Magnetic properties of organic conductor (BEDT-TTF)(3)CuBr4

(BEDT-TTF)(3)CuBr4 is a two-dimensional organic conductor whose structure consists of alternate stacking of BEDT-TTF pi conducting sheets and magnetic Cu2+ sheets. We investigate magnetic susceptibility, magnetization and ESR in order to clarify the correlation between the magnetism and the electronic structure. Above a structural transition T-c = 59 K, the magnetic susceptibility indicates the presence of strong antiferromagnetic interaction between the pi-electrons on BEDT-TTF and the localized d-electrons on Cu2+. The transition brings about the disappearance of the magnetic moments of BEDT-TTF molecules below T-c, which indicates the change from a Mott insulator to a band insulator state. Below T-c, the susceptibility is governed only by Cu2+ spins, and shows an antiferromagnetic transition at T-N = 7.65 K, where the pi-electrons play an important role in the exchange interactions.