Phase Diagrams of Pseudo One-Dimensional Heisenberg Systems

We present the first theoretical results on the anomalous field dependence of the N\'eel temperature for a system of loosely coupled classical Heisenberg chains with orthorhombic anisotropy for different directions of the applied magnetic field. The results compare favorably with the experimental phase diagrams of a series of selected S=5/2 compounds with varying degrees of one dimensionality and anisotropy.