Asymmetric switching behavior in perpendicularly magnetized spin-valve nanopillars due to the polarizer dipole field

TL;DR

This study investigates the asymmetric switching behavior in perpendicularly magnetized spin-valve nanopillars, attributing the asymmetry to the polarizer dipole field and exploring how device modifications can restore symmetry, impacting MRAM stability.

Contribution

The paper demonstrates that the polarizer dipole field causes asymmetry in switching energy barriers, and shows how device geometry can restore symmetry, affecting thermal stability in MRAM devices.

Findings

Switching field distributions are asymmetric with higher energy barriers for certain transitions.

The polarizer dipole field is responsible for symmetry breaking.

Removing the notch pair restores switching symmetry.

Abstract

We report the free layer switching field distributions of spin-valve nanopillars with perpendicular magnetization. While the distributions are consistent with a thermal activation model, they show a strong asymmetry between the parallel to antiparallel and the reverse transition, with energy barriers more than 50% higher for the parallel to antiparallel transitions. The inhomogeneous dipolar field from the polarizer is demonstrated to be at the origin of this symmetry breaking. Interestingly, the symmetry is restored for devices with a lithographically defined notch pair removed from the midpoint of the pillar cross-section along the ellipse long axis. These results have important implications for the thermal stability of perpendicular magnetized MRAM bit cells.