Transfer matrix renormalization group studies on spin chains for molecule-based ferrimagnets

The transfer matrix renormalization group method has been applied to calculate the thermodynamic property of S=1 and S=1/2 alternating chains to show the possibility of a ferrimagnetic ordered state for organic molecule-based magnets. The S=1 site in the chain is composed of two S=1/2 spins coupled by a finite ferromagnetic interaction. From the calculation of the magnetic susceptibility temperature product chiT, we find that ferromagnetic behavior appears when the exchange interactions between the S=1 and S=1/2 moieties are much weaker than the biradical intramolecular interactions, which conclusion is consistent with experimental work. Through the study of the spatial symmetry lowering of intermolecular magnetic interactions, it is shown that the molecular ferrimagnetic alignment relies on the cooperative effects of intermolecular interactions. We also find that the maximum induced by an external field appears in chiT, following the minimum with decreasing temperature. In addition, we discuss the effect of dimerization along the spin chain on the stability of the ferrimagnetic state.