Abstract:
A magnetoresistive random-access memory (MRAM) is disclosed. MRAM device has a magnetic tunnel junction stack having a significantly improved performance of the free layer in the magnetic tunnel junction structure. The MRAM device utilizes a precessional spin current (PSC) magnetic layer in conjunction with a perpendicular MTJ where the in-plane magnetization direction of the PSC magnetic layer is free to rotate.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of Provisional Application No. 62/180,412, filed Jun. 16, 2015. Priority to this provisional application is expressly claimed, and the disclosure of the provisional application is hereby incorporated herein by reference in its entirety. 
     
    
     FIELD 
       [0002]    The present patent document relates generally to spin-transfer torque magnetic random access memory and, more particularly, to a magnetic tunnel junction stack having improved performance of the free layer in the magnetic tunnel junction structure. 
       BACKGROUND 
       [0003]    Magnetoresistive random-access memory (“MRAM”) is a non-volatile memory technology that stores data through magnetic storage elements. These elements are two ferromagnetic plates or electrodes that can hold a magnetic field and are separated by a non-magnetic material, such as a non-magnetic metal or insulator. In general, one of the plates has its magnetization pinned (i.e., a “reference layer”), meaning that this layer has a higher coercivity than the other layer and requires a larger magnetic field or spin-polarized current to change the orientation of its magnetization. The second plate is typically referred to as the free layer and its magnetization direction can be changed by a smaller magnetic field or spin-polarized current relative to the reference layer. 
         [0004]    MRAM devices store information by changing the orientation of the magnetization of the free layer. In particular, based on whether the free layer is in a parallel or anti-parallel alignment relative to the reference layer, either a “1” or a “0” can be stored in each MRAM cell. Due to the spin-polarized electron tunneling effect, the electrical resistance of the cell change due to the orientation of the magnetic fields of the two layers. The cell&#39;s resistance will be different for the parallel and anti-parallel states and thus the cell&#39;s resistance can be used to distinguish between a “1” and a “0”. One important feature of MRAM devices is that they are non-volatile memory devices, since they maintain the information even when the power is off. The two plates can be sub-micron in lateral size and the magnetization direction can still be stable with respect to thermal fluctuations. 
         [0005]    Spin transfer torque or spin transfer switching, uses spin-aligned (“polarized”) electrons to change the magnetization orientation of the free layer in the magnetic tunnel junction. In general, electrons possess a spin, a quantized number of angular momentum intrinsic to the electron. An electrical current is generally unpolarized, i.e., it consists of 50% spin up and 50% spin down electrons. Passing a current though a magnetic layer polarizes electrons with the spin orientation corresponding to the magnetization direction of the magnetic layer (i.e., polarizer), thus produces a spin-polarized current. If a spin-polarized current is passed to the magnetic region of a free layer in the magnetic tunnel junction device, the electrons will transfer a portion of their spin-angular momentum to the magnetization layer to produce a torque on the magnetization of the free layer. Thus, this spin transfer torque can switch the magnetization of the free layer, which, in effect, writes either a “1” or a “0” based on whether the free layer is in the parallel or anti-parallel states relative to the reference layer. 
         [0006]      FIG. 1  illustrates a magnetic tunnel junction (“MTJ”) stack  100  for a conventional MRAM device. As shown, stack  100  includes one or more seed layers  110  provided at the bottom of stack  100  to initiate a desired crystalline growth in the above-deposited layers. Furthermore, MTJ  130  is deposited on top of SAF layer  120 . MTJ  130  includes reference layer  132 , which is a magnetic layer, a non-magnetic tunneling barrier layer (i.e., the insulator)  134 , and the free layer  136 , which is also a magnetic layer. It should be understood that reference layer  132  is actually part of SAF layer  120 , but forms one of the ferromagnetic plates of MTJ  130  when the non-magnetic tunneling barrier layer  134  and free layer  136  are formed on reference layer  132 . As shown in  FIG. 1 , magnetic reference layer  132  has a magnetization direction perpendicular to its plane. As also seen in  FIG. 1 , free layer  136  also has a magnetization direction perpendicular to its plane, but its direction can vary by 180 degrees. 
         [0007]    The first magnetic layer  114  in the SAF layer  120  is disposed over seed layer  110 . SAF layer  120  also has a antiferromagnetic coupling layer  116  disposed over the first magnetic layer  114 . Furthermore, a nonmagnetic spacer  140  is disposed on top of MTJ  130  and a polarizer  150  is disposed on top of the nonmagnetic spacer  140 . Polarizer  150  is a magnetic layer that has a magnetic direction in its plane, but is perpendicular to the magnetic direction of the reference layer  132  and free layer  136 . Polarizer  150  is provided to polarize a current of electrons (“spin-aligned electrons”) applied to MTJ structure  100 . Further, one or more capping layers  160  can be provided on top of polarizer  150  to protect the layers below on MTJ stack  100 . Finally, a hard mask  170  is deposited over capping layers  160  and is provided to pattern the underlying layers of the MTJ structure  100 , using a reactive ion etch (RIE) process. 
         [0008]    Various mechanisms have been proposed to assist the free-layer magnetization switching in magnetic tunnel junction (MTJ) devices. One issue has been that to realize the orthogonal spin transfer effect for in-plane MTJ structures, large spin currents may be required for switching. The need for large switching currents may limit such device&#39;s commercial applicability. One way proposed to reduce switching current is to lower the magnetization of the free layer. However, if the effective magnetization of the free layer is lowered significantly, the orthogonal effect has to be limited so that the free-layer does not go into precessional mode that would make the end state of the free-layer magnetization un-deterministic. This defines the operation window for the in-plane OST structures. In an in-plane device, unlike that shown in  FIG. 1 , the magnetization direction of the reference layer and free layer are in the plane of the layer. Another aspect of in-plane devices is that the thermal stability requirements may limit the size of the MTJ devices to approximately sixty nanometers or higher. 
         [0009]    For perpendicular MTJ structures such as those shown in  FIG. 1 , the precession is not an issue. The orthogonal polarizer acts on the free layer magnetization at the initial state, but when the precession takes hold, the fixed orthogonal polarizer  150  helps only half the cycle of the free-layer magnetization rotation while it harms the other half of the cycle. This is demonstrated with reference to  FIGS. 2-3 .  FIG. 2 a -2 b    shows switching of a free layer  136  of an MTJ. As is seen, free layer  136  has a magnetization direction  200  perpendicular to that of the polarizer  150 . The magnetization direction  200  of the free layer  136  can rotate by 180 degrees.  FIGS. 2 a -2 b    show precession about the axis of the magnetization vector of free layer  136 . During precession, magnetic vector  200  begins to rotate about its axis in a cone-like manner such that its magnetization vector  200 ′ deflects from the perpendicular axis  202  of free layer  136 . Whereas prior to initiating precession, no component of magnetic vector  200  is in the plane of free layer  136 , once precession starts, a component of magnetic vector  200 ′ can be found both in-plane and orthogonal to free layer  136 . As magnetic vector  200 ′ continues to precess (i.e., switch), the rotation of vector  200 ′ extends further from the center of free layer  136 , as is seen in  FIG. 2   b.    
         [0010]    In all prior MTJ devices using a polarizer such as polarizer  150 , the magnetization direction of polarizer  150  is fixed, which is shown in  FIGS. 1 and 3 . See also U.S. Pat. No. 6,532,164, which states that the direction of the magnetization of the polarizing layer cannot vary in the presence of current. Prior to current passing through the MTJ, the free layer  136  has a magnetization direction  200  perpendicular to that of the polarizer  150 . While the magnetization direction  200  of the free layer  136  can rotate by 180 degrees, such rotation is normally precluded by the free layer&#39;s inherent damping ability  205 , which is represented by a vector  205  pointing to axis  202  (shown as a dashed line in  FIG. 2 a    as well as  FIG. 3 ). Axis  202  is perpendicular to the plane of free layer  136 . This damping  205  has value, defined by the damping constant, which maintains the magnetization direction of the free layer  136 . 
         [0011]    Passing a current through polarizer  150  produces a spin-polarized current, which creates a spin transfer torque  210  in the direction of the polarizer  150  on the magnetization vector  200 . This spin transfer torque from the polarizer adds to the main spin transfer torque that causes free layer magnetization direction switching. In devices like those shown in  FIG. 1 , when the spin transfer torque  210  begins to help overcome the damping  205  inherent to the free layer  136 , the magnetic direction  200 ′ begins to precess about its axis, as shown in  FIG. 2 a   . As seen in  FIG. 3 , spin transfer torque  210  helps the magnetization direction of the free layer  136  to precess in a cone-like manner around an axis  202  perpendicular to the plane of the layers. When a spin polarized current traverses the stack  100 , the magnetization of the free layer  136  precesses in a continuous manner (i.e. it turns on itself in a continuous manner as shown in  FIG. 3 ) with maintained oscillations until the magnetic direction of free layer  136  is opposite the magnetic direction prior to the spin torque causing precession, i.e., the magnetic direction of free layer  136  switches by 180 degrees. 
         [0012]      FIG. 3  illustrates precession of a free layer  136  of an MTJ assisted by spin polarized current provided by polarizing magnetic layer  150 . The spin polarized electrons from polarizer  150  provide torque  210  that helps overcome the damping  205  in the first half of the precession  215  because the torque  210  provided by the spin polarized current is opposite that of the inherent damping  205  of the free layer  136 . This is shown on the right-hand side of the middle portion of  FIG. 3 . However, the spin polarized electrons from polarizer  150  actually harm the switching process during the second half of the precession  220 . The reason for this is that the spin of the electrons in the spin polarized current only apply a torque  210  in the direction of their polarization. Thus, when the magnetic vector is in the half of the precession cycle  220  that is opposite the spin of the polarized electrons, the spin transfer torque  210  actually works with the inherent damping  205  of free layer  136  to make rotation more difficult. This is shown in the left-hand side of the middle portion of  FIG. 3 . Indeed, it is the magnetization vector of the reference layer  132  (not shown in  FIG. 3 ) that overcomes the damping of free layer  136  as well as the spin transfer torque  210  during the half of a precession cycle where the spin of the electrons harms precession, and thus it is the reference layer  132  that allows for completion of precession. 
         [0013]    Thus, in prior devices, because magnetization direction of polarizer  150  is fixed, once the precession holds, it has no positive effect on the switching mechanism for a full one-hundred eighty degree precession. This is because polarized electrons will help the spin transfer torque the most when all vectors are closely aligned. 
         [0014]    Thus, there is a need for a spin torque transfer device that reduces the amount of current needed for switching while also switching at high speeds and requiring reduced chip area. 
       SUMMARY 
       [0015]    An MRAM device is disclosed that has a magnetic tunnel junction stack having a significantly improved performance of the free layer in the magnetic tunnel junction structure that requires significantly lower switching currents and which significantly reduces switching times for MRAM applications. 
         [0016]    In one embodiment, a magnetic device includes a synthetic antiferromagnetic structure in a first plane. The synthetic antiferromagnetic structure includes a magnetic reference layer having a magnetization vector that is perpendicular to the first plane and having a fixed magnetization direction. The device also includes a non-magnetic tunnel barrier layer in a second plane that is disposed over the magnetic reference layer. A free magnetic layer is in a third plane and is disposed over the non-magnetic tunnel barrier layer. The free magnetic layer has a magnetization vector that is perpendicular to the third plane and also has a magnetization direction that can precess from a first magnetization direction to a second magnetization direction. The magnetic reference layer, the non-magnetic tunnel barrier layer and the free magnetic layer form a magnetic tunnel junction. The device also includes a non-magnetic spacer in a fourth plane that is disposed over the free magnetic layer. The device includes a precessional spin current magnetic layer in a fifth plane that is physically separated from the free magnetic layer and coupled to the free magnetic layer by the non-magnetic spacer. The precessional spin current magnetic layer has a magnetization vector with a magnetization component in the fifth plane that can freely rotate in any magnetic direction. The device also includes a current source that directs electrical current through the precessional spin current magnetic layer, the non-magnetic spacer, the free magnetic layer, the non-magnetic tunnel barrier layer, and the magnetic reference layer. The electrons of the electrical current are aligned in the magnetic direction of the precessional spin current magnetic layer. The magnetization direction of the precessional spin current magnetic layer follows precession of the magnetization direction of the free magnetic layer, thereby causing spin transfer torque to assist switching of the magnetization vector of the free magnetic layer. 
         [0017]    In another embodiment, the precessional spin current magnetic layer of the magnetic device has a circular shape. 
         [0018]    In another embodiment, the magnetization direction of the magnetization vector of the precessional spin current magnetic layer is in the fifth plane. 
         [0019]    In another embodiment, the magnetization direction of the precessional spin current magnetic layer has a magnetization component in the fifth plane which can freely rotate in the fifth plane. 
         [0020]    In another embodiment, the precessional spin current magnetic layer comprises CoFeB 
         [0021]    In another embodiment, the precessional spin current magnetic layer is magnetically coupled to the free magnetic layer. 
         [0022]    In another embodiment, the precessional spin current magnetic layer is electronically coupled to the free magnetic layer. 
         [0023]    In another embodiment, precession of the precessional spin current magnetic layer is synchronized to precession of the free magnetic layer. 
         [0024]    In another embodiment, the precessional spin current magnetic layer has a rotation frequency greater than zero. 
         [0025]    In another embodiment, a magnetic device includes a precessional spin current magnetic layer in a first plane. The precessional spin current magnetic layer has a magnetization vector with a magnetization component in the first plane which can freely rotate in any magnetic direction. The device includes a non-magnetic spacer layer in a second plane and disposed over the precessional spin current magnetic layer. A free magnetic layer is in a third plane and disposed over the non-magnetic spacer layer. The free magnetic layer has a magnetization vector that is perpendicular to the third plane and also has a magnetization direction that can precess from a first magnetization direction to a second magnetization direction. The device has a non-magnetic tunnel barrier layer in a fourth plane and disposed over the free magnetic layer. A synthetic antiferromagnetic structure is in a fifth plane. The synthetic antiferromagnetic structure includes a magnetic reference layer having a magnetization vector that is perpendicular to the fifth plane. The magnetic reference layer has a fixed magnetization direction. The magnetic reference layer, the non-magnetic tunnel barrier and the free magnetic layer form a magnetic tunnel junction. The device has a current source that directs electrical current through the precessional spin current magnetic layer, the non-magnetic spacer, the free magnetic layer, the non-magnetic tunnel barrier, and the magnetic reference layer. Electrons of the electrical current are aligned in the magnetic direction of the precessional spin current magnetic layer. The magnetization direction of the precessional spin current magnetic layer follows precession of the magnetization direction of the free magnetic layer, thereby causing spin transfer torque to assist switching of the magnetization vector of the free magnetic layer. 
         [0026]    In another embodiment, a magnetic device includes a magnetic tunnel junction in a first plane. The magnetic tunnel junction includes a free magnetic layer and a reference magnetic layer. The free magnetic layer and the reference magnetic layer are separated by a non-magnetic tunneling barrier layer. The free magnetic layer has a magnetization vector that is perpendicular to the first plane, and can precess from a first magnetization direction to a second magnetization direction. The device also has a non-magnetic spacer in a second plane coupled to the free magnetic layer. A precessional spin current magnetic layer is in a third plane and is coupled through the non-magnetic spacer to the free magnetic layer. The precessional spin current magnetic layer is separated from the free magnetic layer by the non-magnetic spacer. The precessional spin current magnetic layer has a magnetization vector with a magnetization component in the third plane which can freely rotate in any magnetic direction. The magnetization direction of the precessional spin current magnetic layer follows precession of the magnetization direction of the free magnetic layer upon application of current to the device. This causes spin transfer torque to assist switching of the magnetization vector of the free magnetic layer. 
     
    
     
       BRIEF DESCRIPTION THE DRAWINGS 
         [0027]    The accompanying drawings, which are included as part of the present specification, illustrate the presently preferred embodiments and, together with the general description given above and the detailed description given below, serve to explain and teach the principles of the MTJ devices described herein. 
           [0028]      FIG. 1  illustrates a conventional MTJ stack for an MRAM device. 
           [0029]      FIGS. 2A and 2B  illustrate the precession of the free layer in an MTJ. 
           [0030]      FIG. 3  illustrates the precession of the free layer in an MTJ used with a polarizing magnetic layer having a fixed magnetization direction. 
           [0031]      FIG. 4  illustrates the precession of the free layer in an MTJ used with a precessional spin current magnetic layer having a magnetization direction that rotates freely. 
           [0032]      FIG. 5  illustrates an MTJ stack for an MRAM device having a precessional spin current magnetic layer. 
           [0033]      FIG. 6  illustrates the magnetic direction of the precessional spin current magnetic layer of an embodiment. 
           [0034]      FIGS. 7A-7E  are graphs of simulations illustrating the improvement in performance of MTJ devices having precessional spin current magnetic layer. 
           [0035]      FIG. 8  illustrates an alternative embodiment of an MTJ stack for an MRAM device having a precessional spin current magnetic layer. 
       
    
    
       [0036]    The figures are not necessarily drawn to scale and the elements of similar structures or functions are generally represented by like reference numerals for illustrative purposes throughout the figures. The figures are only intended to facilitate the description of the various embodiments described herein; the figures do not describe every aspect of the teachings disclosed herein and do not limit the scope of the claims. 
       DETAILED DESCRIPTION 
       [0037]    The following description is presented to enable any person skilled in the art to create and use a precessional spin current structure for a magnetic semiconductor device such as an MRAM device. Each of the features and teachings disclosed herein can be utilized separately or in conjunction with other features to implement the disclosed system and method. 
         [0038]    Representative examples utilizing many of these additional features and teachings, both separately and in combination, are described in further detail with reference to the attached drawings. This detailed description is merely intended to teach a person of skill in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the claims. Therefore, combinations of features disclosed in the following detailed description may not be necessary to practice the teachings in the broadest sense, and are instead taught merely to describe particularly representative examples of the present teachings. 
         [0039]    In the following description, for purposes of explanation only, specific nomenclature is set forth to provide a thorough understanding of the present teachings. However, it will be apparent to one skilled in the art that these specific details are not required to practice the present teachings. 
         [0040]    This present patent document discloses a MRAM device that does not use a polarization layer having a fixed magnetization direction, and is described with reference to  FIGS. 3-7 . Instead of a polarization layer having a fixed magnetization direction, the MRAM device described in this patent document utilizes a precessional spin current (PSC) magnetic layer  350  in conjunction with a perpendicular MTJ where the in-plane magnetization component direction of the PSC layer is free to rotate. In one embodiment, the PSC magnetic layer  350  will rotate with resonant behavior with the free-layer magnetization precessional dynamics. This will significantly improve the impact of the spin current in overcoming the inherent damping of the free layer  336  since the PSC layer will help the spin torque overcome this damping through the entire orbital motion of the precession cycle rather on only half of the precession. This precessional spin current effect throughout the entire one-hundred eighty degree rotation significantly enhances the free-layer magnetization switching. 
         [0041]      FIG. 4  shows the concept behind the MRAM device using a PSC magnetic layer  350  having magnetization vector  270  that rotates instead of a polarization layer  150  having a magnetic vector with a fixed magnetization direction. The free layer  336  in this embodiment is similar to the free layer  136  previously discussed, in that it has an inherent damping characteristic  205  that can be overcome with the assistance of spin transfer torque. However, the embodiment shown in  FIG. 4  replaces polarizing layer  150  with PSC magnetic layer  350 . As seen in the bottom portion of  FIG. 4 , the direction of the spin transfer torque  310  created by spin current passing through free layer  336  changes with the rotation of PSC magnetic layer  350 . As seen in the middle of  FIG. 4 , spin transfer torque  310  causes the magnetization direction  200 ′ of the free layer  336  to precess in a cone-like manner around an axis  202  perpendicular to the plane of the layers.  FIG. 4 , shows a progression of rotation of the magnetic direction  200 ′ about axis  202 . As discussed, when a spin polarized current traverses the device, the magnetization of the free layer  336  precesses in a continuous manner (i.e. it turns on itself in a continuous manner as shown in  FIG. 4 ) with maintained oscillations until the magnetic direction of free layer  336  is opposite the magnetic direction prior to the spin torque causing precession, i.e., the magnetic direction of free layer  136  switches by 180 degrees. The precessional spin current layer  350  and the free-layer  336  are magnetically and/or electronically coupled such that the magnetization direction of the magnetization vector  270  of the PSC magnetic layer  350  follows the precessional rotation of the magnetic vector of the free layer  336 . This can be seen in  FIG. 4 . 
         [0042]    As seen in on the right-hand side of  FIG. 4 , the spin polarized electrons provide torque  310  helps to overcome the damping  205  in the first half of the precession  215  because the torque  310  provided by the spin polarized current is opposite that of the inherent damping  205  of the free layer  336 . As discussed, the magnetization direction of magnetization vector  270  of PSC magnetic layer  350  rotates. Thus, the polarization of electrons of the spin current created by PSC magnetic layer  350  changes as well. This means that the direction of torque  310  exerted on magnetic vector of free layer  336  changes well, which is seen on the bottom of  FIG. 4 . Thus, unlike prior devices having a fixed polarization magnetic layer  150 , the spin of the electrons in the spin polarized current applies a torque  310  in both halves of the precession cycle, including the half of the precession cycle  220  where devices with fixed polarization magnetic layers  150  actually harmed precession. This is seen in the left-hand side of  FIG. 4 . As is seen, the torque  310  continues to help overcome the inherent damping  205  of free layer  136  throughout the entire precession cycle. 
         [0043]    In an embodiment, the precessional vector  270  of the PSC magnetic layer  350  follows the precessional rotation of the magnetic vector of the free layer  336  by being in alignment therewith. In other embodiments, precessional vector  270  of the PSC magnetic layer  350  follows the precessional rotation of the magnetic vector of the free layer  336  by trailing the free layer&#39;s magnetic vector, as will be discussed below. The magnetization direction of the free layer is switched by the spin torque  310  from the reference layer  132  where the direction of the current defines the final state. 
         [0044]    A memory cell with a precessional spin current MTJ structure  300  is shown in  FIG. 5 . MTJ structure  300  includes one or more seed layers  310  provided at the bottom of stack  300  to initiate a desired crystalline growth in the above-deposited layers. Synthetic antiferromagnetic (SAF) layer  320  is disposed over seed layer  310 . SAF layer  320  is comprised of a first SAF layer  332 , anti-ferromagnetic coupling layer  316  and second SAF layer  314 . Second SAF layer  314  is deposited over seed layer  310 , while anti-ferromagnetic coupling layer  316  is placed over second SAF layer  314 . MTJ  330  is deposited over anti-ferromagnetic coupling layer  316 . MTJ  330  includes first SAF layer  332 , which acts as the reference layer of the MTJ, and is also part of SAF layer  320 . A tunneling barrier layer (i.e., the insulator)  334  is over first SAF layer  332  while the free layer  336  is disposed over tunneling barrier layer  334 . As shown in  FIG. 5 , the magnetization vector of first SAF layer  332  has a magnetization direction that is preferably perpendicular to its plane, although variations of a several degrees are within the scope of what is considered perpendicular. As also seen in  FIG. 5 , free layer  336  also has a magnetization vector that is preferably perpendicular to its plane, but its direction can vary by 180 degrees. A nonmagnetic spacer  340  is disposed over of MTJ  330 . PSC magnetic layer  350  is disposed over nonmagnetic spacer  340 . In one embodiment, PSC magnetic layer  350  has a magnetization vector having a magnetic direction parallel to its plane, and is perpendicular to the magnetic vector of the reference layer  132  and free layer  136 . One or more capping layers  370  can be provided on top of PSC layer  150  to protect the layers below on MTJ stack  100 . 
         [0045]    Nonmagnetic spacer  340  has a number of properties. For example, nonmagnetic spacer  340  physically separates the free layer  336  and the PSC layer  350 . Nonmagnetic spacer  340  promotes strong magnetic and/or electronic coupling such that the magnetic direction of the PSC magnetic layer  350  follows the precession cycle of the free layer  336 . In other words, nonmagnetic spacer  340  couples the magnetic direction of the PSC magnetic layer  350  to the magnetic direction of the free layer  336 . Nonmagnetic spacer  340  transmits spin current efficiently from the PSC magnetic layer  350  into the free layer  336  because it preferably has a long spin diffusion length. Nonmagnetic spacer  340  also promotes good microstructure and high tunneling magnetoresistance (TMR) and helps keep the damping constant of the free layer  336  low. 
         [0046]    PSC magnetic layer  350  has at least the following properties. First, in one embodiment, the magnetization direction of PSC magnetic layer  350  is in the plane of the layer but is perpendicular to magnetization direction of free layer  336 . In other embodiments such as shown in  FIG. 6 , the magnetization direction of PSC magnetic layer  350  can have a horizontal component X and perpendicular component Z such that the angle θ between the plane of free layer  336  and the magnetic direction  270  of PSC magnetic layer  350  can be anywhere between 0 and less than 90 degrees. 
         [0047]    PSC magnetic layer  350  preferably has very low coercivity and is therefore manufactured with a very soft magnetic material, e.g., less than fifty (50) Oersteds. PSC magnetic layer  350  should have a strong magnetic coupling to free layer  336  so that its magnetization direction follows magnetic direction of free layer  336  as it precesses about its axis. In one embodiment, PSC magnetic layer  350  is free to rotate near the same frequency as the precessional motion of the free layer  336 . By having nearly the same frequency of the magnetization rotations (PSC magnetic layer  350  magnetization direction and free layer  336  magnetization precession), the free layer switching time is significantly reduced and also tightens the thermal distribution of switching times. In an embodiment, PSC magnetic layer  350  has a rotation frequency greater than zero. Likewise, in an embodiment, PSC magnetic layer  350  has a circular (or near circular) shape so that its magnetization direction has no shape induced anisotropy in the x-y plane (i.e., in the plane of the magnetic film). 
         [0048]    Seed layer  310  in the MTJ structure shown in  FIG. 5  preferably comprises Ta, TaN, Cr, Cu, CuN, Ni, Fe or alloys thereof. Second SAF layer  314  preferably comprises either a Co/Ni or Co/Pt multilayer structure. First SAF layer  332  preferably comprises either a Co/Ni or Co/Pt multilayer structure plus a thin non-magnetic layer comprised of tantalum having a thickness of two to five Angstroms and a thin CoFeB layer ( 0 . 5  to three nanometers). Anti-ferromagnetic coupling layer  316  is preferably made from Ru having thickness in the range of three to ten Angstroms. Tunneling barrier layer  334  is preferably made of an insulating material such as MgO, with a thickness of approximately ten Angstroms. Free layer  336  is preferably made with CoFeB deposited on top of tunneling barrier layer  334 . Free layer  336  can also have layers of Fe, Co, Ni or alloys thereof. Spacer layer  340  over MTJ  330  can be any non-magnetic material such as 2 to 20 Angstroms of ruthenium, 2-20 Angstroms of Ta, 2-20 Angstroms of TaN, 2-20 Angstroms of Cu, 2-20 Angstroms of CuN, or 2-20 Angstroms MgO layer. 
         [0049]    PSC magnetic layer  350  is preferably made from CoFeB. It can also be made with Co, Fe, Ni magnetic layers or can be made out of their alloys. The magnetic alloys can also have boron, tantalum, copper or other materials. Finally capping layer  370  can be any material that provides good interface to PSC layer such as Ta, TaN, Ru, MgO, Cu, etc. 
         [0050]    The manner in which a bit is written using the precessional spin current MTJ structure  300  will now be described. In particular, an electrical current is supplied, for example, by a current source  375 , which passes electrical current through the precessional spin current magnetic layer  350 , the non-magnetic spacer  340 , the free magnetic layer  336 , the non-magnetic tunneling barrier layer  334 , and the reference layer  332 . The electrons of the electrical current passing through the precessional spin current magnetic layer  350  become spin polarized in the magnetic direction thereof, thus creating a spin polarized current that passes through non-magnetic spacer layer  340 , free magnetic layer  336 , tunneling barrier layer  334 , and reference magnetic layer  332 . The spin polarized current exerts a spin transfer torque on free magnetic layer  336 , which helps overcome the inherent damping of the magnetic material making up the free layer  336 . This causes the free magnetic layer  336  to precess about its axis, which is shown in  FIG. 4 . 
         [0051]    Once the magnetic direction of the free magnetic layer  336  begins to precess, the magnetic direction of the PSC magnetic layer  350  begins to rotate, as is also seen in  FIG. 4 . This rotation is caused by the magnetic and/or electronic coupling between the free magnetic layer  336  and the PSC magnetic layer  350  through the non-magnetic spacer  340 . The rotation of the magnetic direction of the PSC magnetic layer  350  causes the spin polarization of the electrons of the electrical current to change in a manner corresponding to the magnetic direction of the PSC magnetic layer  350 . Because the spin of the electrons of the spin polarized current corresponds to the magnetic direction of PSC magnetic layer  350 , and the magnetic direction of PSC magnetic layer  350  follows the precession of free magnetic layer  336 , the spin of the electrons applies spin transfer torque to the free layer  336  in a direction that varies through an entire switching cycle. Thus, devices using PSC magnetic layer  350  can provide spin transfer torque  205  for an entire switching cycle. 
         [0052]    In particular, the structure described herein utilizing PSC magnetic layer  350  and spacer layer  340  creates precessional magnetization that provides spin current to the free layer  336  of an MTJ throughout the whole precession cycle and therefore significantly enhance the free layer switching process, which will result in faster write times. 
         [0053]    The results of simulating a device having the structure described herein are seen in  FIGS. 7 a -7 e   . In  FIGS. 7 a -7 e   , the Y axis is the magnetization in the Z axis of a device  300  from − 1 . 0  to +1.0. The X axis shows the amount of time it takes with switch the magnetization direction of free layer  336  180 degrees. In the simulations, the precession frequency of the magnetization direction of the PSC magnetic layer  350  is designated as (ω) while the precessional frequency of free layer  336  is designated as (ω p ). The results are shown for ω/ω p  ratios of 0 ( FIG. 7 a   ), 0.5 ( FIG. 7 b   ), 0.7 ( FIG. 7 c   ), 0.9 ( FIG. 7 d   ), and 1.0 ( FIG. 7 e   ). In all cases the tilt angle is 30 degrees, which indicates the efficiency of the spin current effect. 
         [0054]    Because the ω/ω p  ratio for the device shown in  FIG. 8 a    is 0, the PSC magnetic layer  350  is not rotating. Thus, the results shown in  FIG. 7 a    actually show the switching time for a device as in  FIGS. 1 and 3 , i.e., a device with a polarizing layer  150  in which the magnetization direction does not rotate. In contrast,  FIGS. 7 b -7 e    show the switching times for ω/ω p  ratios for a device as in  FIGS. 4-6 , i.e., a device with a PSC magnetic layer  350  in which the magnetization direction rotates and thus follows the precession of free layer  336 . In these embodiments, PSC magnetic layer  350  has a rotation frequency greater than zero. Note that the ω/ω p  ratio indicates how closely the precessional vector  270  of the PSC magnetic layer  350  follows precession of the free layer  336 . In other words, as the ω/ω p  ratio approaches unity, the more closely aligned are the precessional vector  270 ′ of the precessing PSC magnetic layer  350  and the magnetic direction of precessing free layer  336 . As is seen in the simulations shown in  FIGS. 7 a -7 e   , the more the precessional vector  270  of the precessing PSC magnetic layer  350  and the magnetic direction of precessing free layer  336  are aligned, the shorter the switching times of the magnetization direction of layer  336 . Thus, in an embodiment, the frequency of rotation of the precessional vector  270  of the precessing PSC magnetic layer  350  is synchronized to be close to the frequency of rotation of free layer  336 .  FIG. 7 a    shows the switching time for a device such as shown in  FIGS. 1 and 3 , in which the magnetization direction of polarizer  150  is fixed and thus has a frequency of rotation of zero. This embodiment has the longest switching time. As the ratio of the precession frequency ω of PSC magnetic layer  350  to the precession frequency of free layer  336  ω p  increases to 0.5, the switching speed has increases. As is seen in  FIGS. 7 c -7 e   , as the ratio of the precession frequency ω of PSC magnetic layer  350  to the precession frequency of free layer  336  ω p  increases to 0.7, 0.9 and then to 1.0, the switching speed has increased significantly, thus demonstrating the significant improvement provided by the various embodiments described herein. 
         [0055]    An alternative embodiment is shown in  FIG. 8 . In this embodiment, magnetic device  400  has had its MTJ stack inverted with respect to the embodiment shown in  FIG. 5 . In particular, magnetic device  400  includes a seed layer  470 . PSC magnetic layer  450  is placed over seed layer  450 . Nonmagnetic spacer  440  is placed over PSC layer  450 . Nonmagnetic spacer  440  has the same properties, construction and characteristics as nonmagnetic spacer  340 , discussed above. PSC magnetic layer  450  has the same properties, construction and characteristics as PSC magnetic layer  350 , discussed above. MTJ  430  is placed over nonmagnetic spacer  440 . MTJ  430  is generally constructed of free layer  436  (which is placed over nonmagnetic spacer  450 ) and reference layer  432 . Free layer  436  and reference layer  432  are spatially separated from each other by tunneling barrier layer  434 , which is made of an insulating material. Tunneling barrier layer  434  also forms part of synthetic antiferromagnetic (SAF) layer  420 . SAF layer  420  is comprised of a first SAF layer  432 , which is also the reference layer of device  400 , anti-ferromagnetic coupling layer  416  and second SAF layer  414 . Anti-ferromagnetic coupling layer  416  is placed over first SAF layer  432 . Finally, capping layer  410  is placed over SAF layer  420 . Current can be provided by a current source  474 . Other than the ordering of the layers, magnetic device operates in the same manner as described with respect to the embodiment shown in  FIG. 5 . Thus, just as shown in  FIG. 4 , PSC magnetic layer  450  rotates in such a way that spin transfer torque  310  is applied in a beneficial manner throughout the entire precession cycle of free layer  436 . 
         [0056]    All of the layers of devices  300  and  400  illustrated in  FIGS. 5 and 8  can be formed by a thin film sputter deposition system as would be appreciated by one skilled in the art. The thin film sputter deposition system can include the necessary physical vapor deposition (PVD) chambers, each having one or more targets, an oxidation chamber and a sputter etching chamber. Typically, the sputter deposition process involves a sputter gas (e.g., oxygen, argon, or the like) with an ultra-high vacuum and the targets can be made of the metal or metal alloys to be deposited on the substrate. Thus, when the present specification states that a layer is placed over another layer, such layer could have been deposited using such a system. Other methods can be used as well. It should be appreciated that the remaining steps necessary to manufacture MTJ stack  300  are well-known to those skilled in the art and will not be described in detail herein so as not to unnecessarily obscure aspects of the disclosure herein. 
         [0057]    It should be appreciated to one skilled in the art that a plurality of MTJ structures  300  can be manufactured and provided as respective bit cells of an STT-MRAM device. In other words, each MTJ stack  300  can be implemented as a bit cell for a memory array having a plurality of bit cells. 
         [0058]    The above description and drawings are only to be considered illustrative of specific embodiments, which achieve the features and advantages described herein. Modifications and substitutions to specific process conditions can be made. Accordingly, the embodiments in this patent document are not considered as being limited by the foregoing description and drawings.