We present the results of the first neutron powder and single crystal diffraction studies of the coupled spin tetrahedra systems \CuTeX (X=Cl, Br). Incommensurate antiferromagnetic order with the propagation vectors kCl≈[0.150,0.422,\half], kBr≈[0.158,0.354,\half] sets in below TN=18 K for X=Cl and 11 K for X=Br. No simple collinear antiferromagnetic or ferromagnetic arrangements of moments within Cu2+ tetrahedra fit these observations. Fitting the diffraction data to more complex but physically reasonable models with multiple helices leads to a moment of 0.67(1)μB/Cu2+ at 1.5 K for the Cl-compound. The reason for such a complex ground state may be geometrical frustration of the spins due to the intra- and inter-tetrahedral couplings having similar strengths. The magnetic moment in the Br- compound, calculated assuming it has the same magnetic structure as the Cl compound, is only 0.51(5)μB/Cu2+ at 1.5 K. In neither compound has any evidence for a structural transition accompanying the magnetic ordering been found.
@article{arxiv.cond-mat/0405513,
title = {Incommensurate magnetic ordering in $Cu_2 Te_2 O_5 X_2$ (X=Cl,Br) studied by neutron diffraction},
author = {O. Zaharko and A. Daoud-Aladine and S. Streule and J. Mesot and P. -J. Brown and H. Berger},
journal= {arXiv preprint arXiv:cond-mat/0405513},
year = {2009}
}