Different High-Temperature Spin Dynamics of Ising Pyrochlore Dy2Sn2O7 and Heisenberg Pyrochlore Gd2Sn2O7

We measured the nuclear magnetic spin-lattice (T-1(-1)) and spin-spin (T-2(-1)) relaxation rates of Sn-119 nuclei of the Ising pyrochlore magnet Dy2Sn2O7 and the Heisenberg pyrochlore magnet Gd2Sn2O7 at high temperatures (120-294 K for Dy2Sn2O7 and 20-284 K for Gd2Sn2O7) to investigate their high-temperature 4f-electron spin dynamics. We found the following two results, which reveal remarkable differences between the spin dynamics of Dy2Sn2O7 and Gd2Sn2O7. First, the ratio T-2(-1)/T-1(-1) for powder samples is almost unity in Dy2Sn2O7, whereas it is about 6 in Gd2Sn2O7. Second, these relaxation rates increase markedly on cooling in Dy2Sn2O7, reflecting thermal activation with a gap of similar to 600 K, whereas they are almost constant with temperature in Gd2Sn2O7. The former result indicates that spin fluctuations of Dy2Sn2O7 are completely restricted to the < 111 > direction, whereas Gd spins fluctuate three-dimensionally and the component of the fluctuations parallel to an externally applied field contributes to the nuclear relaxation rates in Gd2Sn2O7. The latter result indicates that the spin fluctuations of Dy2Sn2O7 are caused by an indirect spin-flip process mediated through an excited state of the crystalline electric field multiplets, whereas those of Gd2Sn2O7 are dominated by the direct mutual spin-flip process by interactions between 4f-electron spins.