Ultra-fast magnetization reversal in magnetic nano-pillars by spin-polarized current

We study the speed limitations of the magnetization switching resulting from spin transfer in pillar-shaped CoFe/Cu/CoFe spin valves. The quasi-static critical currents are Ic-=-2mA for the antiparallel (AP) to parallel (P) configuration and Ic+=+4.6mA for the P to AP transition. Current pulses of duration down to 100 ps and amplitude of 4Ic trigger switching at 300 K. The switching is probabilistic for lower current pulses. The P to AP transition speed is not much temperature dependant from 50 to 300 K. In contrast, the AP to P transition is thermally inhibited and is much faster at 150 K than at 300 K. This thermal inhibition highlights the importance of the macrospin coherency and of the thermally excited spin waves with finite wave vector parallel to the magnetization. Our results validate spin-transfer switching for fast memory applications.