Effect of microstructure on the magnetoresistive properties of NiFe/Co(CoFe)/Al(Ta)-oxide/Co(CoFe) tunnel junctions

The microstructure of the NiFe/Co(CoFe)/Al(Ta)-oxide/Co(CoFe) ferromagnetic tunnel junction was investigated using cross-sectional transmission electron microscopy (TEM). The effect of the insulating layer on the magnetoresistive (MR) properties of the junction was studied. The multilayer junction was formed using magnetron sputtering and the insulating layer was created by plasma oxidation of the deposited metal film. TEM analysis showed that the MR ratio was highly dependent on the insulating layer. For the NiFe/Co/Al-oxide/Co junction, when the Al₂O₃ layer was 13 Å, the oxide layer was flat and the highest MR ratio of 15% was attained. As the Al₂O₃ thickness increased, the interface roughness rapidly increased, and the MR ratio also markedly dropped. In contrast, NiFe/CoFe/Al-oxide/CoFe junction showed a comparatively flatter interface and recorded a higher MR ratio. The Ta-oxide insulating layer remained flat regardless of the thickness; however, the largest MR ratio of only 9% was obtained within a narrow thickness range. We have demonstrated that there exists a direct correlation between the microstructure of the oxide layer and the MR ratio of the junction, which could be utilized to optimize the electrical properties of the ferromagnetic tunneling junction.