Magnetic structure of a multiferroic compound: Cu 2 OCl 2 .

The Cu 2 OCl 2 compound has been shown to be a high-temperature spin-driven multiferroic system, with a linear magneto-electric coupling. In this paper we propose a complete study of its magnetic structure. We derive the low energy magnetic Hamiltonian using ab initio multi-reference configuration interaction and the spin structure using Monte-Carlo simulations. Among the three magnetic structures proposed in the literature from different experimental results, our calculations support the incommensurate cycloid magnetic structure with a q⃑ = ( q a ,0,0) propagation vector. Using symmetry analysis, we show that all experimental results (polarization, magnetic order, magneto-electric coupling) can be accounted for in the Fd ' d '2 magnetic space group (2-fold axis along c⃑ ).