Interacting Electron Spins In Kappa-(Bedt-Ttf)(2)Cu[N(Cn)(2)]I Investigated By Esr Spectroscopy

We performed angular and temperature-dependent electron-spin-resonance measurements in the quasi-two-dimensional organic conductor kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]I. The interlayer spin diffusion is much weaker compared to the Cl and Br analogs, which are antiferromagnetic insulator and paramagnetic metal, respectively;kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]I behaves insulating when cooled below T = 200 K. A spin gap (Delta approximate to 18 K) opens at low temperatures leading to a spin-singlet state. Due to intrinsic disorder a substantial number of spins (similar to 1%) remains unpaired. We observe additional signals below T = 4 K with a pronounced anisotropy indicating the presence of local magnetic moments coupled to some fraction of those unpaired spins.