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

Abstract

We experimentally demonstrate that both quasi-linear and nonlinear self-localized bullet modes of magnetization auto-oscillation can be excited by dc current in the 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 the formation of these modes are fundamentally different. We discuss the relationship between the observed behaviors and the formation of an unstable nonlinear magnon condensate.

Stabilization of a nonlinear bullet coexisting with a Bose-Einstein condensate in a rapidly cooled magnonic system driven by a spin-orbit torque

“…Here, we investigate the effect of the RC mechanism after reaching a quasi-stationary auto-oscillation regime induced by the spin injection via the SOT effect. In accordance to recent studies [15,29], we observe that the STT-induced damping overcompensation over a sufficiently long time triggers quasi-stationary auto-oscillations for the longer pulse duration used. In particular, due to the induced nonlinear frequency shift, the bullet mode is formed exhibiting frequencies below the magnon dispersion.…”

“…[28] resulted in an overpopulation at the bottom of the magnon spectrum. Still, it was kept short enough to prevent the formation of auto-oscillations in the form of the bullet mode in the current density range of interest [15,28,29]. As a result, the magnon population in a broad frequency range at the bottom increased and, consequently, the threshold current shifted.…”

Interplay of Linear and Nonlinear Localization Mechanisms in Spin-Torque Oscillators with a Field Well

“…3, a). In this complex case, only precise timeresolved measurements can distinguish a type of the magnetization dynamics [29].…”

“…It is a consequence of the strong nonuniformity of the internal magnetic field and different nonlinear properties of the edge modes compared to the bulk ones [26][27][28]. In [29], the mode hopping between linearly localized and bullet modes in an SHO with extended active area was found. This means that the excitation thresholds of both these modes are close to each other.…”

Evidence for spin current driven Bose-Einstein condensation of magnons

“…In relatively thick films of yttrium iron garnet (YIG), where microwave-driven BEC has been evidenced 1 – 10 , the magnons at the lowest-energy level exhibit very weak nonlinear interactions resulting in a stabilization of BEC by extrinsic effects 10 , 37 . In contrast, in nanometer-thick YIG films, which can be efficiently driven by spin currents, the energy of the lowest magnon level contains a significant dynamic dipolar contribution, which results in a strong attractive interaction between magnons leading to spatial instabilities and collapses 27 , 38 . The nonlinearities associated with dipolar effects have also been recently shown to be responsible for the strong scattering of magnons out from the lowest-energy state preventing its overpopulation 39 .…”