Incommensurate magnetic ordering in Cu2Te2O5X2 (X=Cl, Br) studied by single crystal neutron diffraction

摘要

Polarized and unpolarized neutron diffraction studies have been carried outon single crystals of the coupled spin tetrahedra systems Cu2Te2O5X2 (X=Cl,Br). A model of the magnetic structure associated with the propagation vectorsk'Cl ~ -0.150,0.422,1/2 and k'Br ~ -0.172,0.356,1/2 and stable below TN=18 Kfor X=Cl and TN=11 K for X=Br is proposed. A feature of the model, common toboth the bromide and chloride, is a canted coplanar motif for the 4 Cu2+ spinson each tetrahedron which rotates on a helix from cell to cell following thepropagation vector. The Cu2+magnetic moment determined for X=Br, 0.395(5)muB,is significantly less than for X=Cl, 0.88(1)muB at 2K. The magnetic structureof the chloride associated with the wave-vector k' differs from that determinedpreviously for the wave vector k~0.150,0.422,1/2 [O. Zaharko et.al. Phys. Rev.Lett. 93, 217206 (2004)].