Accurate measurements of the ferromagnetic resonance linewidth of single crystal BaM hexaferrite spheres employing magnetic plasmon resonance theory

Accurate measurements of the intrinsic ferromagnetic linewidth of a single-crystal Sc-doped hexagonal M-type barium ferrite (BaM) 308-µm-diameter sphere at millimeter-wave frequencies employing a subwavelength cavity are described. The magnetic hysteresis of the resonance frequency and Q-factor of the dominant resonance (which has a plasmonic character) in a hexaferrite sphere are reported. Measurements performed in a fixture open to free space also show that the ferromagnetic linewidth significantly increases as compared to measurements in a closed cavity. This effect can be attributed to radiation losses, as revealed by an electrodynamic magnetic plasmon resonance model, and is relevant for measurements in open structures, such as microstrip lines or coplanar waveguides. The impact of finite resistivity on the linewidth was also analyzed.