Abstract:
Disclosed herein are toggle-mode magnetoresistive random access memory (MRAM) devices having small-angle toggle write lines, and related methods of toggle-mode switching MRAM devices. Also disclosed are layouts for MRAM devices constructed according to the disclosed principles. Generally speaking, the disclosed principles provide for non-orthogonally aligned toggle-mode write lines used to switch toggle-mode MRAM devices that employ a bias field to decrease the threshold needed to switch the magnetic state of each device. While the conventional toggle-mode write lines provide for the desired orthogonal orientation of the applied magnetic fields to optimize device switching, the use of a bias field affects this orthogonal orientation. By non-orthogonally aligning the two write lines as disclosed herein, the detrimental affect of the bias field may be compensated for such that the net fields applied to the device for both lines are again substantially orthogonal, as is desired.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of provisional U.S. Application No. 60/909,235, filed Mar. 30, 2007. The disclosure of this provisional application is hereby incorporated herein by reference in its entirety for all purposes. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure relates to magnetoresistive random access memory devices, and more particularly to toggle-mode magnetoresistive random access memory devices having small-angle toggle write lines. 
       BACKGROUND 
       [0003]    Contrary to conventional memory devices, which use an electric charge to store data, a magnetoresistive random access memory (“MRAM”) device stores data magnetically. The advantages of storing data magnetically include non-volatility, high-speed operation, and durability. Although an MRAM device has these advantages over conventional memory devices, its commercial viability still depends on whether technologies can continue to be developed to improve its power consumption, error rate, and stability of the memory state. 
         [0004]    To store and retrieve data magnetically, an MRAM device is constructed to have a magnetic moment vector that switches between two stable orientations. Each stable orientation corresponds to one of two possible memory states of an MRAM device, and has a different electrical resistance. During a write operation, a magnetic field is applied to switch the magnetic moment vector to the desirable orientation. To read the stored data, a current is passed through the MRAM device to sense the electrical resistance and determine the corresponding memory state. 
         [0005]    Several prior references have already disclosed improvements for the MRAM device. An improved method for writing to an MRAM device known as “toggle-mode” was developed and disclosed by Savtchenko et al. in U.S. Pat. No. 6,545,906. The toggle-mode write operation improves the stability of the memory state and the selectability of the write operation by using a rotating magnetic field to write to an MRAM device. Moreover, toggle-mode MRAM devices with improved power consumption are disclosed in U.S. Pat. No. 6,633,498 and U.S. Pat. No. 6,515,341. A disclosed device comprises an extra bias field that acts on the magnetic material in the MRAM device and reduces the power required to switch the orientation of the magnetic moment vector. 
         [0006]    Despite the improvement in stability of memory state and power consumption, a new problem arises due to the use of an extra bias field in MRAM devices. The bias field not only reduces the switching field but also affects the rotating magnetic field during write operations, and this greatly increases the error rate. Thus, there exists a need for a toggle-mode MRAM device with reduced switching field and low error rate during write operations. 
       BRIEF SUMMARY 
       [0007]    Disclosed herein are toggle-mode magnetoresistive random access memory (MRAM) devices having small-angle toggle write lines, and related methods of toggle-mode switching MRAM devices. Also disclosed are layouts for MRAM devices constructed according to the disclosed principles. Generally speaking, the disclosed principles provide for non-orthogonally aligned toggle-mode write lines used to switch toggle-mode MRAM devices that employ a bias field to decrease the threshold needed to switch the magnetic state of each device. While the conventional toggle-mode write lines provide for the desired orthogonal orientation of the applied magnetic fields to optimize device switching, the use of a bias field affects this orthogonal orientation. By non-orthogonally aligning the two write lines as disclosed herein, the detrimental affect of the bias field may be compensated for such that the net fields applied to the device for both lines are again substantially orthogonal, as is desired. 
         [0008]    In one aspect, a toggle-mode MRAM device is disclosed. In one embodiment, the MRAM devices comprises a first write line extending along a first direction on a first plane, and a second write line extending along a second direction on a second plane that is parallel to the first plane, wherein the second direction is non-orthogonal to the first direction. In addition, this device includes a magnetoresistive memory element having a switchable magnetic state and disposed between the first and second write lines in an intersecting region located where the first and second write lines would intersect if located on the same plane. In such embodiments, the first write line is configured to provide a first magnetic field (H-Field) to the memory element when carrying a first write current, and the second write line is configured to provide a second H-Field to the memory element when carrying a second write current, the first and second H-Fields together sufficient to switch a magnetic state of the element when applied to the element in a predetermined arrangement. 
         [0009]    In another aspect, an array of toggle-mode MRAM devices is disclosed. In one embodiment, the array comprises a plurality of magnetoresistive memory elements arranged in rows and columns, and each having a switchable magnetic state. The array further comprises first write lines extending along corresponding rows on a first plane, as well as second write lines extending along corresponding columns on a second plane that is parallel to the first plane. In such embodiments, each of the plurality of elements are disposed between corresponding pairs of the first and second write lines in a corresponding intersecting region located where a first write line would non-orthogonally intersect a second write line if located on the same plane. Also in such embodiments, each first write line is configured to provide a first H-Field to the memory elements when carrying a first write current, and each second write line is configured to provide a second H-Field to the memory elements when carrying a second write current, where the first and second H-Fields together are sufficient to switch a magnetic state of one of the elements when applied to that element in a predetermined arrangement. 
         [0010]    In yet another embodiment, a method of toggle-mode switching an MRAM device is also disclosed. In one embodiment, the method comprises providing a first write current through a first write line in a first direction and on a first plane for applying a first H-Field to a memory element, and providing a second write current through a second write line in a second direction and on a second plane that is parallel to the first plane, wherein the second direction is non-orthogonal to the first direction, for applying a second H-Field to the memory element. In such methods, the first and second H-Fields together are sufficient to switch a magnetic state of the element when applied to the element in a predetermined arrangement. Also in such methods, the magnetoresistive memory element is disposed between the first and second write lines in an intersecting region located where the first and second write lines would intersect if located on the same plane. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    For a more complete understanding of the principles disclosure herein, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
           [0012]      FIG. 1  illustrates a perspective schematic view of a conventional simplified toggle-mode MRAM device; 
           [0013]      FIG. 2  illustrates a detailed sectional view of the MTJ stack in the MRAM device illustrated in  FIG. 1 ; 
           [0014]      FIG. 3  illustrates a plan view of the conventional toggle-mode MRAM device shown in  FIG. 1 ; 
           [0015]      FIG. 4  illustrates a toggle-mode write operation for switching magnetic moment vectors between their first and second stable orientations in conventional toggle-mode MRAM devices; 
           [0016]      FIG. 5  illustrates the net applied magnetic fields on the conventional MRAM device of  FIG. 1  based on the effect of a bias field introduced along the easy axis; 
           [0017]      FIG. 6  illustrates a perspective view of one embodiment of an MRAM device constructed according to the principles disclosed herein; 
           [0018]      FIG. 7  shows a top view of the MRAM device in  FIG. 6 , as well as the net applied magnetic fields on this novel MRAM device once the affects of the bias field have been compensated for according to the disclosed principles; 
           [0019]      FIG. 8  illustrates a partial schematic view of one embodiment of a memory array comprising SATWL MRAM devices according to the disclosed principles; 
           [0020]      FIG. 9  illustrates another embodiment of a memory array comprising SATWL MRAM devices according to the disclosed principles; and 
           [0021]      FIG. 10  illustrates yet another embodiment of a memory array according to the discloses principles that provides a high density of SATWL MRAM devices. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]      FIG. 1  illustrates a perspective schematic view of a conventional simplified toggle-mode MRAM device  100 , which includes a magnetic tunnel junction (“MTJ”) stack  10 , a first write line  50 , and second write line  60 . In the embodiment illustrated in  FIG. 1 , the MTJ stack  10  does not come in contact with the first and second write lines  50  and  60 , and is simply positioned between the two write lines  50  and  60 . In an alternative embodiment, however, the MTJ stack  10  may be electrically connected to one or both of the first and second write lines  50  and  60 . It is also to be appreciated that although the MTJ stack  10  is illustrated in  FIG. 1  as having a rectangular shape, it may be constructed in other shapes and dimensions depending on specific design requirements. 
         [0023]      FIG. 2  illustrates a detailed schematic view of the MTJ stack  10  in the MRAM device  100  illustrated in  FIG. 1 . The MTJ stack  10  is the magnetic material in an MRAM device that can be used to store data based on its magnetic moment or direction. The MTJ stack  10  may include three layers, depending on the type of write operation used with the MRAM device  100 . In the case of an MRAM device designed for toggle-mode writing (“toggle-mode MRAM device”), the MTJ stack  10  may include a free synthetic antiferromagnetic (“SAF”) layer  20 , a pinned SAF layer  30 , and a tunnel barrier  40 . 
         [0024]    The free SAF layer  20  may include an antiferromagnetically-coupled structure that further includes a first ferromagnetic sublayer  22 , antiferromagnetic coupling spacer sublayer  26 , and a second ferromagnetic sublayer  24 . The first ferromagnetic sublayer  22  has a magnetic moment vector  92 , and the second ferromagnetic sublayer  24  has a magnetic moment vector  94 . The antiferromagnetic coupling spacer sublayer  26  eliminates the ferromagnetic coupling between the ferromagnetic sublayers  22  and  24  so that the magnetic moment vectors  92  and  94  can align in an antiparallel orientation. The magnetic moment vectors  92  and  94  may have a resultant vector (“M f  vector”) that is preferably relatively small compared to the total magnetization of its magnetic moment vectors. This would allow the free SAF layer  20  to be nearly-balanced. 
         [0025]    In the absence of an applied magnetic field, the magnetic anisotropy of a nearly-balanced free SAF layer  20  requires the M f  vector and magnetic moment vectors  92  and  94  to align in a first stable orientation. In the presence of an applied magnetic field, the magnetic moment vectors  92  and  94  of a nearly-balance free SAF layer  20  may respond by rotating to a new orientation that are approximately orthogonal to the applied field. This response contrasts with the response of the magnetization of a single ferromagnetic layer structure or an SAF structure that is not nearly-balanced, in which the M f  vector would simply follow the applied field. In a toggle-mode write operation, magnetic fields of different directions can be generated and manipulated so as to cause the magnetic moment vectors  92  and  94  to rotate 180 degrees. This new orientation is a second stable anisotropy orientation even though the magnetic moment vectors  92  and  94  are antiparallel to their first stable orientation. It is the existence of the first and second stable orientations that allows a toggle-mode MRAM device to have two different memory states. The toggle-mode write operation will later be discussed in greater details. 
         [0026]    The antiferromagnetically-coupled structure in the free SAF layer  20  may include ferromagnetic materials such as Ni, Fe, Co, Ru, Cr, CoFe, CoFeB, CoNiFeB, Gd 23 Fe 77 , Gd 24 Fe 76 , Tb 19 Fe 81 , Tb 21 Fe 79 , Dy 17 Fe 83 , and Dy 21 Fe 79 . It is to be appreciated that this is only a representative list of possible materials and that an antiferromagnetically-coupled structure may include other ferromagnetic materials not explicitly mentioned here. It is also to be appreciated that a free SAF layer  20  may include a plurality of the antiferromagnetically coupled structure. For example, an embodiment of the free SAF layer  20  may have N ferromagnetic sublayers separated by N−1 anti-ferromagnetic coupling spacer sublayers, where N is a whole number greater than 2. Such embodiment would increase the switching volume and might be desirable when the size of the MTJ stack  10  decreases. Additionally, a variety of additional structures may be added to the ferromagnetic sublayers in the SAF layer  20  to meet different design needs. For example, a cladding structure may be added to maintain the magnetic state of the ferromagnetic layer. The cladding structure may comprise a soft magnetic material that provides a mechanism for flux closure, thereby preventing the formation of demagnetization fields. 
         [0027]    The pinned SAF layer  30 , like the free SAF layer  20 , may include an antiferromagnetically-coupled structure that includes a pinned ferromagnetic sublayer  32 , antiferromagnetic coupling spacer sublayer  36 , and a fixed ferromagnetic sublayer  34 . The pinned ferromagnetic sublayer  32  has a magnetic moment vector  96 , and the fixed ferromagnetic sublayer  34  has a magnetic moment vector  98 . The magnetic moment vectors  96  and  98  align in an antiparallel orientation due to the effects of the antiferromagnetic coupling spacer sublayer  36 . The magnetic moment vectors  96  and  98  may have a resultant vector (“M p  vector”); but the M p  vector would be fixed due to a pinning antiferromagnetic layer  5 , which is externally attached to the pinned SAF layer  30  at the bottom of the MTJ stack  10 . Consequently, the M p  vector and magnetic moment vectors  96  and  98  do not rotate in response to an applied field, and the pinned SAF layer  30  does not have to be nearly balanced. 
         [0028]    The fixed orientation of the M p  vector is one of the two stable orientations that are determined by the magnetic anisotropy of the pinned SAF layer  30 . This fixed orientation can either be parallel or antiparallel with respect to the orientation of an M f  vector that is not influenced by an applied magnetic field. 
         [0029]    The antiferromagnetically-coupled structure in the pinned SAF layer  30  may include ferromagnetic materials such as Ni, Fe, Co, Ru, Cr, CoFe, CoFeB, CoNiFeB, Gd 23 Fe 77 , Gd 24 Fe 76 , Tb 19 Fe 81 , Tb 21 Fe 79 , Dy 17 Fe 83 , and Dy 21 Fe 79 . It is to be appreciated that this is only a representative list of possible materials and that an antiferromagnetically-coupled structure may include other ferromagnetic materials not explicitly mentioned here. In addition, there may also be more than one antiferromagnetically-coupled structure, such as NiFe+CoB and other applicable combinations. 
         [0030]    The tunnel barrier  40  is sandwiched between the free and pinned SAF layers  20  and  30 . The materials for the tunnel barrier  40  may include electrically insulating materials that form a tunneling junction. An example of such materials is aluminum oxide, or even MgO, AlN, TaN, Ta 2 O 5 . Electrons tunnel across the tunnel barrier  40  to provide the magnetoresistance that is sensitive to the orientation of magnetic moment vectors in the free SAF layer  20 . The magnetoresistance of the MTJ stack  10  is higher when the magnetic moment vectors  92  and  94  are in one particular stable orientation and is lower when they are switched to the other stable orientation. By measuring the magnetoresistance of the MTJ stack  10  using conventional techniques, one can determine the memory state of a toggle-mode MRAM device. 
         [0031]    The orientation of the magnetization of the MTJ stack  10  is indicated in  FIG. 1  by the easy axis  90 . Easy axis  90  is a magnetic anisotropy axis that is in-line with the stable orientations of the magnetic moment vectors  96  and  98 . Absent an applied magnetic field, the easy axis  90  is also in-line with the stable orientations of the magnetic moment vectors  92  and  94  in the free SAF layer  20 . In the presence of an applied magnetic field, such as magnetic fields  54  or  64 , the magnetic moment vectors  92  and  94  may rotate away from the easy axis  90 ; but once the applied magnetic field is removed, the magnetic moment vectors  92  and  94  would again align along the easy axis  90 , either in parallel or antiparallel orientation to their original alignment. As discussed previously, each of the magnetic moment vectors  96  and  98  in the pinned SAF layer  30  are fixed in one stable orientation so that they do not change their orientation with respect to the easy axis  90  even in the presence of an applied magnetic field. 
         [0032]    The toggle write lines  50  and  60  illustrated in  FIG. 1  are electrical conductors that are capable of producing magnetic fields when currents pass through them. When a current  52  passes through the first write line  50  in the direction illustrated in  FIG. 1 , a magnetic field  54  is produced. Similarly, when a current  62  passes through the second write line  60  in the direction illustrated in  FIG. 1 , a magnetic field  64  is produced. The magnetic fields  54  and  64  are orthogonal to currents  52  and  62 , respectively, and the “Right-hand Rule” used in physics determines the direction of the magnetic fields  54  and  64  on a plane of the MTJ stack  10  extending parallel with the lengths of the first and second write lines  50  and  60  (i.e., in the X and Y directions). On this plane of the MTJ stack  10 , example directions for magnetic fields  54  and  64  are illustrated in  FIG. 1 . 
         [0033]    To produce an applied field that can switch the memory states of a toggle-mode MRAM device, conventional techniques employ the architectural arrangement illustrated in  FIG. 1 . In the illustrated architecture, the first and second write lines  50  and  60  are parallel to the two-dimensional plane mentioned above. Further, the MTJ stack  10  is disposed between the first and second write lines  50  and  60  at an intersecting region of the first and second write lines  50  and  60 . As used herein, the term “intersecting region” refers to the proximate region in three-dimensional space where the first and second write lines  50  and  60  overlap and would intersect if the write lines were formed on the same plane extending in the X-Y directions. In this three-dimensional region, the overlapping first and second write lines  50  and  60  can be orthogonally projected onto a two-dimensional plane that is parallel to the lengths of both write lines  50 ,  60 , and extending in the X-Y directions, so that their “orthogonal projections” are intersecting even though the actual write lines  50 ,  60  do not actually intersect. 
         [0034]    An orthogonal projection is known by a person of ordinary skill in art as the two-dimensional graphic representation of an object formed by the perpendicular intersections of lines drawn from points on the object to a plane of projection. Such orthogonal projections of the first and second write lines  50  and  60  may be illustrated by the plan view of the conventional toggle-mode MRAM device  100  shown in  FIG. 3 . In  FIG. 3 , the conventional architecture in  FIG. 1  is shown two-dimensionally by the orthogonal projections of the write lines  50 ,  60  on a two-dimensional plane that is parallel to both write lines  50 ,  60 , and by the orthogonal projection of the easy axis  90  on the same plane. Thus, the solid lines represent the orthogonal projections of the first write line  50  and second write line  60 , and the dashed line represents the orthogonal projection of the easy axis  90 . To simplify the discussion from hereon, unless otherwise noted, a reference to the orientation of the first and second write lines  50  and  60  relative to each other or to the easy axis  90  in this disclosure should be understood as a reference to the orientations of their orthogonal projections. 
         [0035]    In the architecture shown in  FIG. 3 , the first and second write lines are oriented at a 90° angle with respect to each other. The MTJ stack  10  and the freely rotating magnetic moment vectors  92  and  94  are positioned in-line with the easy axis  90 , which passes through the intersection of the orthogonal write lines and orients at a 45° angle with respect to either of the first and second write lines  50  and  60 . 
         [0036]    In this architecture, current  52  generates magnetic field  54  in the direction depicted in  FIG. 3  when it is conducted through the first write line  50 . Similarly, when conducted through the second write line  60 , current  62  generates the magnetic field  64  in the direction depicted in  FIG. 3 . The magnetic fields  54  and  64  can switch the magnetic moment vectors  92  and  94  between their first and second stable orientations if they are applied in proper sequence, in accordance with typical toggle-mode switching techniques. 
         [0037]    A toggle-mode write operation for switching magnetic moment vectors  92  and  94  between their first and second stable orientations is illustrated in  FIG. 4 . The illustrated write operation includes applying a pulse current  52  through the first write line  50  and a delayed pulse current  62  through the second write line  60 . At the junction of the first and second write lines  50  and  60 , the two pulse currents  52  and  62  together produce a rotating magnetic field over sequential periods of time (t 0 -t 4 ). In response to such an applied field, the magnetic moment vectors  92  and  94  in the free SAF layer  20  would rotate by 180° and switch from a first stable orientation to a second stable orientation. It is to be appreciated that the toggle-mode write operation ensures that the magnetic moment vectors  92  and  94  would switch from one stable orientation to another, regardless of which particular magnetoresistive state (high or low) corresponds to the first stable orientation. As a result, the first and second write lines  50  and  60  do not have to have the capability of conducting currents in more than one direction. 
         [0038]      FIG. 4  also shows the orientations of the applied field and the magnetic moment vectors  92  and  94  during the different time periods. Before the write operation begins (“t 0  period”), the pulse currents  52  and  62  have not been applied and there is no applied field. Consequently, the magnetic moment vectors  92  and  94  remain aligned in their first stable orientation along the easy axis  90 . 
         [0039]    During the first time period of the write operation (“t 1  period”), pulse current  52  is applied and it produces a magnetic field  54 , which solely constitutes the applied field in this time period. The orientation of the applied magnetic field is represented by orientation  70  in the t 1  period of  FIG. 4 . Orientation  70  is identical to the orientation of the magnetic field  54  and it forms a 45° angle with respect to the easy axis  90 . In response to the applied field, the magnetic moment vectors  92  and  94 , while substantially maintaining their antiparallel orientation, rotate away from the easy axis  90  to be approximately orthogonal to orientation  70  of the applied field. It is noted that in the illustrated embodiments, vectors  92  and  94  may be seen as becoming a bit less anti-parallel to each other for period t 0  to period t 1 . This may be caused by less coupling in the free SAF layer ( 92  and  94 ). If this coupling is large enough, however, they will keep their complete anti-parallel orientation. The new orientation of the magnetic moment vectors  92  and  94  is shown in the t 1  period of  FIG. 4 . Although this new orientation of the magnetic moment vectors  92  and  94  reduces the total magnetic energy in an applied magnetic field, it is not in a stable energy state. If the applied field is removed during the t 1  period, the magnetic moment vectors  92  and  94  would return to their first stable orientation in-line with the easy axis  90 . 
         [0040]    During the second time period of the write operation (“t 2  period”), pulse current  62  is applied while pulse current  52  is maintained, as shown in the timing diagram included in  FIG. 4 . They generate magnetic fields  64  and  54  at the same time, respectively, which combine to form the applied magnetic field with orientation  70 ′ as shown in the t 2  period of  FIG. 4 . Orientation  70 ′ is in-line with the easy axis  90  and forms a 45° angle with respect to original orientation  70 . As illustrated, the applied field has rotated clockwise from orientation  70  to orientation  70 ′, and this causes the magnetic moment vectors  92  and  94  to also rotate clockwise in order to maintain their orthogonal orientation with respect to the orientation of the applied magnetic field  70 ′. 
         [0041]    During the third time period of the write operation (“t 3  period”), pulse current  52  is turned off while pulse current  62  is maintained, also as shown in the timing diagram. Consequently, the magnetic field  54  is removed and the magnetic field  64  is left to solely constitute the applied field with a new orientation  70 ″. Orientation  70 ″ forms a 90° angle with respect to orientation  70  on the other side of the easy axis  90 . As the applied field further rotates clockwise from orientation  70 ′ to orientation  70 ″, the magnetic moment vectors  92  and  94  respond by also rotating clockwise to the orientation illustrated in the t 3  period of  FIG. 4 . In the illustrated orientation during this time period, the magnetic moment vectors  92  and  94  are close enough to their second stable orientation (i.e., each anti-parallel to their first stable orientation shown in the t 0  period) that they are not likely to return to their first stable orientation if the applied field is removed. Instead, the magnetic moment vectors  92  and  94  are likely to continue to rotate clockwise to their second stable orientation. 
         [0042]    Thus, to finish the write operation, pulse current  62  is turned off and the magnetic field  64 , which remained as the only applied field in the t 3  period, is removed during the fourth time period (“t 4  period”). By the end of the t 4  period, the magnetic moment vectors  92  and  94  would have rotated 180° as illustrated in  FIG. 4 , and this corresponds to a switch in the memory state of the toggle-mode MRAM device  100 . Therefore, the term “switching field” has been used to describe a rotating magnetic field whose magnitude is large enough to switch the magnetic moment vectors  92  and  94  from a first stable orientation to a second stable orientation by rotating them 180°. 
         [0043]    It is desirable to reduce the magnitude of the toggle-mode switching field because such reduction would decrease the required energy to generate the switching field and lower the power consumption of a toggle-mode MRAM device. One way to accomplish this is the conventional approach of introducing a bias field that orients along the easy axis  90  and acts on the magnetic moment vectors  92  and  94 . Conveniently, such a bias field may be generated without any addition to the MTJ stack  10 . The pinned SAF layer  30 , as discussed previously, has an M p  vector, and the magnitude of the M p  vector may be adjusted so as to generate a magnetic fringing field that aligns along the easy axis  90  and acts as a bias field within the free SAF layer  20 . Some of the possible adjustments include varying the shape and dimension of the MTJ stack  10 , or altering the composition of the pinned SAF layer  30 . 
         [0044]    Although a bias field can reduce the switching field, it can unfortunately also interfere with the applied field during the write operation. As discussed previously, a bias field would be introduced along the easy axis  90  and its orientation is illustrated by orientation  75  in  FIG. 5 . This bias field, however, may interfere with the rotating applied field generated by a conventional toggle-mode MRAM device and cause the applied field to rotate from orientations  70 ,  70 ′, and  70 ″ to orientations  80 ,  80 ′, and  80 ″, respectively. During the t 1  period, the bias field in orientation  75  would combine with the magnetic field  54  in orientation  70  and rotate the applied field clockwise to orientation  80 , rather than the desired orientation  70 . Likewise, during the t 3  period, the bias field in orientation  75  would combine with the magnetic field in orientation  70 ″ and rotate the applied field counterclockwise to orientation  80 ″, rather than the desired orientation  70 ″. During the t 2  period, the bias field would combine with the magnetic field generated by the pulse currents  52  and  62  passing though the orthogonal write lines  50  and  60  and would rotate the applied field to a new orientation  80 ′. If the magnitude of the pulse currents  52  and  62  are substantially similar, the new orientation  80 ′ of the applied field may be similar to its bias-free orientation  70 ′ as shown in  FIG. 5 . For the embodiments discussed in this disclosure, the pulse currents  52  and  62  are assumed to have the same magnitude, and thus orientations  70 ′ and 80′ are assumed to be substantially similar. But it is to be appreciated that the magnitudes of pulse currents  52  and  62  may be different in other embodiments. For example, despite resulting in greater power consumption, the magnitude of pulse current  62  may be adjusted to be higher than that of pulse current  52 . 
         [0045]    As seen from the above, the interference on the intended magnetic field(s) caused by the bias field can result in a net applied field that can adversely affect the toggle-mode write operation, which may result in writing errors. As the bias field causes the applied field to rotate counterclockwise to orientation  80 ″ in the t 3  period, the corresponding magnetic moment vectors  92  and  94  do not fully rotate clockwise to reach their second stable orientation. Rather, they remain in more unstable orientations that correspond to higher energy states. This in turn increases the probability of the magnetic moment vectors  92  and  94  reverting to their first stable orientation rather than switching to their second stable orientations when the applied field is removed. A similar problem may also occur in the clockwise direction during the t 1  period. Therefore, during certain time periods, the probability of writing errors increases as the orientation of the applied field increasingly rotates away from the desired orientations  70  and  70 ″. 
         [0046]    The disclosed principles herein are based on the recognition that the adverse effects of a bias field may be reduced if the applied field can be corrected back to its bias-free desirable orientations  70 ,  70 ′, and  70 ″.  FIG. 6  illustrates a perspective view of one embodiment of an MRAM device  600  constructed according to the principles disclosed herein, that corrects the orientation of the applied field with small-angle-toggle-write-lines (“SATWL”). In this embodiment, some of architecture of the SATWL MRAM device  600 , as shown in X-Y-Z space, is similar to that of the conventional MRAM device  100  shown in  FIG. 1 . More specifically, the MTJ stack  10  for a SATWL MRAM device  600  may be constructed using the same materials and structure as those disclosed for a conventional toggle-mode MRAM device  100 . Moreover, its pinned SAF layer  30  may generate a bias field within the free SAF layer  20 , as discussed above. 
         [0047]    In accordance with the disclosed principles, however, the novel MRAM device  600  shown in  FIG. 6  provides a unique orientation of the first and second writing lines  50  and  60  relative to each other. Rather than being orthogonal to each other along their lengths in the X-Y plane, as found in conventional toggle-mode MRAM devices, the first and second write lines  50  and  60  are not orthogonal and actually create two acute angles along their lengths in this X-Y plane. In the Z-X or Z-Y plane, however, the first and second write lines  50 ,  60  may be parallel as depicted in  FIG. 6 , or may otherwise be non-parallel in an alternative embodiment. 
         [0048]      FIG. 7  shows a top view of the SATWL MRAM device  600  constructed as disclosed herein. In the illustrated embodiment, the first write line  50  has rotated clockwise on the X-Y plane, while the second write line  60  has rotated counterclockwise on this plane so that the angles between a projected intersection of these lines  50 ,  60  are not 90°. The non-orthogonal angles between the write lines may be formed differently other embodiments, For example, the first write line  50  may remain in the original position found in conventional devices, while the second write line  60  rotates counterclockwise, or vice versa. It should further be appreciated that the first and second writes line  50  and  60  are interchangeable, and as a result, the orientation of the disclosed SATWL MRAM architecture may be modified according to the principles discussed in this disclosure. For example, the first write line  50  may rotate counterclockwise while the second write line  60  rotates clockwise to create non-orthogonal angles. 
         [0049]    Due to the non-orthogonal alignment along their lengths on the X-Y plane, the first and second write lines  50  and  60  form two identical acute angles, and their angle measurement with respect to each other is represented by θ. The first and second write lines  50  and  60  also form two identical obtuse angles, whose angle measurement can be represented by the expression 180°−θ. 
         [0050]    As illustrated in  FIG. 7 , the MTJ stack  10  may be oriented in such a way that the easy axis  90  passes through the acute angles formed by the write lines  50 ,  60 , and creates two identical angles whose angle measurements are both θ/2. In the embodiments disclosed herein, the easy axis  90  is assumed to have the same orientation illustrated in  FIG. 7 . It is to be appreciated, however, that in other embodiments, the easy axis  90  may be oriented so that it forms a first angle with the first write line  50  and a different second angle with the second write line  60 . This orientation of the easy axis  90  may be desirable when the pulse currents  52  and  62  have different magnitudes and generate magnetic fields of different magnitudes. 
         [0051]    As illustrated in  FIG. 7 , the non-orthogonal architecture of the SATWL MRAM device  600  alters the orientations of the magnetic fields produced by pulse currents  52  and  62 . The new orientations of the magnetic fields  54  and  64 , in turn, correct the orientations of the applied field at the intersecting region of the first and second write lines  50  and  60 . A pulse current  52  passing through the first write line  50  of a SATWL architecture would now generate a magnetic field  54  in orientation  72  instead of orientation  70 ; orientation  72  represents a counterclockwise rotation from orientation  70 . A pulse current  62  passing through the second write line  60  would similarly generate a magnetic field  64  in a new orientation  72 ″ instead of orientation  70 ″. Unlike orientation  72 , however, orientation  72 ″ represents a clockwise rotation from orientation  70 ″. 
         [0052]    During the toggle-mode write operation, the new orientations  72 ,  72 ″ of the magnetic fields  54  and  64  may compensate for the interference generated by the bias field. During the t 1  period illustrated in  FIG. 7 , the magnetic field  54  in orientation  72  may counteract the clockwise pull of the bias field (orientation  75 ), thus correcting the net applied field on the device  600  from orientation  80  to orientation  82 . Also, the magnetic field  64  in orientation  72 ″ may similarly counteract the counterclockwise pull of the bias field during the t 3  period, and this counteraction would correct the net applied field on the device  600  from orientation  80 ″ to  82 ″. 
         [0053]    It is desirable for the corrected orientations  82  and  82 ″ to be substantially similar to the bias-free orientations  70  and  70 ″. Whether this extent of correction can be achieved depends on the choice of θ, which is the angle measurement between the non-orthogonal first and second write lines  50  and  60  disclosed herein. To illustrate the choice of a desired θ, the X-Y space illustrated in  FIG. 7  is first defined. The X-direction is defined by an axis that is orthogonal to the bias-free orientation  70  of the applied field so that it has no vector-component in the X-direction. The Y-direction, on the other hand, is defined by an axis that is orthogonal to the bias-free orientation  70 ″ of the applied field so that it has no vector-component in the Y-direction. In this X-Y space, the X- and Y-axes are orthogonal and the easy axis  90  forms a 45° angle with each axis. 
         [0054]    For orientation  82  to be substantially similar to orientation  70 , θ is chosen so that the corrected applied field in orientation  82  has nearly-balanced X-component vectors, i.e., the X-components of the bias field in orientation  75  and magnetic field  54  in orientation  72  are nearly balanced. In X-Y space, the X-component vector of the bias field in orientation  75  may be expressed as: B bias  cos 45°, 
         [0000]    wherein B bias  is the magnitude of the bias field. The X-component vector of the magnetic field  54  in orientation  72  may be expressed as: 
         [0000]        B   54  sin(45°−θ/2) 
         [0000]    wherein B 54  is the magnitude of the magnetic field  54 . Accordingly, a desired 0 would be one that satisfies equation (1): 
         [0000]        B   bias  cos 45°= B   54  sin(45°−θ/2)  (1). 
         [0055]    Similarly, for orientation  82 ″ to be substantially similar to orientation  70 ″, θ is chosen such that the corrected applied field in orientation  82 ″ has nearly-balanced Y-component vectors in the X-Y space. The Y-component vector of the bias field in orientation  75  may be expressed as: 
         [0000]      B bias  sin 45°, 
         [0000]    and the Y-component vector of the magnetic field  64  in orientation  72 ″ may be expressed as: 
         [0000]        B   64  sin(45°−θ/2), 
         [0000]    wherein B 64  is the magnitude of the magnetic field  64 . Accordingly, a desired θ would be one that satisfies equation (2): 
         [0000]        B   bias  sin 45 °=B   64  sin(45°−θ/2)  (2). 
         [0056]    Although there are two governing equations for choosing θ, it may be possible to choose one θ that would allow both corrected orientations  82  and  82 ″ to be substantially similar to the bias-free orientations  70  and  70 ″. In an embodiment in which the pulse currents  52  and  62  have the same magnitude, B 54  and B 64  would have the same value as well. This, consequently, reduces the right side of both equations to the expression: 
         [0000]        B  sin(45°−θ/2), 
         [0000]    wherein B would be the value of both B 54  and B 64 . Furthermore, the left side of both equations may be reduced to the expression: 
         [0000]      0.707B bias , 
         [0000]    because sin 45° and cos 45° are both equal to the numeric value of 0.5. Thus, one may choose θ based on the resulting equation (3): 
         [0000]      0.707 B   bias   =B  sin(45°−θ/2)  (3). 
         [0000]    In this case, θ may have values greater zero but less than 90, depending on the magnitudes of the bias and magnetic fields. Of course, in embodiments where the applied currents  52 ,  62  are not equal, the individual equations may be employed. 
         [0057]    In an embodiment in which the pulse currents  52  and  62  have different magnitudes, the choice of a desired θ may be guided by a preference for orientation  82 ″ to be substantially similar to orientation  70 ″ during the t 3  period. This preference is based on the premise that interferences from the bias field during other time periods do not have significant impact on the probability of writing errors. It is the energy state and the stability of the magnetic moment vectors  92  and  94  right before the applied field is removed that mainly determines the probability of writing errors. In an alternative embodiment, however, different design considerations may compel the use of other guiding preferences. 
         [0058]      FIG. 8  illustrates a partial schematic view of a memory array  800  comprising SATWL MRAM devices (one of which is labeled  810 ) and constructed according to the disclosed principles. In this embodiment, each MTJ stack within each device  810  is shown to have an elliptical shape instead of the rectangular shape that has been shown in the previous embodiments. Having a different shape, however, does not change how each MTJ stack orients between the write lines. As in the previous embodiments, the easy axis of each MTJ stack still passes through the acute angles formed by the non-orthogonal write lines. 
         [0059]    In the embodiment illustrated in  FIG. 8 , the devices  810  are aligned in horizontal rows and vertical columns. To create this grid-like array  800 , a set of straight write lines (one of which is labeled  850 ) may be used together with a set of zigzagged or staggered write lines (one of which is labeled  860 ). Along one dimension of the array  800 , either horizontally or vertically, the first set of write lines  850  are straight and parallel, and may keep the devices  810  aligned accordingly. In the other dimension of the array  800 , however, the second set of write lines  860  is zigzagged in order to keep the devices  810  aligned in parallel straight rows/columns. This is because the two sets of write lines are not orthogonally aligned at the intersecting region of each MTJ stack  810 , in accordance with the principles of this disclosure. Therefore, as shown in  FIG. 8 , each vertical write line  860  “zigzags” between each device  810  after it crosses each horizontal straight write line  850  at angle θ so that it does not cross into an adjacent column and so the devices  810  in a single column are maintained in substantially straight alignment. In other embodiments, the straight write lines  850  may be oriented vertically and the zigzagged write lines  860  may be oriented horizontally. 
         [0060]      FIG. 9  illustrates another embodiment of a grid-like memory array  900  that comprises SATWL MRAM devices (on of which is labeled  910 ) aligned in horizontal rows and vertical columns. In this embodiment, two sets of zigzagged write lines (one of each of which are labeled  950  and  960 , respectively) are used to create the array  900 . A first set of write lines  950  would zigzag along one dimension of the array  900 , while a second set of write lines  960  would zigzag along the other dimension of the array  900 . A device  910  would lie in each intersecting region of the first and second sets of write lines  950 ,  960 , where the write lines  950 ,  960  cross each other (their projected orientation) at angle θ. The locations of where the write lines  950 ,  960  zigzag are spaced evenly in both dimensions of the array  900  so that the devices  910  may align in straight vertical columns and straight horizontal rows. Furthermore, in embodiments where neither write line  50 ,  60  is zigzagged, the non-orthogonal sets of write lines  50 ,  60  would not result in an array of devices aligned in straight vertical columns and rows, but instead aligned along the non-orthogonal write lines  50 ,  60 . 
         [0061]    In addition to arrays  800 ,  900  shown in  FIGS. 8 and 9 , the disclosed novel SATWL MRAM devices may be arranged in other types of arrays, in accordance with desired designs. For example,  FIG. 10  illustrates a memory array  1000  with a higher density of SATWL MRAM devices (one of which is labeled  1010 ) than seen in  FIGS. 8  and  9 . In this embodiment, the devices  1010  are aligned along a set of parallel straight write lines (one of which is labeled  1050 ) in one dimension, and along a set of zigzagged write lines (one of which is labeled  1060 ) in the other dimension. In contrast to the embodiment illustrated in  FIG. 8 , the zigzagging write lines  1060  do not zigzag after passing through the intersecting region at each straight write line  1050  (i.e., each row of devices  1010 ); rather, they zigzag after passing through the intersecting region at every two straight write lines  1050 . Aligned in this manner, the straight write lines  1050  may be packed closer together to create a memory array  1000  with a higher density of MRAM devices than perhaps provided by other arrangements. 
         [0062]    While various embodiments of toggle-mode MRAM devices, having small-angle toggle write lines, according to the principles disclosed herein have been described above, it should be understood that they have been presented by way of example only, and not limitation. Accordingly, the following claims should be construed broadly to cover any embodiment tailored to achieve the principles disclosed herein. Thus, the breadth and scope of the invention(s) should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with any claims and their equivalents issuing from this disclosure. Furthermore, the above advantages and features are provided in described embodiments, but shall not limit the application of such issued claims to processes and structures accomplishing any or all of the above advantages. 
         [0063]    Additionally, the section headings herein are provided for consistency with the suggestions under 37 C.F.R. 1.77 or otherwise to provide organizational cues. These headings shall not limit or characterize the invention(s) set out in any claims that may issue from this disclosure. Specifically and by way of example, although the headings refer to a “Technical Field,” such claims should not be limited by the language chosen under this heading to describe the so-called technical field. Further, a description of a technology in the “Background” is not to be construed as an admission that technology is prior art to any invention(s) in this disclosure. Neither is the “Brief Summary” to be considered as a characterization of the invention(s) set forth in issued claims. Furthermore, any reference in this disclosure to “invention” in the singular should not be used to argue that there is only a single point of novelty in this disclosure. Multiple inventions may be set forth according to the limitations of the multiple claims issuing from this disclosure, and such claims accordingly define the invention(s), and their equivalents, that are protected thereby. In all instances, the scope of such claims shall be considered on their own merits in light of this disclosure, but should not be constrained by the headings set forth herein.