  We study a junction of a topological insulator with a thin two-dimensional
(2D) non-magnetic or partially polarized ferromagnetic metallic film deposited
on a 3D insulator. We show that such a junction leads to a finite spin current
injection into the film whose magnitude can be controlled by tuning a voltage
$V$ applied across the junction. For ferromagnetic films, the direction of the
component of the spin current along the film magnetization can also be tuned by
tuning the barrier potential $V_0$ at the junction. We point out the role of
the chiral spin-momentum locking of the Dirac electrons behind this phenomenon
and suggest experiments to test our theory.
