HAXPES studies of FeCoB-MgO-FeCoB tunnel junctions

This work reports on hard X-ray photoelectron spectroscopy of FeCoB-MgO-FeCoB magnetic tunnel junctions. The studies were performed on both the lower part of a tunnel junction only as well as on the full junction. The multilayer structures were deposited on SiO wafers with the sequence Ta/Ru/Ta/FeCoB/MgO/AlOx or Ta/Ru/Ta/FeCoB/MgO/FeCoB/Ta/AlOx. The final samples were annealed at temperatures of 520 K to 920 K. Core level spectroscopy reveals clearly the thermally stimulated interlayer diffusion of boron. The efficiency of the boron transfer between FeCoB and the contiguous Ta layer increases at higher annealing temperatures. Valence band spectroscopy shows strong changes of the electronic structure with increasing annealing temperature and shows a good agreement with the calculated density of states. The dependence of the tunnel magnetoresistance on the annealing temperature is explained for by the combined effects of an improved crystalline structure together with a change in the spin polarization at the Fermi energy caused by the removal of boron from the FeCoB layer.