Cumulant expansion for systems with large spins

A method is proposed for obtaining a systematic expansion of thermodynamic functions of spin systems with large spin S in powers of 1/S. It uses the c umulant technique and a coherent-state representation of the partition func tion Z. The expansion of Z in terms of cumulants yields an effective classi cal I Hamiltonian with temperature-dependent quantum corrections. For the H eisenberg quantum Hamiltonian, they have a non-Heisenberg form. The effecti ve Hamiltonian can be solved by methods familiar for classical systems.