  We propose a nanoscale rotor embedded between two ferromagnetic electrodes
that is driven by spin injection. The spin-rotation coupling allows this
nanorotor to continuously receive angular momentum from an injected spin under
steady current flow between ferromagnetic electrodes in an antiparallel
magnetization configuration. We develop a quantum theory of this angular
momentum transfer and show that a relaxation process from a precession state
into a sleeping top state is crucial for the efficient driving of the nanorotor
by solving the master equation. Our work clarifies a general strategy for
efficient driving of a nanorotor.
