Numerical Analysis of Vortex Spin Torque Oscillator With Modified Free Layer

In this study, we theoretically demonstrate vortex core dynamics in a vortex spin torque oscillator (VSTO) with a modified free layer, which has a smaller radius than the original free layer. We numerically calculate the magnetic vortex dynamics in the VSTO by using the Thiele equation. The deformation potential due to the modified free layer is estimated by using micromagnetic simulation. As a result, the threshold current required to induce self-oscillation can be reduced by modifying the free layer with a 20 nm additional disk, compared to the original free layer. The oscillation frequency in the original VSTO is almost constant as the current density increases above the threshold current of the original ones. In the modified VSTO, the oscillation frequency remains nearly constant beyond the threshold current while below the threshold current, the oscillation frequency becomes nearly proportional to the current density. This characteristic feature enables current control of the oscillation frequency of VSTOs utilizing the proposed free-layer modification.