Uniaxial magnetocrystalline anisotropy of tetragonal Mn Ga100− (50 ≤ x ≤ 75) alloys

Abstract

Abstract Mn-Ga alloys have attracted considerable interests due to the high uniaxial magnetocrystalline anisotropy (MCA) arising from their tetragonal structure, but the evolution and influence mechanism of the uniaxial MCA associated with composition variations is still unclear. In this paper, experiments and first-principles calculations are combined to study the uniaxial MCA over a wide composition range of MnxGa100−x (50 ≤ x ≤ 75) alloys, ranging from MnGa to Mn3Ga. By adopting suitable annealing processes, the single tetragonal phase is obtained in all studied alloys. From the law of approach to saturation, it is deduced that the magnetocrystalline anisotropy constant (K1) firstly increases from 1.59 MJ m−3 for x = 53.5 to a maximum value of 2.24 MJ m−3 for x = 65, and then decreases to 1.3 MJ m−3 for x = 75. By comprehensive first-principles calculations, we find that under the effect of the crystal field, the Mn atom occupying 4d site of the tetragonal lattice (Mn-4d) has easy axis anisotropy, which is positive to the uniaxial MCA; while Mn-2b has a soft in plane tendency, which is negative to the uniaxial MCA. As x increases from 53.5 to 65, the itineracy of the d electrons is increased through d-d hybridization between Mn-2b and Mn-4d, simultaneously the spin-orbit coupling is enhanced due to c axis shortening of the tetragonal lattice, both improve the overall easy axis behavior, resulting in the increase of K1. As x further increasing from 65 to 75, the soft in plane tendency of Mn-2b leads to the decrease of K1. The analysis can be applied to other 3d alloys, and offers a guidance for improving their uniaxial anisotropy, especially for Mn based permanent magnet.