Abstract of Publication No. 349
K. W. Krämer, H. U.
Güdel, B. Roessli, P. Fischer, A. Dönni, N. Wada, F. Fauth, M.
T. Fernandez-Diaz and T. Hauss
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 mB/Er3+ for ErCl3, ErBr3 and