  This letter presents studies of spin-transfer efficiency in electron
transport via discrete electron-in- a-box levels in individual nanometer-scale
Ni particles at 0.06K temperature. In a strong magnetic field, the
spin-transfer rates are estimated by measuring the amplitudes of the Zeeman
splitting of the levels. We find that the spin- and the charge-transfer rates
are comparable, demonstrating significant enhancement of the spin-transfer
efficiency compared to that in larger magnets. In a low magnetic field, we find
an additional energy splitting as evidence that the spin-transfer rate is far
higher than the charge-transfer rate. The effect is explained in terms of the
strong mesoscopic spin- orbit torques, which are exerted on the magnetization
in response to sequential electron tunneling.
