Acher´s constraint on the high-frequency magnetic performance of composites

The paper generalizes the results available from the literature on the constraint on the high-frequency permeability of magnetic composites, with the stress made on the opportunities for obtaining of materials with high microwave permeability. The rigorous derivation is presented of the integral constraint on the permeability. A simple estimation of the effect of eddy currents is given. The applicability limits and opportunities to overcome the constraint are discussed. That the constraint is believed to be invalid in flake particles with hard magnetic axis perpendicular to the flake plane and in tiny magnetic particles possessing exchange resonance modes.