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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 Mn2Ni1.5Ti0.5 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 Mn2Ni1.5Ti0.5, 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.

Phase stability, electronic structure and magnetic properties of the FeRu-based quaternary Heusler alloys, FeRuYZ (Y = Ti, V, Cr, Mn and Z = Al, Ga, In, Si, Ge, Sn), have been investigated theoretically. In one primitive unit cell, these alloys contain 24–26 valence electrons. The formation energy Ef of FeRuYZ increases with increasing atomic number of Z when the Y or Z elements belong to the same group in the periodic table. And in the same period, group-IIIA Z element is not preferable for the stability of FeRuYZ compared with corresponding group-IVA element. This can be explained from the charge density difference. The 24-electron FeRu-based quaternary Heusler alloys are all paramagnets. Among them, FeRuTiSi (Ge, Sn) are all semiconductors with real band gap at the Fermi levels. These alloys may have potential applications as thermoelectric materials. High spin polarization ratio is observed in the 25-electron Heusler alloys, among which FeRuCrAl is predicted to have a 100% spin polarization ratio. The 26-electron FeRuCrSi is found to be a spin gapless semiconductor (SGS) with total spin moment of 2μB. These materials can be interesting for the development of spintronic materials. The origin of SGS character in FeRuCrSi was discussed and compared with half-metallic Fe2CrSi, in which Ru and Fe are isoelectronic. These findings can help to design new SGSs in Heusler alloys.

Transition metal Zn shows a main group element-like behavior in Heusler alloys Ni2MnGa1−xZnx (x = 0, 0.25, 0.50, 075 and 1.00). Zn enters Ga (D) site when it substitutes for Ga in Ni2MnGa chemically. The equilibrium lattice constant decreases with increasing Zn content. But at the same time the magnetic properties do not change too much. Mn partial moment contributes most to the total moment and couples parallel to that of Ni in both austenite and martensite. The partly substitution of Zn for Ga can lead to an increase of martensitic transition temperature due to the competition between cell volume effect and valence electron concentration e/a. Accordingly, the largest energy difference ΔEM between the austenite and martensite was found when x = 0.5. This was further explained based on the electronic structure from the Jahn-Teller effect. Then Zn-doping can be used as a possible way to adjust the phase transition temperatures in Heusler type MSMAs. Zn is much cheaper than Ga or In, this can be quite meaningful for the development of high performance and low cost MSMAs and broaden their industrial applications.