  We observe highly efficient dynamic spin injection from Y$_3$Fe$_5$O$_{12}$
(YIG) into NiO, an antiferromagnetic (AF) insulator, via strong coupling, and
robust spin propagation in NiO up to 100-nm thickness mediated by its AF spin
correlations. Strikingly, the insertion of a thin NiO layer between YIG and Pt
significantly enhances the spin currents driven into Pt, suggesting
exceptionally high spin transfer efficiency at both YIG/NiO and NiO/Pt
interfaces. This offers a powerful platform for studying AF spin pumping and AF
dynamics as well as for exploration of spin manipulation in tailored structures
comprising metallic and insulating ferromagnets, antiferromagnets and
nonmagnetic materials.
