Characterization of interfacial properties in magnetic tunnel junctions by bias-dependent complex impedance spectroscopy

The bias-dependent complex impedance spectra of ion-beam sputtered CoFe-AlO_x-CoFe tunnel junctions have been systematically investigated. An equivalent circuit model, composed of a resistance component and two sets of parallel resistance (R) and capacitance (C) components in series, has been utilized to describe the individual impedance contribution from lead of cross patterns, CoFe-AlO _x interfaces and bulk AlO_x in CoFe-AlO_x-CoFe tunnel junctions. The fitting results reveal that the RC component characterizing the CoFe-AlO_x interface can be explained by a Schottky barrier with imperfectly blocking characteristic, which is attributed to the existence of traps in the depletion region. The tendency of interfacial resistance, R_i, and interfacial capacitance, C_i, as a function of dc bias, V_dc, is expected for the space charge limited current model with exponential trap distribution, which coincides with the analyzing results of dc four-probe current-voltage curves. On the other hand, the R and C components of bulk AlO _x almost remain constant when V_dc increases, in contrast to the electrical behavior of CoFe-AlO_x interfaces. The results suggest the metal-insulator interface, instead of bulk insulator, dominates the electrical transport process of MTJs.