Effect of Carbon Addition on Magnetic Order in Mn–Al–C Alloys

The near-equiatomic Mn-Al alloys are considered for possible applications as rare-earth-free permanent magnets due to the existence of a ferromagnetic τ-phase with L1₀ crystal structure and other attractive physical properties. However, this tetragonal phase is metastable and can only be obtained from a hexagonal E-phase enriched in manganese. The partial atomic disorder can results in antiferromagnetic coupling of the Mn atoms occupying different sublattices. Moreover, the carbon addition, used to improve phase stability and coercivity, leads to the deformation of the unit cell and can affect the Mn-Mn coupling. To clarify this question, a series of magnetic measurements and neutron diffraction (ND) experiments in a wide temperature range was carried out on the Mn₅₅Al₄₅ alloys doped with a small quantity of C. The materials were prepared by melt-spinning and subsequent annealing. Different methods of carbon introduction were tried, the best results were obtained by melting Mn and Al with Mn₂₃C₆. The structural and magnetic properties of the ferromagnetic and paramagnetic τ-phase as well as the precursor E-phase were determined. The average magnetic moments on two sublattices were analyzed using the Weiss plot of the magnetometric data and the Rietveld refinement of the ND data. The dependence of the magnetic moments on the C content and temperature is reported. Ferrimagnetic behavior of the Mn-Al alloys with excess of manganese, caused by antiferromagnetic coupling and responsible for the decrease of magnetization with increase of Mn content, was confirmed.