Microwave magnetic excitations in U-type hexaferrite Sr4CoZnFe36O60 ceramics

Microwave (MW) transmission, absorption, and reflection loss spectra of the ferrimagnetic U-type hexaferrite Sr4CoZnFe36O60 ceramics were studied from 100 MHz to 35 GHz at temperatures between 10 and 390 K. Nine MW magnetic excitations with anomalous behavior near ferrimagnetic phase transitions were revealed. They also change under the application of the weak bias magnetic field (0–700 Oe) at room temperature. Six pure magnetic modes are assigned to dynamics of the magnetic domain walls and inhomogeneous magnetic structure of the ceramics, to the natural ferromagnetic resonance (FMR), and to the higher-frequency magnons. Three modes are considered the magnetodielectric ones with the dominating influence of the magnetic properties on their temperature and field dependences. The presence of the natural FMR in all ferrimagnetic phases proves the existence of the non-zero internal magnetization and magnetocrystalline anisotropy. Splitting of the FMR into two components without magnetic bias was observed in the collinear phase and is attributed to a change in the magnetocrystalline anisotropy during phase transition. The high-frequency FMR component critically slows down to phase transition. At room temperature, FMR splitting and essential suppression of the higher-frequency modes were revealed under the weak bias field (300–700 Oe). The highly nonlinear MW response and FMR splitting are caused by the gradual evolution of the polydomain magnetic structure to a monodomain one. The high number of magnetic excitations observed in the MW region confirms the suitability of using hexaferrite Sr4CoZnFe36O60 ceramics as MW absorbers, shielding materials and highly tunable filters.