Abstract:
A spin-transfer torque magnetic random access memory (STTMRAM) element is configured to store a state when electrical current is applied thereto. The STTMRAM element includes first and second free layers, each of which having an associated direction of magnetization defining the state of the STTMRAM element. Prior to the application of electrical current to the STTMRAM element, the direction of the magnetization of the first and second free layers each is in-plane and after the application of electrical current to the STTMRAM element, the direction of magnetization of the second free layer becomes substantially titled out-of-plane and the direction of magnetization of the first free layer switches. Upon electrical current being discontinued, the direction of magnetization of the second free layer remains in a direction that is substantially opposite to that of the first free layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from previously-filed U.S. Provisional Application No. 61/391,263, filed on Oct. 8, 2010, by Huai et al. and entitled “Magnetic Latch Magnetic Random Access Memory (MRAM)”. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a spin-transfer torque (STT) magnetic random access memory (MRAM), and, more particularly, to an STTMRAM element having magnetic tunnel junctions (MTJs) with a multi-layered free layer. 
     2. Description of the Prior Art 
     Magnetic random access memory (MRAM) is a type of non-volatile memory in which magnetization of magnetic layers in MTJs switches between parallel (corresponding to a low resistance state) and anti-parallel (corresponding to high resistance state) configurations. One type of MRAM is spin-transfer torque magnetic random access memory (STTMRAM) where switching occurs through the application of spin polarized current across the MTJ during programming. 
     STTMRAM has significant advantages over magnetic-field-switched (toggle) MRAM, which has been recently commercialized. The main hurdles associated with field-switched-MRAM are its more complex cell architecture with high write current (currently in the order of milliamps (mA)) and poor scalability attributed to the process used to manufacture these devices. That is, these devices cannot scale beyond 65 nanometer (nm) process node. The poor scalability of such devices is intrinsic to the field writing methods. The current generated fields to write the bits increase rapidly as the size of the MTJ elements shrink. STT writing technology allows directly passing a current through the MTJ, thereby overcoming the foregoing hurdles and resulting in much lower switching current [in the order of microamps (uA)], simpler cell architecture which results in a smaller cell size (for single-bit cells) and reduced manufacturing cost, and more importantly, improved scalability. 
       FIG. 1  shows a prior art STTMRAM element  10  having an anti-ferromagnetic (AFM) layer  6  on top of which is shown formed the a pinned layer (PL) (also known as a “fixed layer”)  5  on top of which is shown formed an exchange coupling layer  4  on top of which is shown formed a reference layer (RL)  3  on top of which is shown formed a barrier layer (BL, also known as a “tunnel layer” or a “MTJ junction layer”)  2  on top of which is shown formed a free layer (FL) (also known as a “storage layer (SL)”)  1 . The layers  3 - 5  are typically referred to as “synthetic antiferromagnetic” (SAF) structure and generally used for providing reference to the free layer  1  during spin torque (ST) switching of the FL  1  and reading of the state of the FL  1  through the resistance down and across the element  10 . The exchange coupling layer (ECL)  4  is typically made of ruthenium (Ru). 
     When electrons flow across the element  10 , perpendicular to the film plane from the RL  3  to the FL  1 , ST from electrons transmitted from the RL  3  to the FL  1  can orientate storage layer or free layer magnetization (as shown by the direction of the arrows in  FIG. 1 ) to a direction parallel to that of RL  3 . When electrons flow from the FL  1  to the RL  3 , ST from electrons reflected from the RL  3  back into the FL  1  can orientate SL magnetization in a direction that is anti-parallel relative to that of RL  3 . With controlling electron (current) flow direction, SL magnetization direction can be switched. Resistance across the element  10  changes between low and high resistance states when the magnetization of the FL  1  is parallel or anti-parallel relative to that of RL  3 . However, the problem with the element  10  as well as other prior art STTMRAM elements is that the level of electric current required to switch the magnetization orientation of FL  1  between parallel and anti-parallel relative to that of RL  3  is still higher than a typical semiconductor CMOS structure can provide, therefore making prior art STTMRAMs&#39; applicability to storage systems not practical.. 
     What is needed is a STTMRAM element that can switch at lower current while still maintaining the same level of stability against thermal agitation. 
     SUMMARY OF THE INVENTION 
     Briefly, a spin-transfer torque magnetic random access memory (STTMRAM) element is configured to store a state when electrical current is applied thereto. The STTMRAM element includes a fixed layer with a magnetization pinned in the plane of the fixed layer and a barrier layer formed on top of the fixed layer. The STTMRAM element further includes a junction layer (JL), and a magnetization layer disposed between the barrier layer and the JL. The magnetization layer is made of a first free layer and a second free layer, separated by a non-magnetic separation layer (NMSL), with the first and second free layers each having in-plane magnetizations that act on each other through anti-parallel coupling. Further included in the STTMRAM element is a perpendicular reference layer (PRL) formed on top of the JL with magnetization in a direction perpendicular to the magnetization of the fixed layer. The direction of the magnetization of the first and second free layers each is in-plane prior to the application of electrical current to the STTMRAM element and after the application of electrical current to the STTMRAM element, the direction of magnetization of the second free layer becomes substantially titled out-of-plane and the direction of magnetization of the first free layer switches. Upon electrical current being discontinued to the STTMRAM element, the direction of magnetization of the second free layer remains in a direction that is substantially opposite to that of the first free layer. 
     These and other objects and advantages of the present invention will no doubt become apparent to those skilled in the art after having read the following detailed description of the various embodiments illustrated in the several figures of the drawing. 
    
    
     
       IN THE DRAWINGS 
         FIG. 1  shows a prior art STTMRAM element  10 . 
         FIG. 2  shows relevant layers of a STTMRAM  20  in accordance with an embodiment of the present invention. 
         FIG. 3(A)  shows the state of the element  20  changing from a parallel magnetic orientation to an anti-parallel magnetic orientation. 
         FIG. 3(B)  shows the state of the element  20  changing from an anti-parallel magnetic orientation to a parallel magnetic orientation. 
         FIG. 4(A)  shows the state of the element  20  during the witching of the FL  24  from a parallel to an anti-parallel orientation relative to the RL  21 . 
         FIG. 4(B)  shows the state of the element  20  during the switching of the FL  24  from an anti-parallel to a parallel orientation relative to the RL  21 . 
         FIG. 5  shows the simulated hard axis transfer curves of a prior art STTMRAM element and a STTMRAM element of the various embodiments of the present invention, such as the element  20 . 
         FIGS. 6(A) and 6(B)  show the actual magnetizations of the free layers at 45 degree state point for S 1  (prior art) and S 2  (t 1 =t 2 =1.5 nm, d=0.5 nm), where S 2  has 60% higher Hk value than S 1 . 
         FIG. 6(A)  shows the magnetization state of S 1  single layer at 45 degree state. 
         FIGS. 6(B) and 6(C)  show the magnetization states of layers  24  and  28  at S 2  tri-layer at 45 degree state. 
         FIG. 7  shows the delta vs. total layer thickness for both the prior art single layer and current invention tri-layer designs. 
         FIG. 8  shows two graphs,  80  and  82 , comparing the switching stability of a STTMRAM element of the embodiments of the present invention that do not include the materials and thicknesses indicated of the FL  28  vs. the switching stability of the STTMRAM element of the embodiments of the present invention that do include the materials and thicknesses indicated herein of the FL  28 . 
         FIG. 9  shows a STTMRAM element  90 , which essentially includes the layers of the element  20 , in a different order, in accordance with another embodiment of the present invention. 
         FIGS. 10-11  show the switching process exhibited by the element  90  of  FIG. 9 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following description of the embodiments, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration of the specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized because structural changes may be made without departing from the scope of the present invention. It should be noted that the figures discussed herein are not drawn to scale and thicknesses of lines are not indicative of actual sizes. 
     In the various STTMRAM elements to follow, a MTJ is employed with perpendicular magnetic anisotropy material(s) with improved stability in non-writing modes and easier switching during writing mode. 
       FIG. 2  shows relevant layers of a STTMRAM element  20  in accordance with an embodiment of the present invention. The STTMRAM element  20  is shown to include a fixed layer (sometimes referred to herein as a “reference layer (RL)”)  21  on top of which is shown formed a barrier layer (sometimes referred to as a “tunneling layer”)  22  on top of which is shown formed a free layer (FL)  24  on top of which is shown formed a non-magnetic separation layer (NMSL)  26  on top of which is shown formed a free layer  28  on top of which is shown formed a junction layer (JL)  30  on top of which is formed a perpendicular reference layer (PRL)  32 . It is understood that the fixed layer  21  is generally formed on top of a substrate with intervening layers therebetween, such as a seed layer, a bottom electrode and other magnetic and non-magnetic layers. The NMSL  26  may be metal or non-metal but is a non-magnetic layer that prevents exchange coupling between its two adjacent layers, the FL  24  and the FL  28 . The FL  24 , NMSL  26  and FL  28  are collectively considered a free layer (or “magnetization layer”) in some embodiments where the free layer is a multi-layered structure. In other embodiments, the free layer may include the same pattern of materials repeated numerous times. For example, the free layer may have a FL/NMSL/FL/NMSL/FL structure. 
     Arrows  131  and  132  show the direction of anisotropy in the FL  28 , in various embodiments. For example, the arrow  131  shows the FL  28  to take on a perpendicular anisotropy and the arrow  132  shows the FL  28  to take on an in-place anisotropy relative to the plane of the film 
     BL  22  is an insulation layer whose resistance changes when the relative magnetization orientations of its two adjacent layers, the FL  22  and the fixed layer  21 , change. 
     In another embodiment, the JL  30  is another barrier layer and in some embodiments, it is made of aluminum (Al) oxide or manganese (Mg) oxide or a conductive layer with Giant Magnetoresistance (GMR) effect between FL  28  and PRL  32 . Electrons flowing between FL  28  and the PRL  32  carry spin torque effect as well and will cause magnetic change on the FL  28  due to spin transfer effects from the PRL  32 . 
     NMSL  26  mainly creates spatial separation between the FL  24  and the FL  28 . No or very weak spin transfer effect between the FL  24  and the FL  28  exists through the NMSL  26 . 
     At non-writing state or when the STTMRAM element  20  is not being written to, the magnetizations of the FL  24  and the FL  28  are anti-parallel and couple to each other through magneto-static coupling field from the edges of these two layers. This coupling enhances thermal stability during non-switching state and increases data retention capability. With additional in-plane anisotropy in the FL  28 , the thermal stability of the tri-layer structure, FL  24 /NMSL  26 /FL  28 , can be further enhanced. Such anisotropy is realized either from shape anisotropy or from crystalline anisotropy. 
     FL  28  can also have a certain perpendicular-to-film plane anisotropy so that when under perpendicular direction spin torque or external field magnetization can further rotate out of plane or even oscillation in-plane due to the perpendicular anisotropy axis. 
     Exemplary materials of which the various layers of the STTMRAM  20  can be made with various associated thicknesses and various characteristics are presented below: 
     PRL  32 : 
     Characteristic: Intrinsic perpendicular anisotropy
         Examplary materials in the case where PRL  32  is a single layer: iron platinum alloy FePtXY, where X and Y each represent a material with X being any of the following materials: boron (B), phosphorous (P), carbon (C), nitride (N) and Y being any of the materials: Cobolt (Co), tantalum (Ta), titanium (Ti), niobium (Nb), zirconium (Zr), tungsten (W), silicon (Si), copper (Cu), silver (Ag), aluminum (Al), chromium (Cr), tin (Sn), lead (Pb), antimony (Sb), hafnium (Hf) and bismuth (Bi), molybdenum (Mo) or ruthenium (Ru).   Alternative Characteristic: Interfacial effect inducing perpendicular magnetic anisotropy   Examplary materials in the case where PRL  32  is made of alloys: Fe-rich FeCoXY or FeNiXY alloys, or alloys CoNiXY.   In the case where PRL  32  is made of multiple layers, exemplary materials are as follows:   [Co/Pt]n, [Co/Pd]n, [Co/Ni]n,   Amorphous ferrimagnetic alloys, such as TbFeCo, GdFeCo
 
JL  30 :
   Non-magnetic metals copper (Cu), silver (Ag), gold (Au);   Non-magnetic materials, aluminum oxide (Al2O3), zinc oxide (ZnO), magnesium oxide (MgO)
 
FL  28 :
   The FL  28  may be made of alloys, such as FeCoXY, FeNiXY, or CoNiXY. The perpendicular anisotropy is tuned while keeping the equilibrium orientation in-plane.   An alloy of one or more of the following material may also comprise the FL  28 : iron (Fe), nickel (Ni), cobalt (Co), platinum (Pt), copper (Cu), boron (B), tantalum (Ta), titanium (Ti), chromium (Cr), niobium (Nb), vanadium (V), zirconium (Zr), terbium (Tb), samarium (Sm), neodymium (Nd), and gadolinium (Gd). May also be comprised of one or more of silicon dioxide (SiO2), titanium dioxide (TiO2), tantalum oxide (Ta2O5), and aluminum oxide (Al2O3).   In other embodiment, the FL  28  is made of magnetic alloys, such as CoFeB-X, where ‘X’ is chosen from the elements having low emissivity into Co and/or Fe, by using one or more of the following elements: Cr, Cu, Ta, Ti, Mo, P, N, and O. ‘X’, in some embodiments, is less than 25 atomic percentage (at %) of Cr, Cu or Mo.   In yet another embodiment, FL  28  is made of CoFeB-Y, where ‘Y’ is chosen from one or more of the oxides and nitrides, such as, SiO2, TiO2, Ta2O5, WO, or ZrO2. ‘Y’, in some embodiments, is less than 20 molar percentage of SiO2 or TiO2.
 
FL  24 :
   An alloy of one or more of iron (Fe), nickel (Ni), cobalt (Co), platinum (Pt), copper (Cu), boron (B), tantalum (Ta), titanium (Ti), chromium (Cr), niobium (Nb), vanadium (V), zirconium (Zr), terbium (Tb), samarium (Sm), neodymium (Nd), and gadolinium (Gd).
 
NMSL  26 :
   Non-magnetic materials, such as titanium dioxide (TiO2), oxide (Al2O3), ruthenium oxide (RuO), strontium oxide (SrO), zinc oxide (ZnO), magnesium oxide (MgO), zirconium dioxide (ZrO2), titanium (Ti), tantalum (Ta), ruthenium (Ru), magnesium (Mg), chromium (Cr), niobium (Nb), nickel niobium (NiNb).   Non-magnetic metals copper (Cu), silver (Ag), gold (Au).   Alternatively, ‘n’ number of interlaced non-magnetic oxide and non-magnetic metallic layers may comprise NMSL  26 , ‘n’ being an integer equal or greater than one.       

     In yet another embodiment, a thin layer, of less than 10 nano meters (nm) in thickness, of Cr, Cu, CrTa, CrTi, CrMo, CrTib, CrZrB, or CrW is deposited on top of the FL  28  prior to deposition of the JL  30 . During manufacturing of the element  20 , and specifically the magnetic annealing process, the foregoing elements segregate along the grain-boundaries and/or along the defect areas to decouple the magnetic grains of the FL  28 . 
     In still another embodiment, a thin layer, typically less than 10 nm, of Cr, Cu, CrTa, CrTi, CrMo, CrTiB, CrZrB, CrW is deposited in the middle of the FL  28 , during manufacturing of the element  20 . 
     In a yet another embodiment, right after deposition of the FL  28 , and before deposition of the JL  30 , an ion implantation process is carried out whereby ions of Cr, Mo, Ta, Ti, Zr, are implanted into the FL  28 . 
     In yet another embodiment, a reactive gas is introduced during the deposition of FL  28 . The gas can be chosen from one or more of the following: O2, N2, Co, Co2, No, NO2, SO2, CF4, or CL2. The flow rate of the inert gas can be kept constant or changed during deposition of the FL  28 . 
     The foregoing approaches and structures desirably result in the formation of largely magnetic and non-magnetic areas in the FL  28  thereby lowering the stiffness of the element  20 . In fact,  FIG. 8  shows graphs showing the affect of the foregoing methods and structure on the behavior of the element  20 , as will be discussed shortly below. 
     It is understood that in various embodiments, any combination of the above-noted material may be employed. 
       FIG. 3(A)  shows the state of the element  20  changing from a parallel magnetic orientation to an anti-parallel magnetic orientation. That is, the direction of magnetization of the FL  24  change relative to the RL  21 .  FIG. 3(B)  shows the state of the element  20  changing from an anti-parallel magnetic orientation to a parallel magnetic orientation. That is, the direction of magnetization of the FL  24  relative to the RL  21 . 
     In  FIG. 3(A) , at the left side of the figure, the element  20  is shown with electrons flowing, as indicated by the arrow  25 , from FL  28  to the direction of BL  22 . These electrons first flow through PRL  32  and then into FL  28 . Due to these transmitted electrons&#39; spin torque (ST) effect between the PRL  32  and the FL  28  through the JL  30 , FL  28 &#39;s magnetization is further rotated out of plane towards the magnetization direction of PRL  32 , and can be substantially perpendicular when spin torque is strong enough to allow such perpendicular configuration. With less in-plane magnetization of FL  28 , FL  24  experiences less coupling field from FL  28  and coupling-induced-stabilization of FL  24  by FL  28  is reduced. Thus, FL  28 -to-FL  24  magnetic latching mechanism is released. Magnetic latching (or “latching” as used herein) refers to the process of weaker coupling of the FL  28  to FL  24  allowing easier switching of the FL  24 . Releasing the latching (or “unlatching” as used herein) refers to the FL  28  coupling to FL  24  to be released. With electrons continuing to travel from FL  24  to RL  21  through BL  22 , reflected electrons from RL  21  lead to switching of FL  24  to an anti-parallel state while FL  28 -FL  24  coupling is unlatched. 
     When FL  24  completes its state switching, electrical current is discontinued. Spin torque from PRL  32  to FL  28  now also discontinues. FL  28 &#39;s magnetization rotates back to an in-plane orientation, as shown by the state of the element  20  on the right side of  FIG. 3(A) . Due to magneto-static field from FL  24 , when magnetization of the FL  28  rotates back in-plane, the magnetization of the FL  28  orients anti-parallel relative to that of FL  24 , which is once again in a latched magnetization configuration. 
     In summary, prior to the flow of electrical current through the element  20 , the direction of magnetization of each of the FLs  24  and  28  is substantially in-plane and after the application of electrical current to the element  20 , with the electrical current flowing through each of the layers thereof, the direction of magnetization of the FL  28  becomes titled out-of-plane, either completely or partially, and the direction of magnetization of the FL  24  switches. When the application of electrical current to the element  20  is discontinued, the direction of magnetization of the FL  28  remains in a direction that is substantially opposite to that of the FL  24 . 
     It is noted that electrical current is applied either from the bottom to the top of the element  20  or from the top to the bottom of the element  20 . 
     With reference to  FIG. 3(B) , switching of FL  24  from anti-parallel to parallel orientation relative to RL  21  is now explained. An analogous operation takes place in  FIG. 3(B)  as that explained relative to  FIG. 3(A) , except that electrons move in the opposite direction, as indicated by the arrow  25  in  FIG. 3(B) . Thus, FL  28  experiences reflected electrons from PRL  32  when current is applied and rotates out of plane opposite to that magnetization of the PRL  32 . FL  28 -to-FL  24  coupling is thus unlatched. FL  24  switches its state to a parallel state relative to RL  21  magnetization due to transmitted electrons from RL  21 . Once switching is completed and current is removed, FL  28   s  magnetization rotates to an in-plane and anti-parallel to the magnetization of FL  24  due to the magneto-static coupling field from FL  24 . 
       FIG. 4(A)  shows the state of the element  20  during the switching of the FL  24  from a parallel to an anti-parallel orientation relative to the RL  21 . The arrow  27  shows the direction of electrons as the arrow  25  in  FIG. 3(A) .  FIG. 4(B)  shows the state of the element  20  during the switching of the FL  24  from an anti-parallel to a parallel orientation relative to the RL  21 . The arrow  27  shows the direction of electrons as the arrow  27  in  FIG. 3(B) . 
     The switching process is analogous to that which is described above relative to  FIGS. 3(A) and 3(B) , except that during current application, FL  28  does not just rotate out of plane. Due to the existence of surface demagnetization field of FL  28 , with possibly a relatively strong perpendicular anisotropy  131  in FL  28 , FL  28  in-plane magnetization component starts to oscillate in-plane and forms a ferromagnetic-resonance (FMR) mode, with a magnetization trajectory depicted as  133 , at the left side of  FIG. 4(A) . Such oscillation can be much higher frequency than the switching speed of the FL  24  and thus produces effectively zero field in the FL  24  from FL  28  over the switching process of the FL  24 . Alternatively, magnetization of FL  28  can become completely perpendicular after the initial oscillation state. After FL  24  switching and current is removed, FL  28  magnetization relaxes back to in-plane orientation anti-parallel to that of the FL  24  due to magneto-static coupling field from FL  24 . 
     In  FIG. 4(B) , the operation of the element  20  is analogous to that of  FIG. 4(A)  discussed above except that the direction of the travel of electrons is in the opposite direction. Thus, FL  28  experiences reflected electrons from the PRL  32  when current is applied and rotates out of plane opposite to the magnetization of the PRL  32  and starts ferromagnetic-resonance (FMR) oscillation. FL  28 -to-FL  24  coupling is thus unlatched. Alternatively, magnetization of the FL  28  can become completely perpendicular after the initial oscillation state. FL  24  switches to a state that is parallel to that of the FL  21  due to transmitted electrons from FL  21 . Once switching is complete and current is removed, FL  28  magnetization rotates back in-plane and anti-parallel to that of FL  24  due to the FL  24 &#39;s magneto-static coupling field. 
     Accordingly, the various embodiments of the present invention realize greater stability than that realized currently by prior art techniques and as discussed hereinabove, use a tri-layer structure, layers  24 - 28 , where FLs  24  and  28 , separated by the NMSL  26 , magneto-statically couple to each other through edge magnetic charges in quiescent state. Such coupling makes the tri-layer structure stable against thermal agitation. Meanwhile, it allows for thinner than usual magnetic layers FL  24  and FL  28  due to stronger thermal stability. In one embodiment of the present invention, the combined thickness of FL  24  and FL  28  is 20% thinner than a single free layer of prior art techniques, at the same thermal stability. 
     Additionally, with magnetic latching FL  28  mainly affected by spin torque from PRL  32 , which is perpendicular in its magnetic state, while coupling between layers FL  24  and FL  28  is affected by NMSL  26 &#39;s thickness, thermal stability through latching effect and easiness of switching with temporarily turning off the latching by spin torque from the PRL  32  to the FL  28  can be individually adjusted with much larger space of optimization than the prior art, where thermal stability and easiness of switching are tightly bonded due to utilizing a single layer FM 2  for the switching and data storage. 
       FIG. 5  shows the simulated hard axis transfer curves of a prior art STTMRAM element with a single free layer (switching layer) and a STTMRAM element of the various embodiments of the present invention, such as the element  20 . An external field is applied to the STTMRAM element structure, where a same size elliptical shape with aspect ratio ˜2 and long axis &lt;200 nm is assumed for all STTMRAM element structures. The single layer case as in  FIG. 5  is for the prior art structures with a single switching layer, i.e. layer  1  in prior art-1 and FM 2  layer in prior art-2. Tri-layer cases consider a switching layer structure composed of layers  24 ˜ 28  as in the embodiments. For the prior art single layer case, the switching layer Ms=1000 emu/cc with thickness t=3 nm. For the embodiment tri-layer cases, layer  24  and  28  have Ms=1000 emu/cc, with thickness t 1 (layer  24 )=t 2 (layer  28 ) and varying from 1 nm to 1.5 nm. The thickness of the spacer layer  26  varies between 0.5 nm and 1 nm in some embodiments. The graph of  FIG. 5  shows external field, in the x-axis, vs. the angle of magnetization of the free layer of the various embodiments of the present invention, in the y-axis. An external field of −1 kOe to +1 kOe is applied in the short axis direction of the ellipse. The free layer magnetization angle relative to the long axis direction of the ellipse is plotted vs. the applied field in  FIG. 5 . At angle of 45 degree, it is regarded the point where the applied field equals intrinsic Hk of the STTMRAM free layer. 
     From  FIG. 5 , it is clearly shown that with tri-layer structure, the field required to reach 45 degree angle is higher than the single layer case. The corresponding Hk values estimated from the transfer curves for each case is also indicated in the legend of  FIG. 5 . By varying the magnetic layer thickness and spacer layer thickness of the tri-layer structure, Hk changes accordingly due to varied coupling strength. However, all cases show a stronger Hk, than single 3 nm layer case as in prior arts, indicating a better thermal stability of the tri-layer structure. 
       FIGS. 6(A) and 6(B)  show the actual magnetizations of the free layers at 45 degree state point for S 1  (prior art) and S 2  (t 1 =t 2 =1.5 nm, d=0.5 nm), where S 2  has 60% higher Hk value than S 1 . 
     In particular,  FIG. 6(A)  is the magnetization state of S 1  single layer at 45 degree state.  FIGS. 6(B) and 6(C)  are the magnetization states of layers  24  and  28  at S 2  tri-layer at 45 degree state.  FIG. 6(A)  and  FIG. 6(B)  are quite similar, only that  FIG. 6(B)  happens at a much higher field. 
       FIG. 7  shows the delta (a measurement of thermal stability) vs. total layer thickness for both the prior art single layer and current invention tri-layer designs. The delta value is defined as magnetic anisotropy energy of the magnetic layer divided by the thermal excitation energy, i.e. K u V/k B T=H k M s V/2k B T, where Hk values are previously obtained from  FIG. 5 , M s =1000 emu/cc for all structures, k B  is Boltzmann constant, T is the absolute temperature of 80 degree Celsius. V for prior art design is the volume of the single switching layer, and the combined volume of layers  24  and  28  in the embodiments of the present invention.  FIG. 7  shows the delta vs. total layer thickness for tri-layer structures with spacer layer thickness of 0.5 nano meters (nm) and 1 nm. For prior art, only t=3 nm is considered as reference. From  FIG. 7 , it is clearly shown that delta for tri-layer design at same total magnetic layer thickness is much higher than prior art single layer. Also to reach same as prior art delta, tri-layer thickness can be &lt;2.4 nm, which is more than 20% thinner than prior art. During switching by spin torque, embodiment type of operation unlatches layer  28  from coupling to layer  24 , and makes switching volume even smaller than the combined volume. Thus a higher stability and easier to switching MRAM MTJ is achieved. 
       FIG. 8  shows two graphs,  80  and  82 , comparing the switching stability of a STTMRAM element of the embodiments of the present invention that do not include the materials and thicknesses indicated of the FL  28  vs. the switching stability of the STTMRAM element of the embodiments of the present invention that do include the materials and thicknesses indicated herein of the FL  28 . Each graph has y-axis representing Mx/Ms (switching stabilization) with ‘Mx’ representing the magnetization of the free layer in the easy axis, i.e. long axis, and ‘Ms’ representing the saturation magnetization of the free layer and x-axis representing time in nano seconds. Graph  80  is the performance of the STTMRAM element  20  without the materials and thicknesses noted in the various embodiments discussed hereinabove at pages 9-12 hereinabove whereas graph  82  is the performance of the STTMRAM element  20  using the materials and thicknesses noted in the various embodiments discussed hereinabove at pages 9-12 hereinabove. The line  131  is the switching behavior of the FL  28  in real-time and when FL  28  has an in-plane magnetization, as does the line  111  relative to another switching state of the FL  28 . Line  132  is the orientation of the FL  28  during switching, as is line  112  when another switching state takes place. Line  100  is a 5 ns pulse starting from 1 ns and ending at 6 ns, applied to the element  20 . As shown, switching of the FL  28 , in graph  82 , is far smoother, with less volatility, than that shown in graph  80 . Switching stabilization improvement desirably causes reduced internal exchange in the FL  28 . It is noted that in graph  80 , the FL  28 &#39;s internal exchange is approximately 1×10 −6  erg/cm whereas in graph  82 , the internal exchange of the FL  28  is 0.2×10 −6  erg/cm. Accordingly, the graphs of  FIG. 8  show reducing the internal exchange of the FL  28 , and applying a current pulse that ends at 6 ns, the FL  24  and the FL  28 , having an in-plane magnetization oscillation, effectively eliminate and render final switched state of the FL  24  and the FL  28  more stable and repeatable. 
       FIG. 9  shows a STTMRAM element  90 , which essentially includes the layers of the element  20 , in a different order, in accordance with another embodiment of the present invention. In  FIG. 9 , the element  90  is shown to have the fixed layer  21 , which is formed on a substrate (not shown) and on top of the layer  21 , is shown formed the BL  22  on top of which is shown formed the FL  24 , on top of which is shown formed the PRL  32 , on top of which is shown formed JL  32 , on top of which is shown formed NMSL  26  and on top of which is shown formed FL  28 . While the direction of the arrows in  FIG. 9  show an in-plane magnetization of the element  90 , a perpendicular magnetization is contemplated, as shown by the lines  131  and  132  of  FIG. 8 , in the FL  28 . 
     During operation, when current is applied to the element  90 , spin transfer torque from the PRL  32  rotates the FL  24  magnetization partially out of plane. With reduced in-plane magnetic moment of the FL  24 , in-plane shape anisotropy of FL  24  reduces and its in-plane switching by the spin transfer torque from fixed layer  21  becomes easier. Once FL  24  is switched and current turned off, FL  24  out of plane magnetization falls back in-plane and magnetizes FL  28  to rotate to opposite direction relative to the FL  24  in-plane magnetization due to the magnetic field from FL  24  acting on FL  28 . 
       FIGS. 10-11  show the switching process exhibited by the element  90  of  FIG. 9 .  FIG. 10  shows the element  90  with the direction of magnetization of the layers  21 ,  24 ,  32  and  28 , as shown by the respective arrows in each layer. The direction of magnetization of the FL  24  is tilted up, for easier switching, and titled up in a direction to pointing to the right of the page. When current  92  is applied to the element  90 , in the direction shown, going from FL  28  down to the layer  21 , the FL  24  switches magnetization direction to point to the left but it still remains tilted, shown at element  90  in the middle of  FIG. 10 . However, the direction of magnetization of the FL  28  remains the same. Next, when current  92  is turned off and no longer applied to the element  90 , as shown by the element  92  appearing at the right-most of  FIG. 10 , the FL  28  switches magnetization direction due to its coupling with the FL  24 . Compared to the MRAM structure without the PRL and JL, the requisite switching voltage, in the embodiment of  FIG. 9 , is lowered by 20%-50%. 
       FIG. 11  shows the switching process exhibited by the element  90  of  FIG. 9  but in a direction that is opposite to that of the  FIG. 10 . In  FIG. 11 , current  94  is shown to be applied in a direction, indicated by the arrow associated with the current  94 , going from the layer  21  to the FL  28 . The FL  28  of the element  90  is initially shown to have a magnetization direction that is opposite to the initial magnetization direction of the FL  28  of  FIG. 10 . The same holds true for the magnetization direction of the FL  24  of  FIG. 11  in association with its counterpart in  FIG. 10  with the FL  24  having a tilted-left direction of magnetization initially. When current  94  is applied, first, as in  FIG. 10 , the direction of magnetization of the FL  24  switches but the direction of magnetization of the FL  28  remains the same, as shown at the element  90  in the middle of the  FIG. 11 . Subsequently, when current  94  is no longer applied to the element  90 , as shown at the left-most of the  FIG. 11 , the direction of magnetization of the FL  28  switches. 
     Although the present invention has been described in terms of specific embodiments, it is anticipated that alterations and modifications thereof will no doubt become apparent to those skilled in the art. It is therefore intended that the following claims be interpreted as covering all such alterations and modification as fall within the true spirit and scope of the invention.