The reason for this is due to a combination of the magnetic or complex permeability and the dielectric properties consisting of permittivity and resistivity of the specific material. So, for each material type there will be an inherent resonant frequency dependent on the minimum dimension’s overall or wall thickness of the core’s cross-section perpendicular to the path of the magnetic field. In terms of impedance, the resonant frequency is the frequency at which peak impedance occurs. The larger this critical dimension, the lower the resonant frequency and the lower the frequency at which the material becomes dominantly lossy µ" less than µ'.