Enhancement of single bullet mode stability in nanowire spin-Hall oscillator with spatially nonuniform current bias

Spin torque oscillators (STO) are compact tunable sources of microwave radiation that serve as a test bed for the studies of nonlinear magnetization dynamics at the nanoscale. It has been shown that spin-Hall torque in ferromagnetic - heavy metal bilayers can provide spin torque necessary for the STO operation. However, multimode generation and low spectral purity of the microwave signals generated in spin-Hall oscillators (SHOs) hinders practical applications of these nanoscale devices.
Here we experimentally demonstrate that tapering of a Pt/Ni80Fe20 nanowire that serves as the SHO active region, results in a sizable extension of the bias current range of single-mode generation (Fig.1) and in the significant decrease of the spectral linewidth of the generated signal.
The observed features are explained in the framework of Ginzburg-Landau auto-oscillator model. The model reveals that in both straight and tapered nanowires the spin torque excites a self-localized spin wave bullet mode, which, owing to the increased role of magnetodipolar interaction, has a micrometer size. The key factor leading to the improvement of the generation characteristics in a tapered nanowire SHO is shown to be a nonuniformity of the bias current density. In particular, this nonuniformity leads to a current-induced displacement of the bullet mode from the nanowire center, which results in the extension of the region of stability of the single-mode generation regime. Also, the nonuniform current density provides a restoring force that reduces the amplitude of thermal fluctuations in the position of the bullet mode along the nanowire, and, thereby, decreases the SHO phase noise. Finally, the current-induced spatial separation of two bullets in 2-mode generation regime results in different nonlinear shifts of the bullets frequency, which leads to a larger intermode frequency separation, in accordance with experiment (Fig.1).