Hysteretic Behavior of the Dynamic Permeability in FeCoB Thin Films

The microwave permeability in a soft magnetic FeCoB film as a function of the external magnetic field is measured in the frequency range from 0.2 to 8 GHz. When the field applied parallel to the orientation of the magnetization varies from 90 to -90 Oe, the permeability spectrum shows hysteretic behavior. Double magnetic resonance peaks come forth and evolve in the imaginary part of the permeability when the field approaches the coercive field of the sample. This phenomenon is attributed to the formation of domains, the resonance frequency of the reversed domains being different than that of the non-reversed domains. Both peak frequencies and peak amplitudes are interpreted quantitatively using simple considerations based on the Landau-Lifshitz equation and the knowledge of the static magnetic properties of the film. The ratio of the reversed magnetic moments to the non-reversed ones is estimated by fitting curves of the permeability spectrum.