  We time-resolve the spin-transfer-torque-induced switching in perpendicularly magnetized tunnel junctions of diameters from 50 to 250 nm in the thermally activated regime. When the field and the spin-torque concur to favor the P to AP transition, the reversal yields monotonic resistance ramps that can be interpreted as a domain wall propagation through the device at velocities of 17 to 30 nm/ns; smaller cells switch hence faster. When the field hinders the P to AP transition, the switching is preceded by repetitive switching attempts, during which the resistance transiently increases until successful reversal occurs. At 50 nm, the P to AP switching proceeds reproducibly in 3 ns, with a monotonic increase of the device resistance. In the reverse transition (AP to P), several reversal paths are possible even in the smallest junctions. Besides, the non uniform nature of the response seems still present at nanoscale, with sometimes electrical signatures of strong disorder during the reversal. The AP to P transition is preceded by a strong instability of the AP state in devices above 100 nm. The resistance becomes extremely agitated before switching to P in a path yielding a slow (20-50 ns) irregular increase of the conductance with variability. Unreversed bubbles of 60 nm can persist a few microseconds in the largest junctions. The complexity of the AP to P switching is reduced but not suppressed when the junctions are downsized below 60 nm. The instability of the initial AP state is no longer detected but the other features remain. In the smallest junctions (50 nm) we occasionally observe much faster (sub-1 ns) switching events. We discuss the origin of the switching asymmetry and its size dependence, with an emphasis on the role of the non uniformities of the stray field emanating from the reference layers, which affects the zones in which nucleation is favored. 
Size-dependence of nanosecond-scale spin-torque switching in   perpendicularly magnetized tunnel junctions