Abstract

Frustrated quantum magnets exhibit a strongly correlated phase over a wide range of temperatures below their Curie-Weiss temperatures, before ordering long range. It is of great interest to study this spin fluid state, but it is always difficult to separate the weak neutron scattering signal from the background. We solve this problem using XYZ polarization analysis, allowing the purely magnetic neutron scattering signal to be isolated. I shall describe recent experiments to study square-lattice Heisenberg antiferromagnets, emphasizing how the spin correlations in the paramagnetic phase give new information on the exchange mechanism. We find that the behaviour of La2CuO4 agrees with the predictions of the quantum non-linear sigma model, and that it is the first electronic material to exhibit ring exchange. We observe quantum order from disorder for Pb2VO(PO4)2, and we find ferromagnetic nearest-neighbour exchange J1 and antiferromagnetic next-nearest-neighbour exchange J2 of comparable magnitude, allowing the exploration of a new region of the J1-J2 phase diagram.