  Efficient spin/charge interconversion is desired to develop innovative
spin-based devices. So far, the interconversion has been performed by using
heavy atomic elements, strong spin-orbit interaction of which realizes the
interconversion through the spin Hall effect and the Edelstein effect. We
demonstrate highly efficient charge-to-spin conversion in a ferromagnetic
metal/Cu/Al2O3 trilayers, which do not contain any heavy element. The resulting
spin torque efficiency is higher than those of conventional spin Hall and
Rashba systems consisting of heavy elements such as Pt and Bi. Our experimental
results qualitatively deviate from typical behaviors arising from spin
transport. However, they are surprisingly consistent with the behaviors arising
from the orbital transport. Our results thus demonstrate a new direction for
efficient charge-to-spin conversion through the orbital transport.
