  We report large enhancement of thermally injected spin current in normal
metal (NM)/antiferromagnet(AF)/yttrium iron garnet(YIG), where a thin AF
insulating layer of NiO or CoO can enhance spin current from YIG to a NM by up
to a factor of 10. The spin current enhancement in NM/AF/YIG, with a pronounced
maximum near the N\'eel temperature of the thin AF layer, has been found to
scale linearly with the spin-mixing conductance at the NM/YIG interface for NM
= 3d, 4d, and 5d metals. Calculations of spin current enhancement and spin
mixing conductance are qualitatively consistent with the experimental results.
