Inverse TMR in the Fe/MgO/Fe(001) Epitaxial System

Both theoretical and experimental studies of the epitaxial Fe/MgO/Fe(001) trilayers have shown new spin dependent tunnelling processes due to the conservation of the (001) crystallo-graphic orientation in the entire stack. In addition the MgO barrier acts as a spin filter : the tunnelling electrons are more or less attenuated in the barrier as a function of their symmetry. The very high tunneling magnetoresistance (TMR) value calculated and obtained at room temperature is a consequence of both this spin filtering effect and the fact that Fe(001) is half metallic as regard s the Delta1 symmetry. The theoretical predictions were performed at the equilibrium (symmetrical barrier). Very recent works have shown that the study of the conductance in Fe/MgO/Fe(001) tunnel junctions with large barrier thickness and out of equilibrium (for biased junctions) reflects the Fe(001) band structure in the Delta direction. At low bias voltage (in the range of plusmn1V) the parallel conductance is governed by the Delta1 Bloch states whereas the antiparallel conductance is governed by the Delta5 symmetry. Here, we are interested in the conductance very far from the equilibrium i.e. at high bias voltage (more than 1.5 V). We observed an inversion of the sign of the TMR for -1.8 V The specific voltage of this crossing point can be understood from the analysis of the Fe band structure well above the Fermi level and can be related to the opening of a new conduction channel.