Compositional correlations to intrinsic magnetic properties in binary and Ti-alloyed MnAl magnetic alloys

Abstract

MnAl rare-earth-free permanent magnets exhibit excellent advantages from economic and resource perspectives, which have attracted extensive attentions in recent decades. We reported the evolution in phase formation and intrinsic magnetic properties of τ-phase in binary MnAl alloys with the variation in Mn:Al ratios. Ferromagnetic τ-phase can be generated within the compositional range of Mn_50+xAl_50−x (x = 1–8), and pure τ-phase can only be obtained in the alloys with x = 4–7. With Mn:Al ratio increasing, saturation magnetization M_s and magnetocrystalline anisotropy constant K₁ are gradually weakened due to the incremental antiferromagnetic Mn-1d atoms, but Curie temperature of τ-phase is gradually increased induced by the strengthened d−d hybridization of Mn_1a−Mn_1d. An attempt of doping traces of Ti was carried out in order to eliminate the negative antiferromagnetic interaction derived from Mn-1d atom. Ti atoms tend to occupy 1d sites and replace the Mn-1d atoms due to the relatively fewer valence electrons compared with Mn, resulting in the reduction in Mn_1a−Mn_1d antiferromagnetic interactions, which is demonstrated by the higher M_s of Mn_55−yAl₄₅Ti_y (y = 1) than that of Mn₅₅Al₄₅. However, with further substitution of Mn by Ti, unfavorable κ-phase is unavoidably generated. Finally, the occupation preference and the corresponding influences on local magnetic interactions as well as the magnetizations of the different alloying atoms including interstitial element C, 3d atoms Ti, Co and Cu, and main-group element Ga are systematically summarized, in order to offer the guidance of designing MnAl permanent magnets with ideal magnetic properties.