Bias-voltage dependence of magnetoresistance in magnetic tunnel junctions grown on Al2 O3 (0001) substrates

Magnetic tunnel junctions with the structure of Al2 O3 (0001)/Pt (111) 20 nm/ Ni80 Fe20 (111) 50 nm/Al 1.6 nm-O/ Co75Fe25 4 nm/ Ir22 Mn78 10 nm/ Ni80 Fe20 30 nm were fabricated using UHV sputtering and photolithography process. As the annealing temperature increased up to 250 °C, tunnel magnetoresistance (TMR) ratio at 1 mV bias increased from 28% to 43% for tox =180 s plasma oxidation and the V±12, 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±12 and the interface of the ferromagnetic electrode and the Al-O insulating layer. V+12, which reflects the bottom ferromagnetic electrode-barrier interface state, changes with plasma oxidation time, while V-12, which corresponds to top ferromagnetic electrode-barrier interface, hardly changes.