We investigated the $5d$ element substitution effect on the thermal conductivity in $\mathrm{F}{\mathrm{e}}_2\mathrm{VAl}$-based superlattice thin films epitaxially grown on a MgO (100) substrate. We found unique crossover behavior in the period dependences of thermal conductivity for Ta-free $\mathrm{F}{\mathrm{e}}_2\mathrm{VAl}$ and $\mathrm{F}{\mathrm{e}}_2\left(\mathrm{V},\mathrm{Ta}\right)\mathrm{Al}$-based superlattices. The $\mathrm{F}{\mathrm{e}}_2\left(\mathrm{V},\mathrm{Ta}\right)\mathrm{Al}$-based superlattices with periods more than 40 nm appeared to have much lower thermal conductivity than Ta-free $\mathrm{F}{\mathrm{e}}_2\mathrm{VAl}$-based superlattices due to the substitution of V by Ta. Unexpectedly, at a shorter periodic length, less than 20 nm, $\mathrm{F}{\mathrm{e}}_2\left(\mathrm{V},\mathrm{Ta}\right)\mathrm{Al}$-based superlattices appeared to have higher thermal conductivity than Ta-free $\mathrm{F}{\mathrm{e}}_2\mathrm{VAl}$-based superlattices despite such a heavy element substitution by Ta. This surprising experimental fact was well accounted for with theoretical calculations, which predicted the dominant contribution of phonons having a shorter mean free path in the Ta substituted samples.

• Received 16 March 2020
• Accepted 1 September 2020

DOI:https://doi.org/10.1103/PhysRevB.102.104301