Abstract
We show that Mn atoms diffuse with two different mechanisms at high and low temperatures in CoFeB and CoFe-based magnetic tunnel junctions. By combining high resolution and scanning transmission electron microscopy, we reveal that below 300\textordmasculine C the amorphous CoFeB and the textured CoFe are equally effective in blocking the diffusion of Mn, contradicting the conventional wisdom that the diffusion occurs primarily along grain boundaries. Below 300\textordmasculine C Mn diffusion in crystalline CoFe occurs
through the bulk and is assisted by oxygen atoms which only diffuse parallel to the bcc close-packed (110) plane. Above 300\textordmasculine C Mn diffuses through vacancies along the grain boundaries of CoFe and in the bulk of amorphous CoFeB. A universal diffusion temperature is proposed based on an isokinetic relation.