Critical Phenomena in Coupled Magnetic Systems -Application to the Organic Antiferromagnet lambda-(BETS)(2)FeCl4-

摘要

In this study, we construct a free-energy functional model for coupled magnetic systems that consist of free large spins and strongly interacting small spins, and apply it to the two-dimensional organic antiferromagnet lambda-(BETS)(2)FeCl4, which has magnetic degrees of freedom of d spins and pi electrons. Here, BETS stands for bis(ethylenedithio)tetraselenafulvalene. The model reproduces the results of previous theories as follows: An effective interaction between pi electrons via d spins drives the system to an antiferromagnetic transition at a finite transition temperature even though neither subsystem exhibits the transition at any temperature if they are decoupled. The sublattice magnetization m of pi electrons is almost constant at temperatures below approximately 0.95T(c). In addition to these corroborations of previous theories, the present theory elucidates a novel mechanism for the first-order transition and the physical origin of the extremely sharp peak of specific heat around T-c that is observed in lambda-(BETS)(2)FeCl4. The critical fluctuation in the pi-electron system plays critical roles in these phenomena. It is revealed that the small peak width is a consequence of the two-dimensionality and the mismatch between T-c and J(1), where J(1) is the original interaction between pi electrons, which imparts rigidity to the long-range order. Hence, the sharp peak is a phenomenon unique to coupled magnetic systems in which T-c J(1). This condition is not satisfied in conventional antiferromagnets. The above results give a physical explanation for the intriguing behaviors observed in lambda-(BETS)(2)FeCl4.