Magnetoelectric transitions of a distorted triangular-lattice antiferromagnet in a magnetic field

Large-scale Monte Carlo simulations are performed to analyze magnetoelectric transitions of a spin-lattice model in a magnetic field. The model explains well an experimental field-temperature phase diagram of RbCoBr₃. The magnetic structure in each phase is determined. Simultaneous magnetoelectric transitions observed in the zero-field case are found to occur only in the low-field region, whereas magnetic transitions and electric transitions are decoupled in the high-field region. When the spin-lattice correlation is weak, an asymmetric ferrimagnetic state and a half-ferrimagnetic state appear. The former is the intermediate state between the partial-disordered state and the ferrimagnetic state. The latter phase is characterized by a sublattice order parameter with a ↑2-↑2-↓ ordering pattern.