Rare Earth Substituted Hexagonal Ferrites for Electromagnetic Absorber Applications. Recent Advances and Critical Issues

摘要

Substitution of rare earths (RE) in hexagonal ferrites may modify the magnetic properties, especially anisotropy, and the constitutive electromagnetic characteristics, permittivity ε and permeability μ. Studies of hexagonal ferrites containing rare earth ions, in view of electromagnetic shielding applications in the microwave frequency range, are critically reviewed. Solid solubility of RE in hexagonal ferrites is low and synthesis of single phase compounds requires a final stage of intensive high temperature annealing. RE ions are more easily accommodated as substitutes for Fe. Dissipation of electromagnetic energy is effected by an absorber material with high dielectric or magnetic losses, while reflectance reduction is a complicated design task, involving shape, thickness and suitable ε and μ values of the material. Although high losses and tunability of resonance frequency are attainable with various RE substituted hexagonal ferrite compositions, the reported results often suffer from measurement errors and ungrounded physical explanations. The need for a better understanding of the microwave measurement methodology and the underlying mechanisms of the material's performance is underlined.