Hysteretic resonance frequencies and magnetization reversal in exchange biased polycrystalline F/AF bilayers

Polycrystalline ferromagnetic(F)/antiferromagnetic(AF) exchange biased bilayers of NiFe/MnNi have been grown by RF sputtering. The measured hysteresis loops are shifted and exhibit a strong asymmetry along the easy axis of the F film. The dynamic properties have been investigated through Complex Permeability frequency Spectra measurements in the 100 MHz-6 GHz range. The dispersion relation of the resonance frequency of imaginary part of the permeability shows hysteretic behavior when measured in presence of a static magnetic field applied along the easy axis of the F layer. A model for magnetization reversal in such bilayer based on the exchange energy development into Fourier-like power series is used. We show that it is possible to fit, with a set of parameters, the experimental asymmetric shape of hysteresis loop of the exchange biased bilayer. The same set of fit parameters are used to calculate the dispersion relation. The hysteretic behavior of the experimental resonance frequencies was found to be in agreement with theoretical results. Such a model reconciles irreversible (such as hysteresis loop) and reversible (such as AC permeability) measurements of the exchange bias.