Limitations on high-frequency permeability of magnetic materials

Engineering of magnetic composites with high microwave permeability is important for EMC/EMI and other applications. One of the challenging problems is to determine constraints on the achievable high-frequency permeability of magnetic composites. The objective of the present paper is to analyze the data available from the literature on the microwave permeability constraints and derive a generalized constraint condition. It is well known that in many practical occasions the actual magnetic frequency dispersion curves differ from the Lorentzian behavior. For these cases, an integral form of the constraint may be useful. Possible applications of the integral constraint are discussed for the cases of large damping, pronounced effect of eddy currents, and inhomogeneous materials. It is shown that the derived constraint can be successfully used in these cases for both estimating microwave performance of devices containing magnetic materials and obtaining additional data on the magnetic structure of materials.