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Abstract:
This work reports on hard x-ray photoelectron spectroscopy (HAXPES) of
CoFeB based tunnel junctions. Aim is to explain the role of the boron
diffusion for the observed improvement of the tunneling
magnetoresistance ratio with increasing annealing temperature. The high
bulk sensitivity of HAXPES was used as a nondestructive technique to
analyze CoFeB-MgO-CoFeB magnetic tunnel junctions. The investigated
samples were processed at different annealing temperatures from 523 to
923 K. Hard x-ray core level spectroscopy reveals an enforced diffusion
of boron from the CoFeB into the adjacent Ta layer with increasing
annealing temperature. The dependence of the tunneling magnetoresistance
on the annealing temperature is explained 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
CoFeB layer and Ta diffusion at high annealing temperature.