Oscillating topological charge for skyrmion-based spin-transfer torque nano-oscillator

Magnetic skyrmions are considered as promising candidates for next generation information carriers. In cubic chiral magnets with bulk Dzyaloshinskii–Moriya interaction, skyrmions trivially extend into the third dimension forming so-called skyrmion string. The third dimension of skyrmion strings introduces an additional degree of freedom that can be utilized in skyrmion-based applications. An example of such application is spin-torque oscillator based on skyrmion breathing, gyrotropic motion and skyrmion deformation mechanisms. Expanding these mechanisms, we propose design of spin-torque oscillator based on topological charge oscillation in confined geometry where skyrmion string breathing, gyrotropic motion and skyrmion-edge interactions are coupled. Micromagnetic calculations were employed to demonstrate topological charge oscillation as a function of model geometries. It is shown that only a small subset of material geometries is suitable to obtain topological charge oscillations, indicating that precise engineering of geometry would be required to realize such a device. Findings presented in this work contribute to a better understanding of a skyrmion string dynamical phenomena, which offer additional route in designing spin-torque oscillators.