We have investigated the microstructure and local chemistry of Ta/Ru/Ta/CoFeB/MgO/CoFeB/Ta/Ru magnetic tunnel junctions with different values of tunneling magnetoresistance (TMR) as a result of annealing at different temperatures. Annealing at 500 °C led to the templated crystallization of the amorphous CoFeB layer having coherent interfaces with MgO grains with an orientation relationship of 〈 001 〉 MgO∥ 〈 001 〉 CoFe, and the B rejected from crystallized CoFeB was found to be dissolved in upper amorphous Ta layers and segregated in the bottom crystalline Ta layer. Annealing at 600 °C led to the dissolution of 3-4 at. % Ta in the MgO barrier, and B was found to be segregated at the CoFeB/MgO and Ta/Ru interfaces as a result of the crystallization of the top amorphous Ta layer. Further degradation in TMR of the samples annealed at 650 °C results from the loss of bcc-CoFe (001) texture in the bottom CoFeB electrode due to the pronounced Ta diffusion into the CoFe/MgO/CoFe layers.
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