Millimeter-Wave Absorption as a Quality Control Tool for M-Type Hexaferrite Nanopowders

Millimeter wave (MMW) absorption measurements have been conducted on commercial samples of large (micrometer-sized) and small (nanometer-sized) particles of BaFe₁₂O₁₉ and SrFe₁₂O₁₉ using a quasi-optical MMW spectrometer and a series of backwards wave oscillators encompassing the 30-120 GHz range. Effective anisotropy of the particles calculated from the resonant absorption frequency indicates lower overall anisotropy in the nano-particles. Due to their high magnetocrystalline anisotropy, both BaFe₁₂O₁₉ and SrFe₁₂O₁₉ are expected to have spin resonances in the 45-55 GHz range. Several of the sampled BaFe₁₂O₁₉ powders did not have MMW absorptions, so they were further investigated by DC magnetization and x-ray diffraction to assess magnetic behavior and structure. The samples with absent MMW absorption contained primarily iron oxides, suggesting that MMW absorption could be used for quality control in hexaferrite powder manufacture.