Raman scattering under structural and magnetic phase transitions in terbium ferroborate

The Raman scattering spectrum of single crystal TbFe3(BO3)4 was studied in the frequency range 3–500 cm−1 at temperatures from 2 to 300 K. It was found that in high- and low-temperature phases there exist additional phonon lines which were not known before. In the high-temperature phase, these lines originate from LO–TO splitting of polar phonons. Appearance of the additional lines in the low temperature phase is due to both a reduction of the crystal symmetry under the phase transition and an increase of the primitive cell volume. It was established that the frequencies of some phonon lines in the magneto-ordered phase are shifted towards the high-energy region upon applying an external magnetic field along the third-order axis. The spectrum of two-magnon Raman scattering was investigated. It was shown that at low temperatures the two-magnon band has a complex shape that reflects specific features in the density of state of the magnon branches. The magnon energy at the Brillouin zone boundary was determined.