High resonance frequencies induced by in-plane antiparallel magnetization in NiFe/FeMn bilayer

Continuous exchange biased bilayers with in-plane antiparallel magnetization were prepared by a pulsed-current, fast-annealing method. The exchange-bias field and ferromagnetic resonance frequency of the bilayers, annealed with different voltages, were obtained using a vibrating sample magnetometer and vector network analyzer, respectively. All annealed bilayers had two exchange-bias fields and two resonance frequencies. The exchange-bias field decreased or even reversed direction, while the resonance frequency increased after annealing. We argue that the transition zone between antiparallel magnetizations had a greater number of rotatable antiferromagnetic spins, and thus, greater rotatable anisotropy, resulting in higher resonance frequencies in the annealed film. Moreover, for samples annealed at different voltages, the narrow area had a greater proportion of rotatable antiferromagnetic spins with a low exchange-bias field and high resonance frequency in comparison with the broad area. A model was proposed to describe these phenomena, which was supported by ion-etching tests.