Stabilization of exponential number of discrete remanent states with localized spin-orbit torques

TL;DR

This study demonstrates that localized spin-orbit torques in bilayer films with crossing elliptical structures can stabilize an exponentially growing number of discrete remanent magnetic states, promising advanced spintronics memory applications.

Contribution

It introduces a method to exponentially increase stable magnetic states using field-free spin-orbit torques in simple bilayer structures with crossing ellipses.

Findings

Number of remanent states grows to 2^N with spin-orbit torques.

States jump to 2^{2N} when currents flow between different ellipses.

Structures exhibit a large variety of stable magnetic states for potential memory devices.

Abstract

Using bilayer films of $\beta$-Ta/Ni$_{0.8}$Fe$_{0.2}$, we fabricate structures consisting of two, three and four crossing ellipses which exhibit shape-induced bi-axial, tri-axial and quadro-axial magnetic anisotropy in the crossing area, respectively. Structures consisting of N crossing ellipses can be stabilized in 2N remanent states by applying (and removing) an external magnetic field. However, we show that with field-free spin-orbit torques induced by flowing currents in individual ellipses, the number of remanent states grows to 2$^\text{N}$. Furthermore, when the current flows between the edges of different ellipses the number of remanent states jumps to 2$^\text{2N}$, including states which exhibit a $\pi$-N\'{e}el domain wall in the overlap area. The very large number of accessible remanent magnetic states that are exhibited by the relatively simple magnetic structures paves the way for intriguing spintronics applications including memory devices.