Pure spin current, a flow of spin angular momentum without flow of any
companying net charge, is generated in two common ways. One makes use of the
spin Hall effect in normal metals (NM) with strong spin-orbit coupling, such as
Pt or Ta. The other utilizes the collective motion of magnetic moments or spin
waves with the quasi-particle excitations called magnons. A popular material
for the latter is yttrium iron garnet, a magnetic insulator (MI). Here we
demonstrate in NM/MI/NM trilayers that these two types of spin currents are
interconvertible across the interfaces, predicated as the magnon-mediated
current drag phenomenon. The transmitted signal scales linearly with the
driving current without a threshold and follows the power-law with n ranging
from 1.5 to 2.5. Our results indicate that the NM/MI/NM trilayer structure can
serve as a scalable pure spin current valve device which is an essential
ingredient in spintronics