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/sub 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 /sub x/ interfaces and bulk AlO/sub x/ in CoFe-AlO/sub x/-CoFe tunnel junctions. The fitting results reveal that the RC component characterizing the CoFe-AlO/sub 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/sub i/, and interfacial capacitance, C/sub i/, as a function of dc bias, V/sub 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 /sub x/ almost remain constant when V/sub dc/ increases, in contrast to the electrical behavior of CoFe-AlO/sub x/ interfaces. The results suggest the metal-insulator interface, instead of bulk insulator, dominates the electrical transport process of MTJs.