  Nano-constriction spin-Hall nano-oscillators (NC-SHNOs) are one of the most
promising alternatives among the microwave spintronics devices. They can
provide highly coherent and widely tuneable microwave signals and can be
fabricated at low temperatures, which makes them compatible with
back-end-of-the-line CMOS processing. For its applications, the frequency
stability of each device is crucial, in particular for synchronization of
oscillator arrays. In this work, we focus on the influence of a realistic grain
structure on the SHNO frequency stability using both measurements as well as
micromagnetic simulations. Grains in the thin ferromagnetic metal films can
influence the output characteristic of the SHNO since the exchange coupling is
reduced locally at the grain boundaries. This work provides a novel
micromagnetic simulation method, for systematic investigation of frequency
instability or variability from device-to-device. Experimentally, a
device-to-device frequency variability of ~270 MHz was found and in some
extreme cases, a double-mode oscillation was observed in the high current
operation range. This oscillation behavior was reproduced in simulations, where
the inclusion of grains resulted in a frequency variability of ~100 MHz and
double-mode oscillations for some particular configurations. The double modes
are consistent with a partial decoupling, or non-coherent operation, of two
oscillating regions located at the nano-constriction edges.
