  A general expression of the current induced spin torque in a magnetic layered
structure in the presence of external dc or ac voltages is derived in the
framework of the scattering matrix approach. A detailed analysis is performed
for a magnetic-nonmagnetic-magnetic trilayer connected to external leads in the
presence of dc voltage bias in the ballistic regime. Alternatively, the
possibility of producing spin torque by means of the adiabatic ac modulation of
external gate voltages (quantum pumping) is proposed and discussed.
