Effect of ferromagnetic layer structure on the magnetization process in NiFe/Co/Al–O/NiFe and Co/NiFe/Al–O/NiFe junctions

Abstract

The effect of ferromagnetic layer structure on the magnetization reversal processes in (NiFe/Co)/Al–O/NiFe (S1) and (Co/NiFe)/Al–O/NiFe (S2) films was investigated. The films were fabricated by sputter deposition, and the Al–O layer was prepared by oxidizing an Al layer in air. Two distinct magnetization processes were observed by transmission Lorentz microscopy (TLM) with increasing in situ applied field. Reversal of the NiFe/Co bilayer in S1 occurs via moment rotation, while reversal of the Co/NiFe bilayer in S2 occurs by domain wall motion, in both cases at higher field than the top NiFe layer. The difference can be ascribed to the difference in the deposition order of the ferromagnetic bilayers (FMBs). High resolution electron microscopy shows that the grains in the top NiFe layer are randomly oriented in both films. In S1, the NiFe grains in the FMB are randomly oriented, with columnar grains present in the Co. In S2, a columnar grain structure of NiFe in the FMB is induced by the Co underlayer. The rough FMB/Al–O interface in S2 leads to weak biquadratic interlayer coupling, which will slightly reduce the field range of the antiparallel magnetization configuration. Hysteresis loops of S1 and S2 show two stage magnetization reversals in each sample, which are consistent with TLM results.