Temperature study of the spin-transfer switching speed from dc to 100 ps

We study the speed of the magnetization switching resulting from spin transfer in pillar-shaped CoFe/Cu/CoFe spin valves and the temperature dependence thereof. The switching speed was investigated with current pulses of durations from 100 ps to dc while the temperature was varied from 50 to 300 K. Quasistatic loops indicate that the reversal events imply transition states with reduced remanences. Their interval of occurrence shrinks gradually to almost null when the temperature is raised to 300 K. The curvature of resistance versus current hysteresis loops is different in the antiparallel and parallel branches, which evidences the influence of the Ampere field on the quasistatic micromagnetic configuration. In the dynamical regime, the pulse-induced parallel to antiparallel transition speed is not much temperature dependent from 50 to 300 K. In contrast, the pulse-induced antiparallel to parallel transition is thermally disfavored and much faster at 150 K than at 300 K. We model the experimental behavior by a competition between thermal fluctuations and the Ampere-field-related C-like bending of the magnetization in the free layer. The contribution of the Ampere field dominates in most cases. These contributions are amplified or damped together by the spin-transfer torque, but since the C-like bending is a response to a magnetic field, it sets in at a gradual pace ruled by the classical Gilbert relaxation. Most of the difference between quasistatic switching and pulse-induced switching results from this complete or incomplete alignment with the total effective field. Our demonstration of 100-ps switching validates spin-transfer switching for fast memory applications. (c) 2005 American Institute of Physics.