Cumulant expansion for ferrimagnetic spin (S1,s2) systems -: art. no. 184416

We have generalized the application of cumulant expansion to ferrimagnetic systems of large spins. We have derived the effective Hamiltonian in terms of classical variables for a quantum ferrimagnet of large spins. A noninteracting gas of ferrimagnetic molecules is studied systematically by cumulant expansion to second order of (Js/T) where J is the exchange coupling in each molecule, s is the smaller spin (S-1,s(2)) and T is temperature. We have observed fairly good results in the convergent regime of the expansion, i.e, T>Js. We then extend our approach to a system of interacting ferrimagnetic molecules. For one-dimensional nearest neighbor interaction we have observed that the correlation of more than two neighboring sites is negligible at moderate and high temperature behavior. Thus the results of a single molecule can be applied to the chain of interacting molecules for temperatures greater than classical energy scale, i.e., T>JS(1)s(2). Finally, we will discuss the effect of spin inhomogeneity on the accuracy of this method.