Time-domain studies of very-large-angle magnetization dynamics excited by spin transfer torques

We describe time-domain measurements which provide 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 steplike dependence on magnetic field, consistent with simulations which predict that the reversal trajectories differ by integer numbers of precession cycles. Storage-oscilloscope measurements of individual experimental traces reveal nonperiodic 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.