Binary switching of gyrotropic modes of skyrmion in a nano-ring structure

Abstract

Magnetic skyrmions are nanoscale swirling like magnetization textures with topological protection which can be stabilized in ferromagnetic systems with Dzyaloshinskii-Moriya interaction1. Currently, skyrmion are being explored for promising candidate for spintronic based memory and logic applications with low power consumption, fast processing and high density. Ultrafast skyrmion dynamics in GHz-band offers potential for high frequency applications. Recent studies on skyrmion dynamics reveal that there are three types of skyrmion resonant modes: clockwise (CW), counter-clockwise (CCW) gyrotropic modes and breathing mode[2]. In this work, we present a systematic investigation on the microwave properties of the skyrmions in concentric and eccentric ring structures using micromagnetic simulations MuMax3 3. We have varied the inner diameter of concentric ring between 50 to 135 nm and a frequency shift of 2.2 GHz is observed as shown in Fig. 1(a). The frequency of skyrmion resonance mode increases when the diameter of the inner circle is increased. This can be attributed to the fact that the skyrmion experiences edge repulsion from the periphery of both inner and outer circles of the ring geometry. We have observed that the skyrmion is annihilated as the repulsion from the edges suppresses the topological stability of the skyrmion.The skyrmion dynamics are found to be extremely sensitive to the edge repulsions and a remarkably large frequency shift of 2 GHz of the skyrmion resonance modes is observed by simply varying the position of a skyrmion in an eccentric ring structure shown in Fig 1(b). It offers a potential route for two (binary) distinct frequency states by simply driving the skyrmion in different positions of the eccentric ring. Such tunable skyrmion modes can be achieved by driving a skyrmion between different locations using an oscillating magnetic field or current (spin torque effect). Such widely tunable resonant skyrmion modes may have potential implications for the realization of on-chip passive microwave devices such as filter and isolators.