Microstructure and atomic order analyses in CoFeCrAl Heusler alloy thin films: Interpretation of spin gapless semiconductor-like transport properties

We report a systematic analysis of transport properties, microstructure and atomic order in the thin films of the CoFeCrAl (CFCA) quaternary Heusler alloy that was predicted to be a spin-gapless semiconductor (SGS). Scanning transmission electron microscopy (STEM) observations revealed that the CFCA films in the as-deposited state and the one annealed at 400°C show single phase, but distinct phase separation occurs in the films annealed above 550°C. Quantitative atomic order analysis performed by the combination of laboratory XRD and anomalous X-ray diffraction (AXRD) shows the CFCA film annealed at 400°C had the B2 structure with large disorders with respect to the Y structure. Based on the site occupation of the elements obtained by the present structural analysis, we predict the electronic structure of the CFCA film by first-principles calculations, which clearly shows the disappearance of the SGS-like band structure. Nevertheless, the CFCA film still exhibits SGS-like transport properties such as negative linear temperature coefficient of resistivity, the change of dominant carrier type with temperature, and positive linear magnetoresistance. The origins of these transport properties are discussed based on the obtained non-SGS-like electronic structure to give a reasonable interpretation other than SGS electronic structure.