Electromagnon in the Z-type hexaferrite $({ m Ba}_{x}{ m Sr}_{1-x})_3 m Co_2Fe_{24}O_{41}$

摘要

We studied experimentally the high-temperature magnetoelectric $({\rmBa}_{x}{\rm Sr}_{1-x})_3\rm Co_2Fe_{24}O_{41}$ prepared as ceramics (x = 0,0.2) and a single crystal (x = 0.5) using inelastic neutron scattering, THztime-domain, Raman and far-infrared spectroscopies. The spectra, measured withvarying temperature and magnetic field, reveal rich information about thecollective spin and lattice excitations. In the ceramics, we observed aninfrared-active magnon which is absent in $E^{\omega}\perp z$ polarized THzspectra of the crystal, and we assume that it is an electromagnon active in$E^{\omega} \| z$ polarized spectra. On heating from 7 to 250 K, the frequencyof this electromagnon drops from 36 to 25 cm$^{-1}$ and its damping graduallyincreases, so it becomes overdamped at room temperature. Applying externalmagnetic field has a similar effect on the damping and frequency of theelectromagnon, and the mode is no more observable in the THz spectra above 2 T,as the transverse-conical magnetic structure transforms into a collinear one.Raman spectra reveal another spin excitation with a slightly differentfrequency and much higher damping. Upon applying magnetic field higher than 3T, in the low-frequency part of the THz spectra, a narrow excitation appearswhose frequency linearly increases with magnetic field. We interpret thisfeature as the ferromagnetic resonance.