Abstract:
A magnetic random access memory (MRAM) element is configured to store a state when electric current flows therethrough. The MRAM element includes a first magnetic tunnel junction (MTJ) for storing a data bit and a reference bit MTJ for storing a reference bit. The data bit MTJ and reference bit MTJ are preferred to be of identical structure that includes a magnetic free layer (FL) having a switchable magnetization with a direction that is perpendicular to a film plane. The direction of magnetization of the FL is determinative of the data bit stored in the at least one MTJ. The identical structure further includes a magnetic reference layer (RL) having a magnetization with a direction that is perpendicular to the film plane, and a magnetic pinned layer (PL) having a magnetization with a direction that is perpendicular to the film plane. The direction of magnetization of the RL and the PL are anti-parallel relative to each other in the data bit MTJ for storing data bit, wherein when electric current is applied to the first MTJ, the magnetization orientation of the FL switches during a write operation, whereas, the direction of magnetization the RL and the PL remain the same. The direction of magnetization of the FL, the RL and the PL are parallel relative to each other in the reference bit MTJ for storing reference bit, the magnetization orientation of the FL does not switch under normal read operations.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation in part of previously-filed U.S. Provisional application Ser. No. 13/360,524, entitled, “PERPENDICULAR MRAM DEVICE AND ITS INITIALIZATION METHOD”, filed by Yuchen Zhou, et al., on Jan. 27, 2012, which claims priority to previously-filed U.S. Provisional Application No. 61/510,025, entitled “PERPENDICULAR MRAM DEVICE AND ITS INITIALIZATION METHOD”, filed by Yuchen Zhou, et al., on Jul. 20, 2011, and is a continuation in part of previously-filed U.S. Provisional application Ser. No. 13/360,553, entitled, “INITIALIZATION METHOD OF A PERPENDICULAR MAGNETIC RANDOM ACCESS MEMORY (MRAM) DEVICE WITH A STABLE REFERENCE CELL”, filed by Yuchen Zhou, et al., on Jan. 27, 2012, which claims priority to previously-filed U.S. Provisional Application No. 61/510,025, entitled “PERPENDICULAR MRAM DEVICE AND ITS INITIALIZATION METHOD”, filed by Yuchen Zhou, et al., on Jul. 20, 2011, and is a continuation in part of previously-filed U.S. Provisional application Ser. No. 13/223,070, entitled, “INITIALIZATION METHOD OF A PERPENDICULAR MAGNETIC RANDOM ACCESS MEMORY (MRAM) DEVICE”, filed by Yuchen Zhou on Jul. 11, 2012, which claims priority to previously-filed U.S. Provisional Application No. 61/510,025, entitled “PERPENDICULAR MRAM DEVICE AND ITS INITIALIZATION METHOD”, filed by Yuchen Zhou, et al., on Jul. 20, 2011. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to non-volatile magnetic memory and particularly to non-uniform switching of non-volatile magnetic based memory. 
     2. Description of the Prior Art 
     Computers conventionally use rotating magnetic media, such as hard disk drives (HDDs), for data storage. Though widely used and commonly accepted, such media suffer from a variety of deficiencies, such as access latency, higher power dissipation, large physical size and inability to withstand any physical shock. Thus, there is a need for a new type of storage device devoid of such drawbacks. 
     There has been an extensive effort in development of alternative technologies such as Ovanic Ram (or phase-change memory), Ferromagnetic Ram (FeRAM), Magnetic Ram (MRAM), Nanochip, and others to replace memories used in current designs such as DRAM, SRAM, EEPROM/NOR flash, NAND flash and HDD in one form or another. Although these various memory/storage technologies have created many challenges, there have been advances made in this field in recent years. MRAM has exceptional advantage when compared to other memory technologies under development in the aspects of speed, write endurance and non-volatility. 
     Perpendicular MRAM is particularly noteworthy because of its adaptability to sub-30 nano meters (nm) size and high density. However, thermal stability has been a continued problem faced in the design of perpendicular MRAM and is described by thermal stability factor, Δ, described as follows:
 
Δ= K   u   V/k   B   T   Eq. (1)
 
     where “K u ” is the perpendicular anisotropy energy density of the storage magnetic layer of the MRAM, “V” is the volume of the storage magnetic layer, “k B ” is the Boltzmann constant, and “T” is the absolute temperature (in Kelvin). 
     This factor inevitably reduces at a given anisotropy energy of the storage magnetic layer resulting in the thermal stability of each bit decreasing. For MRAM applications using extremely high data density, for example dynamic random access memory (DRAM) type of applications, where speed and data capacity are key parameters, lower thermal stability of the data bits may be tolerable, or may be mitigated with reasonable amounts of error correction coding (ECC) to make the overall design function in the targeted regime of application. Perpendicular MRAM currently has a critical dimension of approximately 30 nano meters (nm) heading toward 10 nm. 
     Applications of MRAM generally include a reference MRAM data bit, which provides a reference resistance for comparing the reference bits to the MRAM data bits to indicate whether or not the data bits are in high resistance or low resistance state. The reference bit is desired to be made of identical MRAM cell structure as that of the data bit because it simplifies both the fabrication process and the circuit design than the case where the reference bit is made of a pure resistor without a MTJ structure. 
     With MRAM reference bit being identical to a data bit, the reference bit has the same low thermal stability problem as indicated hereinabove. The ECC is not correcting reference bit errors. Rather, a special data refreshing and assurance mechanism may be needed to make sure the reference bit is always in the correct state before any read operation on the data bits, which is costly both in design and in operation. Additionally, such refresh mechanism may slow down the operation speed of the device considerably and make the device not usable in high data rate applications. 
     What is needed is a perpendicular MRAM that has suitably high thermal stability characteristics in the reference bits allowing it to remain stable with high confidence during long-term operations without special refresh mechanisms required after initialization. Meanwhile, the data bits may still possess a comparatively lower thermal stability for fast and low power write operation. 
     SUMMARY OF THE INVENTION 
     To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a method and a corresponding structure for a spin-transfer-torque magnetic random access memory (STTMRAM) device based on perpendicular MTJ. An STT effect is that in a magneto-resistive device (such as a magnetic tunnel junction (MTJ)), which has a junction layer sandwiched between two magnetic layers, when an electric current flows through the device in the direction perpendicular to the film plane, the conduction electrons can carry the magnetization information from one magnetic layer through the electrons&#39; spins and inject that information into the other magnetic layer and leads to a magnetization orientation change of the other layer. For a magnetic random access memory using this STT effect to switch the free layer (data storage layer) of its magneto-resistive element, it is generally referred to as STTMRAM. 
     Briefly, an embodiment of the invention includes magnetic random access memory (MRAM) element is configured to store a state when electric current flows there through. The MRAM element includes at least one magneto tunnel junction (MTJ) configured to store a data bit with the at least one MJT including, a magnetic free layer (FL) having a switchable magnetization with a direction that is perpendicular to a film plane. The direction of magnetization of the FL is determinative of the data bit stored in the at least one MTJ. The at least one MTJ further includes a magnetic reference layer (RL) having a magnetization with a direction that is perpendicular to the film plane, and a magnetic pinned layer (PL) having a magnetization with a direction that is perpendicular to the film plane. The direction of magnetization of the RL and the PL are anti-parallel relative to each other, wherein when electric current is applied to the MRAM, the magnetization orientation of the FL switches during a write operation, whereas, the direction of magnetization the RL and the PL remain the same. The MRAM element further includes at least another MTJ configured to store a reference bit that is used to compare with the data bit to retrieve the digital information stored in the data bit. 
     These and other objects and advantages of the present invention will no doubt become apparent to those skilled in the art after having read the following detailed description of the preferred embodiments illustrated in the several figures of the drawing. 
    
    
     
       IN THE DRAWINGS 
         FIGS. 1 and 2  show a MRAM element  10  and a MRAM reference element  20  in accordance with one embodiment of the invention. 
         FIGS. 3-7  show a process for initializing the elements  10  and  20 , during manufacturing or as needed, in accordance with a method of the invention 
         FIGS. 8-12  show a process for initializing the elements  10  and  20 , during manufacturing or as needed, in accordance with another method of the invention. 
         FIGS. 13 and 14  show a MRAM element  15  and a MRAM reference element  19  in accordance with another embodiment of the invention. 
         FIGS. 15-19  show a process for initializing the elements  15  and  19 , during manufacturing or as needed, in accordance with a method of the invention. 
         FIGS. 20-24  show a process for initializing the elements  15  and  19 , during manufacturing or as needed, in accordance with another method of the invention 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the following description of the embodiments, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration of the specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized because structural changes may be made without departing from the scope of the invention. 
     As known to those in the art, reference bits are used to read digital information stored in MRAM data storage elements by comparing the state of the MRAM data storage elements to an associated reference MRAM element and determining, based on this comparison, the resistance state of the MRAM element, i.e. the digital information of “1” or “0”. With this in mind,  FIGS. 1 and 2  show a MRAM element  10  and a MRAM reference element  20  in accordance with one of the embodiments of the invention. Each of the elements  10  and  20  are perpendicular in that their respective free layer (or “reference layer”) has a magnetic anisotropy that is perpendicular to the film plane or substrate on which each element is formed. 
     The element  10  is shown to include an underlayer (UL)  6  formed on a substrate (not shown) on top of which is shown formed a perpendicular free layer (FL)  5  on top of which is shown formed a junction layer (JL)  4  (also known herein as “barrier layer” or “tunnel layer” or “barrier tunnel layer”), on top of which is shown formed a perpendicular reference layer (RL)  3  on top of which is shown formed a spacer layer (SL)  2  on top of which is shown formed a perpendicular pinned layer (PL)  1 . 
     Similarly, the element  20  is made of the same layers as that of the element  10  but it serves as the reference bit to aid in reading the state of the element  10 . Accordingly, the elements  10  and  20  are formed on the same die with analogous material and structures. At times, in this document, the element  10  is referred to as “MRAM data bit” or “data bit” and the element  20  is referred to as “MRAM reference bit” or “reference bit” because each of these elements stores one bit of digital information. 
     The FL  5 , JL  4 , RL  3 , SL  2 , and PL  1  generally make up a magnetic tunnel junction (MTJ). This applies to other embodiments of the invention. 
     The FL  5 , RL  3 , and PL  1  of each of the elements  10  and  20 , each have a perpendicular magnetic anisotropy and a magnetization that is perpendicular to the film plane. Different magnetization states of the two bits, i.e. bits  10  and  20 , are achieved after the completion of an initialization process. As shown in  FIGS. 1 and 2  using the arrows  101 ,  301 ,  501 ,  102 ,  302 , and  502 , the PL  5  and the RL  3  of the element  10  each have magnetizations that are anti-parallel relative to each other while the PL  5  and the RL  3  of the element  20  have magnetizations that are parallel relative to each other and in the same direction as that of the magnetization of the FL  5  of the element  20 . Meanwhile, FL  5  of element  10  is switchable in normal operation by programming current/voltage. FL  5  of element  20  stays in parallel to PL  1  and RL  3  direction. 
     In some embodiments, the JL  4  is made of magnesium oxide (MgO), or alumina, or copper (Cu), or Cu nano-pillars dispersed within an oxide layer. In some embodiments, the elements  10  and  20  are each spin transfer torque MRAM (STTMRAM) elements. When a different direction of electric current is applied to and flows through each of the elements  10  and  20 , the magnetization of the FL  5 , and the magnetization of the RL  3  in some embodiments, thereof may be switched into different orientation along the perpendicular direction due to the spin transfer torque between the FL  5  and the RL  3 . 
     In some embodiments, the SL  2  of each of the elements  10  and  20  is made of non-magnetic material such as but not limited to, ruthenium (Ru), tantalum (Ta), titanium (Ti), MgO, Cu, halfnium (Hf), zinc oxide (ZnO), tantalum nitride (TaN), titanium nitride (TiN), IrMn, PtMn, FeRh or alumina. Further, the SL  2  may produce an anti-ferromagnetic coupling between the RL  3  and the PL  1 , particularly when it is made of Ru, Cu or MgO. 
     In operation, with reference to the element  10 , the effective (magnetic) field amplitude from the PL  1  to the FL  5  is Hstatic- 1 , the effective field amplitude from the RL  3  to the FL  5  is Hstatic- 2 . The coercivity field Hc is the magnetic field needed to switch the magnetization of the FL  5  magnetization  501 . Still with reference to the element  10 , due to the anti-parallel orientation of magnetization  101  of PL  1  and magnetization  301  of RL  3 , the total field of the FL  5  to be overcome during switching of the element  10  is (Hstatic- 1 −Hstatic- 2 +Hc=Hc. Although Hstatic- 1  and Hstatic- 2  can be much larger than Hc, when Hstatic- 1  and Hstatic- 2  are similar strength, the direction of magnetization of the FL  5  switches easy due to the magnetic fields from the RL  3  and the PL  2  canceling each other and making Hstatic- 1 −Hstatic- 2 ˜0. 
     With reference to the element  20 , the effective field from the PL  1  to the FL  5  is Hstatic- 1 , and the effective field from the RL  3  to the FL  5  is Hstatic- 2 . The coercivity field, Hc, represents the magnetic field needed to switch the magnetization of the FL  5  magnetization  502 . Relative to the element  10 , the total field on the FL  5  to be overcome during switching is Hstatic- 1 +Hstatic- 2 +Hc−2Hstatic+Hc, when Hstatic- 1  and Hstatic- 2  are similar strength. Even though Hc is small, when FL  5  magnetization  501  is in same direction as magnetizations  102  or PL  1  and  302  of RL  3 , with large Hstatic- 1  and Hstatic- 2 , it is hard to switch the magnetization  502  of the FL  5  to anti-parallel direction to magnetization  302  of RL  3 , due to the high effective field acting on FL  5 . Thus, the parallel state of reference bit, element  20 , is stable against external excitations, including thermal agitation, i.e. with high thermal stability, as well as disturbance caused by read procedure. 
       FIGS. 3-7  show the steps performed during an initialization process of each of the elements  10  and  20 , in accordance with a method of the invention. At step  1 , in  FIG. 3 , a magnetic field  51 , in the direction shown of the arrow, is applied to the elements  10  and  20 . The field  51  is strong enough to magnetize all of the magnetic layers of the elements  10  and  20 , such as the RL  3  and the FL  5  and the PL  1 , in the direction shown as the direction of the arrow showing field  51 . Next, at step  2 , shown in  FIG. 4 , the field  52  is applied, opposing the field  51  but field  52  is not able to switch the PL  1  in either the element  10  or  20  but it is able to switch the magnetization of both of the layers FL  5  and RL  3  such that the magnetizations  301  and  302  of the RL  3  are oriented anti-parallel relative to that of the PL  1  in both the element  10  and the element  20 . 
     As shown in  FIG. 5 , a third (magnetic) field  53  is applied to the element  10  and the element  20 . The field  53  is in the same direction as that of the field  51 . However, field  53  is not strong enough to switch the magnetization of the RL  3  and the PL  1 , but it is suitable to switch the magnetization of the FL  5  to be in the same direction as that of the PL  1  in both the element  10  and the element  20 . Accordingly, as shown in  FIG. 5 , the magnetization direction, shown by the arrow  501  and  502 , is in the same direction as that of the arrow  101  and  102 . However, the field  53  significantly reduces energy barrier to switch the magnetization of the RL  3  in both bits  10  and  20 . 
     Next, in  FIG. 6 , an electric current is applied but only to the reference bit, or element  20 , and not to the data bit, or element  10 , with electrons flow from the FL  5  to the RL  3  as shown by arrow  54 . Even with the field  53  applied to the element  10 , the RL  3  thereof remains unchanged in element  53  in the absence of the electric current. With the field  53  and spin transfer torque (STT) applied to the element  20  by the electric current at the same time, the magnetization  302  of the RL  3  of the element  20  switches to become parallel (or in the same direction) to the magnetizations  502  and  102  of the PL  1  and the FL  5 , and thus a desired and rather solid parallel magnetic state of the element  20  is advantageously achieved 
     Switching of the RL  3  of the element  20  may be monitored by the resistance change across the element  20 . Alternatively, it is not monitored at all when the current  54  is high enough to switch the magnetization of the RL  3  with high confidence. 
     Next, in  FIG. 7 , the field  53  and the current  54  are removed. Preferably the current  54  is removed before the filed  53  is removed, so that the electric current in element  20  does not inadvertently switch the magnetization  502  of FL  5  to antiparallel to  302  of RL  3  when there is no magnetic field  53  to assist FL  5  magnetization orientation. As shown in  FIG. 7 , the state of the element  10  results with the direction of magnetizations of the RL  3  and the PL  1 , shown by the arrows  301  and  101 , respectively, being anti-parallel relative to each other. Similarly, the state of the element  20  results with the direction of magnetizations of the FL  5 , the RL  3 , and the PL  1 , each being parallel relative to the other and therefore advantageously causing a deep or strong parallel state of the element  20 . 
       FIGS. 8-12  show a process for initializing the elements  10  and  20 , during manufacturing or as needed, in accordance with another embodiment of the invention. 
     In step  1 , of  FIG. 8 , a first field  51  is applied, in a direction shown by the arrow associated with the field  51  in  FIG. 8 , to the elements  10  and  20  that is suitably strong enough to magnetize all magnetic layers of the elements  10  and  20  into being in the same direction as the direction of the field  51 . 
     As step  2 , in  FIG. 9 , a second field  52  is applied to the elements  10  and  20 , in a direction shown by the arrow associated with the field  52  and one that opposes the direction of the field  51 . However, the field  52  is not strong enough to switch the magnetization of the RL  3  and the PL  1  but it is capable of switching the magnetization of the FL  5  into a direction opposite to that of the RL  3  and the PL  1  of the elements  10  and  20 . This field  52  also significantly reduces the energy barrier to switch the magnetization of the RL  3  in all bits. 
     Next, as shown in  FIG. 10 , an electric current is applied only to the data bit or element  10  and not to the reference bit, or element  20 , where electrons flow from the FL  5  to the RL  3  as show by arrow  54 . With the absence of the electric current, the magnetization  302  of the RL  3  of the reference bit element  20  remains unchanged. With field  52  and STT by the electric current applied simultaneously to the data bit element  10 , the magnetization  301  of the RL  3  of the data bit switches to a parallel state in the same direction as the magnetization  501  of the FL  5  and anti-parallel relative to the magnetization  101  of the PL  1 . 
     Switching of the RL  3  of the data bit may be monitored by the resistance change across the data bit. Alternatively, it is not monitored at all when the current  54  is high enough to switch the magnetization of the RL  3  of the data bit with high confidence. 
     Next, as shown in  FIG. 11 , the field  52  and the current  54  are removed. Then, a third field  53  is applied that is in same direction as that of the field  51 . Field  53  is not strong enough to switch the magnetizations of the RL  3  and the PL  1  but it is capable of switching the magnetization of the FL  5  to be in the same direction as that of the PL  1  for both elements  10  and  20 . Alternatively, the field  53  switches the magnetization  502  of FL  5  of the element  20 , and not the FL  5  of the element  10 . Optionally, at step  4 , after the field  52  and the current  54  are removed, without applying magnetic field  53 , an electric current  55 , not shown in  FIG. 11 , is applied to the reference bit  20  only with electrons flowing from the RL  3  to the FL  5 , such that the FL  5  is switched along the same direction as that of the PL  1  and the RL  3  in the reference bit  20 . 
     Still optionally, after the field  52  and the current  54  are removed, without applying field  53 , reference bit&#39;s FL  5  switches from an anti-parallel state to parallel state, relative to that of the PL  1  and the RL  3 , due to the combined magnetic field from the RL  3  and the PL  1 . 
     Next, as shown in  FIG. 12 , all fields and current applied to either the data bit or the reference bit is removed. The data bit  10  remains in the state where the magnetization of its RL  3  and PL  1  are anti-parallel relative to each other. The reference bit  20  remains in the state with the magnetization of its FL  5 , RL  3  and PL  1  being parallel relative to each other, i.e. deep (or strong) parallel state. 
       FIGS. 13 and 14  show a MRAM element  15  and a MRAM reference element  19  in accordance with another embodiment of the invention. Each of the elements  15  and  19  are perpendicular in that their respective FL 5 , RL  3  and PL  1  have magnetizations and magnetic anisotropy that are perpendicular to the film plane or substrate on which each element is formed. 
     The element  15  is analogous to the element  10  however, its reference layer  3  and pinned layer  1  are on an opposite side of its junction layer, in contrast to the element  10 . The element  15  is shown to include an UL  6  formed on a substrate (not shown) on top of which is shown formed a perpendicular RL  3  on top of which is shown formed a JL  4  (also known herein as “barrier layer” or “tunnel layer” or “barrier tunnel layer”), on top of which is shown formed a perpendicular FL  5  on top of which is shown formed a SL  2  on top of which is shown formed a perpendicular PL  1 . 
     Similarly, the element  19  is analogous to the element  20 , with the exceptions stated above, and is made of the same layers as that of the element  15  but it serves as the reference bit to aid in reading the state of the element  15 . Accordingly, the elements  15  and  19  are formed on the same die with analogous material and structures. At times, in this document, the element  15  is referred to as “MRAM data bit” or “data bit” and the element  19  is referred to as “MRAM reference bit” or “reference bit” because each of these elements stores one bit of digital information. 
     The FL  5 , RL  3 , and PL  1  of each of the elements  15  and  19 , each have a perpendicular magnetic anisotropy and a magnetization that is perpendicular to the film plane. Different magnetization states of the two bits, i.e. bits  15  and  19 , especially for RL  3 , are achieved after the completion of an initialization process. 
     As shown in  FIGS. 13 and 14  using the arrows  101 ,  301 ,  501 ,  102 ,  302 , and  502 , the PL  1  and the RL  3  of the element  15  each have magnetizations that are anti-parallel relative to each other, as shown by the arrows  101  and  301 , respectively, and the magnetization of the FL  5  is shown to be switchable, as shown by the arrow  501 . The PL  1 , FL  5 , and the RL  3  of the element  19  each have magnetizations that are parallel relative to each other and in the same direction as one another, as shown by the arrows  302 ,  502 , and  102 . 
     Different magnetization states of the two bits,  15  and  19 , are achieved after the completion of the initialization process. As noted, the magnetization of the PL  1  and the RL  3  are each anti-parallel relative to each other in the element  15  and parallel relative to each other and in same direction as that of the FL  5  in the element  19 . The JL  4  may be made of, but not limited to, MgO, alumina, Cu and Cu nano-pillars within an oxide layer. When different direction of electric current is applied through the element  15 , the magnetization of the FL  5  may be switched into a different orientation along with the perpendicular direction due to the spin transfer torque from the RL  3  in the element  15 . 
     The SL  2  of both the element  15  and the element  19  may be made of a non-magnetic layer such as, but not limited to, Ru, Ta, Ti, MgO, Cu, Hf, ZnO, TaN, TiN, IrMn, PtMn, FeRh, or alumina. The SL  2  may produce an anti-ferromagnetic coupling between the FL  5  and the PL  1  and made of Ru, Cu or MgO. 
       FIGS. 15-19  show a process of initialization the elements  15  ad  19 , in accordance with another method of the invention. In  FIG. 15 , a first field  51  is applied to the elements  15  and  19  that are strong enough to magnetize all of the magnetic layers of these elements such that they all have magnetizations that are in the same direction as that of the field  51 . Next, in  FIG. 16 , a second field  52  is applied that opposes the field  51  in direction and is not capable of switching the magnetization of the PL  1  but it is capable of switching the magnetization of both the FL  2  and the RL  3  in all bits—elements  15  and  19 . Thus, the magnetization of the RL  3  is oriented anti-parallel relative to that of the PL  1 . 
       FIG. 17  shows the next step, which is application of a third field  53 , in the same direction as that of the filed  51 , to the elements  15  and  19 . The field  53  is not strong enough to switch the magnetization of the RL  3  and the PL  1  but it is capable of switching the magnetization of the FL  5  to be in the same direction as that of the PL  1  in both the element  19  and the element  15 . Advantageously, the field  53  significantly reduces the energy barrier required to cause switching of the RL  3  in elements  15  and  19 . 
     In  FIG. 18 , while field  53  is being applied, an electric current is applied only to the reference bit, or element  19 , and not to the data bit with electrons flowing from the FL  5  to the RL  3  as shown by the arrow  54 . Even with the field  53  applied, the RL  3  of the element  15  remains unchanged. With the field  53  and current spin transfer torque (STT) applied to the reference bit, element  19 , at the same time, the magnetization  302  of the reference bit the RL  3  switches parallel to the magnetizations of the PL  1  and the FL  5 , and the reference bit is, advantageously, in a deep or very stable parallel state. 
     The switching of the magnetization of the RL  3  of the element  19  may be monitored by the resistance change across the reference bit  19 . Alternatively, it is not monitored at all when the current  54  is high enough to switch the magnetization of the RL  3  with a high degree of confidence. 
       FIG. 19  shows the next step performed after the step of  FIG. 18 , where the field  53  and the current  54  are removed. Preferably the current  54  is removed before the field  53  is removed. The element  15  remains in the state with the magnetization of its RL  3  and PL  1  being in an anti-parallel state relative to each other. The element  19  remains in the state that has the magnetization of its FL  5 , RL  3 , and PL  1  all being in parallel relative to each other, i.e. deep parallel state. 
       FIGS. 20-24  show the steps for initializing the elements  15  and  19 , in accordance with another method of the invention. In  FIG. 20 , a first field  51  is applied to the elements  15  and  19  and it is strong enough to magnetize all magnetic layers into same direction as that of the field  51 . 
     In  FIG. 21 , a second field  52  is applied, opposing the field  51 . The field  52  is not strong enough to switch the magnetization of the RL  3  and the PL  1  but it is capable of switching the magnetization of the FL  5  in a direction that is opposite to that of the RL  3  and the PL  1  in the elements  15  and  19 . The field  52  significantly reduces the energy barrier required to switch the magnetization of the RL  3  in the elements  15  and  19 . 
     In  FIG. 22 , an electron current is applied only to the data bit, and not to the reference bit, where electrons flow from the FL  5  to the RL  3  as shown by arrow  54 . Under the field  52 , the reference bit RL  3 &#39;s magnetization remains unchanged. With the application of the field  52  and the electric current STT, applied simultaneously to the data bit, the magnetization of the RL  3  of the element  15  switches to a parallel state, resembling the magnetization of the FL  5  but being anti-parallel relative to the magnetization of the PL  1 . Switching of the magnetization of the element  15 &#39;s RL  3  may be monitored by monitoring the resistance change across the data bit. Alternatively, it is not monitored at all when the current  54  is high enough to switch the magnetization of the RL  3  of the element  15  with high confidence (or great reliability). 
     In  FIG. 23 , optionally, the field  52  and the current  54  are removed, preferably in the order of removing the current  54  prior to removing the field  52 . Subsequently, the third field  53 , which is in same direction as that of the field  51 , is applied to the elements  15  and  19 . The field  53  is not strong enough to switch the magnetizations of the RL  3  and the PL  1  but it is capable of switching the magnetization of the FL  5  to be in the same direction as that of the PL  1  in the elements  15  and  19  and alternatively, only in the element  19 . 
     Still optionally the current  54  is removed and the field  52  is also removed. An electric current  55  is applied to the reference bit with electrons flowing from the RL  3  to the FL  5 , so that the magnetization of the FL  5  is switched in a direction that is the same as that of the PL  1  and the RL  3  in the reference bit. 
     Still alternatively, the current  54  is removed and the field  52  is also removed allowing the magnetization  502  of the FL  5  of the element  19  to switch to be in a parallel state relative to that ( 102  and  302 ) of the PL  1  and the RL  3  due to the combined magnetic field from the RL  3  and the PL  1  in the element  19 . 
     Next, as shown in  FIG. 24 , the field and current applied to either the element  15  or the element  19  is removed. The element  15  remains in the state where it&#39;s RL  3  and PL  1 &#39;s magnetizations are anti-parallel relative to each other. The element  19  remains in the state where its FL  5 , RL  3 , and PL  1 &#39;s magnetizations are parallel relative to each other resulting in a deep (or reliable) parallel state. 
     It is understood that while only one bit is represented and shown in the various embodiments and methods of the invention, a memory array may employ and typically does employ a large number of data bits and reference bits. In some embodiments, the data bits are stacked on top of each other and the reference bits are stacked on top of each other but other arrangements are contemplated. 
     MTJ structures from layer  1  to layer  5 , in all embodiments as disclosed above can be up-side down from what is shown in the figures. 
     Although the present invention has been described in terms of specific embodiments, it is anticipated that alterations and modifications thereof will no doubt become apparent to those skilled in the art. It is therefore intended that the following claims be interpreted as covering all such alterations and modification as fall within the true spirit and scope of the invention.