Abstract:
An exemplary method for resetting a spin-transfer based random access memory system, the method comprising, inducing a first current through a conductor, wherein the first current is operative to change a direction of orientation of a magnetic reference layer, inducing a second current from the drain terminal to the write terminal via a conductive layer, wherein the second current is operative to change the direction of a magnetic state of a free layer magnet, and inducing a third current through the conductor, wherein the third current is operative to change the direction of magnetic orientation of the reference layer.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present application is co-pending with the concurrently filed applications, entitled “SYSTEMS INVOLVING SPIN-TRANSFER MAGNETIC RANDOM ACCESS MEMORY,” and “METHODS INVOLVING RESETTING SPIN-TORQUE MAGNETIC RANDOM ACCESS MEMORY WITH DOMAIN WALL” assigned to the assignee of the present application, the contents of which are incorporated herein by reference in their entirety. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates generally to magnetic random access memory and specifically to methods involving resetting spin-torque based magnetic random access memory. 
   2. Description of Background 
   A dense, diode-selection based memory architecture has recently been demonstrated for a two-terminal memory device based on phase change materials. However, since spin-RAM usually uses bidirectional current for writing the 0 and 1 states, a dense, diode selection-based memory architecture is difficult to implement with spin-RAM since diodes may limit the use of bidirectional current. 
   A method for resetting a spin-RAM that avoids the use of bidirectional current is desired. 
   SUMMARY OF THE INVENTION 
   The shortcomings of the prior art are overcome and additional advantages are achieved through an exemplary method for resetting a spin-transfer based random access memory system, the method including, inducing a first current through a conductor, wherein the first current is operative to change a direction of orientation of a ferromagnetic film layer, inducing a second current from the drain terminal to the write terminal via a conductive layer, wherein the second current is operative to change the direction of a magnetic state of a free layer magnet, and inducing a third current through the conductor, wherein the third current is operative to change the direction of magnetic orientation of the ferromagnetic film layer. 
   Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with advantages and features, refer to the description and to the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other aspects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
       FIG. 1  illustrates a block diagram of an exemplary method for resetting a three terminal spin-RAM device. 
       FIG. 2  illustrates a front partially cut-away view of an example of an embodiment of a spin-torque based magnetic write random access memory system. 
       FIG. 3  illustrates a perspective view the example of the embodiment of the spin-torque based magnetic write random access memory system of  FIG. 2 . 
       FIGS. 4-8  illustrate a side-view of an example of an embodiment of a spin-torque based magnetic write random access memory system and the method described in  FIG. 1 . 
     The detailed description explains the preferred embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Methods involving resetting spin-torque based magnetic write random access memory are provided. Several exemplary embodiments are described. 
   The embodiments of a spin-torque based magnetic write random access memory allow for more robust operation of the memory device with existing materials combination. It fuirther enables the implementation of a high-density version of spin-RAM, based on diode-selection that makes spin-RAM more economical to produce. This improves cost-to-performance characteristics, while retaining the basic advantages of a spin-torque-based RAM. The ferromagnetic film layer may be a source for changing a direction of spin injection using a unipolar write current for each bit. The ferromagnetic film layer may be relatively large (in excess of 100 nm) and shared by multiple memory bits. 
     FIG. 2  illustrates a front partially cut-away view of a three-tenninal spin-torque based magnetic write random access memory containing electrical connections, T1, T2 and T3 terminals. In the illustrated embodiment, a write portion  100  is a pillar-shaped non-linear selection device, such as, for example, a P/N junction that includes a p-type doped region  110  and an n-type doped region  120  (or in reverse order, depending on the direction of write current for the design). The n-type doped region  120  electrically contacts a ferromagnetic film layer  200 . The p-type doped region  110  electrically contacts a conductor  130  that is connected to theT1 terminal. Though the illustrated embodiment shows a P/N junction, any suitable non-linear selection device may be used for the write portion  100 .  FIG. 2  also includes a second read portion  400   b  having a terminal T3b and a second write portion  100   b  having a terminal T1b. 
   Though  FIG. 2  illustrates two read portions and write portions. The methods described above may be used to reset the memories of embodiments having any additional number of read portions and write portions similar to read portion  400  and write portion  100 . 
   A spin-current generating portion  20  includes the ferromagnetic film layer  200 , and a conductance layer  220 . The ferromagnetic film layer  200  is a magnetic, spin-polarizing layer. The conductance layer  220  is a non-magnetic, spin preserving, high conductance layer such as, for example, copper. A tunnel layer (not shown) may be used to separate the ferromagnetic film layer  200  and the conductance layer  220 . Depending on the specifics of materials properties, the tunnel layer may not be necessary. In some materials combinations, it is possible to allow a direct, high quality interface (usually formed during thin film deposition) between the ferromagnetic film layer  200  and the conductance layer  220 . The conductance layer  220  electrically contacts the T2 terminal. 
   A read portion  400  forms the magnetic state detection device group. The read portion  400  is a pillar shape on the conductance layer  220 . The read portion  400  includes a free layer magnet  410  disposed on the conductance layer  220 , a read non-magnetic layer  420 , and a reference layer  430 . The read non-magnetic layer  420  is a non-magnetic spin-preserving metallic separation layer that may be, for example, a tunnel barrier layer. The reference layer  430  electrically contacts the T3 terminal. The read portion  400  is disposed on the conductance layer  220  with a high quality interface, usually formed during film growth, to allow efficient interaction between the free layer magnet  410  and a spin-current (not shown). 
   The reference layer  430  is magnetically fixed. A direction of magnetization of the reference layer  430  is permanently fixed in the direction indicated by the arrow  402 . In the illustrated embodiment, the arrow  402  points to the right, however, the arrow  402  may point to the left in other embodiments. The free layer magnet  410  is a nanomagnet having a magnetic state illustrated by the arrow  401 . The free layer magnet  410  serves as a memory element and the direction of the arrow  401  (right or left) indicates two bi-stable memory states of the free layer magnet  410 .  FIG. 3  illustrates a perspective view of the three-terminal spin-torque based magnetic write random access memory of  FIG. 2 . 
     FIG. 1  illustrates a block diagram of an exemplary method of resetting a spin-torque-based RAM. Referring to block  151 , a first current is induced through a conductor to change a direction of magnetic orientation of a ferromagnetic film layer. In block  153 , a second current is induced from a drain terminal to a write terminal that changes the magnetic state of a free layer magnet. In block  154 , a third current is induced from the drain terminal to a second write terminal that changes the magnetic state of a second free layer magnet. In block  155 , a fourth current is induced through the conductor to change the direction of magnetic orientation of the ferromagnetic film layer. The method illustrated in  FIG. 1  is shown in detail in  FIGS. 4-7 . 
     FIG. 4  shows an embodiment of a spin-torque-based RAM similar to the embodiment of  FIG. 2 . In  FIG. 4 , the magnetization of the reference layers  430  and  430 b are permanently fixed in the direction indicated by the arrows  402  and  402 b. The magnetic orientation of the free layer magnets  410  and  41 O b , memory elements, are shown by the arrows  401  and  401   b . In the illustrated embodiment the arrows  401  and  401   b  are pointed to the right, representing 1 bits. The direction magnetic orientation of the ferromagnetic film layer  200  is represented by the arrow  201 . 
   To change the free layer magnets  410  and  410   b  from representing 1 bits to 0 bits, the direction of the magnetic orientation of the ferromagnetic film layer  200  is first reversed.  FIG. 5  illustrates a method of reversing the direction of the magnetic orientation of the ferromagnetic film layer  200 . As illustrated in block  151  (of  FIG. 1 ), a first current  101  is induced along the conductor  130 . The conductor  130  is orientated at a right angle to the ferromagnetic film layer  200 . The first current  101  creates a magnetic flux  103  that is effective to change the direction of the magnetic orientation of the ferromagnetic film layer  200  to an opposite direction as indicated by the arrow  201   
   Referring to  FIG. 6 , once the direction of the magnetic orientation of the ferromagnetic film layer  200  is changed, the free layer magnet  410  may be changed from representing a 1 bit to a 0 bit. As illustrated in block  153  (of  FIG. 1 ), a second current  155  is induced between the T2 terminal and the T1 terminal. The second current  155  and the direction of the magnetic orientation of the ferromagnetic film layer  200  result in a spin accumulation. A spin accumulation is a non-equilibrium spin state with conduction electrons having a significant amount of excess spin in the direction defined by the magnetic orientation of the ferromagnetic film layer  200  and the direction of the second current  155 . The region having a spin accumulation is illustrated as region  310 . The spin accumulation of region  310  results in a change in the direction of the magnetic state of the free layer magnet  410  as indicated by the arrow  401 . The direction of the arrow  401  now represents a 0 bit. 
     FIG. 7  illustrates block  154  (of  FIG. 1 ). After the direction of the magnetic state of the free layer magnet  410  is changed, the direction of the magnetic state of the free layer magnet  410  may be changed in a similar manner. A third current  155   b  is induced between the T2 terminal and the T1b terminal resulting in a spin accumulation in a region  130   b . The spin accumulation of region  310   b  results in a change in the direction of the magnetic state of the free layer magnet  410   b  as indicated by the arrow  401   b . The direction of the arrow  401   b  now represents a 0 bit. 
     FIG. 8  illustrates block  155  (of  FIG. 1 ). After the direction of the magnetic state of the free layer magnets  410  and  410   b  are changed, the direction of the magnetic orientation of the ferromagnetic film layer  200  may be returned to the first direction shown in  FIG. 5  by arrow  201 . A fourth current  102  is induced along the conductor  130  in a direction opposite to the first current  101 . The fourth current  102  creates a magnetic flux  109  that is effective to change the direction of the magnetic orientation of the ferromagnetic film layer  200  to an opposite direction as indicated by the arrow  201 . 
   While the preferred embodiment to the invention has been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described.