Spin-Transfer switching with short current pulses in all perpendicular spin valve nanopillars

Spin-transfer switching has been studied in spin-valves with perpendicularly magnetized free and reference layers and a lateral size of 100 nm x 100 nm. We demonstrate 100\% switching probability with high efficiency for current pulses as short as 300 ps, comparable to the magnetization precession time. Experimentally we find that the inverse reversal time $1/\tau= A (I-I_{c0})$ depends linearly on the current overdrive, where $I_ {c0}$ is the zero temperature critical current. The dynamic parameter A depends linearly on the applied easy-axis field, as predicted by a $T=0$ macrospin model for a magnet with uniaxial anisotropy. However, A depends on the applied field much more strongly than predicted by the model. To fit the data we must assume a large initial angle of the magnetization of the free layer. Possible explanations for the discrepancies are a breakdown of the macrospin picture because of domain wall nucleation or propagation. Supported by USARO (Grant No. W911NF0710643).