Polarized inelastic neutron scattering of the partially orderedTb2Sn2O7

Abstract

We present inelastic neutron scattering results on the geometrically frustrated pyrochlore ${\mathrm{Tb}}_{2}{\mathrm{Sn}}_{2}{\mathrm{O}}_{7}$. At high temperature $T>50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, this system resembles the cooperative paramagnet ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$, while at low temperature $T\ensuremath{\sim}60\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$, it displays remarkably different behavior. Powder neutron scattering, susceptibility, and specific heat techniques have shown that below $0.87\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ ${\mathrm{Tb}}_{2}{\mathrm{Sn}}_{2}{\mathrm{O}}_{7}$ 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 $0.1\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, with spins fluctuating at a rate greater than $0.04\phantom{\rule{0.3em}{0ex}}\mathrm{THz}$, resulting in a diffuse magnetic background to the diffraction patterns. A low-lying excitation at $1.2\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ partially softens as short-range correlations build up while cooling in the paramagnetic state.