Comparative experimental and theoretical study on anomalous Nernst effect of Heusler alloy Co ₂ FeSi thin film: estimation of on-site Coulomb interaction at Co site

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 (<i>α</i> _<i>xy</i> ) 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 (<i>U</i> _Co). Our measurements reveal that <i>α</i> _<i>xy</i> is less than 0.1 A m^-1 K^-1 at room temperature and decreases slightly with the temperature. The observed values of <i>α</i> _<i>xy</i> are more than one order of magnitude smaller than the predictions unless a small but finite <i>U</i> _Co 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 <i>U</i> _Co value. By combining these experimental observations with first-principles calculations, we estimate that the appropriate <i>U</i> _Co 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.