Spin-orbit-torque (SOT) based magnetic devices¹ have gained considerable attention due to their non-volatility, speed, and energy efficiency. SOT-induced magnetization switching studies are limited by the rise-time and duration of available pulsed current sources ^2,3. Recently, ultrafast SOT switching has been demonstrated using ~ps current pulses⁴,obtained from an optically excited photoconductive (Auston) switch^4,5, although the switching dynamics have not yet been reported. Here, we report the ultrafast magnetization dynamics measured via the time-resolved polar magneto-optical Kerr effect (MOKE)⁵ in a ferromagnet (inset of Fig. 1), using the ultrafast SOT effect due to a ~ 9 ps long current pulse generated from an Auston switch. The ~ps current pulse, without any in-plane magnetic field, introduces a demagnetization of ~30% and softens the magnet at an ultrafast timescale. In the presence of a 1600 Oe symmetry-breaking in-plane field, SOT tries to rotate the magnetic moments away from the saturation state, for a negative current pulse (blue line in Fig. 1), which when combined with the ultrafast demagnetization, shows a reduced initial demagnetization coupled with SOT-oscillations at longer timescales. However, for positive current, it induces a coherent rotation of the softened magnetic moments toward the negative saturation. Magnetization crosses zero in ~65 ps and switches fully after about ~200 ps (red line in Fig. 1). Sub-ns current pulse-induced SOT dynamics are governed by domain-wall propagation with an additional incubation delay due to domain nucleation ^6-8. The absence of any incubation delay combined with the fast-switching time leads us to believe that the switching is probably governed by the coherent rotation of the softened magnetic moment due to ultrafast heating by the ~ps current pulse excitation. A micromagnetic simulation coupled with 2-temperature ultrafast heating qualitatively agrees well with the observed dynamics (dotted lines in Fig. 1). Our work sheds significant insights on the mechanism of picosecond SOT-induced magnetization dynamics.

References
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Fig. 1. Ultrafast magnetization dynamics with positive and negative ~ps current pulse direction. The theoretical analysis is shown in dotted lines.