Magnetic ordering in the erbium honeycomb lattices of ErX3 (X = Cl, Br, I)

Unusual two-dimensional (2D) and three-dimensional (3D) magnetic ordering p henomena were observed in the insulating, transparent erbium trihalides ErX 3 (X = Cl, Br, I) in the milli-Kelvin temperature range. The layer-type cry stal structures of ErCl3 (AlCl3-type structure) and ErBr3/ErI3 (BiI3-type s tructure) are closely related. All three compounds show a new type of two-s ublattice 120 degrees antiferromagnetic order in the erbium honeycomb layer s with an infinite rotational degeneracy. The magnetic structures were dete rmined 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, l ong-range 2D order between 280 and 400 mK and short-range 3D order below 28 0 mK with a correlation Length of 15 Angstrom along the c-axis and (1/3, 1/ 3, 0). The differences between ErCl3 and ErBr3/ErI3 are due to different ge ometrical layer stackings. Far ErBr3/ErI3 a threefold ambiguity in the magn etic layer stacking causes the disorder along the c-axis. The ordered magne tic moments at saturation increase from 3.3 to 4.7 and 5.5 mu(B)/Ei(3+) for ErCl3, ErBr3 and ErI3, respectively. (C)2000 Elsevier Science B.V. All rig hts reserved.