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

Quasi-two-dimensional organic conductor λ-BETS 2FeCl 4 (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-BETS 2FeCl 4. 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 3 d spin degree of freedom is sustained. These temperature dependences clarify that π and 3 d spins do not cooperatively form the AF order at T MI. In λ-BETS 2Fe xGa 1？xCl 4 system, the increasing Fe 3 d spin density enhances the internal magnetic field caused by π spin antiferromagnetic (AF) ordering, although the 3 d spin itself maintains large entropy against the AF ordering. It was confirmed that the Fe 3 d 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.