referring now to fig5 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 ), 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 fig4 , 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 fig5 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 . 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 bi - directional current in the form of a short pulse at a high current level followed by a long pulse at a lower current level . said bi - directional 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 fig6 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 fig7 . 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 fig8 . 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 .