  We describe time-domain measurements which provide new information about the
large-angle nonlinear dynamics of nanomagnets excited by spin-transfer torque
from a spin-polarized current. Sampling-oscilloscope measurements, which
average over thousands of experimental time traces, show that the mean reversal
time for spin-transfer-driven magnetic switching has a step-like dependence on
magnetic field, because an integer number of precession cycles is required for
reversal. Storage-oscilloscope measurements of individual experimental traces
reveal non-periodic large-amplitude resistance variations at values of magnetic
field and current in a crossover region between the regimes of
spin-transfer-driven switching and steady-state precession. We also observe
directly the existence of time-dependent switching, on the nanosecond scale,
between different precessional modes and between a precessional mode and a
static state, at particular values of magnetic field and current bias.
