We have investigated the crystallization process of an amorphous Co40 Fe40 B20 ribbon and a MgO/ amorphous-Co40 Fe40 B20 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/ Co40 Fe40 B20 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-Co40 Fe40 B20 bilayer film.
ASJC Scopus subject areas