Ramification of complex magnetism in Nd_2Ir_2O_7 observed by Raman scattering spectroscopy

Using Raman scattering spectroscopy, we uncover a complex magnetic behavior of Nd_2Ir_2O_7, which stands out among magnetic pyrochlores by the lowest temperature of the all-in-all-out (AIAO) Ir moments ordering (T^N_Ir=33 K) and the highest temperature at which AIAO order of rare-earth Nd ions is detected (T^*_Nd=15 K). Our work suggests that in the temperature range between 15 K and 33 K, Nd magnetic moments exhibit characteristic spin ice fluctuations, which indicate a strong enhancement of magnetic interactions between Nd ions. This complex behavior emerges from the interplay of strong spin-orbit coupling, electronic correlations, and geometric frustration on two magnetic pyrochlore sublattices of Nd and Ir ions. The ordering of Ir magnetic moments is accompanied by an appearance of one-magnon Raman modes at 26.3 and 29.6 meV compatible with the AIAO order and of a broad mode at 14 meV, characteristic of the spinon continuum originated from Nd spin ice fluctuations. While two one-magnon excitations show minimal temperature evolution with decreasing temperature, the 14 meV mode shifts to higher frequencies as the temperature approaches a crossover to Nd AIAO order, broadens, and disappears below 15 K. An additional two-magnon excitation of the AIAO Nd order at around 33 meV appears in the spectra at low temperatures. These rather high energies of magnetic excitations of Nd moments make Nd_2Ir_2O_7 a particularly attractive playground to study the rare-earth magnetism on the pyrochlore lattice.