  Based on Keldysh's nonequilibrium Green function method, the spin-dependent
transport properties in a ferromagnet-quantum dot (QD)-ferromagnet coupled
system are investigated. It is shown the spin current shows quite different
characteristics from its electrical counterpart, and by changing the relative
orientation of both magnetizations, it can change its magnitude even sign. The
current-induced spin transfer torque (CISTT) is uncovered to be greatly
enhanced when the bias voltage meets with the discrete levels of the QD at
resonant positions. The relationship between the CISTT, the electrical current
and the spin current is also addressed.
