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Magnetic phase diagram of the semi-Heusler alloys from first-principles

The magnetic phase diagram of the Mn-based semi-Heusler alloys is determined at T=0 using first-principles calculations in conjunction with the frozen-magnon approximation. We show that the magnetism in these systems strongly depends on the number of conduction electrons, their spin polarization and the position of the unoccupied Mn 3d states with respect to Fermi energy. Various magnetic phases are obtained depending on these characteristics. The conditions leading to diverse magnetic behavior are identified. We find that in the case of a large conduction electron spin polarization and the unoccupied Mn 3d states lying far above the Fermi level, an RKKY-like ferromagnetic interaction is dominating. On the other hand, the antiferromagnetic superexchange becomes important in the presence of large peaks of the unoccupied Mn 3d states lying close to the Fermi energy. The overall magnetic behavior depends on the competition of these two exchange mechanisms. The obtained results are in very good agreement with the available experimental data.