Initial Permeability of Ferrites at Microwave and Natural Gyromagnetic Resonance

Abstract

At sufficiently high frequencies the initial permeability is attributed to the gyromagnetic motion: Taking into account the effective anisotropy field, the rf field, and the dipolar field we obtain a phenomenon of gyromagnetic resonance called natural resonance (maximum of μ″). As it was shown by Polder and Smit, the off-diagonal component of the permeability creates magnetic charges on the domain walls. The demagnetizing field and the off diagonal permeability give a component of magnetization which is parallel to the applied field and whose average over the ferrite does not cancel. On a simple model of prolate domains, a derivation of this mechanism is presented. The results do not fully explain the shape of the natural gyromagnetic resonance, but they give however the fundamental aspect of the phenomenon. At lower frequency. Snoek's formula is obtained and at higher frequencies this theoretical value of the permeability is perfectly confirmed by several experiments.