Raman scattering study of the rare-earth binary ferroborate Nd_0.75Dy_0.25Fe₃(BO₃)₄ single crystal

We report comprehensive Raman scattering measurements on a single crystal of binary ferroborate Nd_0.75Dy_0.25Fe₃(BO₃)₄ in the temperature range of 7–295 K with 532 nm (18797 cm^-1) laser excitation. The performed analysis of the polarized Raman spectra revealed the bands assigned to phonon, magnetic, and electronic excitations. The temperature evolution of these quasiparticle excitations has allowed us to ascertain the intricate coupling and interplay between lattice, magnetic, and electronic degrees of freedom. Analysis of the measured Raman spectra made it possible to identify all A1 and E phonon modes of predicted by the group-theoretical analysis. The splitting energies between the LO and TO components of the polar E phonons were determined. Below the magnetic ordering temperature of the Fe sublattice, TN, we have revealed a multiple peaked twomagnon excitation. Analyzing the temperature evolution of low-frequency modes in the spectra, we also identified modes that are associated with electronic transitions between the crystal field levels of the Nd³⁺ ions with ground-state ⁴ _I9/2 and of the Dy³⁺ ions with ground-state ⁶H_15/2 multiplets. In addition to the already known temperatures of magnetic transitions, analysis of the temperature behavior of low-frequency phonon and electronic excitations made it possible to establish a temperature T* = 100 K, presumably associated with local distortions of the crystal lattice. The presence of this temperature is confirmed by our ultrasonic study. A group of intense bands observed in the frequency range of 1700–2200 cm^–1 has been associated with the mixed low-lying electronic Raman transitions ⁴I_9/2→ I_11/2 and the high-energy luminescence ones ⁴ G_5/2+²G_7/2→⁴I_9/2 for the Nd³⁺ ion. (author)