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

TL;DR

This paper compares free layer and pinned layer biasing strategies in spin-transfer switching, analyzing their effects on switching duration distribution and reproducibility using a macrospin model with thermal effects.

Contribution

It introduces analytical criteria for optimizing biasing strategies to improve switching reproducibility in spin-transfer devices.

Findings

Free layer biasing reduces switching time variability.

Biasing strategies lead to multiply-stepped switching duration distributions.

Optimal biasing and current conditions enhance reproducibility.

Abstract

We analyze the statistical distribution of switching durations in spin-transfer switching induced by current steps, and discuss biasing strategies to enhance the reproducibility of switching durations. We use a macrospin approximation and model the effect of finite temperature as a Boltzmann distribution of initial magnetization states (adiabatic limit). We compare three model spin-valves: a spin-valve with a free layer whose easy axis is parallel to the pinned layer magnetization (standard geometry), a pinned layer with magnetization tilted with respect to the free layer easy axis (pinned layer biasing), and a free layer whose magnetization is pulled away from easy axis by a hard axis bias (free layer biasing). In the conventional geometry, the switching durations follow a broad regular distribution, with an extended long tail comprising very long switching events. For the two biasing strategies, the switching durations follow a multiply-stepped distribution, reflecting the precessional nature of the switching, and the statistical number of precession cycles needed for reversal. We derive analytical criteria to avoid switching events lasting much longer than the average switching duration, in order to achieve the highest reproducibilities. Depending on the current amplitude and the biasing strength, the width of the switching time distribution can be substantially reduced, the best reproducibility being achieved for free layer biasing at overdrive current of a few times unity.