Abstract:
A method and apparatus for stray magnetic field compensation in a non-volatile memory cell, such as a spin-torque transfer random access memory (STRAM). In some embodiments, a first tunneling barrier is coupled to a reference structure that has a perpendicular anisotropy and a first magnetization direction. A recording structure that has a perpendicular anisotropy is coupled to the first tunneling barrier and a nonmagnetic spacer layer. A compensation layer that has a perpendicular anisotropy and a second magnetization direction in substantial opposition to the first magnetization direction is coupled to the nonmagnetic spacer layer. Further, the memory cell is programmable to a selected resistance state with application of a current to the recording structure.

Description:
BACKGROUND 
     Data storage devices generally operate to store and retrieve data in a fast and efficient manner. Some storage devices utilize a semiconductor array of solid-state memory cells to store individual bits of data. Such memory cells can be volatile (e.g., DRAM, SRAM) or non-volatile (RRAM, STRAM, flash, etc.). 
     As will be appreciated, volatile memory cells generally retain data stored in memory only so long as operational power continues to be supplied to the device, while non-volatile memory cells generally retain data storage in memory even in the absence of the application of operational power. 
     In these and other types of data storage devices, it is often desirable to increase efficiency of memory cell operation, particularly with regard to the writing data to a memory cell. 
     SUMMARY 
     Various embodiments of the present invention are directed to a method and apparatus for stray magnetic field compensation in a non-volatile memory, such as but not limited to a STRAM memory cell. 
     In accordance with various embodiments, a first tunneling barrier is coupled to a reference structure that has a perpendicular anisotropy and a first magnetization direction. A recording structure that has a perpendicular anisotropy is coupled to the first tunneling barrier and a nonmagnetic spacer layer. A compensation layer that has a perpendicular anisotropy and a second magnetization direction in substantial opposition to the first magnetization direction is coupled to the nonmagnetic spacer layer. Further, the memory cell is programmable to a selected resistance state with application of a current to the recording structure. 
     In other embodiments, a memory cell is provided that comprises a first tunneling barrier, a reference structure coupled to the first tunneling barrier that has a perpendicular anisotropy and a first magnetization direction, a second tunneling barrier, a recording structure coupled to the first and second tunneling barriers that has a perpendicular anisotropy, and a compensation layer coupled to the second tunneling barrier that has a perpendicular anisotropy and a second magnetization direction in general opposition to the first magnetization direction. The memory cell is programmed to a selected resistance state by applying a spin polarized current to the recording structure. 
     These and various other features and advantages which characterize the various embodiments of the present invention can be understood in view of the following detailed discussion and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a conventional memory cell. 
         FIG. 2  displays a memory cell operated in accordance with the various embodiments of the present invention. 
         FIG. 3  provides a memory cell operated in accordance with the various embodiments of the present invention with a write current in a first direction. 
         FIG. 4  shows a memory cell operated in accordance with the various embodiments of the present invention with a write current in a second direction. 
         FIG. 5  illustrates a memory cell operated in accordance with the various embodiments of the present invention with a set magnetic field in a first orientation. 
         FIG. 6  displays a memory cell operated in accordance with the various embodiments of the present invention with a set magnetic field in a second orientation. 
         FIG. 7  provides a memory cell operated in accordance with the various embodiments of the present invention with a set magnetic field in a third orientation. 
         FIG. 8  shows a flow diagram for a configuration routine performed in accordance with the various embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a memory cell  124  with a magnetic tunneling junction (MTJ)  158 . The MTJ  158  has a fixed magnetic layer  160  and a free magnetic layer  162  with a tunneling barrier  164  between them. As a write current  166  flows through the MTJ  158 , the interaction between the free electrons and the magnetization of the fixed reference layer  160  polarizes the electric current. The polarized electric current subsequently creates a magnetic torque to set the free layer with a desired magnetization direction. The relationship of the magnetizations of the free layer  162  and the fixed layer  160  correspond to either a high resistance state or a low resistance state. That is, if the free layer  162  and fixed layer  160  have the same magnetic direction, a low resistance state will be present in the MTJ  160 . In contrast, opposing magnetic directions between the fixed layer  160  and the free layer  162  indicate a high resistance state. 
     In  FIG. 2 , a memory cell  168  constructed in accordance with various embodiments of the present invention is displayed. The memory cell  168  comprises a MTJ  170  that has a reference structure  172  and a recording structure  174  coupled to a first tunneling barrier  176 . A second tunneling barrier or a nonmagnetic spacer layer  178  located adjacent a compensation layer  180  is coupled to the recording structure  174 . In some embodiments, a second tunneling barrier layer is chosen to enhance the spin-torque and reduce the switching current of memory cell  168 . The passage of a write current  182  through the memory cell  168  results in the programming of the recording structure  174  with a magnetic direction that dictates either a high resistance state or a low resistance state based on the magnetic relationship with the reference structure  172 . 
     The recording structure  174  and reference structure  172  can be constructed with multiple layers and materials that perform different functions. For example, the reference structure  172  can include a spin polarizing layer with a predetermined magnetization to polarize the spin the electrons of the incoming write current  182 . Further in some embodiments, the spin polarizing layer is exchange coupled to a hard magnetic layer that provides the fixed magnetization of the reference structure  172 . 
     In addition, the compensation layer  180 , recording structure  174 , and reference structure  172  have perpendicular anisotropy (PA). Anisotropy is a state of a material that has different properties along different axes. The PA of the compensation layer  180  is perpendicular to the plane. In some embodiments, the magnetization direction is opposite to that of the reference structure  172  in order to cancel the stray magnetic field generated by the reference layer  172 . The material of the compensation layer  180  and the second tunnel junction or a nonmagnetic spacer layer  178  are selected not only to cancel any stray magnetic fields but to produce negligible, or even zero, spin torque or tunneling magneto resistive effect on the recording structure  174 . 
     In one embodiment, the memory cell  168  is configured with opposing magnetizations between the reference structure  172  and the compensation layer  180  by employing a plurality of set magnetic fields. A first set magnetic field  184  aligns the hard magnet of the reference structure  172 . Subsequently, a second magnetic field  186  is provided to align the magnetization of the compensation layer  180 . In some embodiments, the first magnetic field is greater than the second magnetic field in order to provide magnetizations of opposing directions. There are other ways to align the magnetization directions of the reference structure  172  and the compensation layer  180  in substantially opposing directions  172  and  180  could be made with materials with differing coercivities. Then a two-step process using a large field to align one material and a subsequent set to align the other material can achieve opposing directions. 
       FIG. 3  shows the memory cell  168  of  FIG. 2  operated in accordance with various embodiments of the present invention. The reference structure  172  of the memory cell  168  comprises a plurality of layers including, but not limited to, a PA reference layer  188  and a spin polarizing layer  190 . In certain embodiments, the PA reference layer  188  can comprise a hard magnet, or other suitable materials. The first tunneling barrier  176  is coupled to both a first spin polarizing layer  190  of the reference structure  172  as well as a second spin polarizing layer  192  of the recording structure  174 . A PA recording layer  194  is included in the recording structure  174  to allow stable PA. In addition, the compensation layer  180  and second tunneling junction  178  are coupled to the recording structure  174  in a substantially similar manner as  FIG. 2 . 
     As a write current  182  passes through the memory cell  168 , the recording structure  174  is set in the desired orientation after the electric current has been polarized by the first spin polarizing layer  190  and traversing the first tunneling barrier  176 . In some embodiments, the magnetic relationship between the recording structure  174  and the reference structure  172  correspond to either a high resistance state or a low resistance state by having a parallel or anti-parallel orientation. It should be noted that the high and low resistance states can be matched to a predetermined logical state to allow data to be stored in the memory cell  168 . 
       FIG. 4  displays the memory cell  168  of  FIG. 2  operated in accordance with various embodiments of the present invention. With the flow of the write current  182  in an opposite direction than shown in  FIG. 3 , an anti-parallel magnetic relationship is established between the recording structure  174  and the reference structure  172  creating a high resistance state. In some embodiments, the write current  182  causes the recording structure  174  to switch magnetic directions through the reflection of spin polarized electrons from the PA reference structure  172 . It can be appreciated by one skilled in the art that the compensation layer  180  and second tunneling barrier  178  are configured to provide negligible spin momentum and TMR effect on the recording structure  174  while allowing the compensation layer  180  to cancel the stray magnetic field generated by the reference structure  174 . In some embodiments, the configuration of the second tunneling barrier  178  comprises a nonmagnetic metallic material whose band structure matches either the majority or minority electron band of the compensation layer  180 . 
       FIG. 5  shows the memory cell  168  of  FIG. 2  in accordance with further embodiments. The magnetic direction of the compensation layer  180  is configured in a non-normal orientation to improve the cancellation of stray magnetic fields generated by the reference structure  172 . That is, the direction of the magnetization of the compensation layer  180  is set to an angle with respect to a vertical or horizontal plane. The configuration of the compensation layer  180  to a non-normal orientation allows for the use of a single step for the setting of the magnetizations of the compensation layer  180  and the reference structure  172 . To achieve that a set magnetic field  184  whose direction bisects the angle between the desired orientations of the compensation layer  180  and the reference structure  172  is used to configure the memory cell  168 . 
     In some embodiments, the second tunneling barrier or spacer layer  178  is configured to manipulate the magnetization of the compensation layer  180  so that the PA axis of least resistance is non-normal. In one embodiment, this can be accomplished by proper material for  178 . This material could be CoCrPt, CoPt, or multilayers of Co/Pt or Co/Pd. Although skewed, the respective magnetization directions of the compensation layer  180  and the reference structure  172  remain in opposition. The non-normal magnetization directions can also be achieved by angled deposition during fabrication. In one embodiment, the material used is hexagonally close packed Cobalt. Alternatively, magnetic annealing can be used to achieve non-normal directions. Suitable materials would exhibit phase change transformation during anneal, like forms of FePt or CoPt. 
       FIG. 6  displays the memory cell  168  of  FIG. 2  configured in accordance with yet further embodiments of the present invention. The reference structure  172  is configured to have a non-normal magnetic orientation while the compensation layer  180  maintains an initial magnetization. To achieve this magnetic configuration a set magnetic field  184  whose direction bisects the angle between the desired orientations of the compensation layer  180  and the reference structure  172  is used to configure the memory cell  168 . 
     It should be noted that in some embodiments the set magnetic field  184  is the only set field used to configure the magnetization of the memory cell  168 . Further in some embodiments, the configuration of the memory cell  168  is conducted prior to an initial resistance state being programmed to the recording structure. As before, the respective magnetization directions of the compensation layer  180  and the reference structure  172  remain in general opposition. 
       FIG. 7  provides the memory cell  168  of  FIG. 2  operated in accordance with still further embodiments of the present invention. The set magnetic field  184  is applied to the memory cell as a substantially perpendicular path to the initial magnetic direction of the compensation layer  180  and the reference structure  172 . The set magnetic field  184  affects the magnetic orientation of both the compensation layer  180  and the reference structure  172  to result in non-normal opposing magnetizations. In some embodiments, the second tunneling barrier or spacer layer  178  is configured to manipulate the magnetization of the compensation layer  180  so that the PA axis of least resistance is non-normal. Opposition of the respective magnetization directions of the reference structure  172  and compensation layer  180  is maintained. 
       FIG. 8  displays a flow diagram of a configuration routine  230  performed in accordance with various embodiments of the present invention. The reference structure  172  and recording structure  174  are coupled to the first tunneling barrier at step  232 . The recording structure  174  is further coupled to the second tunneling barrier or spacer layer  178  that is adjacent to the compensation layer  180  at step  234 . The magnetic orientation of the components of the memory cell  168  is configured at step  236  by at least one set magnetic field. 
     It can be appreciated that one or numerous set magnetic fields of equal or different magnitude can be utilized to configure the magnetization of the memory cell. Likewise, the passage of the set current or currents can vary depending on the desired component and magnetization. 
     In step  238 , a resistance state and corresponding logical state is written to the recording structure  174  of the memory cell  168 . In some embodiments, the memory cells  168  are individually programmable to allow for a single bit, or a plurality of bits to written at a single time. Additionally, the individually programmable nature of the memory cells  168  negates any conditioning or initial operation for data to be written to the bit after the configuration routine  230  is completed. 
     As can be appreciated by one skilled in the art, the various embodiments illustrated herein provide advantageous writing of data to a memory cell in a fast and reliable manner. The ability to configure a memory cell to cancel stray magnetic fields allows for consistent data writing and reading. In fact, the required write current is reduced due to improved symmetry of directional current passage through the memory cell. Moreover, a highly consistent data rate can be achieved due to improved magnetic stability of the memory cell. However, it will be appreciated that the various embodiments discussed herein have numerous potential applications and are not limited to a certain field of electronic media or type of data storage devices. 
     It is to be understood that even though numerous characteristics and advantages of various embodiments of the present invention have been set forth in the foregoing description, together with details of the structure and function of various embodiments of the invention, this detailed description is illustrative only, and changes may be made in detail, especially in matters of structure and arrangements of parts within the principles of the present invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.