Spin-Seebeck and anomalous Nernst effects in MnBi and Bi/MnBi composites

The spin-Seebeck effect (SSE) is an advective transport process in a bilayer composed of a ferromagnet (FM) and a nonmagnetic (NM) material with strong spin-orbit coupling. In a temperature gradient, the flux of magnons in the FM transfer spin angular momentum to the electrons in the NM, which, by the ISHE contributes an SSE voltage. In contrast, the intrinsic anomalous Nernst (ANE) conductivity in homogeneous FMs is understood as a non-advective process due to the effect of the temperature gradient on electrons: in MnBi, one can calculate the ANE conductivity from the Berry curvatures using a 32 band tight-binding Hamiltonian with the spin-orbit interaction and magnetization. We show that there can be an additional advective magnonic contribution to the ANE similar to the SSE effect but in uniform metallic FMs. Further, as was done in Ni/Pt ¹, we synthesized composites of aligned MnBi needles in a Bi matrix. In this geometry, we expect an additional SSE contribution from the magnons in MnBi to the Nernst effect in the Bi. In composites with Mn concentration far below the percolation threshold, we observe a large boost in the thermopower and Nernst over the Bi host, which we attribute to advective spin transport.

[1] S. Boona et al., Nat. Comm. 7 (2016)