  Spin-orbit interaction derived spin torques provide a means of reversing the
magnetization of perpendicularly magnetized ultrathin films with currents that
flow in the plane of the layers. A basic and critical question for applications
is the speed and efficiency of switching with nanosecond current pulses. Here
we investigate and contrast the quasistatic (slowly swept current) and pulsed
current-induced switching characteristics of micron scale Hall crosses
consisting of very thin ($<1$ nm) perpendicularly magnetized CoFeB layers on
$\beta$-Ta. While complete magnetization reversal is found at a threshold
current density in the quasistatic case, short duration ($\leq 10$ ns) larger
amplitude pulses ($\simeq 10 \times$ the quasistatic threshold current) lead to
only partial magnetization reversal and domain formation. We associate the
partial reversal with the limited time for reversed domain expansion during the
pulse.
