Spin transport in multilayer systems with fully epitaxial NiO thin films

We report on the generation and transport of thermal spin currents in fully epitaxial {\gamma}-Fe$_2$O$_3$/NiO(001)/Pt and Fe$_3$O$_4$/NiO(001)/Pt trilayers. A thermal gradient, perpendicular to the plane of the sample, generates a magnonic spin current in the ferrimagnetic maghemite ({\gamma}-Fe$_2$O$_3$) and magnetite (Fe$_3$O$_4$) thin films by means of the spin Seebeck effect. The spin current propagates across the epitaxial, antiferromagnetic insulating NiO layer, before being detected in the Pt layer by the inverse spin Hall effect. The transport of the spin signal is studied as a function of the NiO thickness, temperature and ferrimagnetic material where the spin current is generated. In epitaxial NiO grown on maghemite, the spin Seebeck signal decays exponentially as a function of the NiO thickness, with a spin-diffusion length for thermally-generated magnons of {\lambda}$_{MSDL}$ = $1.6 \pm 0.2$ nm, largely independent on temperature. We see no enhancement of the spin current signal as previously reported for certain temperatures and thicknesses of the NiO. In epitaxial NiO grown on magnetite, the temperature-averaged spin diffusion length is {\lambda}$_{MSDL}$ = $3.8 \pm 0.3$ nm, and we observe an enhancement of the spin signal when the NiO thickness is 0.8 nm, demonstrating that the growth conditions dramatically affect the spin transport properties of the NiO even for full epitaxial growth. In contrast to theoretical predictions for coherent spin transport, we do not see vastly different spin diffusion lengths between epitaxial and polycrystalline NiO layers.