THERMODYNAMICS AND CORRELATIONS OF THE EASY-PLANE FERROMAGNET CSNIF3

The ferromagnetic compound CsNiF3 has been the object of plenty of exp erimental and theoretical research, due to its apparent ID behavior (a bove the 3D ordering temperature TN=2.7 K). Indeed it can be modeled a s a ferromagnetic chain with single-site easy-plane anisotropy. Its qu antum Hamiltonian can also be expressed in terms of canonically conjug ate operators by means of the Villain transformation, allowing us to a pply a recently devised method, called the pure-quantum self-consisten t harmonic approximation (PQSCHA), in order to obtain a classical effe ctive Hamiltonian, by means of which the quantum thermal averages can be approximated by phase-space integrals. Comparing with the available quantum techniques, it is apparent that this is an enormous simplific ation. These classical-like formulas are able to describe the quantum behavior of the system, provided that the quantum coupling is not too strong. This is indeed the case for CsNiF3, whose effective Hamiltonia n bears the form of the classical counterpart of the original one, but with suitably renormalized values of applied field, anisotropy, and e xchange constant. An interesting feature is the appearance of an excha nge anisotropy in competition with the easy-plane one, reflecting the effect of the quantum out-of-plane fluctuations. The classical transfe r-matrix method is finally used in calculating various thermodynamic q uantities, static structure factors and correlation lengths. Compariso n with the available experimental data for CsNiF3, as well as with exi sting numerical simulation results, generally shows very good quantita tive agreement.