Influence of boron diffusion on the perpendicular magnetic anisotropy in Ta|CoFeB|MgO ultrathin films

We have studied structural and magnetic properties of Ta vertical bar CoFeB vertical bar MgO heterostructures using cross-section transmission electron microscopy (TEM), electron energy loss spectrum (EELS) imaging, and vibrating sample magnetometry. From the TEM studies, the CoFeB layer is found to be predominantly amorphous for as deposited films, whereas small crystallites, diameter of similar to 5 nm, are observed in films annealed at 300 degrees C. We find that the presence of such nanocrystallites is not sufficient for the occurrence of perpendicular magnetic anisotropy. Using EELS, we find that boron diffuses into the Ta underlayer upon annealing. The Ta underlayer thickness dependence of the magnetic anisotropy indicates that similar to 0.2 nm of Ta underlayer is enough to absorb the boron from the CoFeB layer and induce perpendicular magnetic anisotropy. Boron diffusion upon annealing becomes limited when the CoFeB layer thickness is larger than similar to 2 nm, which coincides with the thickness at which the saturation magnetization M-S and the interface magnetic anisotropy K-I drop by similar to 20%. These results show the direct role which boron plays in determining the perpendicular magnetic anisotropy in CoFeB vertical bar MgO heterostructures. (C) 2015 AIP Publishing LLC.