  Ferromagnetic resonance driven spin pumping of pure spin currents from a
ferromagnet into a nonmagnetic material promises new spin-functional devices
with low energy consumption. The mechanism of spin pumping is under intense
investigation and it is widely believed that exchange interaction between the
ferromagnet and nonmagnetic material is responsible for this phenomenon. We
observe a thousand-fold exponential decay of the spin pumping from 20-nm thick
Y3Fe5O12 films to platinum across insulating barriers, from which the
exponential decay lengths of 0.16 and 0.23 nm are extracted for oxide barriers
with band gaps of 4.93 eV and 2.36 eV, respectively. This prototypical
signature of quantum tunneling through a barrier underscores the importance of
exchange coupling for spin pumping and reveals its dependence on the
characteristics of the barrier material.
