Paramagnetic Metal—Antiferromagnetic Insulator Transition in π-d System λ-BETS₂FeCl₄, BETS = Bis(ethylenedithio)tetraselenafulvalene

Abstract: Quasi-two-dimensional organic conductor λ-BETS₂FeCl₄ (BETS = bis(ethylenedithio)tetraselenafulvalene) transforms from a paramagnetic metal (PM) to an antiferromagnetic insulator (AFI) at a transition temperature, T_MI, of 8.3 K under zero magnetic field. To understand the mechanism of this PM-AFI phase transition, we studied the thermodynamic properties of λ-BETS₂FeCl₄. We observed, below T_MI, a six-level Schottky hump in its specific heat and a broad shoulder in its magnetic susceptibility. Just below the transition temperature T_MI, about 80% of 3d spin degree of freedom is sustained. These temperature dependences clarify that π and 3d spins do not cooperatively form the AF order at T_MI. In λ-BETS₂Fe_xGa_1−xCl₄ system, the increasing Fe 3d spin density enhances the internal magnetic field caused by π spin antiferromagnetic (AF) ordering, although the 3d spin itself maintains large entropy against the AF ordering. It was confirmed that the Fe 3d spin provided favorable conditions for this mysterious PM-AFI phase transition in the π electron system. We propose that this phase transition originates from the magnetic anisotropy introduced by the π-d interaction, which suppressed the low dimensional fluctuation in the π spin system.