Triangular antiferromagnetic order in the honeycomb layer lattice of ErCl3

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) Angstrom, b = 11.745 6(5) Angstrom, c = 6.3187(3) Angstrom and beta = 110.851(3)degrees. The spa ce group is C2/m. Short range, predominantly in-plane, magnetic ordering oc curs above 350 mK up to several Kelvin. Below T-N = 350(5) mK a three-dimen sional 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 Er3ions are located on tare-dimensional honeycomb layers in the a-b plane. The re are two antiferromagnetically coupled 120 degrees triangular sublattices which form right- and left-handed helices along the c-axis. The magnetic m oments are oriented in the a-b plane and amount to 3.3(1) mu (B)/Er3+ at sa turation. From the temperature dependence of the integrated neutron magneti c peak intensity a critical exponent beta = 0.23(2) was derived for the mag netic phase transition.