Abstract
We studied experimentally the high-temperature magnetoelectric (BaxSr1-x)(3)Co2Fe24O41 prepared as ceramics (x = 0, 0.2) and a single crystal (x = 0.5) using inelastic neutron scattering, THz time-domain, Raman, and far-infrared spectroscopies. The spectra, measured with varying temperature and magnetic field, reveal rich information about the collective spin and lattice excitations.
In the ceramics, we observed an infrared-active magnon which is absent in E-omega perpendicular to z polarized THz spectra of the crystal, and we assume that it is an electromagnon active in E-omega parallel to z polarized spectra. On heating from 7 to 250 K, the frequency of this electromagnon drops from 36 to 25 cm(-1) and its damping gradually increases, so it becomes overdamped at room temperature.
Applying external magnetic field has a similar effect on the damping and frequency of the electromagnon, and the mode is no longer 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 different frequency and much higher damping.
Upon applying magnetic field higher than 3 T, in the low-frequency part of the THz spectra, a narrow excitation appears whose frequency linearly increases with magnetic field. We interpret this feature as the ferromagnetic resonance.