Thermodynamics of a binary monolayer of Ising dipolar particles. II. Effect of relative moment

Thermodynamic behaviors of a binary monolayer of Ising dipolar particles are studied using particle dynamics simulation, varying the relative intensity between the upward and downward dipole moments. The orientational order of the solid phase changes from tetragonal to hexagonal as the moment ratio increases. On the basis of the arguments of the candidates for ground state structures, the energy of the structures are well estimated. The transition point is also determined theoretically, which is consistent with the value obtained from the simulation results. Critical condensation is also studied. While the system whose moment ratio is unity does not exhibit the gas-liquid critical condensation, the transition appears as the moment ratio changes. The local structure of the liquid phase is found to be characterized by the ground state of the tetramer. The above-mentioned results imply that the gas-liquid critical point comes close to the melting transition point as the local structure of the liquid phase becomes closer to the structure of the solid phase, and therefore, the critical condensation is vanished.