New magnetic materials with high Curie temperatures for spintronic applications are perpetually sought for. In this paper, we present an ab initio study of the structural, electronic and magnetic properties of Quaternary Heusler compounds CoX′Y′Si where X′ is a transition metal with 4d electrons and Y′ is either Fe or Mn. We find five new half-metallic ferromagnets with spin polarisation nearly 100% with very high Curie temperatures. The variation of Curie temperatures as a function of valence electrons can be understood from the calculated inter-atomic exchange interaction parameters. We also identify a few other compounds, which could be potential half-metals with suitable application of pressure or with controlled doping. Our results reveal that the half-metallicity in these compounds is intricately related to the arrangements of the magnetic atoms in the Heusler lattice and hence, the interatomic exchange interactions between the moments. The trends in the atomic arrangements, total and local magnetic moments, interatomic magnetic exchange interactions and Curie temperatures are discussed with fundamental insights.

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Employing ab initio electronic structure calculations we extensively study ternary Heusler compounds having the chemical formula X2X Z, where X = Mn, Fe or Co; Z = Al or Si; and X changes along the row of 4d transition metals. A comprehensive overview of these compounds, addressing the trends in structural, electronic, magnetic properties and Curie temperature is presented here along with the search for new materials for spintronics applications. A simple picture of hybridization of the d orbitals of the neighboring atoms is used to explain the origin of the half-metallic gap in these compounds. We show that arrangements of the magnetic atoms in different Heusler lattices are largely responsible for the interatomic exchange interactions that are correlated with the features in their electronic structures as well as possibility of half-metallicity. We find seven half-metallic magnets with 100% spin polarization. We identify few other compounds with high spin polarisation as "near half-metals" which could be of potential use in applications as well. We find that the major features in the electronic structures remain intact if a 3d X constituent is replaced with an isoelectronic 4d, implying that the total number of valence electrons can be used as a predictor of half-metallic nature in compounds from Heusler family.

In this work, important insights into the evolution of half‐metallicity in quaternary Heusler alloys are provided. Employing ab initio electronic structure methods we study 18 quaternary Heusler compounds having the chemical formula CoX′Y′Al, where Y′ = Mn, Fe; and X′ a 4d element. Along with the search for new materials for spintronics applications, the trends in structural, electronic, magnetic properties, and Curie temperature are investigated. Comparative studies with the compounds in the quaternary series CoX′Y′Si with X′ materials from the 3d and 4d transition metal series in the periodic table have been made. We observe that the half‐metallic behavior depends primarily on the crystal structure type based on atomic arrangements and the number of valence electrons. As long as these two are identical, the electronic structures and the magnetic exchange interactions bear close resemblances. Consequently, the materials exhibit identical electronic properties, by and large. The roles of different transition metal atoms in affecting the hybridizations and their correlations with the above observations are analyzed. This work, therefore, provides important perspectives regarding the underlying physics of half‐metallic behavior in quaternary Heusler compounds which goes beyond specifics of a given material. This, thus, paves the way for smart prediction of new half‐metals. This work also figures out an open problem of understanding how different ternary Heuslers with different electronic behavior may lead to half‐metallic behavior in quaternary Heuslers with 4d transition metal elements.

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