K. W. Krämer, H. U. Güdel, B. Roessli, P. Fischer, A. Dönni, N. Wada, F. Fauth, M. T. Fernandez-Diaz and T. Hauss
Magnetic ordering in the erbium honeycomb lattices of ErX₃ (X=Cl, Br ,I)
Physica B 276-278, 674-675 (2000)                 

Abstract: Unusual two-dimensional (2D) and three-dimensional (3D) magnetic ordering phenomena were observed in the insulating, transparent erbium trihalides ErX₃ (X=Cl, Br, I) in the milli-Kelvin temperature range. The layer-type crystal structures of ErCl₃ (AlCl₃-type structure) and ErBr₃/ErI₃ (BiI₃-type structure) are closely related. All three compounds show a new type of two-sublattice 120° antiferromagnetic order in the erbium honeycomb layers with an infinite rotational degeneracy. The magnetic structures were determined by powder and single-crystal neutron diffraction. ErCl₃ shows short-range 2D magnetic order above 350 mK to several Kelvin and a transition to 3D magnetic order at 350 mK with a k-vector of (2/3, 0, -1/12). ErBr₃ and ErI₃ display short-range 2D magnetic order from 400 mK to several Kelvin, long-range 2D order between 280 and 400 mK, and short-range 3D order below 280 mK with a correlation length of 15 Å along the c-axis and k = (1/3, 1/3, 0). The differences between ErCl₃ and ErBr₃/ErI₃ are due to different geometrical layer stackings. For ErBr₃/ErI₃ a threefold ambiguity in the magnetic layer stacking causes the disorder along the c-axis. The ordered magnetic moments at saturation increase from 3.3 to 4.7 and 5.5 m_B/Er³⁺ for ErCl₃, ErBr₃ and ErI₃, respectively.