Abstract
We report a detailed analysis of microstructure, magnetic and transport properties of a stoichiometric Mn2CoAl Heusler alloy, which has been predicted to be a spin-gapless semiconductor (SGS). Microstructure analysis revealed that the main phase was Mn1.8Co1.4Al0.8 rather than Mn2CoAl.. Although conventional X-ray diffraction suggested the formation of single phase with XA/L21 strcture, a disk-shaped Mn-rich precipitate phase was observed in both annealed stoichiometric and Co-rich Mn2CoAl alloys along with the Mn1.8Co1.4Al0.8 main phase. Atomically resolved energy-dispersive X-ray spectroscopy mappings revealed that the Mn1.8Co1.4Al0.8 phase has a disordered inverse XA Heusler structure with Mn (A) and Al (C) sites partially substituted by Co and Mn, respectively. First-principles calculations based on the observed disordered structure suggest that the Heusler phase is not SGS but half-metallic. However, both annealed stoichiometric and Co-rich Mn2CoAl exhibit semiconducting-like resistivity and low anomalous Hall conductivity. These transport properties were regarded as the signatures of SGS in the previous studies; however, it could be due to localization of charged carriers due to the intersection of the Fermi level with Mn 3d and Co 3d states in Mn1.8Co1.4Al0.8. This study demonstrates that not only measurements of transport properties but also careful microstructure inspections are essential to validate SGS properties in the theoretically predicted SGS candidates among Heusler compounds.
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