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, TMI, of 8.3 K under zero magnetic field. To understand the mechanism of this PM-AFIphase transition, we studied the thermodynamic properties of λ-BETS2FeCl4. We observed, below TMI, a six-level Schottky hump in its specific heat and a broad shoulder in its magnetic susceptibility. Just below the transition temperature TMI, 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 TMI. In λ-BETS2FexGa1-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-AFIphase 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. © 2011 by the authors; licensee MDPI, Basel, Switzerland.