ANOMALOUS NERNST COEFFICIENT ENHANCEMENT AND BERRY CURVATURE HOTSPOTS IN HALF-METALLIC CO₂MNGA HEUSLER ALLOY THIN FILMS: THICKNESS-DEPENDENT TRANSVERSE THERMOELECTRIC RESPONSE

Objective:  We present a systematic study of the anomalous Nernst effect (ANE) and Berry curvature hotspot distribution in thin films of Co2MnGa Heusler alloys, varying in thickness from the ultrathin-to-intermediate regime (5–70 nm), grown by DC magnetron sputtering on MgO(001) substrates. Method: Using first-principles Berry curvature calculations in combination with the X-ray diffraction, vibrating sample magnetometry, four-probe magnetotransport, and on-chip Nernst measurements, we find the optimal thickness window (40-50 nm) where the anomalous Nernst coefficient |Syx| at room temperature is 2.71 μV/K. Results: This is not related to the scaling of the magnetization with film thickness, which is only 13% different across the entire thickness series, but is related to better L21 chemical ordering, lower interface scattering, the strain relaxation, and the resulting redistribution of Berry curvature weight near the Fermi level. These results are corroborated by first-principles calculations that the anomalous Hall conductivity (AHC) is intrinsic and peaks near EF, and that the transverse thermoelectric conductivity αxy measured in the experiment is 0.704 A/m·K, which is in good agreement with estimates from the Mott-relation for the 50 nm film. The AHC and the ANE are suppressed at particle sizes smaller than 20 nm due to reduced chemical ordering, increased surface scattering, and a magnetic dead layer. Novelty: We have shown the quantitative structure – transport – topology relationship in Co2MnGa and determined the region of parameter space required for maximizing transverse thermoelectric energy conversion in device compatible geometry.