Bias-voltage dependence of magnetoresistance in magnetic tunnel junctions grown on Al_2O_3 (0001) substrates

摘要

Magnetic tunnel junctions with the structure of Al_2O_3 (0001)/Pt (111) 20 nm/Ni_(80)Fe_(20) (111) 50 nm/Al 1.6 nm-O/Co_(75)Fe_(25) 4 nm/Ir_(22)Mn_(78) 10 nm/Ni_(80)Fe_(20) 30 nm were fabricated using UHV sputtering and photolithography process. As the annealing temperature increased up to 250℃, tunnel magnetoresistance (TMR) ratio at 1 mV bias increased from 28% to 43% for t_(ox)=180 s plasma oxidation and the V_(±1/2), at which the zero bias TMR value is halved, is +640 mV and —650 mV for positive and negative bias voltages, respectively. The bias-voltage dependence of TMR could be explained in terms of the relationship with V_(±1/2) and the interface of the ferromagnetic electrode and the Al-O insulating layer. V_(+1/2), which reflects the bottom ferromagnetic electrode-barrier interface state, changes with plasma oxidation time, while V_(-1/2), which corresponds to top ferromagnetic electrode-barrier interface, hardly changes.