Two geometrically frustrated magnets studied by neutron diffraction

In the pyrochlore compounds, Tb 2Ti 2O 7 and Tb 2Sn 2O 7, only the Tb 3 + ions are magnetic. They exhibit quite abnormal—and, in view of their chemical similarity, strikingly different—magnetic behaviour, as probed by neutron diffraction at ambient and applied pressure. Tb 2Ti 2O 7 is a cooperative paramagnet (“spin liquid”), without long-range order at ambient pressure; however, it does become ordered under pressure. By contrast, Tb 2Sn 2O 7 enters an “ordered spin ice” state already at ambient pressure. We analyse a simple model which already clearly exhibits some of the qualitative features observed experimentally. Overall, comparing these two compounds emphasizes the power of small perturbations in selecting low-temperature states in geometrically frustrated systems.