Thermodynamic properties of ferrimagnetic large spin systems

The application of cumulant expansion is generalized to a ferrimagnetic system of large spins. An effective Hamiltonian is obtained in terms of classical variables for a quantum spin chain to describe the thermodynamic properties at moderate and high temperature regimes. We have considered quantum corrections up to second order in the cumulant expansion where fairly good results are observed in the convergent regime, i.e T>Js. (J is the exchange coupling between spins, s is the smaller spin (S1, s(2)) and T is temperature.) Our results show that the correlation of more than two neighbouring sites is negligible at moderate and high temperature. Thus the results of a single ferrimagnet molecule can be applied to the chain of interacting molecules for temperature greater than the classical energy scale, i.e. T>J S(1)s(2).