Abstract
Spin Hall oscillators (SHO) are promising candidates for the generation, detection and amplification of high frequency signals, that are tunable through a wide range of operating frequencies. They offer to be read out electrically, magnetically and optically in combination with a simple bilayer design. Here, we experimentally study the spatial dependence and spectral properties of auto-oscillations in SHO devices based on Pt(7 nm)/Ni80Fe20(5 nm) tapered nanowires. Using Brillouin light scattering microscopy, we observe two individual self-localized spin-wave bullets that oscillate at two distinct frequencies (5.2 GHz and 5.45 GHz) and are localized at different positions separated by about 750 nm within the SHO. This state of a tapered SHO has been predicted by a Ginzburg-Landau auto-oscillator model, but not yet been directly confirmed experimentally. We demonstrate that the observed bullets can be individually synchronized to external microwave signals, leading to a frequency entrainment, linewidth reduction and increase in oscillation amplitude for the bullet that is selected by the microwave frequency. At the same time, the amplitude of other parasitic modes decreases, which promotes the single-mode operation of the SHO. Finally, the synchronization of the spin-wave bullets is studied as a function of the microwave power. We believe that our findings promote the realization of extended spin Hall oscillators accomodating several distinct spin-wave bullets, that jointly cover an extended range of tunability.
Highlights
Spin current-driven magnetization dynamics has a great potential for advancing nano-sized magnetic devices towards practical applications[1,2,3,4,5,6]
We demonstrate synchronization of the auto-oscillation modes to external signals and investigate the corresponding manipulation of the individual spin-wave bullets
The linear spin-wave eigenmodes as a basis of the auto-oscillations are assessed by Brillouin light scattering (BLS) microscopy measurements and micromagnetic simulations
Summary
Spin current-driven magnetization dynamics has a great potential for advancing nano-sized magnetic devices towards practical applications[1,2,3,4,5,6]. Due to nonlinearities in the magnetic system, an intrinsic connection of the frequency to the oscillation amplitude occurs[13] This allows to tune SHOs directly by the strength of the driving spin current and provides an exceptional wide-ranged synchronization to external signals by frequency entrainment[14,15,16]. For higher spin current densities, these SHO switch from a single-mode operation to a spectrally broader and polychromatic output[25] For this regime, the simultaneous excitation of multiple bullet-modes has been suggested based on electrical measurements and modeled by a nonlinear Ginzburg-Landau equation[25], but has not yet been directly observed. We find a steep initial increase in auto-oscillation intensity promising for the use of these SHO as microwave detectors or amplifiers
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