Giant anomalous Nernst effect in the Co_2MnAl_(1-x)Si_x Heusler alloy induced by Fermi level tuning and atomic ordering

摘要

Co_2MnAl has been predicted to have Weyl points near the Fermi level in L2_1-ordered structure, which is expected to give rise to exotic transverse transport properties such as large anomalous Hall (AHE) and Nernst effects (ANE) due to large Berry curvature. In this study, the effects of Fermi level position and atomic ordering on AHE and ANE in Co_2MnAl_(1-x)Si_x were studied systematically. The Co_2MnAl film keeps B2-disordred structure regardless of annealing temperature, which results in much smaller anomalous Hall conductivity σ_(xy) and transverse Peltier coefficient α_(xy) than those calculated for L2_1-ordered Co_2MnAl. Our newly performed calculation of α_(xy) with taking B2 disordering into account well reproduces the experimental resu thus it was concluded that Berry curvature originating from the Weyl points is largely reduced by B2 disordering. It was also revealed Al substitution with Si shifts the position of the Fermi level and greatly improves the L2_1 atomic ordering, leading to strong enhancement of α_(xy). which also agreed with our theoretical calculation. The highest thermopower of ANE of 5.7 μV/K. which is comparable to the recent reports for Co_2MnGa. was observed for Co_2MnAl_(0.63)Si_(0.37) because of dominant contribution of a,,. This study clearly shows the importance of both Fermi level tuning and high atomic ordering for obtaining the effect of topological features in Co-based Heusler alloys on transverse transport properties.