Microscopic investigation of possible multipole ordering in SmRu4P12 by 31P-NMR

To elucidate microscopically the metal-insulator transition (T-MI similar to 16.5 K) and an additional transition (T* similar to 15 K) in Sm-based filled skutterudite compound SmRu4R12, we have carried out P-31-NMR (nuclear magnetic resonance) at various fields from 0.8 to 11 T. The peaks in the spin lattice relaxation rate 1/T-1 observed at T-MI and T* under similar to 7T indicate that there are different magnetic fluctuation modes in SmRu4P12: one freezes below T-MI and the other does below T*. The anomalous field and temperature dependences of the NMR spectrum are explained in terms of the occurrence of multipole ordering. We have examined the two possible scenarios to account for the experimental results, and both cases suggest that the M-I transition involves octapole ordering.