ONSAGER REACTION FIELD-THEORY OF A SPATIALLY ANISOTROPIC HEISENBERG-MODEL

We discuss an Onsager reaction field approach to the paramagnetic phase of a spatially anisotropic Heisenberg model, with nearest-neighbor in-plane (J) and interplane (J(perpendicular to)) exchange couplings. The theory is physically transparent and, together with the use of suitable asymptotic approximations, provides a simple, consistent description of the crossover from effective two- to three-dimensional behavior, and captures well both the pure d=2 (J(perpendicular to)=0) and d=3 (J(perpendicular to)=J) dimensional limits. Application to La2CuO4 is briefly considered.