ESR study on low-dimensional antiferromagnets α-(BEDT-TTF)2PF6 and ζ-(BEDT-TTF)2PF6(THF)

ESR measurements were carried out for α-(BEDT-TTF) 2 PF 6 and ζ-(BEDT-TTF) 2 PF 6 (THF). The temperature dependences of the spin susceptibility of the two salts are very similar to each other and seem to behave as typical paramagnetic insulators with low-dimensional antiferromagnetic interaction. The absolute values of the macroscopic antiferromagnetic interaction, J/k B , are also very close. However, there are obvious differences in their ground states and microscopic behaviors. ζ-(BEDT-TTF) 2 PF 6 (THF) undergoes an antiferromagnetic transition at around 5 K., while α-(BEDT-TTF) 2 PF 6 shows no long-range magnetic ordering down to 2 K. The temperature dependent behaviors of the ESR linewidth, Δ H PP , are quite different: The Δ H PP of ζ-(BEDT-TTF) 2 PF 6 (THF) is almost temperature independent in the paramagnetic region and shows an abrupt increase below about 30 K, while the Δ H PP of ζ-(BEDT-TTF) 2 PF 6 gradually decreases as the temperature decreases. The low temperature electronic states of these salts are discussed from the microscopic point of view.