Magnetic properties of coexistent system of itinerant and localized electrons, (BEDT-TTF)(2)MCo(SCN)(4) (M=K, Rb, Cs)

A coexistent system of itinerant and localized electrons was prepared and the magnetic properties of both electrons were investigated. alpha-(BEDT-TTF)(2)CsCo(SCN)(4) (abbreviated as the CsCo salt), alpha-(BEDT-TTF)(2)RbCo(SCN)(4) (the RbCo salt), and alpha ''-(BEDT-TTF)(2) K1.4Co(SCN)(4) (the KCo salt), undergo metal-insulator transitions at 20 K, 190 K, and 130 K, respectively. The ESR measurement of the RbCo salt indicates that the electronic state below transition temperature, 190 K, is magnetic insulator due to the strong electron correlation. The static magnetic susceptibility of the CsCo salt measured by SQUID is mostly originated from the localized electrons, Co2+ (S = 3/2), and obeys the Curie-Weiss law with theta = -3 K above 20 K, where the weak interaction between Co2+ might be caused by the superexchange through coordinated SCN- anion. Below 20 K, the rapid decrease of chi T is observed because of the crystal field around Co2+.