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, T-a = 250 degrees C, 300 degrees C, 400 degrees C, and 500 degrees 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 T-a >= 250 degrees C. A bcc Fe-Co (110) crystallizes from the fcc Pd (111) interface. The Fe-Co layer is alloyed with Pd layer at T-a = 500 degrees 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 T-a = 300 degrees C. Upon further annealing to T-a >= 400 degrees C, the Ti oxidizes to form amorphous Ti-O-x. The rejected B diffuses back to the CoFe layer at T-a = 500 degrees C that degrades the TMR. On the other hand, the Ta capped MTJs annealed at 300 <= T-a <= 500 degrees 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. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4707964]