  Fast magnetization reversal of uniaxial Stoner particles by spin-transfer
torque due to the spin-polarized electric current is investigated. It is found
that a current with a properly designed time-dependent polarization direction
can dramatically reduce the critical current density required to reverse a
magnetization. Under the condition that the magnitude and the polarization
degree of the current do not vary with time, the shape of the optimal
time-dependent polarization direction is obtained such that the magnetization
reversal is the fastest.
