Correlation between Barrier Width, Barrier Height, and DC Bias Voltage Dependences on the Magnetoresistance Ratio in Ir–Mn Exchange Biased Single and Double Tunnel Junctions

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Abstract
Dual spin-valve-type double tunnel junctions (DTJs) of Ir–Mn/CoFe/AlO x /Co90Fe10/AlO x /CoFe/Ir–Mn and spin-valve-type single tunnel junctions (STJs) of Ir–Mn/CoFe/AlO x /CoFe/Ni–Fe were fabricated using an ultrahigh vacuum sputtering system, conventional photolithography and ion-beam milling. The STJs could be fabricated with various barrier heights by changing the oxidization conditions during deposition and changing the annealing temperature after deposition, while the AlO x layer thickness remained unchanged. There was a correlation between barrier width, height estimated using Simmons' expressions, and dc bias voltage dependence on the MR ratio. The V B dependence on the tunneling magnetoresistance (TMR) ratio was mainly related to the barrier width, and the decrease in the TMR ratio with increasing bias voltage is well explained, taking into account the spin-independent two-step tunneling via defect states in the barrier, as a main mechanism, at room temperature. Under optimized oxidization and annealing conditions, the maximum TMR ratio at a low bias voltage, and the dc bias voltage value at which the TMR ratio decreases in value by half (V 1/2) were 42.4% and 952 mV in DTJs, and 49.0% and 425 mV in STJs, respectively.