Polarized inelastic neutron scattering of the partially ordered Tb2 Sn2 O7

We present inelastic neutron scattering results on the geometrically frustrated pyrochlore Tb2 Sn2 O7. At high temperature T>50 K, this system resembles the cooperative paramagnet Tb2 Ti2 O7, while at low temperature T∼60 mK, it displays remarkably different behavior. Powder neutron scattering, susceptibility, and specific heat techniques have shown that below 0.87 K Tb2 Sn2 O7 enters a partially ordered state that is characterized by two-sublattice ferrimagnetic long-range order which coexists with paramagnetic spin components. We show that (i) the low-temperature state produces a large internal field and collective excitations and (ii) the coexisting paramagnetic state persists down to 0.1 K, with spins fluctuating at a rate greater than 0.04 THz, resulting in a diffuse magnetic background to the diffraction patterns. A low-lying excitation at 1.2 meV partially softens as short-range correlations build up while cooling in the paramagnetic state.