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

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 8.3K 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 13CNMR 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 13CNMR measurements using one-sided-13C-enriched donor molecules, which are the key technique in our experiments. [ABSTRACT FROM AUTHOR]