  Magnetization switching due to a current-pulse in symmetric and asymmetric
spin valves is studied theoretically within the macrospin model. The switching
process and the corresponding switching parameters are shown to depend
significantly on the pulse duration and also on the interplay of the torques
due to spin transfer and external magnetic field. This interplay leads to
peculiar features in the corresponding phase diagram. These features in
standard spin valves, where the spin transfer torque stabilizes one of the
magnetic configurations (either parallel or antiparallel) and destabilizes the
opposite one, differ from those in nonstandard (asymmetric) spin valves, where
both collinear configurations are stable for one current orientation and
unstable for the opposite one. Following this we propose a scheme of ultrafast
current-induced switching in nonstandard spin valves, based on a sequence of
two current pulses.
