Strain-driven fractional spontaneous exchange bias in ferromagnetic/antiferromagnetic thin films with composition-graded ferromagnetic layer

摘要

We experimentally demonstrate that the stress-induced magnetic anisotropy in composition-graded ferromagnetic NiFeTa layers can be employed to trigger exchange bias in NiFeTa/IrMn bilayers without using any deposition field or field cooling procedure. In particular, we found that the NiFeTa/IrMn bilayers exhibit double-shifted loops being composed of both negative and positive exchange biases when the deposition angle is low. As the deposition angle is increased, the magnetization curves change into a single-shifted loop with spontaneous exchange bias aligned along negative or positive direction. The results can be explained by the formation of magnetic domain state under the presence of a strong stress-induced magnetic anisotropy and this domain state is pinned upon the deposition of an antiferromagnetic layer. We also studied the dynamic permeability spectra of the films with respect to temperature and discussed the results in relation with the static magnetic properties.