Computational capability for physical reservoir computing using a spin-torque oscillator with two free layers

TL;DR

This paper investigates the enhanced computational capabilities of spin-torque oscillators with two free layers for physical reservoir computing, revealing complex dynamics and increased memory capacity compared to traditional devices.

Contribution

It introduces a numerical analysis showing that two free layer spin-torque oscillators exhibit richer dynamics and higher memory capacity, advancing physical reservoir computing.

Findings

Oscillators with two free layers show amplitude-modulated oscillations and chaos.

These oscillators have a larger short-term memory capacity than conventional ones.

Enhanced capacity observed near the edge of the echo state property boundary.

Abstract

A numerical analysis on the computational capability of physical reservoir computing utilizing a spin-torque oscillator with two free layers is reported. Conventional spintronics devices usually consist of two ferromagnets, where the direction of magnetization in one layer, called the free layer, can move while that of the other, the reference layer, is fixed. Recently, however, devices with two free layers, where the reference layer is replaced by another free layer, have been developed for various practical applications. Adding another free layer drastically changes the dynamical response of the device through the couplings via the spin-transfer effect and the dipole magnetic field. A numerical simulation of the Landau-Lifshitz-Gilbert equation and a statistical analyses of the Lyapunov exponent and the synchronization index reveal the appearance of an amplitude-modulated oscillation and chaos in the oscillators with two free layers. Such complex dynamics qualitatively change the computational capability of physical reservoir computing because the computational resource is dynamics of the physical system. An evaluation of the short-term memory capacity clarifies that oscillators with two free layers have a larger capacity than those of conventional oscillators. An enhancement in capacity near the edge of echo state property, i.e., the boundary between zero and finite synchronization index, is also found.