First-principles investigation of Mg substitution for Ga on martensitic transformation, magnetism and electronic structures in Ni2MnGa

Abstract The impact of Mg substitution for Ga on the crystal structure, phase stability, magnetism, and electronic structures in the Ni2MnGa1-xMgx (x = 0, 0.25, 0.5, 0.75 and 1) alloys were studied by first-principles calculation. Unlike the elemental substitutions by the transition metals or the main-group elements of III, IV, and V (sp elements) in NiMnGa alloys, very rare knowledge is available on the effect of substitution by the main-group elements of I and II (e.g., Mg) characterized by only s valence electrons. Results show that the substituted Mg prefers to occupy the Ga site directly, and the magnetic atoms tend to be ferromagnetically coupled with each other. The appearance of the minimum global energy with the lattice tetragonality (c/a) larger than one suggests their possibility of being magnetic shape memory alloys. Mg substitution can enhance the austenite stability due to the reduced minority-spin states near the Fermi energy (EF). c/a of martensite decreases linearly when Mg substitutes Ga, which could be meaningful for optimizing transitional hysteresis. For the ferromagnetism, both austenite and martensite are enhanced with Mg substitution, which arises due to the increased moment of Ni owing to its increased eg states above EF. Elastic constant investigations show that Ni2MnGa1-xMgx (x   0.94) alloys are elastically stable. Mg replacement for Ga tends to decrease bulk modulus, Young’s modulus and shear modulus. By contrast, Mg substitution is favorable to improve the ductility of NiMnGa owing to the weaker covalent hybridization of Ni–Mg compared with Ni‒Ga.