Magnetic properties of TTF-type charge transfer salts in the Mott insulator regime

We have investigated the magnetic properties of several TTF-based Mott insu lators consisting of dimerized/trimerized-donor units, which are classified into three subgroups: (1) beta'-(BEDT-TTF)(2)X (X = ICl2, AuCl2) and (BEDT -TTF)(2)GaCl4 as strong dimer systems, (2) alpha'-(BEDT-TTF)(2)IBr2 salt as a weak dimer system, and (3) (BMDT-TTF)(3)ClO4(1,2-dichloroethane) and (BM DT-TTF)3AsFb(1,1,2-trichloroethane) as weak trimer systems. The group (1) c ompounds have localized spin systems with an S = 1/2 localized spin per dim er unit. In group (2), weak dimerization produces a fractional magnetic mom ent, which reflects the delocalization of the electrons, all:hough the magn etism is described in terms of an S = 1/2 low-dimensional antiferromagnet. The ClO4 salt in the group (3) indicates the existence of a localized spin per trimer, whereas the trimerization is quite weak, because the inter-trim er Coulomb interactions generate the charge disproportionation resulting in the localization of electrons. The absence of the charge disproportionatio n brings about the feature of fractional magnetic moments in the AsF6 salts .