The initial susceptibility of ferrites: A quantitative theory

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Abstract
In this contribution a firm foundation to the observed complex permeability spectrum of ferrites (such as NiZn or MnZn spinels) in the gyromagnetic regime (100 MHz, 10 GHz) is provided. Many attributes of the initial permeability of uniaxial ferrites are not correctly reproduced by a straightforward application of the usual Landau–Lifschitz–Gilbert equation. Among those one can cite the observed, non-Lorentzian shape of μ′ and μ″, and the permanence of losses (μ″(ω)>0) up to the frequency ωm=γ4πMs, above which they disappear (‘‘low field loss’’). The problem is recast here in the general framework of effective-medium theory which provides a firm interpretation of the observed complex permeability spectrum of ferrites (such as NiZn or MnZn spinels) in the gyromagnetic regime (100 MHz, 10 GHz).