Time-domain measurements of nanomagnet dynamics driven by spin-transfer torques

We present time-resolved measurements of gigahertz-scale magnetic dynam-ics caused by torque from a spin-polarized current. By working in the time domain, we determined the motion of the magnetic moment throughout the process of spin-transfer–driven switching, and we measured turn-on times of steady-state precessional modes. Time-resolved studies of magnetic relaxa-tion allow for the direct measurement of magnetic damping in a nanomagnet and prove that this damping can be controlled electrically using spin-polarized currents. Spin-polarized electrons traversing a ferro-magnet can transfer spin-angular momentum to the local magnetization, thereby applying a torque that may produce magnetic reversal or steady-state precession (1, 2). This spin-transfer mechanism allows nanomagnets to be manipulated without magnetic fields, and it is the subject of extensive research for ap-plications in nonvolatile memory, programma-ble logic, and microwave oscillators (3–11). However, the gigahertz-scale magnetic dy-namics that can be driven by spin transfer have previously been measured using only frequency-domain techniques (12–16). Here we report direct time-resolved studies of dy-namics excited by spin-transfer torques. By working in the time domain, we are able to characterize the full time-dependent magnetic response to pulses of spin-polarized currents, including transient dynamics. These measure-ments allow a direct view of the process of spin-transfer–driven magnetic reversal, and they determine the possible operating speeds for practical spin-transfer devices. The re-sults provide rigorous tests of theoretical mod-els for spin transfer (1, 9, 17–19) and strongly support the spin-torque model (1, 18) over competing theories that invoke magnetic heat-ing (9, 20). We studied nanopillar-shaped samples con-sisting of two 4-nm-thick permalloy (Py