  We study the adiabatic quantum pumping and quantum size effects of
spin-torque in a magnetic tunnel junction within a scattering matrix approach.
Quantum size effects are predicted in the presence of a dc bias as a function
of the thickness of the normal metal layer inserted between two magnetic layers
and of the fixed magnetic layer. In the presence of ac voltages, the results
for the spin-torque show a peculiar magnification effect and advantages of
spin-torque pumping in actual devices are also discussed.
