Ferromagnetic ordering of S=1/2 Heisenberg ferromagnetic chains in organic magnet β-BBDTA•GaBr4

We have observed the magnetic susceptibility and the specific heat of the beta phase of an organic magnet BBDTA center dot GaBr4 which consists of antiferromagnetic chains and ferromagnetic chains. At low temperatures, the antiferromagnetic chains do not contribute to the magnetic property because of a large intrachain interaction of -197 K. The ferromagnetic chains show a three-dimensional magnetic ordering at 0.4 K. The low-temperature susceptibility and the specific heat in magnetic fields are quantitatively reproduced by calculations for the S = (1)/(2) Heisenberg ferromagnetic chains with 2J(F)=8.7 K interacting only by a ferromagnetic interchain interaction of 2zJ' = 0.29 K. It indicates that this organic magnet realizes a model system of Heisenberg ferromagnets.