Magnetic Properties of Coexistent System of Itinerant and Localized Electrons, (BEDT-TTF)2MCo(SCN)4 (M = K, Rb, Cs)

A coexistent system of itinerant and Iocalized electrons was prepared and the magnetic properties of bath electron~ were investigated. a-(BEDT-TTF)2CSCOjSCN)4 (abbreviated as the Csco Salt ), a-(BEDT-TTF)2RbCOjSCN)4 (the Rbco Salt), and a"-(BEDT-TTF)2 Ki 4Co(SCN)4 (the KCO Salt), undergo metal-insulator transitions at 10 K, 190 K, and 130 K. respectively. The ESR measurement of the Rbco sait indicates that the electronic state below transition temperature, 190 K, is magnetic insulator due to the strong electron correlat,ion. The static magnetic susceptibility of the Csco Salt measured by SQUID is mostly originated from the Iocalized electron~, Co~+ (S = 3/2), and obeys the Curie-Weiss Iaw with 6 = -3 K above 20 K, wheie trie weak interaction between Co~+ might be caused by the superexchange through coordinated SCN~ amen. Below 10 K, the rapid decrease of XT is observed because of the crystal field around Co~+