Ultrafast Spin-Orbit-Torque-Induced Coherent Magnetization Switching with Picosecond Current Pulses

Spin-orbit-torque (SOT)-based magnetization switching offers the promise of fast switching speed at low current density. However, recent SOT switching studies show an incubation delay in the ~ns range due to stochasticity in the nucleation of a magnetic domain during the domain-wall-driven magnetization switching. Here, we experimentally observe ultrafast SOT-induced coherent magnetization switching dynamics with no incubation delay primarily driven by an ultrafast thermal anisotropy torque. We employ an ultrafast photo-conducting switch and a co-planar strip line to generate and guide a ~9 ps current pulse into the heavy metal/ferromagnetic layer stack and thereby induce ultrafast SOT. The magnetization crosses zero in ~70 ps and complete switching occurs in ~250 ps. A good agreement between our experimental results and a macro-spin simulation model confirms that the switching dynamics are coherent and ultrafast heating-induced thermal anisotropy torque is responsible for the coherent switching. These results show a new way to dramatically increase the writing speed of SOT-induced magnetization switching within a coherent regime.