Multifunctional interplay among the conductivity, magnetism and dielectricity in π–d interacting λ-(BETS)2FeCl4

The title BETS salt is a quasi-two-dimensional paramagnetic metal with a high-spin localized at the FeIII site in the tetrahedral anion FeCl4. At TMI = 8.3 K, however, it undergoes a metal-to-insulator transition spontaneously associated by antiferromagnetic (AFM) and ferroelectric-like orders, and also by the structural transition. An application of a high magnetic field makes this insulator unstable against reentering the metallic state, followed at higher fields by a transition to the superconducting state. These unique properties totally indicate noticeable spin- and charge-correlation of π electrons in the crystal lattice with localized d-spins. This article reviews these phenomena focusing on the AFM insulating states and high-temperature metallic states exhibiting a multifunctional interplay among the conductivity, magnetism and dielectricity. Discussions will be made on the anomalous metallic state, in comparison with the isostructural, superconductor GaCl4 salt exhibiting contrasting anomalies in the metallic state.