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

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 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) materials. 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 (Ku) and the current. Indeed, we estimate a critical value of Ku to allow for the excitation of the propagating spin wave. We attribute this mode selectivity between localized and propagating modes to the magnitude and the change of the sign of the nonlinearity of the system from negative to positive at a non-zero Ku, which is supported by analytical calculations. Our results provide deep insight into engineering microwave devices for future magnonic and computational applications.