Local environment dependence of Mn magnetism in bcc iron-manganese alloys: A first-principles study

Magnetic behavior of a 3d solute in a ferromagnetic lattice can be very sensitive to local environment, which is the case of manganese in bcc Fe. Body-centered cubic iron-manganese alloys are studied by means of density functional theory in order to elucidate properties of the lowest-energy magnetic states. Multiple magnetic minima are determined even for the simplest case of an isolated Mn and a Mn dimer in bcc iron. The magnetoenergetic landscape is analyzed within and beyond the collinear magnetic approximation. A direct correlation is identified between the local electronic charge and the local magnetic moment of a Mn solute, being either isolated or forming a small cluster. In particular, the presence of a vacancy near the Mn atom, inducing a local charge decrease, tends to favor the antiferromagnetic Fe-Mn interaction while the presence of an interstitial impurity with a strong electronic hybridization with Mn can favor a ferromagnetic Fe-Mn interaction. Energetic and magnetic properties of Fe-Mn alloys are systematically investigated for a large range of Mn concentrations. An unmixing tendency is clearly noted. A detailed magnetic analysis suggests the Mn-Mn magnetic interactions to be generally dominant over the Fe-Mn interactions, both exhibiting an antiferromagnetic tendency. The average magnetic moment of the Mn atoms in locally random alloys tends to be antiparallel (parallel) to lattice Fe moments for Mn concentrations smaller (larger) than approximately 6 at. %. The transition concentration is shown to be lowered if considering Mn clustering which is energetically favorable. The unsolved discrepancies between experimental and theoretical predictions on the critical concentration for the Mn magnetic behavior change in Fe-Mn solid solution are discussed in the light of the obtained results.