Bias voltage and annealing-temperature dependences of magnetoresistance ratio in Ir-Mn exchange-biased double tunnel junctions

Dual-spin-valve-type double tunnel junctions (DTJs) of sputtered Ir-Mn/Co-Fe/AlO_x/Co₉₀Fe₁₀/AlO_x/Co-Fe/Ir-Mn were fabricated using photolithography and ion-beam milling. The DTJs were annealed at various temperatures (150-400 °C) to introduce interdiffusion. The magnetoresistance (MR) ratio and DC bias voltage value at which the MR ratio decreases in half value (V_1/2) were measured before and after annealing. A maximum MR ratio and V_1/2 obtained after annealing at approximately 320 °C was 42.4% and 952 mV, respectively, at room temperature. There is a correlation between the loss of the MR ratio and that of V_1/2 above 320 °C. The loss of the MR ratio and that of V_1/2 are well explained by considering two phenomena, i.e., interdiffusion of O and Mn at the AlO_x/Co-Fe/Ir-Mn interfaces. The mechanism for the loss of MR ratio is not only related to the loss of interface polarization, but is also related to the barrier properties, taking into account the spin-independent two-steps tunneling via defect states in the barrier. These results are consistent with the X-ray photoelectron spectroscopy and cross-sectional transmission electron spectroscopy measurements, which indicate the existence of an Al-Mn-O barrier above 320 °C.