Crystal-field excitations and spin-phonon interactions in DyNi₂ B₂ C: Raman scattering study

We report polarized Raman results of magnetic superconductor DyNi ₂ B₂ C (T_c =6.2 K, T_N =10.3 K) to explore crystal-field (CF) excitations and spin-phonon interactions. In addition to the Ni- B1g phonon mode at 199 cm^-1, we observed additional Raman modes at 124, 151, and 221 cm^-1. By careful analysis of the temperature evolution of these modes, we attribute the 124 cm^-1 excitation to a CF transition. The 151 and 221 cm^-1 modes correspond to zone-folded phonons because they are completely quenched for T> T _N. With increasing temperature across T_N, the 124 cm ^-1 excitation diminishes rapidly in intensity and, interestingly, an additional mode appears at 119 cm^-1. This excitation grows in intensity with increasing temperature toward 50 K and gradually decreases with increasing temperature further. We attribute the 119 cm^-1 excitation to an excited CF transition from a low-lying CF level at 5 cm^-1 to the higher CF level at 124 cm^-1. Anomalous temperature-dependent behavior of the Ni- B_1g phonon mode was observed in peak energy and in spectral width, i.e., both the phonon energy and the linewidth are enhanced in the vicinity of T_N, suggesting the presence of strong spin-phonon interactions near the antiferromagnetic ordering temperature in DyNi₂ B₂ C.