Lattice distortion, elasticity and antiferromagnetic order in copper-manganese alloys

Alloys of manganese containing 10-20% of copper undergo a tetragonal distortion on cooling through the Néel temperature. This distortion and an anomalous volume expansion have been measured and shown to be proportional to the square of the long-range magnetic order parameter. A simple Weiss molecular field model with strain-dependent exchange is shown to be consistent with these results. The model also predicts anomalies in the elasticity below the Néel temperature which accord well with experiment.