Microstructure and electrical properties of magnetic tunneling junctions: Ta/NiFe/IrMn/M/Al-oxide/M/NiFe (M=Co, NiFe, CoFe)

The microstructure and electrical properties of the Ta/NiFe/IrMn/M/Al-oxide/M/NiFe (M=Co, NiFe, CoFe) ferromagnetic tunnel junctions with different Al-oxide thickness were investigated. The CoFe junction showed the highest magnetoresistance (MR) ratio of 32% at room temperature with the insulation layer of Al (15 Å)-oxide, but also had the highest junction resistance of 34 kΩ (junction area of 200×200 μm) out of the three electrodes. The Co junction had the lowest resistance with reasonably high MR ratio of 24%. Cross-sectional transmission electron microscopy of the junctions showed that the increasing insulation thickness resulted in the rapid increased roughness at the top electrode/Al-oxide interface and the subsequent reduction of the MR ratio for all three electrodes materials. We have demonstrated that the MR effect is not only dictated by the intrinsic properties of the FM electrode materials, but also by the thickness and the microstructure of the oxide layer, which could be utilized to optimize the electrical properties of the ferromagnetic tunneling junction.