Study of ferromagnetic instability in ττ-MnAl, using first-principles

• The material is well known to be a rare-earth free magnet.
• A tetragonal phase of MnAl may be a non-collinear magnet.
• DFT calculations for competing ferromagnetic and antiferromagnetic interactions.
• Substitutions improve the energy factor of MnAl.
• Role sp- hybridization on magnetism properties.

The ττ- phase of the binary Mn-Al system is a promising rare-earth free permanent magnet. However, the experimentally determined figure-of-merit (energy-factor) is significantly lower than the theoretical estimate. This is partly being associated with the low volume fraction of ττ-MnAl during synthesis, chemical disorder driven inter-sublattice antiferromagnetism, and presence of multiple binary phases. In this work, with the help of first-principles linear response calculations, we show that the presumed long range ferromagnetic ordering in ττ-MnAl is unstable. Calculation of Mn magnetic pair-exchange interaction for ττ-MnAl show competing ferromagnetic and antiferromagnetic interactions. As a result, the study infers to a non-collinear magnetic ground state, which then partly explains the low magnetization as observed in experiments. Local probe experiments such as neutron diffraction, corroborating our prediction of the non-collinear state in ττ-MnAl is therefore desired.