Robust Nernst magnetothermoelectricity in the topological spin semimetal FeCrRhX (X=Si, Ge)

We report on the anomalous transverse magnetothermoelectric properties of the topological spin semimetallic equiatomic quaternary Heusler alloys (EQHAs) FeCrRhSi and FeCrRhGe over a wide temperature range between 130and295K. Magnetic measurements reveal ferromagnetic behavior above room temperature with high Curie temperatures (T_C>400K) in these samples, where energetically competing ferromagnetic (FM) and antiferromagnetic (AFM) phases coexist at low temperatures, giving rise to field-induced metamagnetic transitions. The electrical resistivity of both samples exhibits semimetallic characteristics throughout the measured temperature range, with low-temperature resistivity upturns driven by Kondo-like scattering. Throughout this range, the longitudinal Seebeck coefficient is negative in sign for both FeCrRhSi and FeCrRhGe, indicating that electrons are the majority carriers for thermally driven charge transport. Notably, both samples exhibit giant values of the anomalous Nernst coefficient (S^ANE_zy) at room temperature, with S^ANE_zy values of (0.272±0.02)μVK^-1 for FeCrRhSi and (0.295±0.02)μVK^-1 for FeCrRhGe. These values are nearly three times larger than those reported for other EQHAs in the literature. An in-depth analysis reveals that intrinsic Berry curvature drives such large anomalous Nernst effects (ANEs) in our EQHAs. We show that in these samples, the contribution of the transverse thermoelectric conduction to the observed large ANE dominates over that of anomalous Hall effect acting on the charge carrier flow driven by the longitudinal Seebeck effect. The ratio of the anomalous off-diagonal transverse thermoelectric conductivity (α^A_zy) to the anomalous Hall conductivity (σ^AHE_zy) |α^A_zy/σ^AHE_zy| lies between 0.2(^kB_/e) and (^kB_/e) at room temperature, further highlighting the intrinsic Berry curvature as the underlying physical origin of the observed large ANEs.