  We report time-resolved measurements of magnetization switching by spin-orbit
torques in the absence of an external magnetic field in perpendicularly
magnetized magnetic tunnel junctions (MTJ). Field-free switching is enabled by
the dipolar field of an in-plane magnetized layer integrated above the MTJ
stack, the orientation of which determines the switching polarity. Real-time
single-shot measurements provide direct evidence of magnetization reversal and
switching distributions. Close to the critical switching voltage we observe
stochastic reversal events due to a finite incubation delay preceding the
magnetization reversal. Upon increasing the pulse amplitude to twice the
critical voltage the reversal becomes quasi-deterministic, leading to reliable
bipolar switching at sub-ns timescales in zero external field. We further
investigate the switching probability as a function of dc bias of the MTJ and
external magnetic field, providing insight on the parameters that determine the
critical switching voltage.
