  We present results from an X-ray photoemission spectroscopy (XPS) study of
CoFeB/MgO bi-layers where we observe process-dependent formation of B, Fe, and
Co oxides at the CoFeB/MgO interface due to oxidation of CoFeB during MgO
deposition. Vacuum annealing reduces the Co and Fe oxides but further
incorporates B into the MgO forming a composite MgBxOy layer. Inserting an Mg
layer between CoFeB and MgO introduces an oxygen sink, providing increased
control over B content in the barrier.
