  Electrically controllable non-volatile magnetic memories show great potential
for the replacement of semiconductor-based technologies. Recently there has
been strong interest in spin-orbit torque (SOT) induced magnetization reversal
due to the device's increased lifetime and speed of operation. However, recent
SOT switching studies reveal an incubation delay in the ~ns range due to
stochasticity in the nucleation of a magnetic domain during reversal. Here, we
experimentally demonstrate ultrafast SOT-induced magnetization switching
dynamics of a ferromagnet with no incubation delay by avoiding the nucleation
process and driving the magnetization coherently. We employ an ultrafast
photo-conducting switch and a co-planar strip line to generate and guide ~ps
current pulses into the heavy metal/ferromagnet layer stack and induce
ultrafast SOT. We use magneto-optical probing to investigate the magnetization
switching dynamics with sub-picosecond time resolution. Depending on the
relative current pulse and in-plane magnetic field polarities, we observe
either an ultrafast demagnetization and subsequent recovery along with a
SOT-induced precessional oscillation, or ultrafast SOT switching. The
magnetization zero-crossing occurs in ~70 ps, which is approximately an order
of magnitude faster than previous studies. Complete switching needs ~250 ps and
is limited by the heat diffusion to the substrate. We use a macro-magnetic
simulation coupled with an ultrafast heating model to analyze the effect of
ultrafast thermal anisotropy torque and current-induced torque in the observed
dynamics. Good agreement between our experimental results and the macro-spin
model shows that the switching dynamics are coherent and present no noticeable
incubation delay. Our work suggests a potential pathway toward dramatically
increasing the writing speed of SOT magnetic random-access memory devices.
