Self-Modulated Soliton Modes Excited in a Nanocontact Spin-Torque Oscillator

Self-modulated bubble-like solitons, namely droplets, recently observed in spin-torque nanooscillators with a perpendicular free layer, are promising for applications in spintronics, magnonics, and magnetic logic devices. This letter presents a micromagnetic analysis on soliton dynamics. Our results identify the role of physical parameters (i.e., external field, saturation magnetization, and exchange constant) in achieving experimental findings such as hysteretic, linear and nonlinear spin-wave excitations. The modes with different excitation frequencies have nonuniform spatial distributions of power, and the power of frequency sidebands is mostly located near the outer border of the nanocontact. At high currents, a wavelet-based analysis highlights that the magnetoresistive signal due to the sideband modes is not stationary, providing a possible origin of the asymmetric sidebands observed in spintronic self-modulators. We also identify the thermal field as the key ingredient for the excitation of linear modes in a subcritical regime.