First-principles study of electronic and magnetic structures of CoFeB|Ta and CoFe|TaB heterostructures

The electronic and magnetic structures of the CoFeB|Ta and CoFe|TaB heterostructures were investigated from first principles in an attempt to clarify the effect of the Ta layer on the energetic stability and magnetisation of the heterostructures, which are used in magnetic random access memory devices. The electronic structures of these heterostructures were calculated by using the density functional pseudopotential method in the generalised gradient approximation. Calculation clarified that the Ta layer of the CoFeB|Ta heterostructure energetically stabilised the heterostructure, chemically absorbing the B atoms present in its CoFeB layer, and intensified the magnetisation of the heterostructure. The result is consistent with the conventional understanding that the Ta layer works as a getter of B atoms. The d electronic states of the heterostructures were investigated by calculating d electron numbers of metal atoms of the heterostructure. Interestingly, it was found that the Ta layer of both the heterostructures was spin polarised and the spin- polarisation direction reversed by the B atom absorption by the Ta layer. The results suggest that the Ta layer actively participates in the magnetisation of the heterostructure, contrary to the conventional understanding that the Ta layer is irrelevant to the magnetisation of the heterostructure.