Patent Publication Number: US-10783944-B2

Title: Magnetic memory and a method of operating magnetic memory

Description:
RELATED APPLICATIONS 
     The present application claims priority to Singapore Patent Application No. 10201800918W, titled “Magnetic Memory and a Method of Forming Magnetic Memory,” filed by Applicant National University of Singapore on Feb. 2, 2018, the contents of which are incorporated by reference herein in their entirety. 
     BACKGROUND 
     Technical Field 
     The present disclosure relates generally to magnetic memory and more specifically to magnetic memory that utilizes a spin-orbit torque (SOT) writing scheme without an assist magnetic field. 
     Background Information 
     Magnetic memory (e.g., magnetic random access memory (MRAM)) is a type of non-volatile memory technology with a high operation (read and write) speed. In contrast to other memory technologies, which use electric charges or current flow to store binary data and are therefore volatile, magnetic memory uses magnetic elements to store binary data, which do not degrade over time, giving non-volatile characteristics. In addition, magnetic memory exhibits low power consumption and is suited for high-speed operation. Accordingly, magnetic memory is a promising candidate for use as a universal memory (i.e. a memory that has multiple desirable properties typically only found in different memory types). 
     Generally, magnetic memory includes a magnetic tunnel junction (MTJ) and electrical current lines. A MTJ generally includes a tunnel layer and two magnetic layers, namely, a free layer and a reference layer. The magnetization direction of the reference layer is generally fixed, while that of the free layer is generally switchable between two stable states. The electrical resistance of a MTJ is sensitive to the relative magnetization alignment between the free layer and the reference layer. Using this property, binary information can be written to a magnetic memory by switching the magnetization direction of the free layer, and later read therefrom based on electrical resistance. For example, a high electrical resistance from an anti-parallel alignment between the free layer and the reference layer can represent a binary “1”, and a low electrical resistance from a parallel alignment between the free layer and the reference layer can represent a binary “0”. 
     Conventionally, in a magnetic memory, spin-transfer torque (STT) is used to switch the magnetization direction of the free layer.  FIG. 1  is a schematic diagram of a conventional MTJ  100  that utilizes a STT writing scheme. The MTJ includes a reference layer  120  separated from a free layer  140  by a tunnel barrier  130 . A write line  150  is adjacent to the free layer  140  and a bit line is adjacent to the reference layer  120 . The magnetization direction of the reference layer  120  is fixed (e.g., in the “up” direction in  FIG. 1 ) and that of the free layer  140  can be switched between two states (e.g., “up” or “down” direction in  FIG. 1 ) by applying a positive or negative writing current across the MTJ  100 . A reading current, which senses the MTJ electrical resistance from the relative alignment of the two magnetic layers, also flows across the MTJ  100 . Since the writing and reading current share the same path, a STT magnetic memory is typically a two-terminal memory. 
     Conventional STT switched magnetic memories, such as shown in  FIG. 1  may suffer several issues. Since the writing and reading currents flow along the same path, a STT switched magnetic memories may suffer from the issue of read disturb, i.e. the inadvertent switching of a free layer from a reading current. Moreover, repeated application of a large writing current across a MTJ may cause the tunnel barrier to breakdown, which eventually destroys the MTJ of a STT switched magnetic memory. 
     In an attempt to mitigate these issues, spin-orbit torque (SOT) writing schemes have been proposed.  FIG. 2  is a schematic diagram of a conventional MTJ  200  that uses a SOT writing scheme Like a STT magnetic memory, the SOT magnetic memory includes a reference layer  120  separated from a free layer  140  by a tunnel barrier  130 . However, a SOT layer is disposed adjacent to the free layer  140 . In contrast to a STT writing scheme where the writing current is applied across the MTJ, a SOT writing current is not applied across the MTJ, but along a SOT layer  250 . The SOT layer  250  is configured to exert SOT on the free layer  140  when a current is passed through the SOT layer  250 . The exerted SOT may switch the free layer  140  (e.g., to an “up” or “down” direction in  FIG. 2 ). The reference layer  120  is fixed in one direction (e.g., in the “up” direction in  FIG. 2 ). Since the writing current and the reading current paths are separated, SOT magnetic memory is typically three-terminal memory, and may be free from the issue of read disturb. Further, since the writing current does not flow across the MTJ, the issue of tunnel barrier breakdown may be avoided. These aspects, in addition to an ability to achieve high-speed operation, low power consumption, and non-volatile characteristics, make a SOT magnetic memory a promising candidate for universal memory. 
     Despite the characteristic described above, however, conventional SOT magnetic memories are not perfect. In a conventional SOT writing scheme, not only the writing current, but also an external assist magnetic field, is necessary to deterministically switch the magnetization direction of a free layer, especially for magnetic layers with perpendicular magnetic anisotropy (PMA) that are required for high-density applications. However, applying an assist magnetic field requires additional components and added complexity. A reliable SOT writing scheme without the need of an assist magnetic field could greatly increase the density and scalability of magnetic memory devices, as applying magnetic field typically requires additional, complex circuits/components. 
     Some attempts have been made to develop a SOT writing scheme that does not need of an assist magnetic field. In the conventional STT and SOT magnetic memories depicted in  FIGS. 1 and 2 ), STT or SOT induced rotational motion of magnetization in the free layer  140  is utilized to achieve the switching. Attempts have been made to instead use current induced domain wall motion driven by SOT to switch the magnetization direction of a free layer  140 . The domain walls in PMA magnetic layers may be driven solely by the current flow in an adjacent SOT layer and do not require an external assist magnetic field. The domain wall motion direction depends on the current polarity and the spin-orbit interaction in a SOT layer (e.g., the domain wall can move along or against current flow direction depending on the SOT layer configuration). Therefore, the magnetization of a domain wall device may be switched by current induced domain wall motion, and the resultant domain expansion, in the absence of an external assist magnetic field. 
     However, using conventional techniques, current induced domain wall motion cannot be used to subsequently switch magnetization direction of a free layer (e.g., from an “up” to a “down” direction, and then, subsequently, from a “down” to an “up” direction, or vice versa), and therefore is not practical for a commercial magnetic memory. 
       FIGS. 3 a -3 d    are a progression of schematic diagrams showing a conventional switching technique that uses current induced domain wall motion driven by SOT, and showing limitations thereof. Once a domain wall is created within a free layer, the magnetization direction can be switched into one direction from SOT driven domain wall motion. Referring to  FIGS. 3 a -3 c   , it can be seen that applying a positive current along the SOT layer  250  results in an “up” to “down” magnetization direction switch in the free layer  140 . However, as the domain wall is annihilated after the switching, applying opposite polarity current does not lead to subsequent switching into the opposite direction. Referring to  FIG. 3 d   , the magnetization direction is not switched back to the “up” state, but it remains in the “down” state when a negative current is applied along the SOT layer  250 . As can be seen, using conventional techniques, current induced domain wall motion can cause a switching event without an assist magnetic field, but only a single switching event, making it impractical for a commercial magnetic memory. 
       FIG. 4  is a graph  400  showing an example of a series of applied currents with opposite polarities and corresponding MTJ resistance change for a conventional current induced domain wall motion device. As can be seen again, only a single MTJ resistance change occurs, and therefor only one switching event. 
     Accordingly, there is a need for new devices and methods that may address at least one of the above noted problems. 
     SUMMARY 
     In example embodiments, a three-terminal SOT magnetic memory and operation method are provided that utilize SOT driven domain wall motion to achieve subsequent switching without the need for an external assist magnetic field. The magnetic memory includes a MTJ having a reference layer, a tunnel barrier layer and a free layer, where the tunnel barrier layer is positioned between the reference layer and the free layer. A SOT layer is disposed adjacent to the free layer. A pair of pinning site are positioned at a longitudinal end of the free layer and each has an opposite magnetization direction from the other. The SOT layer is configured to exert SOT and switch a magnetization direction of the free layer via domain wall motion in a direction of current flow, when an electric current is passed through a length of the SOT layer. Example embodiments of the present disclosure provide reliable, subsequent switching even in the absence of an external assist magnetic field. Since an assist magnetic field typically requires additional complex components, example embodiments of the present disclosure may advantageously greatly increase the density and scalability of magnetic memory. 
     It should be understood that a variety of additional features and alternative embodiments may be implemented other than those discussed in this Summary. This Summary is intended simply as a brief introduction to the reader, and does not indicate or imply that the examples mentioned herein cover all aspects of the disclosure, or are necessary or essential aspects of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The description below refers to the accompanying drawings of example embodiments, of which: 
         FIG. 1  is a schematic diagram of a conventional MTJ that utilizes a STT writing scheme; 
         FIG. 2  is a schematic diagram of a conventional MTJ that uses a SOT writing scheme; 
         FIGS. 3 a -3 d    are a progression of schematic diagrams showing a conventional switching technique that uses current induced domain wall motion driven by SOT, and showing limitations thereof; 
         FIG. 4  is a graph showing an example of a series of applied currents with opposite polarities and corresponding MTJ resistance change, for a conventional current induced domain wall motion device; 
         FIG. 5  is a graph showing an example of a series of applied currents with opposite polarities and corresponding MTJ resistance change, for an example magnetic memory based on SOT driven domain wall motion; 
         FIG. 6  is a schematic diagram showing a first example embodiment of a MTJ that may be subsequently switched using SOT driven domain wall motion in the absence of an external assist magnetic field; 
         FIG. 7  is a schematic diagram showing a second example embodiment of a MTJ that may be subsequently switched using SOT driven domain wall motion in the absence of an external assist magnetic field; 
         FIGS. 8 a -8 d    are a progression of schematic diagrams showing an example writing process using spin-orbit interaction in the absence of an external assist-field, using the example embodiment of  FIG. 6 ; 
         FIGS. 8 e - h    are contrast enhanced magneto-optic Kerr effect images that show an example writing processes similar to  FIGS. 8 a -8 d   , now in a specific example device; 
         FIGS. 9 a -9 d    are a progression of schematic diagrams showing another example writing process using spin-orbit interaction in the absence of an external assist-field, using the example embodiment of  FIG. 6 ; and 
         FIGS. 9 e - h    are contrast enhanced magneto-optic Kerr effect images that show an example writing processes similar to  FIGS. 9 a -9 d   , now in a specific example device. 
     
    
    
     DETAILED DESCRIPTION 
     Definitions 
     As used herein, the term “substrate” should be interpreted broadly to refer to a structure to which one or more materials, or one or more layers of material, may be deposited thereon. A substrate may include the one or more layers of material deposited thereon. A substrate may comprise a wafer comprising one or more layers of material such as, but not limited to, e.g. dielectric layers, metal layers, etc. 
     As used herein, the term “layer”, when used to describe a structure, should be interpreted broadly to refer to a level or thickness of the structure that is distinguishable from another level or thickness of another structure. A layer may comprise the same, or different materials, from the other structure. The layer and the other structure may be the same or different in properties (e.g., size, shape, etc.), as long as they are distinguishable from each other. A layer is not limited to a single material, but may comprise one or more sub-layers or intermediate layers of one or more materials, which may themselves also be distinguishable from adjacent layers. When a layer is formed by individual sub-layers or intermediate layers, the dimensions of each of individual sub-layer or intermediate layer may be the same or different. 
     As used herein, the terms “coupled” or “connected” are intended to cover both directly connected, or connected through one or more intermediate structures, unless otherwise stated. 
     As used herein, the term “adjacent”, when referring to two elements, should be interpreted to refer to one element being in close proximity to another element. Adjacent elements may contact each other, but the term shall not be limited to contact. Adjacent elements may also be elements separated by one or more further elements that are disposed therebetween. 
     As used herein, the term “and/or” (e.g., as in “X and/or Y”) should be interpreted to mean either “X and Y” or “X or Y”, and should be taken to provide explicit support for both meanings or for either meaning. 
     Further, as used herein, the term “substantially” should be understood to include, “entirely” or “completely”, and also to include within a reasonable variation, defined as, a variation of no more than +/−5% when in reference to a value. Further, as used herein, the terms “about” and “approximately” should be understood to mean within a reasonable variation, defined as a variation of no more than +/−5% when in reference to a value. 
     Example Embodiments 
     In one example embodiment, a SOT magnetic memory and associated operation method are provided that utilize SOT driven domain wall motion to achieve subsequent switching without the need for an external assist magnetic field. 
       FIG. 5  is a graph  500  showing an example of a series of applied current with opposite polarities and corresponding MTJ resistance change for an example magnetic memory based on SOT driven domain wall motion. In contrast to  FIG. 4 , one can see subsequent switching events with repeated MTJ resistance change in response to polarity changes of the applied current. 
       FIG. 6  is a schematic diagram showing a first example embodiment of a MTJ  600  that may be subsequently switched using SOT driven domain wall motion in the absence of an external assist magnetic field. In this example, the MTJ  600  is part of a three-terminal SOT magnetic memory. For clarity, it should be understood that  FIG. 6  is not to scale and that the selection devices, such as transistors or diodes, are not shown. The MTJ  600  includes a reference layer  610 , a tunnel barrier  620 , and a free layer  630 . A SOT layer  640  is positioned to be adjacent to the free layer  630  and is configured to exert SOT on the free layer  630  when the writing current is applied through the SOT layer  640 . The magnetization of the free layer  630  may be switched via SOT driven domain wall motion (e.g., from an “up” to a “down” direction, and vice versa, in  FIG. 6 ), while the magnetization of the reference layer  610  is fixed (e.g., the magnetization direction is “up” in  FIG. 6 ). The free layer  630  and the reference layer  610  are depicted as a single layer in  FIG. 6 . However, it should be understood that the free layer  630  and the reference layer  610  may be configured as multilayers, for example, as one or more magnetic layers and one or more nonmagnetic layers. 
     In the first example embodiment shown in  FIG. 6 , the free layer  630  and the SOT layer  640  are designed to have tapered widths, when viewed from an overhead perspective (i.e., when viewed from along a z-axis normal to the layers  610 - 630  of the MTJ  600  that fall in an x-y plane). The tapered width is formed to provide at least two pinning sites (pinning site A  650  and pinning sight B  660 ) at longitudinal ends of a free layer  630  (i.e. at end along the x-axis). The pinning sites  650 ,  660  are connected by tapered regions  670 ,  675  to a narrower portion of the free layer  630  (and in this example, also the SOT layer  640 ) disposed therebetween. The magnetization in the two pinning sites  650 ,  660  are in opposite directions (e.g., the magnetization direction of pinning site A  650  is “down” and magnetization direction of the pinning site B  660  is “up” in  FIG. 6 ). The tapered regions  670 ,  675  or the pinning sites  650 ,  660  may stop domain wall propagation. 
       FIG. 7  is a schematic diagram showing a second example embodiment of a MTJ  700  that may be subsequently switched using SOT driven domain wall motion in the absence of an external assist magnetic field. In this example, the MTJ  700  is part of a three-terminal SOT magnetic memory. For clarity, it should be understood that  FIG. 7  is not to scale and that the selection devices, such as transistors or diodes, are not shown. The MTJ  700  includes a reference layer,  710  a tunnel barrier  720 , and a free layer  730 . A SOT layer  740  is positioned to be adjacent to the free layer  730  and is configured to exert SOT on the free layer  730  when writing current is applied through the SOT layer  740 . The magnetization of the free layer  730  can be switched via SOT driven domain wall motion (e.g., from an “up” to a “down” direction, and vice versa, in  FIG. 7 ), while the magnetization of the reference layer  710  is fixed (e.g., the magnetization direction is “up” in  FIG. 7 ). The free layer  730  and the reference layer  710  are depicted as a single layer  FIG. 7 . However, it should be understood that the free layer  730  and the reference layer  710  may be configured as multilayers, for example, as one or more magnetic layers and one or more nonmagnetic layers. 
     In the second example embodiment shown in  FIG. 7 , the free layer  730  and the SOT layer  740  are designed to have tapered thicknesses, when viewed from a side perspective (i.e., from along a y-axis parallel to the layers  710 - 730  of the MTJ  700  that fall in a x-y plane). The tapered thickness is formed to provide at least two pinning sites (pinning site A  750  and pinning sight B  760 ) at longitudinal ends of free layer  730  (i.e. i.e. at end along the x-axis). The pinning sites  750 ,  760  are connected by tapered regions  770 ,  775  to a narrower portion of the free layer  730  disposed therebetween. The magnetization in the two pinning sites  750 ,  760  are in opposite directions (e.g., magnetization direction of the pinning site A  750  is “down” and magnetization direction of the pinning site B  760  is “up” in  FIG. 7 ). The tapered regions  770 ,  775  or the pinning sites  750 ,  760  may stop domain wall propagation. 
     It should be understood that tapered width as shown in the first example embodiment of  FIG. 6 , and tapered thickness as shown in the second example embodiment of  FIG. 7 , may be used individually, or combined in the same SOT magnetic memory. As shown in both  FIG. 6  and  FIG. 7 , an example SOT magnetic memory may include three terminals, e.g., terminal  1 , terminal  2 , and terminal  3 . A reading current may be applied across the MTJ  600 ,  700  between terminal  1  and terminal  2  or between terminal  1  and terminal  3 . To write the SOT magnetic memory, a writing current is applied through the SOT layer  640 ,  740  between terminal  2  and terminal  3 . The applied writing current results in SOT which is exerted on the free layer  630 ,  730 , and the free layer can be switched between two magnetization directions (e.g., from an “up” to a “down” direction, and vice versa). The switching direction is determined by a combination of the applied current polarity and the magnetization direction in the pinning sites  650 - 660 ,  750 - 760 . 
       FIGS. 8 a -8 d    are a progression of schematic diagrams showing an example writing process using spin-orbit interaction in the absence of an external assist-field, using the example embodiment of  FIG. 6 . The magnetization direction of the free layer  630  is switched from “up” (along the +z-direction) to “down” (along the −z-direction) under a positive applied current (in the +x-direction).  FIG. 8 a    shows the magnetization direction of each element before applying the writing current. In this example, the magnetization directions of the pinning site A  650  and the pinning site B  660  is in the “down” and “up” directions, respectively, and the magnetization direction of the free layer  630  is in the “up” direction. Therefore, a domain wall is formed in the tapered region  675  between pinning site A  650  and the free layer  630 . When positive writing current is applied along the SOT layer  640 , a SOT or an equivalent SOT field is exerted on the domain wall, which pulls the nearby magnetic domains to “down”. Therefore, under the writing current, the “down” domain expands and the “up” domain shrinks, which results in domain wall motion along the current flow direction (in the +x-direction) as shown in  FIGS. 8 b , 8 c   , and  8   d.    
     When the domain wall reaches another tapered region  670  on the opposite end (near pinning site B  660 ) of the free layer  630 , the switching process is completed and the magnetization direction of the free layer  630  is switched from “up” to “down”. Because of the non-homogeneous magnetic anisotropy and the non-uniform current density arising from the tapered region  670  of the free layer  630  and the SOT layer  640 , the domain wall does not move toward the pinning site  660 , but is pinned at the tapered region  670  between the pinning site B  660  and the narrower portion of the free layer  630 , as shown in  FIG. 8 d   . Therefore, the domain wall does not annihilate, and is protected even after the switching process. The domain wall between the pinning site B  660  and the narrower portion of the free layer  630  can be used for subsequent reverse switching. 
       FIGS. 8 e - h    are contrast enhanced magneto-optic Kerr effect images that show an example writing processes similar to  FIGS. 8 a -8 d   , now in a specific example device constructed from Tantalum (Ta)/cobalt-iron-boron (CoFeB) alloy/magnesium oxide (MgO) layers.  FIGS. 8 e , 8 f , 8 g , and 8 h    correspond to the switching process in  FIGS. 8 a , 8 b , 8 c , and 8 d   , respectively. For this example device, a SOT layer of Ta (e.g., with a thickness of about 1 nm to about 30 nm, for example 6 nm), a free layer of Co 40 Fe 40 B 20  (with a thickness of about 0.7 nm to about 10 nm, for example 0.9 nm), a tunnel barrier of a MgO (with a thickness of about 0.6 nm to about 3 nm, for example, 2 nm) and a device protection layer of silicon dioxide (SiO 2 ) (with a thickness of about 1 nm to about 8 nm, for example, 3 nm) were prepared on a silicon (Si)/SiO 2  substrate by magnetron sputtering and annealed at 200° C. for 30 minutes. The structure has been patterned into a tapered nanowire using electron beam lithography and argon (Ar) ion etching. The nanowire serves as a free layer  630  and the tapered regions at two ends of the nanowire also act as pinning site A  650  and pinning site B  660 . 
       FIG. 8 e    shows the magnetization direction of each element before applying the current. For this example writing process, the magnetization direction of the pinning site A  650  and the pinning site B  660  is in the “down” direction (indicated by light shading) and the “up” direction (indicated by dark), respectively, and the magnetization direction of the free layer  630  is in the “up” direction. With applying the positive writing current (in the +x-direction), the “down” domain expands and the “up” domain shrinks, which results in domain wall motion along the current flow direction, as shown in  FIGS. 8 f , 8 g   , and  8   h . The current may be applied for a duration of 0.1 ns to 50 ns. When the domain wall reaches the tapered region on the opposite end of free layer  630 , the switching process is completed and the free layer  630  is switched from “up” to “down”. Further increasing the magnitude or duration of applied current does not move the domain wall toward the pinning site B  660 . Therefore, the domain wall does not annihilate and is protected even after the switching process. This is in contrast to what occurs in a conventional device, where the domain wall is annihilated after a first switching process and is unavailable for subsequent switching, as discussed above in reference to  FIG. 3 . 
       FIGS. 9 a -9 d    are a progression of schematic diagrams showing another example writing process using spin-orbit interaction in the absence of an external assist-field, using the example embodiment of  FIG. 6 . Here, the magnetization direction of the free layer  630  is switched from “down” (along the −z-direction) to “up” (along the +z-direction) under negative applied current (in the −x-direction).  FIG. 9 a    shows the magnetization direction of each element before applying the current. The magnetization directions of the pinning site A  650  and the pinning site B  660  are in “down” and “up” directions, respectively, and the magnetization direction of the free layer  630  is in the “down” direction. Therefore, a domain wall is formed in the tapered region  670  between the pinning site B  660  and the narrower portion of the free layer  630 . This writing process can be considered as an opposite switching process, compared the “up” to “down” switching process illustrated in  FIG. 8 , because the magnetization configuration before applying current (e.g., the magnetization configuration in  FIG. 9 a   ) is identical to the magnetization configuration after “up” to “down” switching (e.g., the magnetization configuration in  FIG. 8 d   ). When the negative writing current is applied along the SOT layer  640 , a SOT or an equivalent SOT field is exerted on the domain wall, which pulls the nearby magnetic domains into the “up” state. Therefore, under the writing current, the “up” domain expands and the “down” domain shrinks, which results in the domain wall motion along the current flow direction, as shown in  FIGS. 9 b , 9 c   , and  9   d.    
     When the domain wall reaches another tapered region  675  at the opposite end of the free layer  630  near pinning site A  650 , the switching process is completed and the free layer  630  is switched from “down” to “up”. Because of the non-homogeneous magnetic anisotropy and the non-uniform current density arising from the tapered region  675  of the free layer  630  and the SOT layer  640 , the domain wall does not move toward the pinning site A  650 , but is pinned at the tapered region  675  between the pinning site A  650  and the narrower portion of the free layer  630 , as shown in  FIG. 9 d   . By utilizing this domain wall and a negative applied current, the opposite “up” to “down” switching can be achieved again (e.g., as already described above in reference to  FIG. 8 ), allowing the device to be repeatedly switched between states. 
       FIGS. 9 e - h    are contrast enhanced magneto-optic Kerr effect images that show an example writing processes similar to  FIGS. 9 a -9 d   , now in a specific example device constructed from Ta/CoFeB/MgO layers.  FIGS. 9 e , 9 f , 9 g , and 9 h    correspond to the switching process in  FIGS. 9 a , 9 b , 9 c , and 9 d   , respectively. This example device is identical to that described above in relation to  FIGS. 8 e - h   .  FIG. 9 e    shows the magnetization direction of each element before applying the current. The magnetization directions of the pinning site A  650  and the pinning site B  660  are in the “down” direction (as indicated by light shading) and the “up” direction (as indicated by dark shading), respectively, and the magnetization direction of the free layer  630  is “down”. With applying the negative writing current (in the −x-direction), the “up” domain expands and the “down” domain shrinks, which results in domain wall motion along the current flow direction as shown in  FIGS. 9 b , 9 c , and 9 d   . When the domain wall reaches another tapered region on the opposite end of free layer  630 , the switching process is completed and the free layer is switched from “down” to “up”. Further increasing the magnitude or duration of applied current does not move the domain wall toward the pinning site A  650 . Therefore, the domain wall does not annihilate and is protected after the switching process. 
     In summary, example embodiments of the present disclosure describe a SOT magnetic memory and operation method that utilize SOT driven domain wall motion to achieve subsequent switching without the need for an external assist magnetic field. It should be understood that various adaptations and modifications may be made to the above-discussed techniques. For example, while it is discussed that the domain wall motion direction is along the current direction, it should be understood that the domain wall motion direction with respect to the current flow direction may be altered by a choice of the SOT layer and/or a configuration of SOT and the free layer. While it is discussed that the magnetization directions of the pinning site A and the pinning site B are in the “down” direction and “up” direction, respectively, it should be understood that this may be reversed (i.e. the magnetization directions of the pinning site A and the pinning site B may be in the “up” direction and “down” direction, respectively). Likewise, the magnetization direction of the reference layer may be in the “down” or “up” direction. 
     While it is discussed above that the free layer may be a CoFeB alloy, such as Co 40 Fe 40 B 20 , it should be understood that other alloys, including other CoFeB alloys such as Co 60 Fe 20 B 20 , cobalt-iron (CoFe) alloy, cobalt-iron (CoFe), and the like may be used. In general, the material choices for the free layer and reference layer can be, but are not restricted to, any of a variety of ferromagnetic materials and their alloys and multilayers, ferrimagnetic materials and their alloys and multilayers, and ferromagnetic or ferrimagnetic insulators. The free and reference layers can be a magnetic single layer or a combination of magnetic layers and non-magnetic layers. 
     While it is discussed above that the SOT layer may be Ta, it should also be understood that the SOT layer may alternatively be tungsten (W), platinum (Pt), or other materials. In general, the SOT layer can be any material with a sufficient ability to exert SOT, and the material choices for the SOT layer can be, but not restricted to, normal metals, heavy metals, metallic alloys, topological insulators, 2-dimensional electron gas, Weyl semi-metal, metallic multilayers, or a combination of the above. The tunnel barrier can be, but is not restricted to, any oxide or other insulating or nonmagnetic materials. Further, while it was discussed above that the materials in a specific example device were grown using sputtering, it should be understood that a variety of other metal and oxide growth techniques may be utilized. Annealing conditions may also vary. While, a patterning technique using electron beam lithography and Ar ion etching is mentioned above, it should be understood that various other techniques, including photolithography, focused ion beam etching, etc. may be used. 
     Furthermore, it should be understood that in the above description when value ranges are discussed, it is intended that the ranges cover and teach all possible sub-ranges as well as individual numerical values within the ranges. That is, the end points of a range should not be interpreted as inflexible limitations. For example, a description of a range of 1% to 5% should be interpreted to disclose sub-ranges such as 1% to 2%, 1% to 3%, 1% to 4%, 2% to 3% etc., as well as individually values within the range such as 1%, 2%, 3%, 4% and 5%. 
     In generally, it should be understood that the various elements described above may be made from differing materials, implemented in different combinations or otherwise formed or used differently without departing from the intended scope of the disclosure. Example embodiments are not necessarily mutually exclusive as some may be combined with one or more embodiments to form new example embodiments. It will be appreciated by a person skilled in the art that other variations and/or modifications may be made to the specific embodiments without departing from the scope of the invention. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.