Instability threshold versus switching threshold in spin-transfer-induced magnetization switching

We study the magnetization dynamics induced by spin-angular momentum transfer in pillar-shaped $\mathrm{Co}\mathrm{Fe}∕\mathrm{Cu}∕\mathrm{Co}\mathrm{Fe}$ spin valves in zero-applied magnetic field. The voltage noise generated by the spin flow is used to identify the microwave magnetic excitations. In the bistable region of resistance $\left(R\right)$ versus current $\left(I\right)$ traces, a resonant magnetic excitation at $8\mathrm{GHz}$ is pumped above a first threshold identified as the instability current. Higher currents are required to induce switching. Between the instability current and the switching current, a reversible rounding of the $R\left(I\right)$ hysteresis loop is observed. Numerical modeling indicates that it arises from a current-induced dynamic instability of the magnetization: When a state with collinear magnetizations is driven unstable, the magnetization of the thinnest ferromagnetic layer undergoes a sustained precession along a large orbit. The simulated precession orbit is stationary at $0\mathrm{K}$ and randomly perturbed at $300\mathrm{K}$, and its main characters are in quantitative agreement with the experiment.