Electronic and Magnetic Properties of Organic Conductors (DMET)2MBr4 (M = Fe, Ga)

(DMET)2MBr4 (M = Fe, Ga) are isostructural organic conductors whose crystal structure consists of an alternate stacking of quasi one-dimensional chain-based donor layers and anion square lattices. The resistivity, ESR, magnetic susceptibility, magnetization, and magnetoresistance of these salts were investigated in order to clarify the correlation between the electronic structure and the magnetism. The electronic structures of both salts are metallic down to TMI ~40 K, below which a Mott insulating state is stabilized, accompanied by an SDW transition at TSDW ~25 K. The FeBr4 salt with Fe^3+ (S = 5/2) localized spins undergoes an antiferromagnetic transition at TN = 3.7 K. In the FeBr4 salt, the magnetization curves, which show field-direction-dependent anomalies in addition to a spin-flop transition, are demonstrated to have a participation of donor (pi)-electron spins in the magnetization processes. The field dependence of the magnetoresistances below TN tracks faithfully that of the magnetization, where the donor (pi)-electrons and Fe^3+ d-electrons are responsible for the former and the latter, respectively. This clearly demonstrates the presence of the (pi)-d interaction that plays an important role in the interplay between electron transport and magnetism.