Comparative experimental and theoretical study on anomalous Nernst effect of Heusler alloy Co2FeSi thin film: estimation of on-site Coulomb interaction at Co site

Abstract

Co2FeSi is considered a half-metallic ferromagnet and a Weyl semimetal, however, its predicted properties have been shown to be heavily influenced by how the on-site Coulomb interaction is incorporated, which remains controversial. In this study, we measure the anomalous Nernst conductivity (αxy) and anisotropic magnetoresistance (AMR) effect of a Co2FeSi thin film from low temperature to room temperature, and compare the results with those of first-principles calculations using different values for on-site Coulomb interaction at the Co site (UCo). Our measurements reveal that αxy is less than 0.1 A m−1 K−1 at room temperature and decreases slightly with the temperature. The observed values of αxy are more than one order of magnitude smaller than the predictions unless a small but finite UCo is incorporated. The AMR effect exhibits a notable sign change from negative to positive with increasing temperature, which is inconsistent with the predicted band structure calculated using a large UCo value. By combining these experimental observations with first-principles calculations, we estimate that the appropriate UCo value is approximately 1–2 eV. Our findings provide valuable insight into the correlation effect in Co2FeSi, emphasizing the critical role of the on-site Coulomb interaction in accurately describing the transport properties of Co-based Heusler alloys.