First-principles study of ferromagnetic Ni 2 CoGa(Zn) alloys in the Heusler and the inverse Heusler structure
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1200-G04-03
First-principles study of ferromagnetic Ni2CoGa(Zn) alloys in the Heusler and the inverse Heusler structure Antje Dannenberg1, Markus E. Gruner1, Manfred Wuttig2, and Peter Entel1 1
Faculty of Physics and Center for Nanointegration, CeNIDE,
University of Duisburg-Essen, 47048 Duisburg, Germany 2
Department of Materials Science and Engineering, University of Maryland, College Park, MD
20742-2115, USA ABSTRACT We performed an ab initio characterization of ferro- and nonmagnetic Ni2CoGa and Ni2CoZn compounds with respect to their potential application as new ferromagnetic shape memory alloys. The calculation of structural energy differences and mixing energies in the common X2YZ Heusler structure and the inverse (XY)XZ structure revealed, that both alloys are stable in the tetragonal distorted Heusler structure with a c/a ratio of 1.38 and show ferromagnetic ordering. The Curie temperatures are of the order of 250 K. Exchanging Ga with Zn improves the magnetic properties of the alloy without qualitative modification of the structural energy landscape, but at the expense of a reduced mixing energy. INTRODUCTION Currently, extensive work is spend on ferromagnetic shape memory alloys (FSMA) because of their potential use as smart materials. The prototype material is Ni2MnGa in which the martensitic transition occurs for off-stoichiometric, Mn-excess compositions slightly above room temperature, which, however, is still too low for many applications, as, e.g., in the automotive sector. A further complication is the fact that with increasing martensite temperature the Curie temperature can not be enhanced simultaneously. Thus, the interplay of high Curie and high martensite temperatures could not yet be obtained in the Ni-Mn-based systems. In the search for innovative magnetic materials for suitable magneto-mechanical devices numerous studies have been done in the last decade for various systems as Ni-Mn-(Al, Sn, Sb, In) [1, 2], Co-Ni-(Al, Ga) [3–11], Ni-Fe-(Al, Ga) [12] and Cu-Mn-(Al, Ga) [13, 14]. A detailed investigation of the structural and magnetic properties of Fe2CoGa and Fe2CoZn alloys in the Heusler and the inverse Heusler structure has been carried out recently by the authors, predicting comparatively high Curie temperatures of the order of 800 K [15]. In off-stoichiometric Ni-Mn-Ga the valence electron concentration e/a has been used for describing the martensitic transition temperature dependency on the chemical composition [16,17]. Therefore, CoNiGa(Zn), with a similar e/a, is a reasonable candidate for a new ferromagnetic shape memory alloy. A martensitic transformation has in fact been observed experimentally for Co2NiGa [5, 7]. Thus, a systematic investigation of the Ni2CoGa and the Ni2CoZn compound is a matter of course. Concerning the structural ordering, a general trend can be formulated: Transition metal atoms to the left of Fe prefer to occupy the Co site (as Mn in Ni2MnGa) while those to the right of Fe preferentially sit on the Ni sublattice [18]. In Ni2CoGa(Zn) both, Ni and Co, are s
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