Spin-lattice model of magnetoelectric transitions in RbCoBr₃

Extensive Monte-Carlo simulations are performed to analyze a recent neutron-diffraction experiment on a distorted triangular lattice compound RbCoBr₃. We consider a spin-lattice model where both spin and lattice are Ising variables. This model explains well successive magnetic and dielectric transitions observed in the experiment. The exchange interaction parameters and the spin-lattice coupling are estimated. It is found that the spin-lattice coupling is important to explain the slow growth of a ferrimagnetic order. The present simulations were made possible by developing a modified Monte-Carlo algorithm, which accelerates slow Monte-Carlo dynamics of quasi-one-dimensional frustrated systems.