We present a proposal for a fully electrically controllable quantum dot based spin
current injector. The device consists of a quantum dot that is strongly coupled to
a ferromagnetic electrode on one side and weakly coupled to a nonmagnetic electrode
on the other side. The presence of the ferromagnetic electrode results in an exchange
field that splits the dot level. We show that this exchange-induced splitting can
lead to almost full spin polarization of the current flowing through the device. Moreover,
we also demonstrate that the sign of polarization can be changed by the gate or the
bias voltage within a switching time in the nanosecond range. Thus, the proposed device
can operate as an electrically controlled, rapidly switchable spin current source,
which can be realized in various state-of-the-art nanostructures.
