Influence of chemical and magnetic interface properties of Co-Fe-B/MgO/Co-Fe-B tunnel junctions on the annealing temperature dependence of the magnetoresistance

The knowledge of chemical and magnetic conditions at the Co40Fe40B20/MgO interface is important to interpret the strong annealing temperature dependence of tunnel magnetoresistance of Co-Fe-B/MgO/Co-Fe-B magnetic tunnel junctions, which increases with annealing temperature from 20% after annealing at 200 degrees C up to a maximum value of 112% after annealing at 350 degrees C. While the well defined nearest neighbor ordering indicating crystallinity of the MgO barrier does not change by the annealing, a small amount of interfacial Fe-O at the lower Co-Fe-B/MgO interface is found in the as grown samples, which is completely reduced after annealing at 275 degrees C. This is accompanied by a simultaneous increase of the Fe magnetic moment and the tunnel magnetoresistance. However, the tunneling magnetoresistance of the MgO based junctions increases further for higher annealing temperature which cannot be caused by Fe-O reduction. The occurrence of an x-ray absorption near-edge structure above the Fe and Co L edges after annealing at 350 degrees C indicates the recrystallization of the Co-Fe-B electrode. This is a prerequisite for coherent tunneling and has been suggested to be responsible for the further increase of the tunneling magnetoresistance above 275 degrees C. Simultaneously, the B concentration in the Co-Fe-B decreases with increasing annealing temperature, at least some of the B diffuses toward or into the MgO barrier and forms a B2O3 oxide. (C) 2007 American Institute of Physics.