We demonstrate highly efficient spin Hall nano-oscillators (SHNOs) based on NiFe/beta-W bilayers. Thanks to the very high spin Hall angle of beta-W, we achieve more than a 60% reduction in the auto-oscillation 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 beta-W. Published by AIP Publishing.

We demonstrate low-operational-current W/Co20Fe60B20/MgO spin Hall nano-oscillators (SHNOs) on highly resistive silicon (HiR-Si) substrates. Thanks to a record high spin Hall angle of the beta-phase W (theta(SH) = -0.53), a very low threshold current density of 3.3 x 10(7) A/cm(2) can be achieved. Together with their very wide frequency tunability (7-28GHz), promoted by a moderate perpendicular magnetic anisotropy, HiR-Si/W/CoFeB based SHNOs are 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.

We investigate the effect of hafnium (Hf) dusting on the magnetodynamical properties of NiFe/Pt bilayers using spin-torque-induced ferromagnetic resonance measurements on 6 μm wide microstrips on high-resistive Si substrates. Based on two series of NiFe(t_NiFe)/Hf(t_Hf)/Pt(5) stacks, we first demonstrate that the zero-current magnetodynamic properties of the devices benefit from Hf dusting: (i) the effective magnetization of the NiFe layer increases by 4%–8% with Hf present and (ii) the damping α decreases linearly with t_Hf by up to 40%. The weaker anisotropic magnetoresistance (AMR≈0.3%–0.4%) of the 3 nm NiFe series is largely unaffected by the Hf, while the stronger AMR of the 5 nm NiFe series drops from 0.7% to 0.43% with increasing t_Hf. We find that the spin Hall efficiency ξ_SH is independent of the NiFe thickness, remaining unaffected (ξ_SH = 0.115) up to t_Hf = 0.4 nm and then decreasing linearly for higher t_Hf. The different trends of α and ξ_SH suggest that there is an optimum Hf thickness (≈0.4 nm) for which the threshold current for auto-oscillation should have a minimum, while the much lower damping should improve mutual synchronization. Our results also indicate that the spin-orbit torque is entirely damping-like with no field-like torque component. Finally, the internal spin Hall angle of Pt is estimated to be θ_SH = 0.22 by calculating the transparency of the interface.

We experimentally study the auto-oscillating spin-wave modes in Ni₈₀Fe₂₀/β-W constriction-based spin Hall nano-oscillators as a function of bias current, in-plane applied field strength, and azimuthal field angle, in the low-field range of 40-80 mT. We observe two different spin-wave modes: i) a linear-like mode confined to the minima of the internal field near the edges of the nanoconstriction, with weak frequency dependencies on the bias current and the applied field angle, and ii) a second, lower frequency mode that has significantly higher threshold current and stronger frequency dependencies on both bias current and the external eld angle. Our micromagnetic modeling qualitatively reproduces the experimental data and reveals that the second mode is a spin-wave bullet and that the SHNO mode hops between the two modes, resulting in a substantial increase in linewidths. In contrast to the linear-like mode, the bullet is localized in the middle of the constriction and shrinks with increasing bias current. Utilizing intrinsic frequency doubling at zero eld angle we can reach frequencies above 9 GHz in fields as low as 40 mT, which is important for the development of low-eld spintronic oscillators with applications in microwave signal generation and neuromorphic computing.

We employ electrical measurements complemented by systematic micromagnetic simulations to reveal the complex dynamics of nanoconstriction-based spin Hall nano-oscillators. In particular, depending on the strength and out-of-plane angle of the applied magnetic field, we observe three distinct types of magnetization auto-oscillation: (a) a linear-like mode localized in the vicinity of the nanoconstriction by the demagnetizing field, (b) a further localized “regular” spin wave bullet, and(c) a “large” bullet that fills the entire area of the nanoconstriction. Although it has been assumed for some time that bullets only emerge if the nonlinearity of the system is negative (corresponding to the attraction of magnons), our results demonstrate that, in patterned films, they could be sustained even if the nonlinearity of the system is positive (corresponding to the repulsion of magnons). So, in contrast to the regular spin wave bullet, the auto-oscillation volume of its novel large counterpart enlarges, with the amplitude enhancing their drift stability and, correspondingly, reducing their linewidth. We demonstrate that tuning can be achieved between the observed modes at a fixed external field by changing only the drive current, thanks to the amplitude-dependent nonlinearity of the auto-oscillations. This flexibility of nanopatterned spin Hall nano-oscillators is desirable to achieve synaptic functionality in oscillator-based neuromorphic computing devices.

We study mutual synchronization phenomena in multiple nanoconstriction-based SHNOs under weak in-plane fields down to μ₀H = 30 mT and investigate the angular dependence of the synchronization condition. We compare double nanoconstriction and multiple nanoconstrictions with different spacings of 300 and 900 nm between the constrictions. For all the tested devices, we observe clear evidence of mutual synchronization of individual nanoconstrictions (NCs) only for angles smaller than a critical angle. This critical angle is higher for the 300 nm spacing than for the 900 nm spacing as a result of the stronger synchronization arising from the shorter distance. Direct inspection of the spin waves using μ-BLS maps confirms synchronization of the double nanoconstrictions. Alongside the synchronization, we observe a strong second harmonic that could be interpreted as a sign that the synchronization is mediated by the propagation of the second harmonic of the spin waves. Micromagnetic simulation explains the synchronization at the lower angles by the direction of the spatial profile of the modes and confirms the role of exchange coupling in the synchronization of nanoconstriction-based SHNOs.