Temperature-dependent Mn-diffusion modes in CoFeB- and CoFe-based magnetic tunnel junctions: Electron-microscopy studies

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 $300°\mathrm{C}$, 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 $300°\mathrm{C}$, 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 $300°\mathrm{C}$, 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.

Figure 1

A typical cross-sectional HRTEM image of CoFeB-D. The parallel bars indicate the 110 planes of CoFe. (b), (c), and (d) show the electron diffraction patterns by Fourier transforms from the top layer, the CoFeB layer, and the bottom layer in (a), respectively.

Figure 2

(Color online) TMR as a function of annealing temperature for four MTJs.

Figure 3

(Color online) Analysis of elemental (Mn, Co) distributions within a CoFeB-D sample. (a) The HAADF image. (b) Line profiles for elements Mn and Co extracted from (a) along the marked direction in the middle.

Figure 4

(Color online) Analysis of elemental (Mn, Co) distributions for a CoFe-D sample. (a) The HAADF image. (b) Line profiles for elements Mn and Co extracted from (a) along the marked direction in the middle for $260°\mathrm{C}$ annealed sample. (c) Line profiles for elements Mn and Co extracted from (a) along the marked direction in the middle for as-grown sample.