We present inelastic neutron scattering results on the geometrically frustrated pyrochlore ${\mathrm{Tb}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$. At high temperature $T>50\mathrm{K}$, this system resembles the cooperative paramagnet ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, while at low temperature $T\sim 60\mathrm{mK}$, it displays remarkably different behavior. Powder neutron scattering, susceptibility, and specific heat techniques have shown that below $0.87\mathrm{K}$ ${\mathrm{Tb}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$ 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 $0.1\mathrm{K}$, with spins fluctuating at a rate greater than $0.04\mathrm{THz}$, resulting in a diffuse magnetic background to the diffraction patterns. A low-lying excitation at $1.2\mathrm{meV}$ partially softens as short-range correlations build up while cooling in the paramagnetic state.

• Received 17 August 2007

DOI:https://doi.org/10.1103/PhysRevB.76.212405