Transmission electron microscopy study on the effect of various capping layers on CoFeB/MgO/CoFeB pseudo spin valves annealed at different temperatures

The microstructure of pseudo spin-valve magnetic tunnel junctions (MTJs) comprising a stacking structure of Ta/Ru/Ta/CoFeB/MgO/CoFeB/ with and without X = Pd, Ti, Ta fabricated on thermally oxidized Si wafer with different annealing temperatures, Ta = 250 °C, 300 °C, 400 °C, and 500 °C, has been investigated. The as-deposited MTJs exhibit an amorphous CoFeB structure that crystallizes into bcc Fe-Co (001) from the MgO (001) interface upon annealing at Ta ≥ 250 °C. A bcc Fe-Co (110) crystallizes from the fcc Pd (111) interface. The Fe-Co layer is alloyed with Pd layer at Ta = 500 °C to form an (Fe, Co)-Pd alloy layer, which causes a drastic reduction in the tunneling magnetoresistance (TMR) from 171% to −2.7%. In the Ti capped MTJs, bcc Fe-Co (001) crystallizes from the hcp (001) Ti interface at Ta = 300 °C. Upon further annealing to Ta ≥ 400 °C, the Ti oxidizes to form amorphous Ti-Ox. The rejected B diffuses back to the CoFe layer at Ta = 500 °C that degrades the TMR. On the other hand, the Ta capped MTJs annealed at 300 ≤ Ta ≤ 500 °C show a perfect grain-to-grain epitaxy with an orientation relationship of (001)[110]MgO//(001)[100]CoFe without interdiffusion or oxidation, resulting in the highest TMR value among all the MTJs with various capping layers.