Distribution of the magnetization reversal duration in subnanosecond spin-transfer switching

Abstract

We study the experimental distribution of switching times in spin-transfer switching induced by sub-ns current pulses in a pillar-shaped spin valve, whose free layer easy axis is parallel to the spin polarization of the current. The pulse durations leading to successful switching events follow a multiply stepped distribution. The step positions reflect the precessional nature of the switching. Modeling indicates that the switching proceeds through an integer number of gradually amplified precession cycles. This number is determined by the initial magnetization state. The switching probability distribution can be modeled considering the thermal variance of the initial magnetization orientation and by analyzing the occurrence of a vanishing total torque condition in the set possible magnetization trajectories. Modeling helps us to understand why switching cannot happen with a reproducible sub-ns duration when the free layer easy axis is parallel to the spin polarization of the current. To circumvent that problem, we propose to bias the spin valve with a hard axis field, which could provide an increased reproducibility of the switching duration.