Synthesis of New Nitride Material Manganese Cobalt Nitride by RF Co-Sputtering

Transition metal (<i>TM</i>) nitrides have historically been used as cutting tools due to their outstanding properties such as high hardness and strength, excellent thermal conductivity, and unique electrochemical properties. In 2019, Sun <i>et al. </i>[1] constructed a stability map of inorganic ternary metal nitrides using high-throughput computational methods based on a data-mined structure prediction algorithm. While this work predicted hundreds of new ternary nitrides, only one new chemical space was predicted to contain a previously unknown compound comprising nitrogen and two <i>TM</i>s: MnCoN₂. In this study, a series of Mn-Co-N thin films are synthesized by RF magnetron sputtering and characterized for structural and magnetic properties.<br/>Combinatorial Mn-Co-N thin films are deposited using reactive co-sputtering at different temperatures ranging from 25 – 450^oC and various process pressures. The phase of the as-grown material is checked by X-ray diffraction. Survey density function theory total energy calculations are performed in parallel on six prototype structures based on zincblende or rocksalt lattices. Comparing the calculated structures with experimental diffraction patterns, the synthesized films better match the rocksalt-derived structures, which also exhibit lower formation energy than the zincblende candidates. However, only the primary diffraction peaks are seen, suggesting a large amount of cation anti-site disorder. Manganese and cobalt concentrations are measured by X-ray fluorescence and nitrogen concentration by Rutherford backscattering spectrometry, confirming nearly 1:1:2 concentrations of Mn:Co:N with some reduced N due to O impurities. Scanning electron microscopy and energy dispersive x-ray spectroscopy are also performed, which illustrated the presence of all elements at their respective energy levels. Magnetic properties for MnCoN₂ films have been studied by SQUID magnetometry, which demonstrates that the film possesses a weak moment of remanant magnetization 0.01 emu/gm and coercive field of 0.5 T. The study of moment vs temperature shows a transition temperature at ~10 K and overall antiferromagnetic correlations. To better understand the origin of the magnetic moment in the compound and its correlation with structure and stoichiometry (both anion and cation), we perform X-ray Absorption Spectroscopy (both near edge and extended fine structure). Our experimental confirmation of this new <i>TM₁–TM₂–</i>N ternary nitride motivates renewed effort in new materials prediction and discovery in similar ternary spaces, which we are actively pursuing. For example, in our ongoing computational search, several more <i>TM₁–TM₂–</i>N have been predicted as new stable nitrides.<br/><br/><br/>1. Sun, W. <i>et al,</i> A map of the inorganic ternary metal nitrides. <i>Nat. Mater.</i> <b>18</b>, 732–739 (2019)