Patent Publication Number: US-7911829-B2

Title: Scalable magnetic memory devices

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a continuation of U.S. application Ser. No. 11/481,541 filed on Jul. 6, 2006, now U.S. Pat. No. 7,433,225 the disclosure of which is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to data storage, and more particularly, to data storage devices and techniques for use thereof. 
     BACKGROUND OF THE INVENTION 
     Semiconductor devices, such as magnet random access memory (MRAM) devices, use magnetic memory cells to store information. Information is stored in the magnetic memory cells as an orientation of the magnetization of a free layer in the magnetic memory cell as compared to an orientation of the magnetization of a fixed (e.g., reference) layer in the magnetic memory cell. The magnetization of the free layer can be oriented parallel or anti-parallel relative to the fixed layer, representing either a logic “1” or a logic “0.” The orientation of the magnetization of a given layer (fixed or free) may be represented by an arrow pointing either to the left or to the right. When the magnetic memory cell is sitting in a zero applied magnetic field, the magnetization of the magnetic memory cell is stable, pointing either left or right. The application of a magnetic field can switch the magnetization of the free layer from left to right, and vice versa, to write information to the magnetic memory cell. 
     One of the objectives of MRAM is to have a low operating power and small area. This objective requires a low switching field for the magnetic memory cell, because a low switching field uses a low switching current, which uses less power, and because smaller currents require smaller switches, which occupy less space. However, as the area of the magnetic memory cells becomes increasingly smaller, a process referred to as “scaling” due to the fact that the area of the magnetic memory cell is scaled down to allow for more magnetic memory cells in the same area, the switching field actually increases. 
     U.S. Pat. No. 6,545,906 issued to Savtchenko, et al., entitled “Method of Writing to Scalable Magnetoresistance Random Access Memory Element” (hereinafter “Savtchenko”) describes a toggle free layer for use in MRAM devices. Prior to Savtchenko, MRAM devices employed a single free layer design. Both of these approaches, however, are prone to the problems associated with scaling that are described above. Namely, as the size of the magnetic memory cell is scaled down, an increased amount of power is required to switch the magnetic memory cell. For example, as much as 80 Oersteds (Oe) can be required to switch a 150 nanometer (nm) toggle magnetic memory cell. 
     Thus, scalable magnetic memory devices having reduced switching fields would be desirable. 
     SUMMARY OF THE INVENTION 
     The present invention provides techniques for reducing the switching fields in magnetic memory devices. In one aspect of the invention, a magnetic memory cell is provided. The magnetic memory cell comprises at least one fixed magnetic layer and a plurality of free magnetic layers, separated from the at least one fixed magnetic layer by at least one barrier layer. The free magnetic layers include a first free magnetic layer adjacent to the barrier layer, a second free magnetic layer separated from the first free magnetic layer by at least one spacer layer, and a third free magnetic layer separated from the second free magnetic layer by at least one anti-parallel coupling layer. A magnetic moment of the first free magnetic layer is greater than both a magnetic moment of the second free magnetic layer and a magnetic moment of the third free magnetic layer. 
     In another aspect of the invention, a magnetic random access memory (MRAM) device is provided. The MRAM device comprises a plurality of word lines oriented orthogonal to a plurality of bit lines, and a plurality of magnetic memory cells configured in an array between the word lines and bit lines. At least one of the plurality of magnetic memory cells comprises at least one fixed magnetic layer, and a plurality of free magnetic layers, separated from the at least one fixed magnetic layer by at least one barrier layer. The free magnetic layers include a first free magnetic layer adjacent to the barrier layer, a second free magnetic layer separated from the first free magnetic layer by at least one spacer layer, and a third free magnetic layer separated from the second free magnetic layer by at least one anti-parallel coupling layer. A magnetic moment of the first free magnetic layer is greater than both a magnetic moment of the second free magnetic layer and a magnetic moment of the third free magnetic layer. 
     In yet another aspect of the invention, a method of writing data to an MRAM device having a plurality of word lines oriented orthogonal to a plurality of bit lines, and a plurality of magnetic memory cells configured in an array between the word lines and bit lines, comprises the following steps. A word line current is provided to a given one of the word lines to select all of the magnetic memory cells along the given word line. At least one of the selected magnetic memory cells comprises at least one fixed magnetic layer and a plurality of free magnetic layers, separated from the at least one fixed magnetic layer by at least one barrier layer. The free magnetic layers include a first free magnetic layer adjacent to the barrier layer, a second free magnetic layer separated from the first free magnetic layer by at least one spacer layer, and a third free magnetic layer separated from the second free magnetic layer by at least one anti-parallel coupling layer. A magnetic moment of the first free magnetic layer is greater than both a magnetic moment of the second free magnetic layer and a magnetic moment of the third free magnetic layer. A bit line current is provided to each of the bit lines corresponding to the selected magnetic memory cells. The word line current is removed. The bit line current is removed. 
     In still another aspect of the invention, a magnetic memory cell is provided. The magnetic memory cell comprises at least one fixed magnetic layer, and a plurality of free magnetic layers, separated from the at least one fixed magnetic layer by at least one barrier layer. The free magnetic layers include a first free magnetic layer adjacent to the barrier layer, a second free magnetic layer separated from the first free magnetic layer by at least one first parallel coupling layer, and a third free magnetic layer separated from the second free magnetic layer by at least one second parallel coupling layer. A magnetic moment of the second free magnetic layer is greater than both a magnetic moment of the first free magnetic layer and a magnetic moment of the third free magnetic layer. 
     A more complete understanding of the present invention, as well as further features and advantages of the present invention, will be obtained by reference to the following detailed description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating an exemplary magnetic memory cell according to an embodiment of the present invention; 
         FIG. 2  is a diagram illustrating an exemplary magnetic memory cell array according to an embodiment of the present invention; 
         FIGS. 3A-B  are diagrams illustrating top-down views of stable magnetic states of the exemplary magnetic memory cell of  FIG. 1  in a zero applied magnetic field according to an embodiment of the present invention; 
         FIG. 4  is a diagram illustrating another exemplary magnetic memory cell according to an embodiment of the present invention; 
         FIGS. 5A-B  are diagrams illustrating top-down views of stable magnetic states of the exemplary magnetic memory cell of  FIG. 4  in a zero applied magnetic field according to an embodiment of the present invention; 
         FIG. 6  is a diagram illustrating an exemplary methodology for writing data to a magnetic memory cell array according to an embodiment of the present invention; 
         FIGS. 7A-B  are graphs representing current pulse sequences used to write data to an individual magnetic memory cell according to an embodiment of the present invention; 
         FIG. 8  is a graph illustrating an exemplary critical switching curve for a magnetic memory cell according to an embodiment of the present invention; 
         FIGS. 9A-D  are graphs illustrating magnetic moment rotations in magnetic layers of a magnetic memory cell according to an embodiment of the present invention; 
         FIGS. 10A-B  are graphs illustrating moment rotations for a magnetic memory cell when a field is applied in an easy axis direction according to an embodiment of the present invention; and 
         FIGS. 11A-B  are graphs illustrating moment rotations for a magnetic memory cell when a field is applied in a hard axis direction according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  is a diagram illustrating exemplary magnetic memory cell  100 . Magnetic memory cell  100  comprises tunnel barrier  106  deposited on fixed layer  104 , and free layer  102  deposited on tunnel barrier  106 . 
     According to the exemplary embodiment depicted in  FIG. 1 , magnetic memory cell  100  can be configured, i.e., patterned, to have, when viewed from top-down view  126 , a circular (or elliptical) shape. See also the top-down depictions of magnetic memory cell  100  having a circular shape in  FIGS. 3A-B , described below. It is to be understood, however, that any other patternable shapes and/or configurations can be employed in accordance with the present teachings. As will be described in detail below, magnetic memory cell  100  may be used in conjunction with a magnetic random access memory (MRAM) device. 
     As shown in  FIG. 1 , free layer  102  is a multiple-layer structure. Namely, free layer  102  comprises magnetic layer  108  adjacent to tunnel barrier  106 . Spacer layer  114  is deposited on a side of magnetic layer  108  opposite tunnel barrier  106 . Magnetic layer  112  is deposited on a side of spacer layer  114  opposite magnetic layer  108 . Anti-parallel coupling (AP-coupling) layer  110  is deposited on a side of magnetic layer  112  opposite spacer layer  114 . Finally, magnetic layer  116  is deposited on a side of AP-coupling layer  110  opposite magnetic layer  112 . 
     Each of magnetic layers  108 ,  112  and  116  can comprise any suitable ferromagnetic material, including, but not limited to, one or more of a nickel/iron alloy (NiFe alloy) and alloys containing one or more of Ni, Fe, Cobalt (Co) and a rare earth element. Further, any one of magnetic layers  108 ,  112  and  116  can have a same, or a different, composition as any of the other magnetic layers  108 ,  112  and  116 . For example, according to one exemplary embodiment, all three magnetic layers  108 ,  112  and  116  comprise the same NiFe alloy. 
     Each of magnetic layers  108 ,  112  and  116  has a magnetic moment μ associated therewith, wherein μ equals the product of a magnetization of the layer M s , a thickness of the layer T and an area of the layer A, i.e.,
 
μ= M   s   ·T·A.   (1)
 
As will be described below, μ points in a particular direction, i.e., a direction of magnetization. Magnetic layers  108 ,  112  and  116  have magnetic moments μ 1 , μ 2 , and μ 3 , respectively, associated therewith. According to the present teachings, μ 1  is greater than both μ 2  and μ 3 , i.e., μ 1 &gt;μ 2  and μ 1 &gt;μ 3 . Further, according to one exemplary embodiment, μ 2  is equal to μ 3 , i.e., μ 2 =μ 3 .
 
     When magnetic layers  108 ,  112  and  116  are configured to have a circular shape, as described above, it can be assumed that each of magnetic layers  108 ,  112  and  116  occupies the same area, i.e., magnetic layers  108 ,  112  and  116  have areas A 1 , A 2  and A 3 , respectively, associated therewith wherein A 1 =A 2 =A 3 . Further, when magnetic layers  108 ,  112  and  116  have the same composition, as described above, it can be assumed that each of magnetic layers  108 ,  112  and  116  has the same magnetization, i.e., magnetic layers  108 ,  112  and  116  have magnetizations M s1 , M s2  and M s3 , respectively, associated therewith wherein M s1 =M s2 =M s3 . Accordingly, when both of the above conditions apply, i.e., when magnetic layers  108 ,  112  and  116  are circular and each layer has the same composition, T is the sole variable in μ differences between the layers. According to this exemplary embodiment, magnetic layer  108  is adapted to have a greater thickness than either magnetic layer  112  or magnetic layer  116 , i.e., magnetic layers  108 ,  112  and  116  have thicknesses T 1 , T 2  and T 3 , respectively, associated therewith wherein T 1 &gt;T 2  and T 1 &gt;T 3 . See, for example,  FIG. 1 . Further, according to one exemplary embodiment, magnetic layer  112  has the same thickness as magnetic layer  116 , i.e., T 2 =T 3 . 
     Magnetic layers  108  and  112  are separated by spacer layer  114 . Spacer layer  114  mediates zero coupling between magnetic layers  108  and  112 . According to an exemplary embodiment, spacer layer  114  comprises one or more of tantalum nitride (TaN), Ta, titanium (Ti), tungsten (W) and niobium (Nb). Materials that can give zero coupling, such as ruthenium (Ru), can also be used if the thickness of the layer is adjusted to give zero coupling. 
     Magnetic layers  112  and  116  are separated by AP-coupling layer  110 . AP-coupling layer  110  mediates anti-parallel magnetic coupling between magnetic layers  112  and  116 . According to an exemplary embodiment, AP-coupling layer  110  comprises one or more of Ru, osmium (Os), copper (Cu), chromium (Cr), molybdenum (Mo), rhodium (Rh), rhenium (Re) and iridium (Ir). 
     As described above, tunnel barrier  106  separates free layer  102  from fixed layer  104 . Tunnel barrier  106  comprises any suitable tunnel barrier material, including, but not limited to, one or more of aluminum oxide (AlOx) and magnesium oxide (MgO). 
     As shown in  FIG. 1 , fixed layer  104  can also be a multiple-layer structure. Namely, fixed layer  104  comprises magnetic layer  118  adjacent to tunnel barrier  106  and separated from magnetic layer  120  by AP-coupling layer  122 . Anti-ferromagnetic (AF) layer  124  is adjacent to a side of magnetic layer  120  opposite AP-coupling layer  122 . 
     Each of magnetic layers  118  and  120  can comprise any suitable ferromagnetic material, including, but not limited to, one or more of an alloy containing Ni, Fe or Co, and can have a same, or a different, composition as the other of magnetic layers  118  and  120 . AP-coupling layer  122  mediates anti-parallel magnetic coupling between magnetic layers  118  and  120 , and can comprise one or more of Ru, Os, Cu, Cr, Mo, Rh, Re and Ir. 
     AF layer  124  serves to pin the direction of magnetization of magnetic layers  118  and  120 . AF layer  124  can comprise a metal alloy, including, but not limited to, one or more of a platinum/manganese alloy (PtMn) and an iridium/manganese alloy (IrMn). 
     While  FIG. 1  depicts magnetic memory cell  100  as having tunnel barrier  106  being deposited on top of fixed layer  104  and free layer  102  being deposited on top of tunnel barrier  106 , this configuration is merely exemplary, and other configurations can be employed. By way of example only, the layers may be deposited in the reverse order to that shown in  FIG. 1 , i.e., with tunnel barrier  106  being deposited on top of free layer  102  and fixed layer  104  being deposited on top of tunnel barrier  106 , so long as the free magnetic layer having the greatest magnetic moment (e.g., magnetic layer  108 ) is adjacent to tunnel barrier  106 . 
       FIG. 2  is a diagram illustrating exemplary magnetic memory cell array  200 . Magnetic memory cell array  200  can be employed as an MRAM device. 
     Magnetic memory cell array  200  comprises bit lines  202  and word lines  204  running orthogonal to each other above and below a plurality of magnetic memory cells  100 . The configuration of magnetic memory cell array  200  shown in  FIG. 2  is merely exemplary, and other configurations are possible. By way of example only, magnetic memory cell array  200  can be configured to have bit lines  202  run below magnetic memory cells  100  and word lines  204  run above magnetic memory cells  100 . 
     Methods for writing data to magnetic memory cell array  200  will be described in detail below. In general, however, each word line  204  applies a magnetic field H word  along a y-axis of each magnetic memory cell  100 , and each bit line  202  applies a magnetic field H bit  along an x-axis of each magnetic memory cell  100 . The y-axis comprises a hard switching axis of each magnetic memory cell  100  and the x-axis comprises an easy switching axis of each magnetic memory cell  100 . 
       FIGS. 3A-B  are diagrams illustrating top-down views of stable magnetic states of exemplary magnetic memory cell  100  in a zero applied magnetic field. Arrows  302 ,  304  and  306  represent magnetic moments μ 1 , μ 2  and μ 3  of magnetic layers  108 ,  112  and  116 , respectively. Since μ 1 &gt;μ 2  and μ 1 &gt;μ 3 , as described above, a heavier arrow is used for arrow  302  than for arrows  304  and  306 . According to the present teachings, at least magnetic layer  108  will have an intrinsic anisotropy set along the x-axis direction (i.e., along the easy switching axis). Further, by virtue of the fact that μ 1 &gt;μ 2  and μ 1 &gt;μ 3 , μ 1  will point along the intrinsic anisotropy axis of magnetic layer  108 . As such, there are two stable magnetic states for magnetic memory cell  100  when in a zero applied magnetic field. The first stable magnetic state is when μ 1  points in the positive x-axis direction. See, for example,  FIG. 3A  wherein arrow  302  is pointing to the right. The second stable magnetic state is when μ 1  points in the negative x-axis direction. See, for example,  FIG. 3B  wherein arrow  302  is pointing to the left. 
     The dipole fields from each of magnetic layers  108 ,  112  and  116  make μ 1 , μ 2  and μ 3  spread out in a triangle. See  FIGS. 3A-B  wherein arrows  302 ,  304  and  306  spread out in a triangle. 
     As shown in  FIG. 3A , when arrow  302  (μ 1 ) points to the right, arrow  304  (μ 2 ) points to the lower left and arrow  306  (μ 3 ) points to the upper left. Similarly, as shown in  FIG. 3B , when arrow  302  (μ 1 ) points to the left, arrow  304  (μ 2 ) points to the upper right and arrow  306  (μ 3 ) points to the lower right. 
     It is notable that in the two stable magnetic states shown in  FIGS. 3A and 3B , it is immaterial which of μ 2  or μ 3  points to the upper/lower left/right, respectively. For example, the operation of magnetic memory cell  100  would not change if in the configuration shown in  FIG. 3B  arrow  304  (μ 2 ) pointed to the lower right instead of the upper right and arrow  306  (μ 3 ) pointed to the upper right instead of the lower right. 
       FIG. 4  is a diagram illustrating exemplary magnetic memory cell  400 . Magnetic memory cell  400  comprises tunnel barrier  406  deposited on fixed layer  404 , and free layer  402  deposited on tunnel barrier  406 . 
     Free layer  402  is a multiple-layer structure. Namely, free layer  402  includes three magnetic layers  408 ,  412  and  416 , each of which can comprise a ferromagnetic material, including, but not limited to, one or more of a NiFe alloy and alloys containing one or more of Ni, Fe, Co and a rare earth element. Further, any one of magnetic layers  408 ,  412  and  416  can have a same, or a different, composition as any one of the other magnetic layers  408 ,  412  and  416 . 
     Magnetic layer  412  is separated from, and is parallel coupled (P-coupled) to both magnetic layers  408  and  416  by P-coupling layers  410  and  414 , respectively. Each of P-coupling layers  410  and  414  can comprise any suitable P-coupling materials, including, but not limited to, one or more of Ru, Os, Cu, Cr, Mo, Rh, Re and Ir, and may have a same, or a different, composition as each other. 
     Each of magnetic layers  408 ,  412  and  416  has a magnetic moment μ associated therewith. The value of u can be calculated according to Equation 1, above. Magnetic layers  408 ,  412  and  416  have magnetic moments μ 4 , μ 5  and μ 6 , respectively, associated therewith. According to the present teachings, μ 5  is greater than μ 4  and μ 6 , i.e., μ 5 &gt;μ 4  and μ 5 &gt;μ 6 . Further, according to one exemplary embodiment, μ 4  is equal to μ 6 , i.e., μ 4 =μ 6 . As described in detail above, if magnetic layers  408 ,  412  and  416  occupy the same area A and have the same magnetization M s , then layer thickness T is the sole variable in moment μ differences between the magnetic layers. Such an instance is depicted in  FIG. 4 , wherein magnetic layer  412  is shown to have a greater thickness than either of magnetic layers  408  and  416 . 
     Fixed layer  404  is a multiple-layer structure. Namely fixed layer  404  comprises magnetic layer  418  adjacent to tunnel barrier  406  and separated from magnetic layer  420  by AP-coupling layer  422 . AF layer  424  is adjacent to a side of magnetic layer  420  opposite AP-coupling layer  422 . Magnetic layers  418  and  420  can have a same, or a different, composition as each other, and can comprise any suitable ferromagnetic material, including, but not limited to, one or more of a NiFe alloy and alloys containing one or more of Ni, Fe, Co and a rare earth element. AP-coupling layer  422  mediates anti-parallel magnetic coupling between magnetic layers  418  and  420  and can comprise one or more of Ru, Os, Cu, Cr, Mo, Rh, Re and Ir. 
     AF layer  424  serves to pin the direction of magnetization of magnetic layers  418  and  420 . AF layer  424  can comprise a metal alloy, including, but not limited to, one or more of a PtMn alloy and an IrMn alloy. 
     A plurality of magnetic memory cells  400  may be used in a magnetic memory cell array. For example, a plurality of magnetic memory cells  400  can be used in magnetic memory cell array  200 , described in conjunction with the description of  FIG. 2  above, in place of magnetic memory cells  100 . 
       FIGS. 5A-B  are diagrams illustrating top-down views of stable magnetic states of exemplary magnetic memory cell  400  in a zero applied magnetic field. Arrows  502 ,  504  and  506  represent magnetic moments μ 4 , μ 5  and μ 6  of magnetic layers  408 ,  412  and  416 , respectively. Since μ 5 &gt;μ 4  and μ 5 &gt;μ 6 , as described above, a heavier arrow is used for arrow  504  than for arrows  502  and  506 . According to the present teachings, at least magnetic layer  412  will have an intrinsic anisotropy set along the x-axis direction (i.e., along the easy switching axis). Further, by virtue of the fact that μ 5 &gt;μ 4  and μ 5 &gt;μ 6 , μ 5  will point along the intrinsic anisotropy axis of magnetic layer  412 . As such, there are two stable magnetic states for magnetic memory cell  400  in a zero applied magnetic field. The first stable magnetic state is when μ 5  points in the positive x-axis direction. See, for example,  FIG. 5A  wherein arrow  504  points to the right. The second stable magnetic state is when μ 5  points in the negative x-axis direction. See, for example,  FIG. 5B  wherein arrow  504  is pointing to the left. 
     As shown in  FIG. 5A , when arrow  504  (μ 5 ) points to the right, arrow  502  (μ 4 ) points to the lower left and arrow  506  (μ 6 ) points to the upper left. Similarly, as shown in  FIG. 5B , when arrow  504  (μ 5 ) points to the left, arrow  502  (μ 4 ) points to the upper right and arrow  506  (μ 6 ) points to the lower right. As explained above, in  FIGS. 5A-B  it is immaterial which of μ 4  or μ 6  points to the upper/lower left/right, respectively. 
       FIG. 6  is a diagram illustrating exemplary methodology  600  for writing data to a magnetic memory cell array, such as magnetic memory cell array  200 , described in conjunction with the description of  FIG. 2 , above, having a plurality of word lines oriented orthogonal to a plurality of bit lines and a plurality of magnetic memory cells (e.g., magnetic memory cells  100  or magnetic memory cells  400 , both described above) therebetween. As described above, in each magnetic memory cell in the array, the free magnetic layer having the greatest moment μ has an intrinsic anisotropy pointing along the x-axis direction and thus aligned with the direction of field applied by the bit line. 
     In step  602 , a current is passed along a given one of the word lines (a word line current) thereby selecting all of the magnetic memory cells on that given word line (i.e., the word line current destabilizes the magnetic memory cells, which essentially erases all pre-existing data and makes the magnetic memory cells easier to write). Namely, all of the magnetic memory cells on that given word line are selected to be written together at the same time. According to one exemplary embodiment, there are 128 magnetic memory cells per word line. 
     In step  604 , each of the magnetic memory cells selected in step  602 , above, is written by sending a small current through each corresponding bit line (a bit line current). For example, if 128 magnetic memory cells are selected in step  602  above, then a bit line current is sent through each of the 128 corresponding bit lines to write data to those 128 magnetic memory cells. 
     The bit line current can be either a positive current or a negative current. A positive current will write a logic “1” to the corresponding magnetic memory cell, and a negative current will write a logic “0” to the corresponding magnetic memory cell. 
     In step  606 , the word line current is removed. In step  608 , the bit line current is removed. As a result, data (i.e., either a logic “1” or a logic “0”) is written to each of the magnetic memory cells selected in step  602 , above. 
       FIGS. 7A-B  are graphs representing current pulse sequences used to write data to an individual magnetic memory cell. Specifically,  FIG. 7A  illustrates the current pulse sequence used to write a logic “0” to a magnetic memory cell, and  FIG. 7B  illustrates the current pulse sequence used to write a logic “1” to a magnetic memory cell. The arrows in  FIGS. 7A and 7B  represent the current pulses. 
     Referring to  FIG. 7A , the sequence starts with the magnetic memory cell being in a zero applied magnetic field. As described in conjunction with the description of  FIG. 6 , above, a current is first applied to the word line, i.e., the word line is turned on. In this instance, a negative current is then applied to the bit line, i.e., the bit line is turned on. The current to the word line is then removed, i.e., the word line is turned off, followed by the current to the bit line being removed, i.e., the bit line is turned off. As a result, a logic “0” will have been written to the magnetic memory cell. 
     Referring to  FIG. 7B , the sequence again starts with the magnetic memory cell being in a zero applied magnetic field. The word line is first turned on, followed by the bit line. In this instance, however, a positive current is applied to the bit line. The word line is then turned off, followed by the bit line being turned off. As a result, a logic “1” will have been written to the magnetic memory cell. 
       FIG. 8  is a graph illustrating critical switching curve  800  for magnetic memory cell  100 . The values of H bit  and H word  are presented in Oersteds (Oe). Switching curve  800  is analogous to the common Stoner-Wohlfarth astroid. 
     For magnetic fields inside curve  800 , magnetic memory cell  100  is stable. For magnetic fields outside of curve  800 , magnetic memory cell  100  can be written. 
     Switching curve  800  was calculated with a single domain model using the following parameters for magnetic memory cell  100 . Magnetic layer  108  has a thickness of six nanometers (nm); magnetic layer  112  has a thickness of five nm; and magnetic layer  116  has a thickness of five nm. Each of magnetic layers  108 ,  112  and  116  has a M s  equal to 1,500 electromagnetic units/cubic centimeter (emu/cc), an intrinsic anisotropy H i  of 25 Oe, and a diameter of 150 nm. The exchange coupling J between magnetic layers  112  and  116  equals −0.05 erg/square centimeter (erg/cm 2 ), wherein a negative J value denotes AP-coupling. 
     From switching curve  800  it is notable that the bit and word line fields, e.g., H bit  and H word , respectively, required to write magnetic memory cell  100  are very small, for example, less than 50 Oe for the word line (which, as described above, may be distributed over 128 magnetic memory cells per word line), and less than 10 Oe for the bit line. Thus, for an exemplary array having 128 magnetic memory cells per word line, the total field per magnetic memory cell is 10+50/128˜10 Oe. 
     By comparison, for toggle switching devices, such as those described in Savtchenko, one needs H bit =H word =80 Oe. Thus, the total field per toggle switching device is 80+80/128=81 Oe. Therefore, magnetic memory cell  100  uses eight times less power than a typical toggle switching device. 
     Another important feature of magnetic memory cell  100  is that when H word  is zero, a very large bit line field is required to switch magnetic memory cell  100 . See, for example, switching curve  800  wherein an H bit  of greater than 300 Oe is needed to switch magnetic memory cell  100  when H word  is zero. Thus, magnetic memory cell  100  is very stable under half select. It is only when the word line is selected that it is then easy to write magnetic memory cell  100  with a small bit line field, as described above. 
       FIGS. 9A-D  are graphs illustrating magnetic moment rotations in magnetic layers  108 ,  112  and  116  of magnetic memory cell  100 . Specifically, in  FIGS. 9A and 9B , the rotations of magnetic moments μ 1 , μ 2 , and μ 3  in magnetic layers  108 ,  112  and  116 , respectively, are depicted when magnetic memory cell  100  starts in a “0” logic state, and a logic “0” or a logic “1,” respectively, is written. In  FIGS. 9C and 9D , the rotations of magnetic moments μ 1 , μ 2 , and μ 3  in magnetic layers  108 ,  112  and  116 , respectively, are depicted when magnetic memory cell  100  starts in a “1” logic state, and a logic “0” or a logic “1,” respectively, is written. In each of  FIGS. 9A-D , μ 1  corresponding to magnetic layer  108  is depicted as a line ending in a black circle, μ 2  corresponding to magnetic layer  112  is depicted as a line ending in a gray circle and μ 3  corresponding to magnetic layer  116  is depicted as a line ending in a white circle. Circles have been used for ease of depiction and identification of each magnetic moment with the corresponding magnetic layer. However, it is to be understood that each circle can be replaced with an arrowhead, e.g., as in  FIGS. 3A-B , described above wherein μ 1 , μ 2 , and μ 3  are represented by arrows  302 ,  304  and  306 , respectively. 
     The graphs shown in  FIGS. 9A-D  were all calculated with a single domain theory, using the same parameter values as in switching curve  800 , above. Namely, magnetic layer  108  has a thickness of six nm; magnetic layer  112  has a thickness of five nm; and magnetic layer  116  has a thickness of five nm. Each of magnetic layers  108 ,  112  and  116  has a M s  equal to 1,500 emu/cc, an H i  of 25 Oe, and a diameter of 150 nm. Finally, the exchange coupling J between magnetic layers  112  and  116  equals −0.05 erg/cm 2 , wherein a negative J value denotes AP-coupling. 
     In  FIG. 9A , as mentioned above, magnetic memory cell  100  starts in a “0” logic state, and a logic “0” is written. According to this exemplary embodiment, μ 1  initially points to the left in the zero applied magnetic field. As described above, when μ 1  points to the left in a zero applied magnetic field, magnetic memory cell  100  registers a logic “0.” Beginning at the point of zero applied magnetic field, a current pulse sequence then occurs. 
     First, a 50 Oe word line field is applied. As the word line field is applied, μ 1  rotates up to point along the hard axis direction. A small negative bit line field is then applied. This negative bit line field rotates μ 1  slightly to the left. As the word line field is then decreased, the negative bit line field keeps μ 1  biased to the left, so that when H word  has been decreased to zero again, μ 1  points to the left. μ 1  stays pointing to the left as the bit line field is reduced to zero. 
     In  FIG. 9B , as mentioned above, magnetic memory cell  100  starts in a “0” logic state, and a logic “1” is written. According to this exemplary embodiment, μ 1  initially points to the left in the zero applied magnetic field. As described above, when μ 1  points to the left in a zero applied magnetic field, magnetic memory cell  100  registers a logic “0.” As above, beginning at the point of zero applied magnetic field, a current pulse sequence then occurs. 
     First, a 50 Oe word line field is applied. As the word line field is applied, μ 1  rotates up to point along the hard axis direction. A small positive bit line field is then applied. This positive bit line field rotates μ 1  slightly to the right. As the word line field is then decreased, the positive bit line field keeps μ 1  biased to the right, so that when H word  has been decreased to zero again, μ 1  points to the right. μ 1  stays pointing to the right as the bit line field is reduced to zero. 
     In  FIG. 9C , as mentioned above, magnetic memory cell  100  starts in a “1” logic state, and a logic “0” is written. According to this exemplary embodiment, μ 1  initially points to the right in a zero applied magnetic field. As described above, when μ 1  points to the right in a zero applied magnetic field, magnetic memory cell  100  registers a logic “1.” As above, beginning at the point of zero applied magnetic, a current pulse sequence then occurs. 
     First, a 50 Oe word line field is applied. As the word line field is applied, μ 1  rotates up to point along the hard axis direction. A small negative bit line field is then applied. This negative bit line field rotates μ 1  slightly to the left. As the word line field is then decreased, the negative bit line field keeps μ 1  biased to the left, so that when H word  has been decreased to zero again, μ 1  points to the left. μ 1  stays pointing to the left as the bit line field is reduced to zero. 
     In  FIG. 9D , as mentioned above, magnetic memory cell  100  starts in a “1” logic state, and a logic “1” is written. According to this exemplary embodiment, μ 1  initially points to the right in a zero applied magnetic field. As described above, when μ 1  points to the right in a zero applied magnetic field, magnetic memory cell  100  registers a logic “1.” As above, beginning at the point of zero applied magnetic field, a current pulse sequence then occurs. 
     First, a 50 Oe word line field is applied. As the word line field is applied, μ 1  rotates up to point along the hard axis direction. A small positive bit line field is then applied. This positive bit line field rotates μ 1  slightly to the right. As the word line field is then decreased, the positive bit line field keeps μ 1  biased to the right, so that when H word  has been decreased to zero again, μ 1  points to the right. μ 1  stays pointing to the right as the bit line field is reduced to zero. 
     In general, the three moments μ 1 , μ 2 , and μ 3  bodily rotate together, largely maintaining their original triangular relative orientation. The purpose of the AP-coupling between magnetic layers  112  and  116  is to promote this bodily rotation. 
     Thus, in conclusion, when magnetic memory cell  100  starts in either a “0” or “1” logic state, a word line field followed by a small negative bit line field will write a logic “0.” Similarly, when magnetic memory cell  100  starts in either a “0” or “1” logic state, a word line field followed by a small positive bit line field will write a logic “1.” 
       FIGS. 10A-B  are graphs illustrating moment rotations for magnetic memory cell  100  when a field is applied in the easy axis direction. Specifically,  FIG. 10A  is a hysteresis graph illustrating magnetic moment μ 1  of magnetic layer  108  (normalized to 1) in the easy axis direction and  FIG. 10B  is a graph illustrating the rotations of magnetic moments μ 1 , μ 2 , and μ 3  in magnetic layers  108 ,  112  and  116 , respectively, when a field is applied in the easy axis direction. In  FIG. 10A , the magnetic field applied by the easy axis (H easy ) is plotted on the x-axis and the component of μ 1  along the easy axis (normalized to 1) is plotted on the y-axis. In  FIG. 10B , H easy  is again plotted on the x-axis and the directions of the moments corresponding to magnetic layers  108 ,  112  and  116  are shown. As with  FIGS. 9A-D , described above, in  FIG. 10B , μ 1  corresponding to magnetic layer  108  is depicted as a line ending in a black circle, μ 2  corresponding to magnetic layer  112  is depicted as a line ending in a gray circle and μ 3  corresponding to magnetic layer  116  is depicted as a line ending in a white circle. Circles have been used for ease of depiction and identification of each magnetic moment with the corresponding magnetic layer. However, it is to be understood that each circle can be replaced with an arrowhead, e.g., as in  FIGS. 3A-B , described above wherein μ 1 , μ 2 , and μ 3  are represented by arrows  302 ,  304  and  306 , respectively. 
     The progression of the magnetic moment rotations occurs in the sequence indicated by arrow  1002 . For example, starting at H easy  equals −400 Oe and following the sequence indicated by arrow  1002 , it is shown that μ 1  starts pointing to the left, and subsequently flips to pointing to the right, near +400 Oe, and then as the field is reversed the moment initially stays pointing to the right and then flips to pointing to the left near −400 Oe. 
     The graphs in  FIGS. 10A-B  were all calculated with a single domain theory, using the same parameter values as in, for example, switching curve  800 , above. Namely, magnetic layer  108  has a thickness of six nm; magnetic layer  112  has a thickness of five nm; and magnetic layer  116  has a thickness of five nm. Each of magnetic layers  108 ,  112  and  116  has a M s  equal to 1,500 emu/cc, an H i  of 25 Oe, and a diameter of 150 nm. Finally, the exchange coupling J between magnetic layers  112  and  116  equals −0.05 erg/cm 2 , wherein a negative J value denotes AP-coupling. 
     As can be seen from  FIGS. 10A-B , when a field is applied along the easy axis, magnetic layer  108  does not respond initially, and magnetic layers  112  and  116  become stabilized by the field. This is why such a large field is required to switch magnetic memory cell  100  when the field is applied along the easy axis. 
       FIGS. 11A-B  are graphs illustrating moment rotations for magnetic memory cell  100  when a field is applied in the hard axis direction. Specifically,  FIG. 11A  is a hysteresis graph illustrating magnetic moment μ 1  of magnetic layer  108  (normalized to 1) in the hard axis direction, and  FIG. 11B  is a graph illustrating the rotations of magnetic moments μ 1 , μ 2 , and μ 3  in magnetic layers  108 ,  112  and  116 , respectively, when a field is applied in the hard axis direction. In  FIG. 11A , the magnetic field applied by the hard axis (H hard ) is plotted on the x-axis and the component of μ 1  in the hard axis direction (normalized to 1) is plotted on the y-axis. In  FIG. 11B , H hard  is again plotted on the x-axis and the directions of the moments corresponding to magnetic layers  108 ,  112  and  116  are shown. As with  FIGS. 9A-D , described above, in  FIG. 11B , μ 1  corresponding to magnetic layer  108  is depicted as a line ending in a black circle, μ 2  corresponding to magnetic layer  112  is depicted as a line ending in a gray circle and μ 3  corresponding to magnetic layer  116  is depicted as a line ending in a white circle. Circles have been used for ease of depiction and identification of each magnetic moment with the corresponding magnetic layer. However, it is to be understood that each circle can be replaced with an arrowhead, e.g., as in  FIGS. 3A-B , described above wherein μ 1 , μ 2 , and μ 3  are represented by arrows  302 ,  304  and  306 , respectively. 
     The progression of the magnetic moment rotations occurs in the sequence indicated by arrow  1102 . For example, starting at H hard  equals −50 Oe and following the sequence indicated by arrow  1102 , it is shown that μ 1  starts pointing down, and subsequently flips to pointing up, near +30 Oe, and then as the field is reversed the moment initially stays pointing up and then flips to pointing down near −30 Oe. 
     The graphs in  FIGS. 11A-B  were all calculated with a single domain theory, using the same parameter values as in, for example, switching curve  800 , above. Namely, magnetic layer  108  has a thickness of six nm; magnetic layer  112  has a thickness of five nm; and magnetic layer  116  has a thickness of five nm. Each of magnetic layers  108 ,  112  and  116  has a M s  equal to 1,500 emu/cc, an H i  of 25 Oe, and a diameter of 150 nm. Finally, the exchange coupling J between magnetic layers  112  and  116  equals −0.05 erg/cm 2 , wherein a negative J value denotes AP-coupling. 
     By way of comparison with  FIGS. 10A-B , described above, as can be seen from  FIGS. 11A-B , when a field is applied along the hard axis, moments  302 ,  304  and  306  bodily rotate so that magnetic layer  108  points in the direction of the applied field. 
     Although illustrative embodiments of the present invention have been described herein, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be made by one skilled in the art without departing from the scope of the invention.