Tunable mechanical and half-metallic properties of Mn_2-XFe_1+XSi alloys: A first principles investigation

Highlights

• The results show the Mn_2-XFe_1+XSi alloys possess tunable meachanical and halfmetallic properties.

• It analyse the origin of the gap in the minoritiy bands of the compounds.

Abstract

The structural, mechanical, electronic, and magnetic properties of Mn2-XFe1+XSi (X = 0, 0.25, 0.5, 0.75, 1) Heusler alloys were studied in the framework of the density functional theory (DFT). The results show that Mn2-XFe1+XSi compounds are mechanically and dynamically stable with Hg2CuTi type (X = 0, 0.25 and 0.5) and Cu2MnAl type (X = 0.75 and 1); the volume of the series compounds decreases and they can change from brittle to ductile as Fe content increases. All the compounds have half-metallic properties and their Fermi levels gradually move to the middle of the spin-down band gap as X increases. The magnetic moment of Mn2-XFe1+XSi mainly arises from more Mn-d orbitals than Fe-d orbitals and satisfies the extended Slater-Pauling rule; the energy gaps are generated from the split in the energy level of d-orbitals of atoms on A, C sites and then hybrid with the d-orbitals of atoms on B sites.

Introduction

Heusler alloys have great research value and broad prospects because of their diverse components, polymorphic structures with good controllability, as well as rich physical properties such as ferromagnetism, shape memory effects, half-metallicity and thermoelectricity [[1], [2], [3], [4], [5], [6]]. Half-metallic Heusler alloys are very promising for application. These materials present a metallic property in one spin band and an energy gap in the other spin band around the Fermi level (EF). Therefore, half-metallic Heusler alloys are half metals as they can be regarded as hybrids of normal metals and semiconductors. These half-metals are perfect spin injectors that can maximize the efficiency of spin-dependent devices due to the electrons at EF forming a completely spin-polarized current [7]. Although many other materials like some oxide (e.g. CrO2, La2/3Sr1/3MnO3, Fe3O4) [[8], [9], [10]], double perovskites (e.g. Sr2FeReO6) [11], pyrites (e.g. CoS2) [12], zinc blende or wurtzite structures compounds (e.g. CrAs, CrTe, CrSe, etc) [13,14] were found to be half-metallic, the Heusler alloys are still prominent for technological applications as they have relatively high Curie temperature and significant spin transport effects.

Full Heusler alloys have the stoichiometric formula X2YZ, in which X and Y are usually transition metals, and Z is a ⅢA, ⅣA or ⅤA element in the periodic table. They are found with the Cu2MnAl type structure or the Hg2CuTi structure. Cu2MnAl type structure (space group no. 225: $\text{Fm}\begin{array}{l}\_\hfill \\ 3\hfill \end{array}m$) is the fully ordered, in which the X atoms are located on the Wyckoff position A (0, 0, 0) and C (½, ½, ½), the Y and Z atoms occupy the B (¼, ¼, ¼) and D (¾, ¾, ¾) sites, respectively. For Hg2CuTi type structure (space group no 216: $\text{F}\begin{array}{l}\_\hfill \\ 4\hfill \end{array}3m$), the X atoms are located on A (0, 0, 0) and B (¼, ¼, ¼) position, the Y and Z atoms occupy the C (½, ½, ½) and D (¾, ¾, ¾) sites, respectively. When the X atom has more valence electrons than the Y atom, the Heusler alloy takes the Cu2MnAl type structure. Otherwise the alloy takes the Hg2CuTi type structure [[15], [16], [17], [18]]. This phenomenon is a rule of thumb of atom preferential occupancy in Heusler alloys and the effectiveness has been verified by calculating the minimal total energy of Heusler alloys with different atomic configurations [19]. Many half-matallic ferromagnets have been predicted in Mn2- and Fe2-based full Heusler compounds by first principle calculations, e.g. Mn2CoZ [20], Mn2CuGe [21], Mn2RhGe [22], Fe2Xal [23], Fe2XY [24] and Fe2CoSn [25]. Fe combined Mn based Heulser Fe2MnZ and Mn2FeZ were heavily studied topics [[26], [27], [28], [29]]. These studies showed that the stoichiometric Mn2FeSi and MnFe2Si are both half metallic ferromagnets and crystallize in the Hg2CuTi type and Cu2MinAl type, respectively. The electronic structures, magnetism and spin polarization have been investigated. Aryal et al. [27] confirmed that the experimental crystal structure of Mn2FeSi consists with the predicted Hg2CuTi type, and its antiferromagnetic behavior reported in the literature. They also observed compensated magnetic states in their experiments. However, systematic experimental and theoretical studies on the non-stoichiometric alloys of Mn2FeSi to MnFe2Si are so far not available.

All the constituent elements of Mn2-XFe1+XSi are abundant in the earth. It is a significant investigation on these compounds in terms of low cost and potential applications. The microscopic mechanism of the magnetic moment and atom coupling would be attractive to identify as Mn, Fe and Si atoms are the nearest or second nearest neighbors of each other in the structures of Hg2CuTi type and Cu2MinAl type. Moreover, compounds with stable physical properties are expected to obtain by changing the concentration of constituent elements because the electronic, mechanical and magnetic properties of the Heusler alloys can be modified by the orbital hybridization between the ⅢA, ⅣA or ⅤA atoms and transition metal atoms. Additionally, the series Mn2-xFe1+xSi alloys provide a very good case study of the origin of band gap in the structural phase transition between Cu₂MnAl type and Hg2CuTi type. These are the motivations that support us to investigate the series Mn2-xFe1+xSi alloys in this paper.

The paper is arranged in the following way: Section 2 is the detailed methods that were adopted in this paper. Section3 presents the results and analysis which are: (1) the stable structures screened out from all the possible structures of the series Mn2-XFe1+XSi with the Hg2CuTi type and Cu2MnAl type, and the verification; (2) the mechanical properties, electronic structure and magnetism; (3) the analysis of the origin of the band gap. The paper is finally summarized in the conclusion section.