Fluctuation-Induced Magnetic Order of RbFeCl3in a Magnetic Field

Mean-field theory is insufficient to explain the magnetic order of RbFeCl 3 in a magnetic field applied in the c plane. Specifically, quantum and thermal fluctuations induce magnetic structures which do not otherwise appear, as found also for CsCuCl 3 . We derive microscopically an expression for the free energy which describes both kinds of fluctuations on an equal basis. Our result includes mean-field theory, but goes beyond it to include terms which lift the nontrivial degeneracy. In addition to the modified 120° structure at low fields and the aligned state at high fields, we find an up-up-down-type collinear structure and a coplanar structure at intermediate fields. Our theory explains qualitatively experimental results at low temperatures, but detailed confirmation requires further experiments including the neutron scattering.