Phase transitions, partial disorder and multi-k structures in Gd2Ti2O7

The geometrically frustrated antiferromagnet Gd2Ti2O7 exhibits magnetic behaviour of such complexity that it poses a challenge to both experiment and theory. Magnetic ordering commences at T-N = 1.1 K and there is a further magnetic phase transition at T' = 0.7 K. Here we use neutron diffraction to definitively establish the nature of the phase transition at T' and the magnetic structure adopted below this temperature. Between T' and T-N the structure is partly ordered, as previously reported. Below T' the remaining spins order, but only weakly. The magnetic structure in this temperature range is shown to be a 4-k structure, closely related to the 1-k structure previously suggested. The 4-k and 1-k variants of the structure are distinguished by analysis of the diffuse scattering, which we believe represents a new method of solving the 'multi-k' problem of magnetic structure determination.