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

Exchange-biased MnF₂/Fe bilayers, examined by variable angle and temperature ferromagnetic resonance (FMR), exhibit a sudden onset of a unidirectional and fourfold anisotropy below the MnF₂ N\'eel temperature. This unexpected fourfold symmetry arises from frustrated perpendicular coupling between the MnF₂ 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 HFC=13kOe, 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 N\'eel point due to the ferromagnetic/antiferromagnetic coupling.