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°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 T a≥250°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°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°C. Upon further annealing to T a 400°C, the Ti oxidizes to form amorphous Ti-O x. The rejected B diffuses back to the CoFe layer at T a=500°C that degrades the TMR. On the other hand, the Ta capped MTJs annealed at 300≤T a≤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.
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