Large anomalous Nernst and inverse spin-Hall effects in epitaxial thin films of Kagome semimetal Mn$_3$Ge

Synthesis of crystallographically well-defined thin films of topological materials is important for unraveling their mesoscale quantum properties and for device applications. Mn$_3$Ge, an antiferromagnetic Weyl semimetal with a chiral magnetic structure on a Kagome lattice, is expected to have enhanced Berry curvature around Weyl nodes near the Fermi energy, leading to large anomalous Hall / Nernst effects and a large spin-Hall effect. Using magnetron sputtering, we have grown epitaxial thin films of hexagonal D0$_{19}$ Mn$_3$Ge that are flat and continuous. Large anomalous Nernst and inverse spin-Hall effects are observed in thermoelectric and spin-pumping devices. The anomalous Nernst signal in our Mn$_3$Ge films is estimated to be 0.1 $\mu$V / K, and is comparable to that in ferromagnetic Fe, despite Mn$_3$Ge having a weak magnetization of ~3.5 m$\mu_B$ at room temperature. The spin mixing conductance is 90.5 nm$^{-2}$ at the Py / Mn$_3$Ge interface, and the spin-Hall angle in Mn$_3$Ge is estimated to be about 8 times of that in Pt.