Metastable states in the triangular-lattice Ising model studied by Monte Carlo simulations: Application to the spin-chain compound Ca₃Co₂O₆

It is well known that the spin-chain compound Ca₃Co₂O₆ exhibits interesting plateaus in the magnetization as a function of the magnetic field at low temperatures. The origin of them is still controversial. In this paper, we study the thermal behavior of this compound with a single-flip Monte Carlo simulation on a triangular lattice and demonstrate the decisive influence of metastable states on the splitting of the ferrimagnetic 1/3 plateau below 10 K. We consider the [Co₂O₆]_n chains as giant magnetic moments described by large Ising spins on planar clusters with open boundary conditions. With this simple frozen-moments model we obtain stepped magnetization curves which agree quite well with the experimental results for different sweeping rates. We describe particularly the out-of-equilibrium states that split the low-temperature 1/3 plateau into three steps. They relax thermally to the 1/3 plateau, which has long-range order at equilibrium. Such metastable states are further analyzed with snapshots unveiling an interlinked mobile domain walls structure that is responsible for the observed behavior of the 1/3 plateau. A comparison is also given of our classical Monte Carlo results with exact diagonalization results in small triangular quantum clusters, providing further support for our thermal description of this compound.