INTERPRETATION OF THE MAGNETIC-BEHAVIOR OF AN HEXAKIS MANGANESE(II) NITROXIDE OLIGONUCLEAR COMPLEX

A theoretical approach to the magnetic behaviour of an oligonuclear metal-nitroxide complex formed by six Mn (II) ions (S = 5/2) and six free radical ligands (s = 1/2) is performed. The temperature dependence of the magnetic susceptibility is studied and the complexity of an exact calculation is underlined. The low temperature behaviour is, however, shown to be compatible with the existence of a ground energy multiplet with total spin S(t) = 12 corresponding to the minimal energy of the complex for a strong antiferromagnetic coupling. For the high temperature region we develop a model of a cyclic alternate finite chain of classical Mn spins and quantum nitroxide spins. Through the evaluation of the different correlation functions between the various spins it is possible to obtain an analytical expression of the susceptibility leading to the determination of the exchange integral J = 480 K and to a good fit with experiment for 150 K < T < 300 K.