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 electr on microscopy (TEM). The effect of the insulating layer on the magnetoresis tive (MR) properties of the junction was studied. The multilayer junction w as formed using magnetron sputtering and the insulating layer was created b y plasma oxidation of the deposited metal film. TEM analysis showed that th e MR ratio was highly dependent on the insulating layer. For the NiFe/Co/Al -oxide/Co junction, when the Al2O3 layer was 13 A, the oxide layer was flat and the highest MR ratio of 15% was attained. As the Al2O3 thickness incre ased, the interface roughness rapidly increased, and the MR ratio also mark edly dropped. In contrast, NiFe/CoFe/Al-oxide/CoFe junction showed a compar atively flatter interface and recorded a higher MR ratio. The Ta-oxide insu lating layer remained flat regardless of the thickness; however, the larges t MR ratio of only 9% was obtained within a narrow thickness range. We have demonstrated that there exists a direct correlation between the microstruc ture of the oxide layer and the MR ratio of the junction, which could be ut ilized to optimize the electrical properties of the ferromagnetic tunneling junction. (C) 2001 American Institute of Physics.