Unveiling the structure-property relationship in metastable Heusler compounds by systematic disorder tuning

Abstract

Heusler compounds represent a unique class of materials that exhibit a wide range of fascinating and tuneable properties such as exotic magnetic phases, superconductivity, band topology, or thermoelectricity. An exceptional, but for Heusler compounds common, feature is that they are prone to antisite defects and disorder. In this regard, the $\mathrm{Fe}{}_{2}\mathrm{VAl}$ Heusler compound has been a particularly interesting and disputed candidate. Even though various theoretical scenarios for the interplay of physical properties and disorder have been proposed, the metastable disordered A2 phase hitherto precluded experimental investigation in bulk samples. Here, we report experimental results on disorder-tuned $\mathrm{Fe}{}_{2}{\mathrm{VAl}}_{0.9}{\mathrm{Si}}_{0.1}$ alloys all the way toward the A2 phase, which we realized via rapidly quenching our samples from high temperatures. We measured the thermoelectric properties of these materials in a wide temperature range (4 to 700 K); they suggest a gradual semimetal/narrow-gap semiconductor $\ensuremath{\rightarrow}$ metal transition upon increasing the disorder. We also find a large anomalous Hall effect in the disordered A2 phase, arising from the side-jump scattering of charge carriers at the antisite magnetic moments. This is corroborated by measurements of the temperature- and field-dependent magnetization, which increases dramatically up to $\ensuremath{\approx}2.5\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{\text{B}}/\mathrm{f}.\mathrm{u}.$ as compared to the ordered compound ($<0.1\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{\text{B}}/\mathrm{f}.\mathrm{u}.$). This study provides an experimental realization of the metastable A2 structure in bulk $\mathrm{Fe}{}_{2}\mathrm{VAl}$-based alloys and grants insight into the structure-property relationship of these materials. Our work confirms that temperature-induced antisite disorder, occurring during thermal heat treatment, can be a precisely tuneable parameter in the family of Heusler compounds.