Magnetic torque measurement of antiferromagnetic transition in molecular conductors using a microcantilever

A series of BETS salts, where BETS is bis(ethylenedithio)tetraselenafulvalene, is a good candidate for a systematic study of π-d interaction between the conduction electrons and local magnetic moments in organic molecular conductors. The anion part, MX4 (M = Ga, Fe; X = Cl, Br), can be continuously replaced to form mixed crystals with compositions, FexGa1-xBryCl4y. In case of the replacement of magnetic Fe with nonmagnetic Ga, only iron-rich mixed crystals show antiferromagnetic transition at low temperature. Since κ-(BETS)2FeCl4 was reported to exhibit successive antiferromagnetic and superconducting transitions (TN = 0.45 K, TC = 0.1 K) without magnetic field[1], we can expect to elucidate an interplay between magnetism and superconductivity in the π-d system. However, the magnetic easy axis of κ-(BETS)2FeCl4 is not clear. Although Otsuka reported that the easy axis is almost parallel to the a axis [1], there were several reports in which the easy axis was parallel to the b axis.[2-4] So the determination of magnetic easy axis was the most crucial problem. Therefore, we tried to measure magnetic torque using AFM microcantilever, which method is so sensitive that it only needs a tiny crystal less than 1 μg.[5]