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

Abstract

Quasi-two-dimensional organic conductor λ-BETS 2 FeCl 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 2 FeCl 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 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-BETS 2 Fe x Ga 1−x Cl 4 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.

Temperature Dependence of Internal Field by Analysis of Specific Heat on an Organic Conductor λ-BETS2FeCl4

“…Recently, we found a six-level Schottky hump in the specific heat of -BETS2FeCl4 system in the AFI phase [7][8][9], where Tokumoto reported that the magnetic susceptibility decreases rapidly with an anomalous shoulder [5]. Just below the TMI, approximately 80 % of the 3d spin degree of freedom remain free.…”

Antiferromagnetic Ordering in Organic Conductor λ-(BEDT-TTF)2GaCl4 Probed by 13C NMR

“…However, Akiba et al performed specific heat measurements for λ-(BETS) 2 FeCl 4 and revealed that the 3d spin system shows a Schottky-like anomaly in the magnetic heat capacity below T N , and they suggested that the localized π electron system undergoes an AF ordering at T N , while the 3d spin system maintains a paramagnetic state under the effective magnetic field arising from the π−d interaction. 27 . However, the AF ordering state had not been found in λ-type systems with nonmagnetic anions.…”

Magnetic and Electronic Properties of π-d Interacting Molecular Magnetic Superconductor κ-(BETS)2FeX4 (X = Cl, Br) Studied by Angle-Resolved Heat Capacity Measurements

“…Such situation characteristic in the -d interacting system changes the three-dimensional ordered state to the unconventional magnetic state such that the nature of the one-dimensional direct magnetic interaction remains even below TN. Interestingly, the similar hump structure is also observed more clearly in the -(BETS)2FeCl4, which is another-d interacting system, which is reviewed in [56]. Recently, some theoretical studies on the emergence of the hump structure in the -d interacting system have been reported [57].…”