Spin wave excitations in a nanowire spin-torque oscillator with perpendicular magnetic anisotropy

TL;DR

This study uses micromagnetic simulations to explore spin wave excitations in nanowire spin-torque oscillators with perpendicular magnetic anisotropy, revealing conditions for localized and propagating modes and their potential for reconfigurable microwave devices.

Contribution

It demonstrates the feasibility of exciting localized and propagating spin wave modes in PMA nanowire STOs and identifies the critical anisotropy strength for mode transition, supported by analytical calculations.

Findings

PMA enables excitation of both localized and propagating spin wave modes.

A critical anisotropy value allows for propagating mode excitation.

Mode selectivity is linked to a sign change in system nonlinearity.

Abstract

Spin torque oscillators (STOs) are emerging microwave devices that can potentially be used in spin-logic devices and the next-generation high-speed computing architecture. Thanks to their non-linear nature, STOs are easily tunable by the magnetic field and the dc current. Spin Hall nano-oscillators (SHNOs) are promising types of STOs and most of the current studies focus on localized modes that can be easily excited. Here, we study using micromagnetic simulations, the nature of the spin-torque-induced excitations in nanowire devices made of perpendicular magnetic anisotropy (PMA) material. Our results showed that upon including PMA the excitation of localized and propagating spin wave modes is feasible. We study the nature of the mode excitations as a function of the PMA strength (\text{K}$_u$), and the current. Indeed, we estimate a critical value of \text{K}$_u$ to allow for the excitation of the propagating spin wave. We attribute this mode selectivity between localized and propagating modes to a switch in the sign of the nonlinearity of the system from negative to positive at a non-zero \text{K}$_u$ which is supported by analytical calculations. Our results provide deep insight into engineering reconfigurable microwave devices for future magnonic and computational applications.