Abstract:
A magnetic tunnel junction (MTJ)-based magnetic random access memory (MRAM) cell with a thermally assisted switching (TAS) writing procedure and methods for manufacturing and using same. The TAS MTJ-based MRAM cell includes a magnetic tunnel junction that is formed with an anisotropic shape and that comprises a ferromagnetic storage layer, a reference layer, and an intermediate insulating layer. The ferromagnetic storage layer has a magnetization that is adjustable above a high temperature threshold; whereas, the reference layer has a fixed magnetization. The ferromagnetic storage layer is provided with a magnetocrystalline anisotropy that is oriented essentially perpendicular to a long axis of the anisotropic shape of the magnetic tunnel junction. The TAS MTJ-based MRAM cell advantageously limits the effects of dispersion in the magnetic tunnel junction shape anisotropy coming from the fabrication process and features a lower power consumption when compared with conventional MTJ-based MRAM and TAS MTJ-based MRAM cells.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to European patent application, Serial No. EP08290468.1, filed on May 20, 2008. Priority to the European application is expressly claimed, and the disclosure of which is hereby incorporated herein by reference in its entirety. 
       FIELD 
       [0002]    The present disclosure relates to magnetic memories, especially non-volatile random-access magnetic memories used to store and read data in electronic systems. More particularly, the present disclosure relates to Magnetic Random Access Memories, referred to as MRAM, based on magnetic tunnel junctions and an improvement of the shape of the memory cell used in a tunnel junction based MRAM using a thermally assisted write scheme. 
       BACKGROUND 
       [0003]    Magnetic random access memories (MRAMs) have been the object of a renewed interest with the discovery of magnetic tunnel junctions (MTJ) having a strong magnetoresistance at ambient temperature. These MRAMs present many advantages such as speed (a few nanoseconds of duration of writing and reading), nonvolatility, and insensitivity to ionizing radiation. Consequently, they are increasingly replacing memory that uses more conventional technology based on the charge state of a capacitor (DRAM, SRAM, FLASH). 
         [0004]    In conventional MTJ based MRAM, the memory cell includes a magnetic tunnel junction that comprises a stack of several alternatively magnetic and non-magnetic metallic layers. Examples of conventional MTJ-based MRAM devices are described in U.S. Pat. No. 5,640,343. In their simplest forms, the magnetic tunnel junctions of MTJ-based MRAM are formed from two magnetic layers of different coercivity that are separated by an insulating thin layer. The first layer (or reference layer) of the magnetic tunnel junction is characterized by a fixed magnetization; whereas, the second layer (or storage layer) is characterized by a magnetization direction that can be changed. When the respective magnetizations of the reference layers and the storage layer are antiparallel, the resistance of the magnetic tunnel junction is high. On the other hand, when the respective magnetizations are parallel, the resistance of the magnetic tunnel junction becomes low. 
         [0005]    Preferentially, the reference layer and the storage layer are made of 3d metals such as Fe, Co or Ni or their alloys. Eventually, boron can be added in the layer composition in order obtain an amorphous morphology and a flat interface. The insulating layer typically comprises alumina (Al 2 O 3 ) or magnesium oxide (MgO). Preferentially, the reference layer itself can be formed from several layers as described, for instance, in U.S. Pat. No. 5,583,725, in order to form a synthetic antiferromagnetic layer. A double tunnel junction as described in the paper by Y. Saito et al., Journal of Magnetism and Magnetic Materials Vol. 223 (2001), p. 293, can also be used. In this case, the storage layer is sandwiched between two thin insulating layers with respective reference layers located on the opposite sides of the thin insulating layers. 
         [0006]      FIG. 1  shows a memory cell  1  of a conventional MTJ-based MRAM. The memory cell  1  includes a magnetic tunnel junction  2  that comprises a storage layer  21 , an insulating layer  22  and a reference layer  23 . The magnetic tunnel junction  2  is illustrated as being disposed between a selection CMOS transistor  3  and a word current line  4 . A bit current line  5  is placed orthogonal with the word current line  4 . When electrical currents flow in the word and bit current lines  4 ,  5 , word and bit magnetic fields  41  and  51  are respectively produced. Electrical currents are typically short current pulses from 2 to 5 nanoseconds having a magnitude on the order of 10 mA. An additional control current line  6  is applied to control the opening and/or the closing of the transistor  3  to address each memory cell  1  individually. 
         [0007]    During a writing process, the transistor  3  is in the blocked mode (OFF), and no current flows through the magnetic tunnel junction  2 . The intensity of the current pulses and their synchronization are adjusted so that only the magnetization of the storage layer  21  located at the crossing of the word and bit current lines  4 ,  5  can switch under the combined effect of the word and bit magnetic fields  41  and  51 . 
         [0008]    During a reading process, the transistor  3  is in the saturated mode (ON) and a junction current will flows through the magnetic tunnel junction  2  allowing the measurement of the junction resistance of the memory cell  1 . The state of the memory cell  1  is determined by comparing the measured resistance with the resistance of a reference memory cell. For example, a low junction resistance will be measured when the magnetization of the storage layer  21  is parallel to the magnetization of the reference layer  23  corresponding to a value of “0.” Conversely, a magnetization of the storage layer  21 , antiparallel to the magnetization of the reference layer  23 , will yield a high junction resistance corresponding to a value of “1.” 
         [0009]    Basic structural details for this type of conventional MTJ-based MRAM are described in U.S. Pat. Nos. 4,949,039 and 5,159,513; while, U.S. Pat. No. 5,343,422 discloses an implementation of a random-access memory (RAM) based on a MTJ based MRAM structure. 
         [0010]    To help ensure that this architecture is working properly during the writing process, it is necessary to use memory cells  1  with an anisotropic form having high aspect ratios, typically 1.5 or more. Such geometry is required to obtain bi-stable functioning of the memory cell  1 , a good writing selectivity between the selected memory cell and the half-selected cells located on the same line/column, and good thermal/temporal stability of the information. 
         [0011]    According to U.S. Pat. No. 5,959,880, the aspect ratio of a memory cell can be reduced by increasing the magnetocrystalline anisotropy of the material that forms the storage layer. By doing this, the system is stable in time and temperature, and both states of the memory cell are well defined. On the other hand, the writing field required to reverse the magnetization of the memory cell from one stable state to another is significant and therefore the power consumed during the writing process is large. Conversely, if the magnetocrystalline anisotropy is low, the power consumed at writing is also low, but thermal and temporal stability of the storage layer are no more ensured. In other words, U.S. Pat. No. 5,959,880 teaches that it is not possible to simultaneously ensure low power consumption and thermal and temporal stability. 
         [0012]    A thermally assisted writing switching (TAS) process for the above-referenced MTJ-based MRAM structure is described in United States Patent Application Publication No. US 2005/0002228 A1. The particularity of the magnetic tunnel junction of the TAS MTJ based MRAM is that both the reference layer and the storage layer are exchange biased. More precisely, the reference and storage layers are pinned by interaction with an adjacent antiferromagnetic reference layer and antiferromagnetic storage layer respectively. During a thermally assisted writing process, for example, a junction current pulse is sent through the magnetic tunnel junction rising the temperature of the magnetic tunnel junction and the magnetic coupling between the ferromagnetic storage layer and antiferromagnetic storage layer disappears. The magnetic tunnel junction is then cooled while a moderate magnetic field is applied by making a current to flow in the word current line, allowing for the reversal of the magnetization of the storage layer. 
         [0013]    In contrast with the conventional MTJ-based MRAM, the TAS MTJ based MRAM structure is characterized by a considerably improved thermal stability of the storage layer due to the pinning of the antiferromagnetic storage layer. An improved writing selectivity is also achieved due to the selective heating of the memory cell to be written in comparison with the neighboring memory cells remaining at ambient temperature. The TAS MTJ-based MRAM structure also allows for a higher integration density without affecting its stability limit, and reduced power consumption during the writing process since the power required to heat the memory cell is less than the one needed to generate magnetization in the conventional MTJ-based MRAM structure. 
         [0014]    A further improvement of the TAS MTJ-based MRAMs in terms of power consumption has been described in United States Patent Application Publication No. US 2006/0291276 A1. Here, the writing field is further reduced by selecting a circular geometry of the memory cell junction. In this case, the writing field is only given by the magnetocrystalline anisotropy of the storage layer and there is no contribution from the shape anisotropy. However, the use of a circular geometry does not allow for simultaneously low power consumption and thermal and temporal stability of the storage layer. 
         [0015]    The benefit of using a circular magnetic tunnel junction can be better understood by expressing the energy of the magnetic barrier height that has to be overcome to write the cell from a state “0,” of low electrical resistance, to a state “1,” of high electrical resistance. The barrier energy per volume unit, E b , can be expressed as set forth in Equation 1. 
         [0000]    
       
         
           
             
               
                 
                   
                     E 
                     b 
                   
                   = 
                   
                     K 
                     + 
                     
                       
                         
                           
                             A 
                              
                             
                                 
                             
                              
                             R 
                           
                           - 
                           1 
                         
                         L 
                       
                        
                       
                         tM 
                         s 
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                      
                     
                         
                     
                      
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
         [0016]    In Equation 1, the first term, K, is the magnetocrystalline anisotropy and the second term corresponds to the shape anisotropy. In the second term, AR is the aspect ratio of the magnetic tunnel junction, defined as the ratio of the length to the width L of the magnetic tunnel junction; t is the thickness of the storage layer; and M s  its saturation magnetization. The ellipticity can be defined as (AR-1), expressed in percentage terms. 
         [0017]    The limitations of the prior art can be understood by considering that the barrier energy E b  increases with decreasing the size of the magnetic tunnel junction (L decreases and AR is constant), resulting in a significant increase in power consumption. In the other hand, the barrier energy E b  decreases with decreasing AR (L being constant), resulting in a loss of thermal and temporal stability. 
         [0018]    In the case of a TAS MTJ-based MRAM with an exchange-biased storage layer, the storage layer stability at working temperatures is ensured by the pinning of the ferromagnetic storage layer with the antiferromagnetic layer, while, at writing temperatures, the pinning disappears and the memory cell can be written with a low writing field. In the case of a circular cell junction, a low writing field is obtained only by the low magnetocrystalline anisotropy. A low writing field and good thermal stability can then be obtained simultaneously by combining the junction geometry with the TAS MTJ-based MRAMs. 
         [0019]    However, usual MRAM fabrication processes cannot guarantee perfectly circular magnetic tunnel junctions over a large array of memory cells, due to, for example, accuracy limitations in the pattering of the different junction layers. In addition, the magnitude of writing fields is strongly dependant on variations in the junction ellipticity.  FIG. 2  shows the dependence of the writing field H R  of the storage layer, on the aspect ratio of the magnetic tunnel junction for a conventional TAS MTJ-based MRAM cell. For example, the magnitude of the writing field more than doubles when the junction aspect ratio is increased from AR=1.0 to 1.1, representing a 10% variation typical from a usual fabrication process. The inset of  FIG. 2  shows a top view of magnetic tunnel junctions with aspect ratio comprised between 1.0 and 1.1. 
         [0020]    Such a variation of the aspect ratio results in a large dispersion of the writing field and a significant increase of the power consumption in a magnetic memory device containing an array of memory cells with circular junctions. In addition, electromigration effects in the current lines that occur for large electrical currents at high writing field may not be avoided. 
       SUMMARY 
       [0021]    The disclosed magnetic tunnel junction (MTJ)-based magnetic random access memory (MRAM) cell with a thermally assisted switching (TAS) writing procedure advantageously provides a reduced dependence of the dispersion of the writing field on the junction aspect ratio due to the fabrication process. 
         [0022]    As illustrated by the preferred embodiments, the TAS MTJ-based MRAM cell can include a magnetic tunnel junction. The magnetic tunnel junction can be formed from a ferromagnetic storage layer, a reference layer, and an insulating layer that is disposed between the storage and reference layers. The ferromagnetic storage layer can have a magnetization that is adjustable above a high temperature threshold; whereas, the reference layer can be provided with a fixed magnetization. Preferably, the magnetic tunnel junction is formed with an anisotropic shape, and a magnetocrystalline anisotropy of the ferromagnetic storage layer can be oriented essentially perpendicular to a long axis of the anisotropic shape of the magnetic tunnel junction. In one embodiment, the magnetocrystalline anisotropy of the ferromagnetic storage layer and the long axis of the anisotropic shape of the magnetic tunnel junction can form an angle within a preselected range, such as within a range between eighty degrees (80°) and ninety degrees (90°), inclusive. 
         [0023]    The magnetic tunnel junction can be formed with any suitable anisotropic shape and/or dimension, such as an aspect ratio. In one illustrative embodiment, for example, the anisotropic shape of the magnetic tunnel junction can comprise an elliptical shape, a rectangle shape, a crescent shape, a semi-ellipse shape, and/or a diamond shape without limitation. In another illustrative embodiment, the magnetic tunnel junction can have an anisotropic shape with an aspect ratio comprised between approximately 1.0 and 1.5. For instance, the aspect ratio of the magnetic tunnel junction can be comprised within a first range between 1.0 and 1.1 and/or a second range between 1.0 and 1.05. 
         [0024]    The present application likewise discloses a method of writing data in the TAS MTJ-based MRAM cell, the cell further comprising a select transistor being coupled with the magnetic tunnel junction and controllable via a word line, a connecting current line electrically connected to the magnetic tunnel junction, and a word current line; the method comprising: 
         [0025]    heating the magnetic tunnel junction until it has reached a high temperature threshold; 
         [0026]    aligning the magnetization of the ferromagnetic storage layer in a direction essentially parallel or antiparallel with the magnetization orientation of the reference layer; and 
         [0027]    cooling down the magnetic tunnel junction to a low temperature threshold at which the magnetization of the ferromagnetic storage layer is pinned. 
         [0028]    In the context of the patent application, the expressions “ellipse,” “elliptical,” and “ellipticity” generally refer to any closed shapes having an anisotropic form such as ellipses, crescents, semi-ellipses, diamonds, rectangles, etc. 
         [0029]    Advantages of the TAS MTJ-based MRAM cell comprise limiting the effects of dispersion in the magnetic tunnel junction shape anisotropy coming from the fabrication process, a lower power consumption, and facilitated cell scaling down, compared with the MTJ-based MRAM and TAS MTJ-based MRAM cells of the prior art. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    The preferred embodiments will be better understood with the aid of the description of an embodiment given by way of example and illustrated by the figures, in which: 
           [0031]      FIG. 1  shows a schematic view of a conventional magnetic tunnel junction (MTJ)-based magnetic random access memory (MRAM) cell; 
           [0032]      FIG. 2  shows a dependence of the writing field of the storage layer on the aspect ratio of the magnetic tunnel junction for the TAS MTJ-based MRAM cell of  FIG. 1 ; 
           [0033]      FIG. 3  illustrates an embodiment of a TAS MTJ-based MRAM cell comprising an exemplary magnetic tunnel junction; 
           [0034]      FIG. 4  illustrates an exploded view of the exemplary magnetic tunnel junction of the TAS MTJ-based MRAM cell of  FIG. 3 ; 
           [0035]      FIG. 5  illustrates an exemplary variation of the writing field with the magnetic tunnel junction aspect ratio for a magnetocrystalline anisotropy axis of the ferromagnetic storage layer being parallel and perpendicular to the long axis of the magnetic tunnel junction; and 
           [0036]      FIG. 6  illustrates a top view of a conventional magnetic tunnel junction with an aspect ratio of 1.5, and two magnetic tunnel junctions with aspect ratios of 1.0 and 1.05 respectively. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0037]      FIG. 3  illustrates one embodiment of a thermally assisted switching (TAS) magnetic tunnel junction (MTJ)-based magnetic random access memory (MRAM) memory cell  1 . The TAS MTJ-based MRAM cell  1  comprises a magnetic tunnel junction  2  placed between a selection CMOS select transistor  3  and a connecting current line  7  for passing a junction current pulse  31  flowing through the magnetic tunnel junction  2  when the transistor  3  is in the saturated or open mode (ON). A control current line  6  is used to control the opening and the closing of the transistor  3  to address each memory cell individually. The TAS MTJ-based MRAM cell  1  also comprises a word current line  4 , shown above and perpendicular to the connecting current line  7  in the example of  FIG. 3 . Other configurations of the word current line  4  are however possible. For example the word current line  4  can be placed parallel with the connecting current line  7  and/or on the side of or below the magnetic tunnel junction  2 . 
         [0038]    An exploded view on the exemplary magnetic tunnel junction  2  is shown in  FIG. 4 . The magnetic tunnel junction  2  contains a storage layer  21  preferably comprising a ferromagnetic storage layer  21   a  and an antiferromagnetic storage layer  21   b . The ferromagnetic storage layer  21   a  has a thickness typically of the order of 1 to 10 nm and is made of a material having a planar magnetization, typically selected from the group Permalloy (Ni 80 Fe 20 ), CO 90 Fe 10  or other alloys containing Fe, Co or Ni. The ferromagnetic storage layer  21   a  is exchange-coupled by the antiferromagnetic storage layer  21   b  made of a manganese-based alloy, for example, of IrMn or FeMn. The antiferromagnetic storage layer  21   b  has a blocking temperature T BS  sufficiently high to ensure that at a low temperature threshold below T BS , for example, at standby temperature, i.e., in the absence of heating, magnetization of the ferromagnetic storage layer  21   a  is sufficiently pinned to be able to preserve its magnetization over a period of several years but not so high as to make it necessary to heat the magnetic tunnel junction  2  excessively during every the writing process that could yield to material degradation and high power consumption. Here, a T BS  in the range of, for example, 120 to 220° C. is suitable. 
         [0039]    The magnetic tunnel junction  2  also contains a reference layer  23  preferably comprising a first ferromagnetic reference layer  23   a  and a second ferromagnetic reference layer  23   c , both formed of a Fe, Co or Ni based alloy. The two ferromagnetic reference layers  23   a ,  23   c  are antiferromagnetically coupled by inserting between them a non-ferromagnetic reference layer  23   b  made, for example, of ruthenium. An antiferromagnetic reference layer  24 , preferably formed of a Mn based alloy such as PtMn or NiMn and characterized by a blocking temperature T BR  higher than T BS , is provided below the second ferromagnetic reference layer  23   c . The antiferromagnetic reference layer  24  orients the magnetic moment of the first ferromagnetic reference layer  23   a , and a pinning field is generated that fixes the magnetic moment of the second ferromagnetic reference layer  23   c . The reference layer structure described above is well known in the state of the art under the name of synthetic antiferromagnet pinned layer. Other configurations of the storage layer  23  are also possible. For example, the reference layer  23  can comprise a single ferromagnetic reference layer pinned by the antiferromagnetic reference layer  24  described above. 
         [0040]    An insulating layer  22  playing the role of a tunnel barrier and preferably made of a material selected from the group comprising Al 2 O 3  and MgO is inserted between the storage layer  21  and the reference layer  23 . The tunneling resistance of a magnetic tunnel junction  2  depends exponentially on the insulating layer thickness and is measured by the resistance-area product (RA) of the magnetic tunnel junction  2 . The RA should be sufficiently small to flow the junction current  31  through the magnetic tunnel junction  2 , sufficiently high to raise the temperature of the antiferromagnetic storage layer  21   b  above its blocking temperature T BS . In order to force a current density in the range of 10 5  A/cm 2  to 10 7  A/cm 2 , typically required to raise the temperature of the magnetic tunnel junction  2  up to 100° C., the RA value should be of the order of 1 to 500 Ω·μm 2 . 
         [0041]    In another embodiment, the magnetic tunnel junction  2 , at least one thermal barrier layer (not shown) made typically of BiTe or GeSbTe and having a very low thermal conductivity can be added at the top and at the bottom of the magnetic tunnel junction  2 . The purpose of these additional layers is to increase the heating efficiency of the junction current  31  flowing through the magnetic tunnel junction  2  while limiting the diffusion of the heat towards the electrode (not shown) ensuring the electrical connection between the magnetic tunnel junction  2  and the connecting current line  7 . Here, the thermal barrier itself is electrically connected to the electrode directly or via a conductive layer, for example made of TiN or TiWN. 
         [0042]    During the thermally assisted writing process, the junction current pulse  31  having a magnitude comprised between 10 5  A/cm 2  and 10 7  A/cm 2  and lasting several nanoseconds is sent through a connecting current line  7  and the magnetic tunnel junction  2  (with transistor ON), rising the temperature of the magnetic tunnel junction  2  to a high temperature threshold of about 120 to 220° C., lying between T BS  and T BR  where the magnetic coupling between the ferromagnetic storage layer  21   a  and antiferromagnetic storage layer  21   b  disappears and the magnetization of the ferromagnetic storage layer  21   a , being no more pinned, can be freely adjusted. The magnetic tunnel junction  2  is then cooled while a moderate word magnetic field  41  is applied by flowing a current in the word current line  4 , allowing for the aligning of the magnetization of the ferromagnetic storage layer  21   a  in a direction according to the magnetic field  41  orientation, essentially parallel or antiparallel with the magnetization orientation of the reference layer  23 . The magnetic tunnel junction  2  is then cooled down at a low temperature threshold below the blocking temperature T BS  of the antiferromagnetic storage layer  21   b , where the magnetization of the ferromagnetic storage layer  21   a  becomes pinned in its reversed direction, or written state. 
         [0043]    The magnetic tunnel junction  2  preferably has an anisotropic shape, such as an elliptical shape, and the ferromagnetic storage layer  21   a  has a magnetocrystalline anisotropy that is oriented essentially perpendicular to the long axis of the anisotropic shape of the magnetic tunnel junction  2 . In other words, in the TAS MTJ-based MRAM cell  1 , the magnetization of the ferromagnetic storage layer  21   a  is oriented in a direction that is essentially perpendicular with the long axis, or easy axis, of the anisotropic shape of the magnetic tunnel junction  2  at a high temperature threshold, when the magnetization of the ferromagnetic storage layer  21   a  can be freely adjusted. In one embodiment, the anisotropic shape of the magnetic tunnel junction  2  can have an irregular shape with a long axis that may not be well defined such that the long axis of the anisotropic shape may be not be strictly perpendicular to the magnetocrystalline anisotropy. Accordingly, the magnetocrystalline anisotropy of the ferromagnetic storage layer and the long axis of the anisotropic shape of the magnetic tunnel junction  2  can form an angle within a preselected range, such as within a range between eighty degrees (80°) and ninety degrees (90°), inclusive. In other words, the term “perpendicular” (or “perpendicularly”) as used herein can generally refer to forming an angle within the preselected range. 
         [0044]    An advantage of using such a magnetic tunnel junction geometry and magnetocrystalline orientation can be seen from the variation in the writing (or coercive) field, H R , of the ferromagnetic storage layer  21   a  with the junction aspect ratio.  FIG. 5  compares the variation of the word magnetic field  41 , or writing field H R , with the junction aspect ratio AR for a conventional memory cell, where the ferromagnetic storage layer  21   a  has a magnetocrystalline anisotropy axis parallel to the long axis of the ellipse (filled circles), and for the memory cell  1 , where the ferromagnetic storage layer  21   a  has a magnetocrystalline anisotropy axis perpendicular to the long axis of the ellipse (open circles). Here, the variation of the writing field H R  has been calculated for the junction anisotropic shapes having aspect ratios AR comprised between 1.0 and 1.1. The calculations were performed by means of micromagnetic simulations assuming standard material parameters corresponding to the ferromagnetic storage layer  21   a  employed in a usual TAS MTJ-based MRAM cell  1  and assuming that the writing field H R  is applied parallel to the magnetocrystalline anisotropy axis. The calculations also assumed that the writing field H R  is not influenced by the dispersion in ellipticity and is essentially given by the magnetocrystalline anisotropy value corresponding, for example, to the writing field of a circular junction (AR=1.0). 
         [0045]    The shape of the magnetic tunnel junction  2  is not limited to an elliptical shape but can have any shape that is anisotropic, such as a rectangle, crescent, semi-ellipse, diamond, etc., where the magnetocrystalline anisotropy axis is essentially perpendicular to the long axis of the anisotropic shape of the magnetic tunnel junction  2 . 
         [0046]    As can be seen in  FIG. 5 , the writing field H R  increases approximately linearly with the memory cell aspect ratio AR, for a magnetocrystalline anisotropy axis of the ferromagnetic storage layer  21   a  that is essentially parallel to the long axis of the ellipse. In this case, a variation in the memory cell aspect ratio AR due to the manufacturing process will result in an overall increase in the writing field H R  and a larger power consumption of the magnetic memory device. Conversely, in the case of a magnetocrystalline anisotropy being essentially perpendicular to the long axis of the ellipse, the writing field H R  decreases approximately linearly with the aspect ratio AR, and a variation in the memory cell aspect ratio AR will tend to diminish the overall writing field H R  and power consumption of the magnetic memory cell  1 . 
         [0047]    In a preferred embodiment, the magnetic tunnel junction  2  of the memory cell is characterized by an aspect ratio AR equal or above a value of 1.0, corresponding to a circular (or square, etc.) magnetic tunnel junction  2 , but preferably comprised between 1.0 and 1.5, and a magnetocrystalline anisotropy axis of the ferromagnetic storage layer  21   a  perpendicular to the long axis of the ellipse. 
         [0048]    A magnetic memory device (not represented) can be formed by assembling a matrix comprising a plurality of TAS MTJ-based MRAM cells  1 , where each magnetic tunnel junction  2  of each memory cell  1  is connected on the side of the storage layer  21 , or ferromagnetic storage layer  21   a , to the connecting current line  7 , and on the opposite side to the control current line  6 , placed perpendicular with the connecting current line  7 . When one of the memory cells  1  is to be written, a current pulse is sent in one or several control lines  6  to put at least one of the transistors  3  of the corresponding control lines  6  in mode ON, and a junction current pulse  31  is sent to each connecting lines  7  corresponding to the memory cells  1  to be written, i.e., the memory cells  1  placed at the intersection of the active connecting current lines  7  and active control lines  6 . 
         [0049]    Using today&#39;s lithographic fabrication processes a maximal variation, Δ c , in the shape anisotropy of the magnetic tunnel junction  2  of about ±5% can be typically obtained. This corresponds, for example, to an aspect ratio AR of the magnetic tunnel junction  2  varying from 1.0 to 1.1 with an average aspect ratio of 1.05, for the memory cells  1  of the magnetic memory device. In the exemplary calculations of  FIG. 5 , an anisotropic shape with an aspect ratio AR of 1.05 corresponds to a writing field H R  of about 25 Oe, for the TAS MTJ-based MRAM cell  1 . This represents a decrease of about 30% in the writing field value compared to the one calculated for a magnetic tunnel junction  2  with an aspect ratio AR of 1. 
         [0050]    In a preferred embodiment, the magnetic tunnel junction  2  of the memory cell  1  has an aspect ratio AR comprised within the maximum shape anisotropy variations allowed by the fabrication process used for the memory cell fabrication, and has a magnetocrystalline anisotropy axis of the ferromagnetic storage layer  21   a  perpendicular to the long axis of the magnetic tunnel junction anisotropic shape. For example, the magnetic tunnel junction  2  of the memory cell  1  has an aspect ratio AR comprised between 1.0 and 1.1. 
         [0051]    Continuous improvements in the fabrication processes may equally allow for smaller variations in the aspect ratio AR of the magnetic tunnel junctions  2  within the memory device. For example, using such advanced fabrication technologies, the magnetic tunnel junction  2  of the memory cell  1  could be characterized by an aspect ratio AR comprised between 1.0 and 1.05, or even smaller. 
         [0052]      FIG. 6  compares schematically the top view of a conventional magnetic tunnel junction  2  with a field induced magnetic switching (FIMS) architecture having an aspect ratio AR of 1.5, with two magnetic tunnel junctions  2  of the TAS MTJ-based MRAM cell  1  having aspect ratios AR of 1.0 and 1.05 respectively. 
         [0053]    In another embodiment, the variation of the junction aspect ratio AR is minimized by using an appropriate fabrication process and/or by a careful control of the fabrication process and/or by selecting fabricated memory cells  1  having the least variation possible in their aspect ratio AR. Here, the magnetic memory device containing such magnetic tunnel junctions  2  with an aspect ratio AR of about 1.0 or any other value, can be fabricated with no or a very small dispersion in the aspect ratio AR. Such memory device can have minimal variations of the writing field H R  due to the combined effect of the small or inexistent dispersion, in the junction aspect ratios AR, and in the magnetocrystalline anisotropy axis of the ferromagnetic storage layer  21   a  being essentially perpendicular to the long axis of the anisotropic shape of the magnetic tunnel junctions  2 . 
         [0054]    In yet another embodiment, the magnetic tunnel junctions  2  of the memory device have a magnetocrystalline anisotropy axis of the ferromagnetic storage layer  21   a  essentially parallel to the long axis of the junction anisotropic shape, the latter having a very small or no dispersion of the aspect ratio AR. 
         [0055]    The fact that the magnetocrystalline anisotropy axis of the ferromagnetic storage layer  21   a  is perpendicular to the long axis of the anisotropic shape of the magnetic tunnel junction  2 , gives rise to a competition between the magnetocrystalline anisotropy and shape anisotropy terms of the barrier energy, E b . For example, in the absence of an external applied magnetic field, the magnetic moments of the ferromagnetic storage layer  21   a  may be tilted with respect to the magnetic moments of the reference layer  23 . This tilt can increase with increasing aspect ratios AR, translating in an important dispersion in the resistance value during the reading operation, and resulting in a loss in the read margin that corresponds to the difference between low and high resistance states. 
         [0056]    With the TAS MTJ-based MRAM cell  1 , however, the writing sequence comprises a last cooling stage of the magnetic tunnel junction  2 , performed under the word magnetic field  41 , corresponding to the writing field H R . This word magnetic field  41  “freezes” the magnetic state of the ferromagnetic storage layer  21   a  resulting in a much reduced tilt of the magnetic moments of the ferromagnetic storage layer  21   a  with respect to the reference layer  23 , yielding to a much lesser influence in the read margin. For example, a loss of less than 20% for the read margin is expected in the case of the magnetic tunnel junction  2  with a shape anisotropy variation of 10%. 
         [0057]    The disclosed embodiments are susceptible to various modifications and alternative forms, and specific examples thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the disclosed embodiments are not to be limited to the particular forms or methods disclosed, but to the contrary, the disclosed embodiments are to cover all modifications, equivalents, and alternatives. 
         [0058]    For example, other configurations of the TAS MTJ-based MRAM cell  1  can be used in the context provided the magnetic tunnel junction  2  is fabricated with an isotropic (circular, square, etc.) or anisotropic (elliptical, rectangular, etc.) geometry and has a magnetocrystalline anisotropy axis of the ferromagnetic storage layer  21   a , essentially parallel to the long axis of the anisotropic shape of the magnetic tunnel junction  2 . An example of another TAS MTJ-based MRAM cell  1  configuration is the memory cell described in unpublished European patent application Serial No. EP07291520 by the present applicant, where the magnetic tunnel junction  2  comprises a writing layer added on top of the storage layer  21 . Another example is the magnetic tunnel junction described in the above-referenced United States Patent Application Publication No. US 2005/0002228 A1 of a general thermally assisted MRAM architecture, where the writing process is ensured by the combination of one magnetic field and a local heating, and the storage layer is exchanged biased with an antiferromagnetic layer. 
       REFERENCE NUMBERS 
       [0000]    
       
           1  memory cell 
           2  magnetic tunnel junction 
           21  storage layer 
           21   a  ferromagnetic storage layer 
           21   b  antiferromagnetic storage layer 
           22  insulating layer 
           23  reference layer 
           23   a  first ferromagnetic reference layer 
           23   b  non-ferromagnetic reference layer 
           23   c  second ferromagnetic reference layer 
           24  antiferromagnetic reference layer 
           3  select transistor 
           31  junction current pulse 
           4  word current line 
           41  word magnetic field 
           5  bit current line 
           51  bit magnetic field 
           6  control current line 
           7  connecting current line 
       
     
       REFERENCE SYMBOLS 
       [0000]    
       
         AR aspect ratio of the memory cell 
         AR-1 ellipticity of the memory cell 
         E b  barrier energy 
         H R  writing (coercive) field of the ferromagnetic storage layer 
         L width of the magnetic tunnel junction 
         M s  saturation magnetization of the memory cell 
         RA resistance-area product of the insulating layer 
         t thickness of the storage layer 
         T BS  blocking temperature of the antiferromagnetic storage layer 
         T BR  blocking temperature of the antiferromagnetic reference layer 
         Δ e  maximal variation in the junction anisotropy