Steady-state skyrmion oscillations in a spin valve (SV) nanopillar based on hybrid polarizer: A skyrmion-based spin torque nano-oscillator (STNO)

Abstract

The skyrmion-based spin torque nano-oscillators (STNOs) have received a lot of interest due to their prospects as microwave signal generators. These oscillators are accessible by achieving consistent circular spinning of the skyrmion in a circular disk, caused by the spin transfer torque (STT) effect. However, the Magnus force effect, that relates to the non-trivial topological part of the skyrmion and driven on by the higher current densities, leads the skyrmion to accumulate in the center or annihilate toward the edge of the device. To counterbalance the Magnus force, we propose a skyrmion-based STNO model built on a circular nanopillar-shaped spin valve having a hybrid polarizer, which is divided into two regions, each with a different polarization angle. Using Thiele equation-based analytical framework in parallel with micromagnetic simulations, we illustrate that the proposed hybrid polarizer system is intended to ensure steady circular motion of the skyrmion without skyrmion annihilation or accumulation, by strengthening the boundary-induced force. Moreover, we also investigate the current density and polarizer parameters effect on skyrmion oscillation frequency. The results show that the frequency that can be limited for a skyrmion-based STNO with a hybrid polarizer is up to 3.35 GHz, which is higher in terms of magnitude than that of the homogeneous polarizer-based STNO. Our findings provide an insight into the phenomenon of skyrmion dynamics in a circular nanopillar-shaped spin valve with hybrid polarizer, which may open up new avenues for overcoming the frequency limit of the skyrmion-based STNOs.