Antiferromagnetic Ground State of Molecular π-d System λ-(BEDT-STF)2FeCl4 Studied by Site-selective Magnetization and Thermodynamic Measurements

Abstract Organic conductors consisting of donor and anion molecules provide various important study fields such as superconductivity and electron correlation. π electrons are responsible for the emergence of these phenomena. On the other hand, the coexistent systems of π electrons and localized 3d spins have been realized by introducing magnetic anions. Such coexistent systems are called π-d systems. In π-d systems, various fascinating magnetic and conducting phenomena are observed due to the coupling of π electrons and localized 3d electrons (π-d interaction). One of the most striking phenomena is magnetic field-induced superconductivity, which is first observed in λ-(BETS)2FeCl4, where BETS is bis(ethylenedithio)tetraselenafulvalene. On the other hand, λ-(BETS)2FeCl4 shows a metal-insulator transition accompanying an antiferromagnetic ordering at TN of 8.3 K and the unconventional antiferromagnetic state that 3d spins behave as if they stay paramagnetic even below TN. These phenomena are also considered to be caused by the π-d interaction, but their mechanisms have not been demonstrated. Recently, a new λ-type π-d system of λ-(BEDT-STF)2FeCl4, where BEDT-STF is bis(ethylenedithio)dithiadiselenafulvalene, has been developed. In this review, the influence of bandwidth control by donor substitution and the dilution effect of magnetic ions on the magnetic properties of the π-d system will be discussed. We will demonstrate that site-selective magnetization measurements by 13C NMR and 57Fe Mössbauer measurements and heat capacity measurements provide important information to understand the mechanism of unconventional magnetic properties in π-d systems. We would also like to introduce the advantages of the 13C NMR measurements using one-sided-13C-enriched donor molecules, which are the key technique in our experiments.