O. Veits, R. Oppermann, M. Binderberger, J. Stein: Extension of the Popov-Fedotov method to arbitrary spin, J. Phys. I France 4 (1994) 493
The partition function and thermodynamic expectation values of a spin system with arbitrary spin quantum number are expressed by an exact fermionic representation on Fock space. Extending a suggestion of Popov and Fedotov, this is achieved by introducing a set of imaginary chemical potentials. Spin operators are replaced by usual bilinear combinations of Fermi operators satisfying the spin commutation relations.