Abstract:
The free layer in a magneto-resistive memory element is stabilized through being pinned by an antiferromagnetic layer. A control valve layer provides exchange coupling between this antiferromagnetic layer and the free layer. When writing data into the free layer, the control valve layer is heated above its curie point thereby temporarily uncoupling the free layer from said antiferromagnetic layer. Once the control valve cools, the free layer magnetization is once again pinned by the antiferromagnetic layer.

Description:
FIELD OF THE INVENTION 
     The invention relates to the general field of magneto-resistive memory arrays with particular emphasis on how information is stored and, more particularly, with overcoming high programming current and scaling problems. 
     BACKGROUND OF THE INVENTION 
     Magnetic tunneling junction (MTJ) or Giant magneto-Resistance (GMR)/Spin Valve (SV) with two ferromagnetic layer separated by a non-magnetic layer—a tunneling oxide layer for MTJ or a transitional metal for GMR/SV—have been widely studied for use as a memory element in, for example, a Magnetic Random Access Memory (MRAM). Usually, one of these ferromagnetic layers (reference or pinned layer) is magnetized in a fixed direction while the other layer is free to switch its magnetization direction (free layer). 
     For MRAM applications, the magnetizations of both free and reference layers are in the film plane, as illustrated by  FIG. 1   a . The anisotropy field that keeps the free layer magnetization parallel or anti-parallel to the reference layer is usually generated through shape anisotropy that occurs when the shape deviates from a circle, e.g. as an ellipse. In the quiescent state, the free layer magnetization lies along the long axis of the cell (see the ellipse in  FIG. 1   a ) oriented in the direction of magnetization of the reference layer, either parallel or anti-parallel thereto. This long axis is referred to as the easy axis (x), while the direction perpendicular to it is the hard axis (y). The cross section of  FIG. 1   a  is given in  FIG. 1   b.    
     The digital information stored in the MTJ is thus encoded as the direction of magnetization of the free layer.  FIG. 2  shows resistance R of such a MTJ element as a function of external field Hs along the orientation of the pinned layer magnetization. When the field is off, the two states with minimum and maximum resistances correspond to the free layer magnetization being parallel and anti-parallel to the pinned layer magnetization respectively. The field required to switch between the two states (Hs) is determined by the anisotropy energy of the element. 
     In a conventional MRAM application, two orthogonal external fields are used to program an MRAM cell such as  35 . These are provided by current lines  31  and  32 , as shown in  FIG. 3 . The bit line provides the easy axis field while the word line provides the hard axis field. To program a cell, both bit and word line currents are applied, the combination of these two fields overcoming the shape anisotropy to set the magnetization of the selected cell into a desired direction. Although cell  35  is the one that was selected, many other cells, along either a powered bit or word line, such as  33  or  32 , also experience a field from either a bit line current or from a word line current, albeit smaller than the combined field that is experienced by the selected cell. Such cells are referred to as half-select cells. They are susceptible to being accidentally programmed and thereby causing an error. 
     Another shortcoming of this approach is the scaling difficulty: as dimensions grow smaller, thermal agitation may perturb stored information. This thermal effect is described by 
     
       
         
           
             f 
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                 f 
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                 exp 
                 ⁡ 
                 
                   ( 
                   
                     - 
                     
                       
                         B 
                         · 
                         
                           H 
                           s 
                         
                         · 
                         
                           M 
                           s 
                         
                         · 
                         V 
                       
                       
                         k 
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                         T 
                       
                     
                   
                   ) 
                 
               
             
           
         
       
     
     where f is the thermal switching frequency, f 0  and B are constants, k is the Boltzman constant, T is the temperature. To have a thermally stable stored information in the MRAM cell, the Δ=BH s M s tA has to be higher than a certain constant value. As dimension scales down, the area A is decreased, to maintain constant value of Δ, the 
               H   s     =     Δ       BM   s     ⁢   tA             
has to be increased, hence requiring higher switching current to write.
 
     These two shortcomings can be avoided by thermally assisting the switching of the magnetization during the write operation, as described in Ref [1] as well as in U.S. Pat. Nos. 6,385,082, 6,704,220, and 6,771,534. The latter propose using joule heating to reduce the Ms value, and hence to lower Hs, while maintaining thermal stability when this heat is absent. However, in this scheme the choice of free layer material is limited by the requirement that a large enough magneto-resistance has to be achieved for there to be enough read signal for the detection of stored information. Thus the choice of free layer is usually limited to Co, Fe, Ni, and their mutual alloys. These all have high Curie temperatures so the current required to obtain a significant reduction of Ms is very high. They suggested use of rare earth ferromagnetic materials that have low Curie temperatures and lower magneto-resistance values than Co, Fe, Ni, and their mutual alloys. But these materials and are highly corrosive which makes for great process difficulties. 
     The other approach to overcoming half-select and scaling issues is an exchange biased design, described in Ref. [1], for current field writing, and in U.S. Pat. No. 7,110,287 for spin torque transfer writing, to couple the free layer to a low blocking temperature (Tb) AFM layer (separate from the AFM used to pin the reference layer which has a high blocking temperature). 
     A schematic drawing of this design is shown in  FIG. 4 . Shown there are second AFM layer  41  (which has lower Tb), seed layer  42 , bottom electrode  43 , and diode/transistor  44 . Data storage is achieved by changing the direction of the exchange-coupling field (He) of second AFM  41  at free layer  11 . This is achieved by sending a current pulse through the MTJ to heat AFM layer  41  above Tb so He goes to zero. The word line is then energized to provide a directional magnetic field at the free layer, following which the heating current is turned off so that the magnetized free layer of the MTJ cools in the presence of the word line field. This sets the magnetization of the free layer in the desired direction, either parallel or antiparallel to the reference layer&#39;s magnetization. 
     The anisotropic field to maintain stored data against thermal agitation is provided by the unidirectional field 
               H   e     =     J     2   ⁢           ⁢     M   s     ⁢   t             
where J is the exchange coupling energy per unit area which is determined by the AFM and free layer material properties.
 
     The problem with this design is that, in order to have low Tb, the AFM&#39;s thickness has to be small; but He drops, and its variability increases, rapidly with AFM thickness so one ends up with a wide range of He values distributed among the various memory elements. The temperature generated by the heating current has to overcome the AFM with the highest Tb, which means that a high current will be needed. 
     REFERENCES 
     
         
         [1] I. L. Prejbeanu et al. “Thermally Assisted Switching in Exchange-Biased Storage Layer Magnetic Tunnel Junctions”, IEEE Trans. Mag. 40 No. 4, July 2004, 2625-2627. 
       
    
     A routine search of the prior art was performed with the following references of interest being found: 
     In U.S. 2008/0180991, Wang discloses a free layer containing nano-channels. In U.S. Pat. No. 7,375,405, Fukuzawa et al. show an AFM coupling layer of Rh, Ru, Cr, or Ir where magnetic pinning is performed at a temperature no higher than 300° C. Parkin, in U.S. Pat. No. 7,357,995, teaches a coupling layer comprising alloys of Cr with V, Nb, W, and Fe. 
     In U.S. Pat. No. 7,309,617, Reuhrig et al. disclose a reference layer oriented by cooling to below the Curie temperature. Their coupling layer comprised Ru, Au, or Co while Deak, in U.S. Pat. No. 7,230,844, teaches heating to near or above the Curie temperature for spin transfer. 
     SUMMARY OF THE INVENTION 
     It has been an object of at least one embodiment of the present invention to provide a method for eliminating the half-select problem in magnetic memory arrays. 
     Another object of at least one embodiment of the present invention has been for said method to increase the robustness of stored information in magnetic memory elements, despite size reductions of said memory elements. 
     Still another object of at least one embodiment of the present invention has been for said method to enable information to be stored in said memory elements in at least two different ways. 
     A further object of at least one embodiment of the present invention has been to provide a structure that is suitable for the implementation of said method. 
     These objects have been achieved by pinning the free layer magnetization via an AFM or antiferromagnetic layer (unrelated to the AFM that serves to pin the reference layer magnetization). As a key feature of the invention, a control valve layer (CV) is inserted between this AFM and the free layer (referred to as NF since it is a normal free layer). CV comprises ferromagnetic material that exchange couples this AFM to the free layer. 
     When CV is heated above its Curie temperature it becomes paramagnetic and no longer exchange-couples the AFM to the free layer so the magnetization of the latter is no longer pinned and can be switched, if so desired. Once switching is complete, CV is allowed to cool below its Curie temperature so that the AFM pins free layer NF once more. The necessary heating of CV is supplied by Joule heating that is generated by passing a current through the MTJ (or GMR) device. This Joule heating can be generated in the body of the device or in CV itself by giving CV the form of a matrix of ferromagnetic nano-channels embedded within an insulator. 
     Storing of information while free layer NF is in its unpinned state can be accomplished by generating magnetic fields in an additional wire having a bi-directional current capability which can provide a bi-directional setting field parallel or anti-parallel to the pinned layer, after passing a uni-directional heating current through the cell, or by a bi-directional current through the cell which provides heating and also sets the magnetization of the free layer through spin-torque transfer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1   a ,  1   b , and  2  illustrate the basic operation of an MTJ device. 
         FIG. 3  shows how, in the prior art, certain memory cells may be half-selected. 
         FIG. 4  is a schematic cross section of two Spin Torque MRAMs of the prior art. 
         FIG. 5   a  illustrates how, in the present invention, a coupling layer has been inserted between a second AFM layer and the normal ferromagnetic free layer. 
         FIG. 5   b  illustrates using an anti-parallel coupling layer, between CV and the free layer, to further increase the robustness of the stored data 
         FIG. 6  is a plot of Curie temperature vs. dopant concentration for NiX 
         FIG. 7  plots both Curie temperature and saturation induction of a NiFe alloy as a function of nickel concentration. 
         FIG. 8  is a schematic cross-section of a coupling valve layer having the form of magnetic nano-conducting channels embedded in insulation. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to  FIG. 5   a , the invention discloses a MRAM design utilizing a “coupling valve” switching mechanism. In this design, the MTJ&#39;s free layer is a composite of three layers: 
     1) A normal ferromagnetic layer  51  (NF) immediately adjacent to tunneling layer  12  (for highest MR value), 
     2) a coupling valve layer  52  (CV), and 
     3) a 2 nd  antiferromagnetic (AFM) layer  53  having a high blocking temperature. 
     As a key feature of the invention, the coupling valve layer is engineered to behave in the following manner: 
     during storage (including read operations) it is ferromagnetic and is exchange coupled to both free layer NF and the 2 nd  AFM layer so that the unidirectional field He from second AFM  53  acts on free layer NF via CV to maintain the magnetizations of both the NF and CV layers along the desired direction. 
     During a write operation, the CV layer has been heated above its Curie point by a current through the MTJ, causing it to lose its ferromagnetic property, so it can no longer provide exchange coupling between AFM layer  53  and free layer NF  51  whereby free layer NF no longer experiences the exchange field He of the AFM layer. Since free layer NF has intrinsically low or zero magnetic anisotropy, its magnetization can be easily set in the desired direction. Two methods are available for setting the magnetization of free layer NF  53 : 
     1) by an external magnetic field generated by current flowing through the word line or the bit line, similar to the approach portrayed in  FIG. 4 , or 
     2) by spin torque transfer from a spin polarized current flowing through MTJ itself. 
     For (1), the external field case, CV layer  52  becomes ferromagnetic again when the heating current is turned off and it is magnetized in the direction set by the external field and free layer NF&#39;s exchange coupling. Then, when the current that generates the external field is turned off, the magnetizations of NF and CV will be set to be parallel or anti-parallel to the pinned layer, depending the word line (or bit line) current direction. 
     For (2), the spin torque transfer writing case, the direction of current flowing through the MTJ sets the magnetization direction of the free layer NF. The CV layer is chosen to be a magnetic layer with low Curie temperature (Tc) so, when the MTJ temperature is above Tc, the CV layer transforms from a ferromagnetic to a paramagnetic phase whereby exchange coupling is reduced to zero, thereby decoupling the free layer NF from AFM layer  53  and allowing easy writing of the free layer&#39;s magnetic direction through the spin-torque-transfer effect. 
     The blocking temperature of 2 nd  AFM layer  53  needs to exceed the maximum temperature experienced by the MTJ so that its magnetic properties can remain unchanged during and after the write operation. 
     For the current generated (i.e. external) field writing case, the writing procedure can be further detailed as follows: 
     1) turn on the word (or bit) line current appropriate for the desired field direction, 
     2) send a pulse current through the MTJ via a diode (or transistor) so as to raise the MTJ free layer temperature above the CV layer&#39;s Tc, 
     3) allow the free layer to cool down with the word or bit line current field still present in order to set the NF and CV layers&#39; magnetizations in their desired directions. 
     For the spin torque transfer writing case, the writing procedure can be further detailed as follows:
         (a) To set the free layer magnetization parallel (from antiparallel) to the reference layer&#39;s magnetization:   (1) the CV is deactivated by passing a relatively high current pulse through the MTJ, so heat, diffusing mainly from the barrier layer, raises the CV&#39;s temperature above its Tc.   (2) an excess of electrons flows from the reference layer into the free layer and are magnetically aligned with the reference layer, causing, through spin torque, free layer NF to be magnetized in the same direction as the reference layer,   (3) once the CV temperature falls below Tc, the CV becomes exchange coupled to free layer NF and pins NF in its existing direction (parallel to the reference layer).   (b) The same sequence as above is used to set the free layer magnetization antiparallel (from parallel) to the reference layer&#39;s magnetization except that, in step 2, the electrons flow from the free layer into the reference layer so a majority of them are magnetically aligned parallel to the reference layer. The minority electrons with spin antiparallel to the pinned layer and the free layer will be reflected back from the pinned layer to free layer NF, through spin torque, causing free layer NF to be magnetized anti-parallel to the reference layer.       

     Since the coupling valve layer acts like a switching valve that turns exchange coupling between AFM and free layer NF on and off, so MRAMs having this type of free layer structure can be referred to as “coupling valve RAMs”. 
     To further increase thermal stability, a synthetic antiferromagnetic structure (SAF) can be employed, as shown by  FIG. 5   b . During a data storage or reading operation, the CV layer is strongly coupled anti-parallel to free layer NF  51  via Ru, Rh, Re, Cu, or Cr layer  54 . The magnetic moment of CV can be matched to that of NF, if so desired. Since the top and bottom layers of a SAF are anti-aligned to each other, He from the 2 nd  AFM on SAF is greatly enhanced so there is no residual de-magnetizing field, making this structure thermally robust and thus capable of being scaled down to very small dimensions. During writing, when the CV layer has temporarily entered a paramagnetic state, its magnetic moment becomes zero. Hence the remaining magnetic moment of the SAF derives from the NF layer which can be easily set by a word/bit line current field or by spin torque transfer, as discussed above. 
     Implementation Details: 
     For the spin torque transfer version discussed above, the invention requires that the MTJ be accessed through a transistor able to provide, in addition to its normal service, a bidirectional current in the form of a short pulse at a high current level followed by a long pulse at a lower current level. Said bidirectional current is required to cause electrons to flow from the reference layer into the free layer when writing the free layer magnetization parallel to the reference layer&#39;s magnetization, and vice versa. 
     Note, too, that the invention can be implemented so that magnetization in both the free layer NF and the reference layers lies in the plane of the deposited film or the magnetization may be perpendicular to the film plane. In a perpendicular design, both magnetizations of the free layer and reference (pinned) layer are perpendicular to the film plane. The free layer magnetization can be set to be along or against the reference layer magnetization. 
     The perpendicular configuration is achievable in magnetic films such as FePt, CoPt, CoFeTb, CoFeGd, etc or in multilayer structures such as Fe/Pt, Co/Pt, Co/Ni, Fe/Pd, and Co/Pd, which have high perpendicular anisotropy; this overcomes the de-magnetization field enabling the magnetization to be stable perpendicular to the film plane. The advantages of the perpendicular configuration are that a very low current is needed and the MTJ cell can be given a circular shape which is smaller than the more conventional elliptical shape. 
     To construct the invention, a first preferred embodiment is to have the coupling valve layer made of magnetic material with low curie temperature, ranging from 85-˜300° C. The free layer NF can be made of Co, Fe, Ni or their alloys, all of which have high curie temperatures (Tc of pure Ni ˜358° C., pure bcc Co ˜1130° C., pure Fe ˜770° C.). The coupling valve layer can be any conducting, semiconducting or weakly insulating magnetic material with a curie temperature between 85 and ˜300° C. Some examples are given below: 
     Ni, Fe, Co or alloys of form XY (where X=Ni, Fe, or Co, Y=Mo, Pt, V, Cr, Si, Al, Zn, Mn, Cu, Pd, C, Ce, B, S, or P, etc.) as shown in  FIG. 6  for Ni. 
     A NiFe alloy with a Ni concentration around 30-35% can have a curie temperature around 120 to 250° C., as shown in  FIG. 7 . Other low curie temperature materials can be rare earths like CrTe (Tc ˜100° C.); magnetic oxides like BeFeO 4  (Tc ˜190° C.), Er 2 O 3 *Fe 2 O 3  (Tc ˜275° C.); semi-metals like Heusler alloys (Cu 2 MnX where X=Al, In, Sn, Ga) or CoCrFeAl. The second AFM material can be any metallic antiferromagnetic material with a high blocking temperature such as MnX (X=Pt, Ir, Ru, Rh, Os, Ni, Fe) or MnXY (X or Y=Pt, Cr, Pd, Fe, Rh, Ru) and TbCo. 
     A second preferred embodiment is illustrated in  FIG. 8 . Here the coupling valve layer is a plurality of magnetic nano-conducting channels (NCC)  81  embedded in an insulating material (such as an oxide), these micro-channels being Co, Fe, Ni, or an alloy of these elements. During storage or read operations, AFM layer  53  continues to be exchange coupled to the free NF layer  51  through the magnetic nano-channels but during writing, the current will be concentrated within those nano-channels generating more heat to raise their temperature to be above their curie temperature thereby making them paramagnetic or super-paramagnetic which will decouple second AFM  53  from free layer NF. 
     Note that the use of a nano-channel in this environment is quite different from its role in a CPP GMR spacer layer. In the latter, the nano-channel serves to make the resistance component of the resistance area product as high as possible whereas in the device that forms the present invention the resistance of the NCC should be as low as possible while still concentrating the current to a sufficient degree to raise the local temperature above the Tc of the MAGNETIC material used to form the channels. This is unlike a GMR micro-channel, which need not be, and preferably shouldn&#39;t be, magnetic.