Increased ferromagnetic resonance linewidth and exchange anisotropy in NiFe/FeMn bilayers

Abstract

In the past investigations, the exchange-biased bilayers show much larger ferromagnetic resonance (FMR) linewidth than that of single ferromagnetic layer films. However, the mechanism of the large linewidth remains controversial. In this paper, the FMR linewidths of NiFe/FeMn bilayers prepared by dc magnetron sputtering system are systematically studied. Besides the intrinsic damping and magnetic inhomogeneity, the extrinsic relaxation based on two-magnon scattering process should be also considered to explain the strong in-plane angular dependence of the linewidths. The fitting of the out-of-plane angular dependence of the linewidths shows that the intrinsic Gilbert damping effect plays a major role in the increased linewidth in the bilayers. The value of the g factor increases due to the effect of the exchange coupling at the NiFe/FeMn interface, resulting in an enhancement in the Gilbert damping factor G. The fitting results indicate that the line broadening of the exchange-biased films is related to the exchange anisotropy. The effects of FeMn layer thickness, growth sequence of FeMn layer, and temperature on the linewidths further approve this interpretation.