We have investigated the crystallization process of an amorphous Co40Fe40B20 ribbon and a MgO/amorphous-Co40Fe40B20 thin film to simulate the segregation of B in the magnetic tunneling junctions that use CoFeB ferromagnetic electrodes and a MgO tunneling barrier. Both types of samples show the same crystallization sequence: a primary crystallization of bcc-FeCo from an amorphous precursor followed by boride crystallization at a higher annealing temperature. Three-dimensional atom probe analysis has shown that very little boron is dissolved in the bcc-FeCo crystals (∼1 at. %). A transmission electron microscopy observation of an annealed MgO/Co40Fe40B20 bilayer thin film has shown clear evidence of MgO serving as heterogeneous nucleation sites for bcc-FeCo grains. Energy filtering transmission electron microscopy has shown strong boron segregation at the MgO/FeCo interface after the crystallization of the MgO/amorphous-Co40Fe40B20 bilayer film.
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You et al. (2008) studied this question.
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