Nd_2Sn_2O_7: An all-in-all-out pyrochlore magnet with no divergence-free field and anomalously slow paramagnetic spin dynamics

摘要

We report measurements performed on a polycrystalline sample of the pyrochlore compound Nd_2Sn_2O_7. It undergoes a second order magnetic phase transition at T_c ≈ 0.91 K to a noncoplanar all-in-all-out magnetic structure of the Nd~(3+) magnetic moments. The thermal behavior of the low temperature specific heat fingerprints excitations with linear dispersion in a three-dimensional lattice. The temperature independent spin-lattice relaxation rate measured below T_c and the anomalously slow paramagnetic spin dynamics detected up to ≈30T_c are suggested to be due to magnetic short-range correlations in unidimensional spin clusters, i.e., spin loops. The observation of a spontaneous field in muon spin relaxation measurements is associated with the absence of a divergence-free field for the ground state of an all-in-all-out pyrochlore magnet as predicted recently.