Sub-ns spin-transfer switching: compared benefits of free layer biasing and pinned layer biasing

摘要

We analyze the statistical distribution of switching durations inspin-transfer switching induced by current steps, and discuss biasingstrategies to enhance the reproducibility of switching durations. We use amacrospin approximation and model the effect of finite temperature as aBoltzmann distribution of initial magnetization states (adiabatic limit). Wecompare three model spin-valves: a spin-valve with a free layer whose easy axisis parallel to the pinned layer magnetization (standard geometry), a pinnedlayer with magnetization tilted with respect to the free layer easy axis(pinned layer biasing), and a free layer whose magnetization is pulled awayfrom easy axis by a hard axis bias (free layer biasing). In the conventionalgeometry, the switching durations follow a broad regular distribution, with anextended long tail comprising very long switching events. For the two biasingstrategies, the switching durations follow a multiply-stepped distribution,reflecting the precessional nature of the switching, and the statistical numberof precession cycles needed for reversal. We derive analytical criteria toavoid switching events lasting much longer than the average switching duration,in order to achieve the highest reproducibilities. Depending on the currentamplitude and the biasing strength, the width of the switching timedistribution can be substantially reduced, the best reproducibility beingachieved for free layer biasing at overdrive current of a few times unity.