Pulsed spin-torque switching has been studied using single-shot time-resolvedelectrical measurements in perpendicularly magnetized magnetic tunnel junctionsas a function of pulse amplitude and junction size in 50 to 100 nm diametercircular junctions. The mean switching time depends inversely on pulseamplitude for all junctions studied. However, the switching dynamics is foundto be strongly dependent on junction size and pulse amplitude. In 50 nmdiameter junctions the switching onset is stochastic but the switching oncestarted, is fast; after being initiated it takes less than 2 ns to switch. Inlarger diameter junctions the time needed for complete switching is stronglydependent on the pulse amplitude, reaching times less than 2 ns at large pulseamplitudes. Anomalies in the switching rate versus pulse amplitude are shown tobe associated with the long lived (> 2 ns) intermediate junction resistancestates.
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