Magnon Spin Nernst Effect: Current Progress, Challenges, and Outlooks

Magnons in antiferromagnetic insulators are promising information carriers for energy-efficient spintronic applications because they can transport spin angular momenta over long distances without incurring Joule heating. However, due to the lack of electrical charge and net magnetization, generating (hence manipulating) antiferromagnetic magnons remains a fundamental challenge. It has been recently proposed that the magnon spin Nernst effect (SNE), which is the thermomagnonic counterpart of the spin Hall effect, is an efficient way to generate magnonic pure spin currents without accompanying heat currents. The magnon SNE can be realized in a collinear antiferromagnet on 2D honeycomb lattice, in which the Dzyaloshinskii–Moriya (DM) interaction plays a crucial role in breaking symmetry. To reconcile with real experimental setup, we analyze detectable signals arising from the magnon SNE in the presence of spin diffusion in a finite system. We find that by proper device engineering, spin diffusion effects can substantially enhance the detectable signal and that optical measurements are generally more favorable than electrical measurements. In addition to 2D honeycomb antiferromagnets, we review a series of recent progress in the magnon SNE with different ground states such as noncollinear antiferromagnets, paramagnets, and magnets with other crystal structures. We also review alternative physical mechanisms that can lead to magnon SNE other than the DM interaction such as magnon-phonon hybridization. Finally, we briefly mention recent experimental attempts in verifying the magnon SNE.