltage differential is illustrated in  FIG. 3 . The resistance value of device  100  depends upon the polarity of the applied voltage from voltage source  140 , which makes device  100  well suited for memory applications. Line  301  illustrates the performance of device  100  as configured in  FIG. 1 , while line  302  illustrates the performance of device  100  as configured in  FIG. 2 .  
         [0020]     Referring now to  FIG. 4 , a further device  400  in accordance with the present invention is illustrated. Device  400  may comprise a first ferromagnetic metallic layer  410  and a second ferromagnetic metallic layer  420  separated by a nonmagnetic layer  450 . First ferromagnetic metallic layer  410  may be pinned to an anti-ferromagnetic magnetoelectric thin film  430 . First ferromagnetic metallic layer  410  and second ferromagnetic metallic layer  420  may comprise CrO 2  while anti-ferromagnetic magnetoelectric thin film  430  may comprise Cr 2 O 3 . One skilled in the art will realize, however, that other materials with similar electrical properties may be used in conjunction with the present invention. A magnetic field may be induced in anti-ferromagnetic magnetoelectric thin film  430  by applying a voltage differential from voltage source  440  between electrical contact  442  and electrical contact  444 . One skilled in the art will realize that electrical contacts  442 ,  444  need not be positioned as illustrated and may be indirect. A voltage differential V controls the magnetization of anti-ferromagnetic magnetoelectric thin film  430 , which tunes the exchange coupling with the pinned first ferromagnetic metallic layer  410 . A maximum exchange field M o H e  exists due to coupling between first ferromagnetic metallic layer  410  and anti-ferromagnetic magnetoelectric thin film  430 , which allows switching of the field value of first ferromagnetic metallic layer  410  between −M o H e -M o H i  and M o H e -M o H i  for the half-hysteresis state after reaching a positive saturation field, where M o H i  is the small intrinsic switching field of the free first ferromagnetic metallic layer  410 . The exchange field M o H e  may have a magnitude corresponding to typical pinning values of spin valve devices, which can be several tens of mT.  FIG. 4  illustrates device  400  at a first voltage applied by voltage source  440  such that the magnetic field within first ferromagnetic metallic layer  410  is illustrated.  
         [0021]      FIG. 5  illustrates device  400  with a second voltage applied by voltage source  440  to induce a second magnetic field in anti-ferromagnetic magnetoelectric thin film junction  430 . As illustrated in  FIG. 4  and  FIG. 5 , the magnetoresistivity of device  400  may be measured between second electrical contact  444  and a third electrical contact  446 , positioned so as to measure the resistance of device  400  along a nonlateral direction of device  400 . Intrinsic longitudinal properties of anti-ferromagnetic magnetoelectric thin film  430  makes a non-lateral characterization of the magnetoresistance of device  400  desirable. One skilled in the art will appreciate that electrical contacts  444 ,  446  could be positioned differently than illustrated and may be indirect. One skilled in the art will further realize that a fourth contact (not shown) could be used instead of second contact  444  to measure the magnetoresistance of device  400 .  
         [0022]     The resulting magnetoresistance curves of device  400  are illustrated in  FIG. 6 . Line  404  illustrates performance of device as configured in  FIG. 4 , while line  405  illustrates performance of device as configured in  FIG. 5 . Devices such as device  400  illustrated in  FIG. 4  and  FIG. 5  may be particularly useful as a mechanism for current induced switching in magnetic memories. Further, device  400  may be used as a logic device, with the voltage applied by voltage source  440  serving as one logical input and the direction of an external magnetic field serving as a second logical input, and with the resulting high or low resistance serving as the logical output. Logical XOR and NXOR gates could be fabricated in this fashion.  
         [0023]     Referring now to  FIG. 7 , a method  700  of utilizing a device such as device  100  is illustrated. Method  700  begins by providing a hard magnetic first ferromagnetic metallic layer in step  710 . Step  700  further includes the step of providing a soft magnetic second ferromagnetic metallic layer as step  720 . Method  700  further provides an anti-ferromagnetic magnetoelectric thin film junction between the first and second ferromagnetic metallic layers in step  730 . A voltage differential is applied between the first ferromagnetic metallic layer and the second ferromagnetic metallic layer in step  740 . As a result of step  740 , a magnetic field is induced in the anti-ferromagnetic magnetoelectric thin film junction, leading to varying magnetoresistance of the device  100  that can indicate whether a stray-field (for example, from a bit of MRAM) is present. The magnetoresistance of device  100  may be measured in step  750 .  
         [0024]     A further method  800  in accordance with the present invention of utilizing a device such as device  100  is illustrated in  FIG. 8 . In method  800  a first ferromagnetic metallic layer and an adjacent pinning layer are provided in step  810 . Method  800  thereafter continues in a fashion similar to method  700 , with the step of providing a soft magnetic second ferromagnetic layer  820  corresponding to step  720 ; the step of providing a anti-ferromagnetic magnetoelectric thin film junction in step  830  corresponding to step  730 ; the application of a voltage differential of step  840  corresponding to step  740 ; and the measurement of the magnetoresistance of the device in step  850  corresponding to step  750 .  
         [0025]     Referring now to  FIG. 9 , a further method  900  in accordance with the present invention is illustrated. A first ferromagnetic metallic layer is provided in step  910 . A magnetic second ferromagnetic layer is provided in step  920 . An anti-ferromagnetic magnetoelectric thin film is pinned to the first ferromagnetic metallic layer in step  930 . A voltage differential is applied to the device in step  940 . The nonlateral magnetoresistance properties of the device are measured in step  950 . The nonlateral magnetoresistance properties measured in step  950  may be used for various purposes, such as described in conjunction with  FIGS. 4-6 .  
         [0026]     Many different arrangements of the various components depicted, as well as components not shown, are possible without departing from the spirit and scope of the present invention. Embodiments of the present invention have been described with the intent to be illustrative rather than restrictive. Alternative embodiments will become apparent to those skilled in the art that do not depart from its scope. A skilled artisan may develop alternative means of implementing the aforementioned improvements without departing from the scope of the present invention.  
         [0027]     It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations and are contemplated within the scope of the claims. Not all steps listed in the various figures need be carried out in the specific order described.