K. W. Krämer, H. U. Güdel, P. Fischer, F. Fauth, M. T. Fernandez-Diaz and T. Hauss
Triangular antiferromagnetic order in the honeycomb layer lattice of ErCl₃
Eur. Phys. J. B 18, 39-47 (2000)                 

Abstract: ErCl₃ crystallizes in the AlCl₃-type layer structure. The crystal structure was refined in the paramagnetic state by powder neutron diffraction. The monoclinic lattice parameters at 1.5 K are a = 6.8040(3) Å, b = 11.7456(5) Å, c = 6.3187(3) Å and b = 110.851(3)°. The space group is C2/m. Short-range, predominantly in-plane, magnetic ordering occurs above 350 mK up to several Kelvin. Below T_N = 350(5) mK a three-dimensional antiferromagnetic order with a propagation vector of k = (2/3,0,-1/12) sets in. The magnetic structure of ErCl₃ was determined by powder and single-crystal neutron diffraction at temperatures down to 45 mK. The Er³⁺ ions are located on two-dimensional honeycomb layers in the a-b plane. There are two antiferromagnetically coupled 120° triangular sublattices which form right- and left-handed helices along the c-axis. The magnetic moments are oriented in the a-b plane and amount to 3.3(1) m_B/Er³⁺ at saturation. From the temperature dependence of the integrated neutron magnetic peak intensity a critical exponent b = 0.23(2) was derived for the magnetic phase transition.