Neutron Diffraction Study of the Magnetic Properties of MnBr2

Abstract

The antiferromagnetic structure of Mn${\mathrm{Br}}_2$ ($T_N=2.16\mathrm{}°$K) has been determined by single-crystal neutron diffraction measurements at temperatures down to 1.35°K and with magnetic fields (0-13 kilo-oersteds) applied to the sample. The antiferromagnetic structure appeared to have hexagonal symmetry in the absence of a magnetic field but this was found to be associated with a structure domain-growth property. The true magnetic structure was found to be associated with one or another of three hexagonal axes, the particular growth direction being determined by the direction of an applied magnetic field. Information relating to indirect magnetic exchange via the bromine ions in this crystal was obtained from a study of the domain growth properties. The antiferromagnetic ordering transition in this compound was found to be nearly first order rather than of the usual second order type. A small lowering (∼2%) of $T_N$ was observed for an applied field of 13.1 kilo-oersteds. Short-range order data were obtained from powder patterns.