Spin correlations in the frustrated square lattice Pb2VO(PO4)2

Abstract The new frustrated square-lattice system, Pb 2 VO(PO 4 ) 2 , has been investigated using polarised neutron scattering. From these studies, made on powdered samples, we have determined the nature of the exchange interactions and the magnetic ordering for this novel quantum magnet. Quantum order from disorder occurs at low temperature, and the ground state observed below the Neel temperature T N ∼3.7 K is a collinear antiferromagnet. At room temperature there are no magnetic correlations and it is possible to model the scattering with the V 4+ magnetic form factor. However, at T ∼20 K, a temperature well into the paramagnetic phase, magnetic correlations are observed, and these spin correlations have been modelled using a high-temperature series expansion. Ferromagnetic nearest-neighbour exchange J 1 ∼−2 K and antiferromagnetic next-nearest-neighbour exchange J 2 ∼6.5 K are obtained, and this is of particular interest theoretically, since it corresponds to a new region of the J 1 – J 2 phase diagram.