Spin Hall nano-oscillators (SHNOs) utilize pure spin currents to drive local regions of magnetic films and nanostructures into auto-oscillating precession. If such regions are placed in close proximity to each other they can interact and sometimes mutually synchronize, in pairs or in short linear chains. Here we demonstrate robust mutual synchronization of two-dimensional SHNO arrays ranging from 2 x 2 to 8 x 8 nano-constrictions, observed both electrically and using micro-Brillouin Light Scattering microscopy. The signal quality factor, Q = f /∆f , increases linearly with number of mutually synchronized nanoconstrictions (N ), reaching 170,000 in the largest arrays. While the microwave peak power first increases as N 2 , it eventually levels off, indicating a non-zero relative phase shift between nano-constrictions. Our demonstration will enable the use of SHNO arrays in two-dimensional oscillator networks for highquality microwave signal generation and neuromorphic computing. arXiv:1812.09630v1 [cond-mat.mes-hall]

We demonstrate low-operational-current W/Co 20 Fe 60 B 20 /MgO spin Hall nanooscillators (SHNOs) on highly resistive silicon (HiR-Si) substrates. Thanks to a record high spin Hall angle of the β-phase W (θ SH = -0.53), a very low threshold current density of 3.3 × 10 7 A/cm 2 can be achieved. Together with their very wide frequency tunability (7-28 GHz), promoted by a moderate perpendicular magnetic anisotropy, this makes HiR-Si/W/CoFeB based SHNOs potential candidates for wide-band microwave signal generation. Their CMOS compatibility offers a promising route towards the integration of spintronic microwave devices with other on-chip semiconductor microwave components.

Low operational current spin Hall nano-oscillators based on NiFe/W bilayers. Letters, 109(24) We demonstrate highly efficient spin Hall nano-oscillators (SHNOs) based on NiFe/b-W bilayers. Thanks to the very high spin Hall angle of b-W, we achieve more than a 60% reduction in the autooscillation threshold current compared to NiFe/Pt bilayers. The structural, electrical, and magnetic properties of the bilayers, as well as the microwave signal generation properties of the SHNOs, have been studied in detail. Our results provide a promising path for the realization of low-current SHNO microwave devices with highly efficient spin-orbit torque from b-W. Published by AIP Publishing. Applied Physics

Magnetic droplets are nontopological dynamical solitons that can be nucleated in nanocontact based spin torque nano-oscillators (STNOs) with perpendicular magnetic anisotropy free layers. While theory predicts that the droplet should be of the same size as the nanocontact, its inherent drift instability has thwarted attempts at observing it directly using microscopy techniques. Here, we demonstrate highly stable magnetic droplets in all-perpendicular STNOs and present the first detailed droplet images using scanning transmission X-ray microscopy. In contrast to theoretical predictions, we find that the droplet diameter is about twice as large as the nanocontact. By extending the original droplet theory to properly account for the lateral current spread underneath the nanocontact, we show that the large discrepancy primarily arises from current-in-plane Zhang-Li torque adding an outward pressure on the droplet perimeter. Electrical measurements on droplets nucleated using a reversed current in the antiparallel state corroborate this picture.