Non-local spin Seebeck effect in the bulk easy-plane antiferromagnet NiO

TL;DR

This paper reports the first observation of magnon spin currents generated by the Spin Seebeck effect in bulk NiO, revealing how magnetic field and temperature gradients influence magnon populations and spin transport in antiferromagnets.

Contribution

It demonstrates the generation and measurement of magnon spin currents via SSE in bulk NiO and models the magnetic field dependence affecting magnon modes.

Findings

Magnon spin currents are generated by SSE in bulk NiO at 5 K.

Magnetic field induces non-degeneracy in magnon modes, affecting the SSE signal.

Non-local magnon signals saturate with current and do not decay exponentially with distance.

Abstract

We report the observation of magnon spin currents generated by the Spin Seebeck effect (SSE) in a bulk single crystal of the easy-plane antiferromagnet NiO. A magnetic field induces a non-degeneracy and thereby an imbalance in the population of magnon modes with opposite spin. A temperature gradient then gives rise to a non-zero magnon spin current. This SSE is measured both in a local and a non-local geometry at 5$\,$K in bulk NiO. The magnetic field dependence of the obtained signal is modelled by magnetic field splitting of the low energy magnon modes, affecting the spin Seebeck coefficient. The relevant magnon modes at this temperature are linked to cubic anisotropy and magnetic dipole-dipole interactions. The non-local signal deviates from the expected quadratic Joule heating by saturating at a current from around 75$\,\mu A$ in the injector. The magnon chemical potential does not decay exponentially with distance and inhomogeneities may be the result of local magnon accumulations.