  We report intrinsic spin decay length of an antiferromagnetic insulator. We
found that at an antiferromagnetic/ferromagnetic interface, a spin current
generated by spin pumping is strongly suppressed by two-magnon scattering. By
eliminating the two-magnon contribution, we discovered that the characteristic
length of spin decay in NiO changes by two-orders of magnitude through the
paramagnetic to antiferromagnetic transition. The spin decay length in the
antiferromagnetic state is longer than 100 nm, which is an order of magnitude
longer than previously believed. These results provide a crucial piece of
information for the fundamental understanding of the physics of spin transport.
