Electronic and Vibrational Properties of Fe₂NiAl and Co₂NiAl Full Heusler Alloys: A First-Principles Comparison

Abstract

We provide a comparative first-principles investigation of the structural, electronic, vibrational and thermodynamic properties of the full Heusler compounds, cubic Fe2NiAl and tetragonally distorted Co2NiAl. In both cases, we find the inverse Heusler structure to be in close competition with a layered arrangement of the elements, which breaks cubic symmetry and is accompanied by significant absolute values for the magnetocrystalline anisotropy energy. While for Fe2NiAl the layered arrangement is predicted as a new ground state, we show evidence that it might become dynamically stable around room temperatures also for Co2NiAl. We identify in both systems subtle differences in the electronic and vibrational density of states between the different structures, which might be related to their particular stability.Index Terms-Density functional theory, Heusler alloys, magnetocrystalline anisotropy, vibrational densities of states I. INTRODUCTIONP ermanent magnets are of vital interest for modern industry since they are essential for the miniaturization of electronic devices, robotics, electrical vehicles, magnetic data storage and cooling technologies [1]- [10]. Nowadays, the most powerful commercial permanent magnets, such as (Nd,Pr) 2 Fe 14 B and Sm 2 (Co,Cu,Fe,Zr) 17 , are based on rareearth elements, which are comparatively scarce and expensive. Therefore the search for rare-earth free permanent magnets is a central topic for modern materials science.Among the distinguished candidates is tetratenite L1 0 -FeNi, whose magnetocrystalline anisotropy energy (MAE) of ≈ 1.1−1.3 MJ/m 3 [11]-[13] approaches the order of Nd-Fe-B. Tetratenite is mainly found in meteorites [14], [15] according to the rather low order-disorder phase transition temperature (200 − 300 o C) leading to extremely slow atomic diffusion, which makes its synthesis very challenging on the large scale. However, L1 0 -FeNi can be grown as a thin film [16]-[20], whereas other current strategies also include the effect of doping, interstitials, pressure, or simultaneous application of stress and magnetic field under an inert atmosphere on the order-disorder transformation [21]-[24]. In recent years, full Heusler alloys based on Ni [25], [26], Fe [26], [27], Co [26], [27], Rh, Au, and Mn [26] were considered for application as permanent magnets with a mediumrange energy product. The advantages of Heusler alloys are their comparatively low price and flexibility in properties, which can be tuned by structural ordering, composition and various defects. Fe-and Co-based Heusler alloys were recently studied by Matsushita et al. in terms of whether one may obtain a large MAE in alloys that adopt a low symmetry tetragonal structure [27]. Among 30 considered compositions, the authors found 15, which undergo tetragonal distortion and have MAE from −12 MJ/m 3 (Co 2 PtAl) to +5.19 MJ/m 3 (Fe 2 PtGe). It was proposed in theoretical studies [27]-[30] that Fe 2 NiAl