Observation of Spin-flop Transition in Antiferromagnetic Organic Molecular Conductors using AFM Micro-cantilever

A series of (BETS)$_{2}$Fe$_{1-x}$Ga$_{x}$Cl$_{1-y}$Br$_{y}$ salts is a good candidate for a systematic study of $\pi $-d interaction between the conduction electrons and local magnetic moments in organic conductors. Some of them show antiferromagnetic ground state at low temperatures. A torque magnetometry is useful for precise determination of the easy axis as well as the spin-flop field. In this work we will report on the measurements of spin-flop transitions in antiferromagnetic organic molecular conductors including $\lambda $-(BETS)$_{2}$FeCl$_{4}$[1], using a commercial self-sensing piezo-resistive microcantilever for Atomic Force Microscopy (AFM) developed by Seiko Instruments Inc. We have succeeded in observation of spin-flop transitions of tiny single crystals including $\lambda $-(BETS)$_{2}$FeCl$_{4}$ as small as less than 1$\mu $g[2]. The results are consistent with the capacitive magnetic torque study[3]. [1] M. Tokumoto et al. Synth. Metals 86, 2161 (1997). [2] M. Tokumoto et al., ICMM2004, Polyhedron in press. [3] T. Sasaki et al., Synth. Metals 120, 759 (2001).