Understanding the magnetic structural transition in all-d-metal Heusler alloy Mn2Ni1.25Co0.25Ti0.5

Mn-Ni-Co-Ti alloys are newly reported all-d-metal Heusler alloys with magnetic field induced martensitic transformation and large volume discontinuity. Here we investigated the electronic structure and magnetic structural transition in Mn2Ni1.25Ti0.5 and Mn2Ni1.25Co0.25Ti0.5 theoretically. In these alloys, the same number of Mn atoms enter D (0.75, 0.75, 0.75) site when extra Ni/Co substitute for Ti. Mn (D) spin moment is parallel to that of Mn at B (0.25, 0.25, 0.25) site in cubic austenitic phase but becomes antiparallel in tetragonal martensitic phase. Then during the martensitic transition, the total magnetic moment decreases obviously. Compared with Mn2Ni1.25Ti0.5, the doping of Co in Mn2Ni1.25Co0.25Ti0.5 can enlarge the energy difference between the antiparallel and parallel configurations of Mn (B) and Mn (D) moments and thus stabilize the ferromagnetic coupling between them in austenite. DOS structure and charge density difference reveal this is related to the strong d-d hybridization between Co-Mn nearest neighbors. That is the origin of the FM activation effect of Co-doping in literature. The volume-conserving assumption may need improvement in all-d-metal Heusler alloys. The lowest total energy for Mn2Ni1.25Co0.25Ti0.5 martensitic was obtained when its cell volume is smaller than the equilibrium cell volume of the austenite. The predicted volume contraction is as large as 3.5% with a c/a ratio of 1.45. These results give reasonable explanations for existing experimental observations. (C) 2018 Elsevier B.V. All rights reserved.