The interface stability and microstructure between amorphous and crystalline ferromagnetic (FM) layers Fe56CO24B20 and Co70Fe30 (at. %) and oxide barrier layers (AlO) deposited by physical vapor deposition, in both as-deposited and annealed states, have been studied using hysteresis loops for magnetic measurement, x-ray photoelectron spectroscopy for elemental depth profiling, and transmission electron microscopy for atomic-level microstructure. AlO was found to be amorphous on both amorphous Fe56Co24B20 and polycrystalline Fe30Co70 FM layers substantial Fe diffusion towards the AlO layer and Al diffusion towards the FM layer are clearly observed for the Fe56Co24B20/AlO system when-annealed above 360 degrees C and will likely cause magnetic tunneling junction devices made from this system to fail. (c) 2006 American Vacuum Society.