Ultrahigh frequency spin-torque nano-oscillator based on bilayer-skyrmions

Abstract

A skyrmion in a nanodisk with a single free layer (Single-layer Skyrmion) can be excited by the spin-polarized current to a steady state of circular motion for high-frequency applications. However, limited by the threshold of the driving current owning to the skyrmion Hall effect (SkHE), the maximum frequency of the skyrmion oscillation is usually 1 ∼ 3 GHz. In this work, the dynamics of skyrmions in the nanodisk with two antiferromagnetic-coupled free layers (Bilayer-skyrmions) are investigated. Our simulations show that the maximum frequency of Bilayer-skyrmions can be increased up to above 10 GHz, about one order of magnitude larger than that of Single-layer Skyrmion because of the disappearance of the SkHE in the antiferromagnetic-coupled system. In addition, it is found that the oscillation frequency of Bilayer-skyrmions linearly increases with the current density. Finally, spin-torque nano-oscillators based on multiple pairs of Bilayer-skyrmions are further demonstrated to increase the frequency limit up to 36.5 GHz. Our results may be useful for the design and applications of current-induced skyrmion-based oscillators for ultrahigh-frequency applications.