Order in the Heisenberg pyrochlore: The magnetic structure of Gd₂Ti₂O₇

The rare-earth pyrochlore material Gd₂Ti₂O₇ is considered to be an ideal model frustrated Heisenberg antiferromagnet with additional dipolar interactions. For this system there are several untested theoretical predictions of the ground state ordering pattern. Here we establish the magnetic structure of isotopically enriched ¹⁶⁰Gd₂Ti₂O₇, using powder neutron diffraction at a temperature of 50 mK. The magnetic structure at this temperature is a partially ordered, noncollinear antiferromagnetic structure, with propagation vector k = 1/2 1/2 1/2. It can be described as a set of "q=0" ordered kagomé planes separated by zero interstitial moments. This magnetic structure agrees with theory only in part, leaving an interesting problem for future research.