Elsevier

Journal of Alloys and Compounds

Volume 632, 25 May 2015, Pages 822-829
Journal of Alloys and Compounds

Effect of substitution on elastic stability, electronic structure and magnetic property of Ni–Mn based Heusler alloys: An ab initio comparison

https://doi.org/10.1016/j.jallcom.2015.01.255Get rights and content

Highlights

  • Effect of substitution on elastic and other properties of some MSMA has been studied.

  • We predict that Pt2MnGa is inherently the least brittle one among all the materials.

  • Cu substitution at Mn site of Pt2MnGa leads to decrease of its inherent brittleness.

  • We report the magnetic moment and Curie temperature for all the materials.

  • A double peak structure and a pseudo-gap near Fermi level are seen in stable materials.

Abstract

First-principles density functional theory based calculations have been used to predict the bulk mechanical properties of magnetic shape memory Heusler alloy Ni2MnGa substituted by copper (Cu), platinum (Pt), palladium (Pd) and manganese (Mn) at the Ni site. The elastic constants of Ni2MnGa alloy with and without substitution are calculated. We analyze and compare in detail the bulk mechanical properties for these alloys, in particular, the ratio between the calculated bulk and shear modulii, as well as the Poisson’s ratio and Young’s modulii. This analysis further based on an empirical relation, indicates that Pt2MnGa may inherently be the least brittle material, among the above-mentioned alloys. Interesting difference has been observed between the shear modulii calculated from Voigt’s and Reuss’s method. This has been explained in terms of the values of the tetragonal shear constant C of the materials. Study of Heisenberg exchange coupling parameters and Curie temperature as well as density of states of the materials shows the effect of substitution at the Ni site on the magnetic and electronic properties, respectively.

Introduction

One important signature of shape memory alloys (SMAs) is a thermoelastic transition from a high temperature cubic austenite phase to a low temperature martensite phase of lower symmetry. Most of these SMAs also show interesting electronic and magnetic properties [1], [2], [3], [4], [5]. Magnetic shape memory alloys (MSMAs) are considered to be technologically important following the discovery of 10% magnetic field induced strain (MFIS) in the martensite phase of Ni2MnGa [1], [2], [3], and 4% MFIS in the martensite phase of Mn2NiGa [6]. Besides MFIS, magnetocaloric effect (MCE) [7], [8] and inverse MCE [9], [10], [11], [12] are observed in some of these MSMA including Ni2MnGa and Mn2NiGa, suggesting possible practical applications of these well known Heusler alloys. Partial or complete substitution of atoms by various elements or changing of relative concentrations of manganese (Mn), gallium (Ga) and nickel (Ni) of these well known compounds can help in tuning of their various properties [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]. Changes in the properties by partial replacement of Mn by copper (Cu) as well as the effect of the Cu-substitution at the Ga and Ni site in Ni2MnGa with small to substantial degree of substitution have already been studied in the literature both from theoretical and experimental methods [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]. Substitution up to 100% at different sites in both Ni2MnGa and Mn2NiGa has been studied from first principles calculation, the substituent being a Cu atom [13]. It has been predicted that large substitution of Cu at Ni site, leads to a more stable austenite phase, rendering a martensite transition impossible unlike the cases of substitution at Mn or Ga sites.

Recently, a maximum MFIS of 14% has been predicted for Pt doped Ni2MnGa (composition Ni1.75Pt0.25MnGa) [18] on the basis of first-principles calculations. Neutron diffraction study establishes the existence of modulated structure in Ni1.8Pt0.2MnGa, which is a prerequisite for a large MFIS in small magnetic fields [31]. It is also shown very recently [32] that Ni1.8Pt0.2MnGa exhibits sizable MCE near room temperature. A large increase in martensite transition temperature has also been reported earlier for Pt-doped Ni–Mn–Ga [33]. Hence, by studying the literature it appears that, Pt-doped Ni2MnGa is expected to have similar or better properties in comparison to Ni2MnGa for technological applications.

The disadvantage related to poor mechanical properties of Ni2MnGa in terms of its technological suitability has been already addressed in the literature and studies aiming to improve the ductility of SMA alloys have been carried out earlier [34]. The elastic properties are amongst the most important physical properties relating to the structure and mechanical stability of materials. These properties are also associated with some other fundamentally important properties, e.g. hardness, specific heat and Debye temperature. There are two elastic modulii important for applications, namely, bulk modulus and shear modulus. First-principles calculations can give reliable values of the elastic constants and modulii of materials. In the literature, there are some studies of first-principles calculations of elastic and related properties of materials, including Heusler alloys [34], [35]. However, in spite of the outstanding, both experimental and theoretical, work on the magnetic Heusler alloys and their functional properties in conjunction with the structural martensitic transformation, relatively less studies have been carried out on the elastic properties of these alloys. In this paper, we carry out in depth study of the bulk mechanical stability in terms of the elastic stability criteria [36] and elastic constants of Ni2MnGa, substituted at the Ni site by Cu and isoelectronic elements Pt as well as Pd. Though the material parameters at a larger length scale, such as grain boundaries, structural defects like vacancies and dislocations influence the mechanical properties substantially, the non-directional metallic bonding is thought to contribute significantly to the ductility of most of the metallic solids. In crystalline materials in the cubic phase a high value of ratio of shear and bulk modulii gives a fairly good phenomenological indication of the inherent brittleness of the material [37]. This inherent brittleness is probed here using the empirical relationships proposed by Pettifor [38] and Pugh [39].

Both Cu and Pt are highly ductile materials. Further, because Pt and also Pd are isoelectronic with Ni while Cu has only one electron more than Ni, we expect interesting results if we substitute Ni with Pt, Pd and Cu. Hence, we study and compare here the elastic properties of Ni2MnGa and its substituted alloys. We also study the electronic structures in terms of the density of states (DOS). In addition, we evaluate the magnetic properties of these alloy materials in terms of the Heisenberg exchange coupling parameters from which we derive the Curie temperature (TC) following the literature [40]. In what follows, first, we give a brief account of the methodology used in this work and then we present our results on elastic, electronic and magnetic properties of the above-mentioned materials along with the related discussion. Finally, the results of our work are summarized in the last section.

Section snippets

Method

Ni2MnGa is known to assume an ordered X2YZ type (conventional Heusler alloy structure) of structure while Mn2NiGa has a XYXZ type (inverse Heusler alloy structure) of structure (X, Y typically elements with d-electrons and Z typically elements with s, p electrons). In the austenite phase, Ni2MnGa has cubic L21 structure that consists of four interpenetrating face-centered-cubic (fcc) lattices with origin at fractional positions (0.25, 0.25, 0.25), (0.75, 0.75, 0.75) (0.5, 0.5, 0.5), and (0.0, 0.0, 

Elastic stability of substituted Ni2MnGa

First we discuss the mechanical stability of the cubic austenite phase of Ni2MnGa and its substituted alloys. For cubic lattices, there are only three independent elastic constants, C11,C12 and C44, where, from symmetry, we have the following conditions: C11=C22=C33; C12=C23=C13 and C44=C55=C66. Table 1 lists all the relevant elastic constants along with the lattice parameters calculated for all the materials studied here using the CASTEP programme package [46]. The optimized geometries are

Summary and conclusion

Using first-principles density functional theory based calculations, we investigate the effect of Pt, Pd, Cu and Mn substitution at the Ni site in Ni2MnGa alloy system. We study in depth the mechanical and electronic stability of these substituted compounds. The elastic constants agree well with the literature wherever the data are available. It is observed that the tetragonal shear constant, C, has values close to and/or below zero for all the compounds studied here indicating that, for all

Acknowledgments

Authors thank S.R. Barman, S.W. D’Souza and S. Singh for useful discussions and Manfred Wuttig for providing results of unpublished work. P.D. Gupta, S.K. Deb and G.S. Lodha are thanked for facilities and encouragement throughout the work. The scientific computing group and the computer centre of RRCAT, Indore are thanked for help and support in running the codes. TR thanks M. Baral, K. Mondal and C. Kamal for discussions and HBNI, RRCAT for financial support.

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