Mysterious Paramagnetic States of Fe 3d Spin in Antiferromagnetic Insulator of λ-BETS₂FeCl₄ System

Abstract

In this study, we have investigated a quasi-two-dimensional organic conductor, λ-BETS 2 FeCl 4 , that exhibits a phase transition from a paramagnetic metal to an antiferromagnetic (AF) insulator at a critical temperature ( T MI ∼8.3 K). To investigate the mechanism of a notable phase transition, we studied the thermal properties of the conductor for temperatures down to 0.2 K at zero magnetic field. We observed that the specific heat of the conductor showed a sharp peak at T MI and a Schottky-type hump. The temperature dependence of the excess specific heat clarifies that π and 3d spins do not cooperatively form an AF order at T MI . We concluded that a 3d spin system keeps a paramagnetic state even at the lowest studied temperature.