Thermodynamics of layered Heisenberg magnets with arbitrary spin
Abstract
We present a spin-rotation-invariant Green-function theory of long- and short-range order in the ferro- and antiferromagnetic Heisenberg model with arbitrary spin quantum number S on a stacked square lattice. The thermodynamic quantities (Curie temperature TC, N\'eel temperature TN, specific heat CV, intralayer and interlayer correlation lengths) are calculated, where the effects of the interlayer coupling and the S dependence are explored. In addition, exact diagonalizations on finite two-dimensional (2D) lattices with S>=1 are performed, and a very good agreement between the results of both approaches is found. For the quasi-2D and isotropic 3D magnets, our theory agrees well with available quantum Monte Carlo and high-temperature series-expansion data. Comparing the quasi-2D S=1/2 magnets, we obtain the inequalities TN>TC and, for small enough interlayer couplings, TN<TC. The results for CV and the intralayer correlation length are compared to experiments on the quasi-2D antiferromagnets Zn2VO(PO4)2 with S=1/2 and La2NiO4 with S=1, respectively.
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