Controllable excitation of quasi-linear and bullet modes in a spin-Hall nano-oscillator

We experimentally demonstrate that both quasi-linear and nonlinear self-localized bullet modes of magnetization auto-oscillation can be excited by dc current in a nano-gap spin Hall nano-oscillator, by utilizing the geometry with an extended gap. The quasi-linear mode is stable at low driving currents, while the bullet mode is additionally excited at larger currents and becomes increasingly dominant with increasing current. Time-resolved measurements show that the formation of the bullet mode is delayed relative to the quasi-linear mode by up to 100 nanoseconds, demonstrating that the mechanisms of formation of these modes are fundamentally different. We discuss the relationship between the observed behaviors and the formation of an unstable nonlinear magnon condensate.