Mechanisms of Magnetic Order in a Charge Transfer Complex Mediated by Virtual Interactions Due to the Charge Fluctuation

We study mechanisms where a magnetic order appears by means of thermal or quantum fluctuations, motivated by the magnetic phase transition in charge-transfer systems, e.g., ( n -C 3 H 7 ) 4 N [Fe II Fe III (dto) 3 ], where dto = C 2 O 2 S 2 in which the low temperature phase has no direct magnetic interaction among magnetic atoms. In a mean-field analysis, it was pointed out that thermally activated structure of the high temperature phase may play an important role. By means of Monte Carlo method, we do not found the magnetical phase transition predicted by the mean field theory in the two dimensional model. As mechanisms to realize the fluctuation induced magnetic phase transition, we disuss effects of the dimensionality, and also effects of quantum mixing of magnetic states into the ground state.