First-principle study on effects of Zn-doping on electronic structure, magnetism and martensitic transformation of Heusler type MSMAs Ni<sub>2</sub>FeGa<sub>1–</sub><i><sub>x</sub></i>Zn<i><sub>x </sub></i>(<i>x</i> = 0–1)

Abstract

The magnetic shape memory alloys (MSMAs) have both martensitic transformation and ferromagnetism in the same material, thus external magnetic field can be used to induce/control the phase transformation or the reorientation of martensite variant. MSMAs have received considerable attention for their interesting properties and wide applications in different fields. For practical applications, the martensitic transformation temperature <i>T</i><sub>M</sub> is an important factor and a high <i>T</i><sub>M</sub> is preferable. Recently, Zn-doping has been found to be a possible way to elevate the value of <i>T</i><sub>M</sub> of Ni-Mn based MSMA, but this effect on other kinds of MSMAs is not very clear yet. Heusler alloy Ni<sub>2</sub>FeGa is a typical MSMA with unique properties, however, its <i>T</i><sub>M</sub> is relatively low. So it can be meaningful to find possible ways to increase its phase transition temperature. In this paper, the influences of Zn-doping on the electronic structure, martensitic transformation and magnetic properties of Heusler-type magnetic shape memory alloy Ni<sub>2</sub>FeGa are investigated by first-principle calculations. Total energy calculation and charge density difference indicate that Zn atom prefers to occupy the Ga (D) site when substituting for Ga in Ni<sub>2</sub>FeGa<sub>1–<i>x</i></sub>Zn<sub><i>x</i></sub> (<i>x</i> = 0, 0.25, 0.5, 0.75, 1). This main-group-element-like behavior is related to the closed 3d shell of Zn. Due to the similar atomic radii of Ga and Zn, Zn-doping does not lead the lattice constant to change greatly. The variation of the energy difference Δ<i>E</i><sub>M</sub> between the martensite and austenite with Zn content increasing is calculated, and the result shows that Δ<i>E</i><sub>M</sub> increases with Zn-doping increasing, and thus conducing to increasing the stability of the martensite phase and to evaluating the transformation temperature <i>T</i><sub>M</sub> in Ni<sub>2</sub>FeGa<sub>1–<i>x</i></sub>Zn<sub><i>x</i></sub>. This trend can be explained by the Jahn-Teller effect observed in the DOS structure. The Zn-doping does not change the magnetic structure of Ni<sub>2</sub>FeGa. A ferromagnetic coupling between Fe spin moment and Ni spin moment can be observed within the whole range studied. The calculated total spin moment increases with Zn content increasing. The variation of formation energy <i>E</i><sub>f</sub> with Zn-doping is investigated. In Ni<sub>2</sub>FeGa<sub>1–<i>x</i></sub>Zn<sub><i>x</i></sub> a negative <i>E</i><sub>f</sub> is retained within the whole range studied, though it increases slightly with the doping of Zn. It is also found that the Zn-doping can increase the stability of L2<sub>1</sub> Heusler phase in Ni<sub>2</sub>FeGa<sub>1–<i>x</i></sub>Zn<i><sub>x</sub></i> and suppress the formation of the FCC L1<sub>2</sub> phase.