Collective dynamics in the Heisenberg pyrochlore antiferromagnet Gd ₂Sn₂O₇

Gd₂ Sn₂ O₇ is believed to be a good approximation to a Heisenberg antiferromagnet on a pyrochlore lattice with exchange and dipole-dipole interactions. The system is known to enter a long-range ordered ground state (the "Palmer Chalker" state) below T_c =1K with k_ord = (000). However, persistent electronic spin fluctuations have been observed as T→0. Using inelastic neutron scattering, we have studied the buildup of short-range spin-spin correlations as the temperature is lowered, and the eventual formation of a gapped long-range ordered state that is able to sustain spin waves below T_c. As a magnetic field is applied, new magnetic phases develop and the gap widens. These measurements show that Gd₂ Sn₂ O₇ completely relieves itself of frustration, but the self-selected ground state is very delicate.