TMRG studies on spin alignment in molecule-based ferrimagnetics

A physical picture of spin alignment in organic molecule-based ferrimagnets is presented from studying the thermal effective magnetic moment of the sublattice by use of the transfer matrix renormalization group. We conclude that the classical antiparallel spin alignment is not the most stable state. The three-spin system tends to parallel alignment when the exchange interaction between the biradical and the monoradical molecules is much weaker than that within the biradical, which can result in the decrease of the effective magnetic moment upon lowering the temperature. More importantly, we give the theoretical evidence that even the antiparallel spin alignment in the biradical-monoradical alternating chain does not necessarily lead to ferrimagnetic spin ordering due to the formation of the spin singlet pairs, which suppresses the ferrimagnetic spin alignment.