Exchange-biased magnetic tunnel junctions fabricated with in situ natural oxidation

Exchange-biased magnetic tunnel junctions with a Ta/NiFe/FeMn/NiFe/Al–oxide/NiFe/Ta structure have been fabricated. The tunnel barrier was formed by the in situ natural oxidation of an Al metal layer under controlled oxygen pressure. Photolithography and ion milling were used to pattern the multilayer into junction structures of 2×2 μm²–20×20 μm² dimensions. Magnetoresistance (MR) curves show spin-valve-like characteristics, in which an antiparallel configuration of magnetizations in both ferromagnetic layers is observed between 50 and 240 Oe, and the hysteresis loops for both the free and pinned layers exhibit sufficient separation. An evaluation of the MR curves shows the exchange-bias field to be 340 Oe and coercivity levels in the free layer to become as low as 13 Oe. At room temperature normalized junction resistance is ${\text{2×10}}^{\text{−5}}\mathrm{ \Omega }{\mathrm{cm}}^2,$ with MR ratios still being maintained at 13%. This resistance value is much lower than previously reported values for junctions produced either with plasma oxidation or thermal oxidation in air. Maximum variation in junction resistance is only ±5% for 10×10 μm² junctions over a 2 in. wafer. The MR ratio decreases by half when the bias voltage is raised from 0 to 440 mV, approximately the same ratio of decrease as has been previously reported for other successful junctions.