Magneto-Thermal Transport in Weyl Semimetal Co2MnGa Thin Films INVITED

Abstract

The ferromagnetic Heusler compound Co2MnGa is a promising Weyl semimetal with the Fermi energy in the vicinity of the Weyl nodes. Bulk Co2MnGa recently attracted attention because of an exceptionally large anomalous Nernst effect [1,2] and exotic surface states [3]. Simultaneously, Co2MnGa thin films were successfully prepared and showed interesting transport properties [4]. Beside others, the non-trivial topology of the band structure of Weyl semimetals leads to unexpected magneto-thermoelectic transport phenomena which will be discussed in this talk. We report on our observation of a record large anomalous Nernst coefficient in Co2MnGa thin films - up to -3µV/K [5, 6], and discuss the procedure for the quantitative determination of the anomalous Nernst coefficient. Several samples with comparable saturated magnetization were studied, yielding significantly different anomalous Nernst responses. We conclude that the microscopic origin of the anomalous Nernst effect in Co2MnGa is complex and contains contributions from the intrinsic Berry phase and surface states. We further employ a series of Co2MnGa thin films to study the validity of the Mott relation by measuring all four transport coefficients - the longitudinal resistivity, anomalous Hall, Seebeck and anomalous Nernst coefficients [6]. We show that the measured anomalous Nernst signal can be connected to the remaining three magneto-thermal transport coefficients in the whole sample series. This is strong evidence that the Mott relation holds also in Weyl semimetals, where the intrinsic Berry curvature dominates the transport. In the last part of the talk we focus on another magneto-transport phenomena and its thermal counterpart – anisotropic magnetoresistance and anisotropic magneto-thermopower. We show that due to high epitaxial quality of the Co2MnGa films both non-crystalline and crystalline contributions to the anisotropic magnetoresistance can be measured. Interestingly, we do not observe the same crystalline contributions in the magneto-thermopower measurements [7].