While tetragonal MnAl with the L10-type structure (P4/mmm #123) is metastable, it displays good intrinsic hard magnetic properties such as large magnetization (Ms=110 emu/g), high magnetocrystalline anisotropy field (HA=40 kOe), and moderate Curie temperature (Tc=600 K),1 which make it highly attractive as a sustainable rare-earth free permanent magnet. However, the metastable behavior of the MnAl L10-phase promotes the formation of structural defects that degrade its intrinsic magnetic properties such as the local magnetocrystalline anisotropy (MCA). In this work, we study the effects of a partial replacement of Al by Ga on the stability of the L10-phase, and both the magnetization and the MCA using a first-principles density functional calculation. We find that increasing the Ga content, enhances the phase stability while the total magnetic moment per formula unit (f.u.) remains almost unchanged. The site-, atomic- and spin-resolved MCA has been estimated by evaluating the spin-orbit coupling energy using a second-order perturbation approximation. Structural distortion, chemical effects and different spin components in Mn(Al1-xGax) all contribute to the MCA. The site- and atomic-resolved MCA calculations show that the MCA energy (MAE) mainly comes from the Mn atoms. With increasing Ga content, the MAE at both Mn and Al(Ga) sites increases and the total MAE increases from 0.2 meV/f.u. for x=0 to 0.34 meV/f.u for x=1 (Fig. 1). However, the structural distortion induced by the partial replacement of Al by Ga reduces the MCA. The spin-resolved MCA and band structure calculations indicate that the microscopic origin of high MCA is mainly associated with the spin flipping behavior near the Fermi level, induced by the spin-orbit coupling. The derived effective magnetic anisotropy field increases from 37 kOe (x=0) to 46 kOe (x=1), in agreement with experiments. Doping with Ga improves the stability of the L10 structure and enhances the magnetic anisotropy field, which facilitates the development of high coercivity Mn-Al based permanent magnets.
1 L Pareti, F. Bolzoni, F. Leccabue, A.E. Ermakov, J. Appl. Phys. 59, 3824 (1986)
Fig. 1 Dependence of MAE on Ga content, x, in Mn(Al1-xGax).