Anomalous temperature behaviour of the elastic constants of antiferromagnetic Mn-Invar alloys

The temperature dependences of the elastic constants of the antiferromagnetic Invar alloys Fe₆₀Mn₄₀ and Co₄₆Mn₅₄ have been investigated between 4.2 K and 800 K. Anomalies in the temperature behaviour of the sound velocities have been found in the vicinity of the magnetic phase transition indicating a strong magnetoelastic coupling in these antiferromagnetic Invar systems. The Landau theory has been adapted to the particularities of the systems under study for a phenomenological interpretation of the experimental data. Comparing our results with data obtained on ferromagnetic Invar alloys, we conclude that our phenomenological model is able to describe consistently the elastic anomalies in ferromagnetic and antiferromagnetic Invar systems.