Transport property studies on magnetic tunnel junctions with wedge-shaped barrier

Systematic studies on the transport property were carried out in FM (NiFe, Co, FeCo)/AlOx/NiFe magnetic tunnel junctions (MTJs) by oxidizing a wedge-shaped Al layer, resulting in over-, optimized, and under-oxidized barriers. An interesting resistance peak can be generally observed in the over-oxidized region, which was attributed to the significant change in oxidation level of the bottom electrode. This argument was supported by the barrier thickness and height fittings. Combining the studies of tunnel magnetoresistance (TMR) and the bias dependence as a function of the Al layer thickness, we can conclude that, in the MTJ exhibiting maximum TMR, the Al layer has not been fully oxidized to reach stoichiometric Al2O3, and the bottom electrode has already been slightly oxidized. A sizable TMR ratio can still be observed in under-oxidized region, even with very thick Al layer and short oxidation time, which implies that no pure metallic Al exists near the bottom ferromagnet/insulator (FM/I) interface. This is consistent with the grain boundary oxidation model for post-oxidizing the Al layer in MTJs