Induced anisotropy and positive exchange bias: A temperature, angular, and cooling field study by ferromagnetic resonance

Exchange-biased (formula presented) bilayers, examined by variable angle and temperature ferromagnetic resonance (FMR), exhibit a sudden onset of a unidirectional and fourfold anisotropy below the (formula presented) Néel temperature. This unexpected fourfold symmetry arises from frustrated perpendicular coupling between the (formula presented) and the Fe overlayer in the presence of twinning in the antiferromagnet layer. These data are consistent with earlier polarized-neutron-reflectometry results. The FMR data show a clear reversal in the direction of the unidirectional anisotropy as a function of cooling field, switching sign at (formula presented) which is consistent with the onset of positive exchange bias observed in conventional magnetometry experiments. The low-temperature FMR linewidth reflects the in-plane symmetry of the resonance itself, exhibiting surprising divergence in the hard directions. Temperature-dependent FMR measurements reveal a sharp reduction in the resonance field below the Néel point due to the ferromagnetic/antiferromagnetic coupling.