Spin and lattice excitations of a BiFeO 3 thin film and ceramics

We present a comprehensive study of polar and magnetic excitations in ${\mathrm{BiFeO}}_{3}$ ceramics and a thin film epitaxially grown on an orthorhombic (110) ${\mathrm{TbScO}}_{3}$ substrate. Infrared reflectivity spectroscopy was performed at temperatures from 5 to 900 K for the ceramics and below room temperature for the thin film. All 13 polar phonons allowed by the factor-group analysis were observed in the ceramic samples. The thin-film spectra revealed 12 phonon modes only and an additional weak excitation, probably of spin origin. On heating towards the ferroelectric phase transition near 1100 K, some phonons soften, leading to an increase in the static permittivity. In the ceramics, terahertz transmission spectra show five low-energy magnetic excitations including two which were not previously known to be infrared active; at 5 K, their frequencies are 53 and $56\phantom{\rule{0.16em}{0ex}}{\text{cm}}^{\ensuremath{-}1}$. Heating induces softening of all magnetic modes. At a temperature of 5 K, applying an external magnetic field of up to 7 T irreversibly alters the intensities of some of these modes. The frequencies of the observed spin excitations provide support for the recently developed complex model of magnetic interactions in ${\mathrm{BiFeO}}_{3}$ [R. S. Fishman, Phys. Rev. B 87, 224419 (2013)]. The simultaneous infrared and Raman activity of the spin excitations is consistent with their assignment to electromagnons.