Micro-focused Brillouin light scattering study of the magnetization dynamics driven by Spin Hall effect in a transversely magnetized NiFe nanowire

We employed micro-focused Brillouin light scattering to study the amplification of the thermal spin wave eigenmodes by means of a pure spin current, generated by the spin-Hall effect, in a transversely magnetized Pt(4nm)/NiFe(4nm)/SiO2(5nm) layered nanowire with lateral dimensions 500x2750 nm2. The frequency and the cross section of both the center (fundamental) and the edge spin wave modes have been measured as a function of the intensity of the injected dc electric current. The frequency of both modes exhibits a clear redshift while their cross section is greatly enhanced on increasing the intensity of the injected dc. A threshold-like behavior is observed for a value of the injected dc of 2.8 mA. Interestingly an additional mode, localized in the central part of the nanowire, appears at higher frequency on increasing the intensity of the injected dc above the threshold value. Micromagnetic simulations were used to quantitatively reproduce the experimental results and to investigate the complex non-linear dynamics induced by the spin-Hall effect, including the modification of the spatial profile of the spin wave modes and the appearance of the extra mode above the threshold.

💡 Research Summary

The authors investigate how a pure spin current generated by the spin‑Hall effect (SHE) can amplify thermally excited spin‑wave eigenmodes in a laterally confined NiFe nanowire. The device consists of a Pt(4 nm)/NiFe(4 nm)/SiO₂(5 nm) stack patterned into a 500 nm × 2.75 µm wire. A dc charge current flowing through the Pt layer produces a transverse spin current via SHE, which exerts a spin‑transfer torque on the NiFe magnetization. The nanowire is magnetized transversely by an external field, establishing a well‑defined equilibrium direction.

Micro‑focused Brillouin light scattering (BLS) with sub‑300 nm spatial resolution is employed to probe the local spin‑wave spectra at the wire centre and at the two lateral edges. In the absence of current, two thermally populated modes are observed: a fundamental (center) mode near 7.8 GHz and an edge‑localized mode near 7.2 GHz. As the dc current is increased, both modes display a clear red‑shift of their frequencies and a strong increase of the BLS intensity, indicating reduced effective damping and partial amplification.

A pronounced threshold is found at I ≈ 2.8 mA. Below this value the intensity grows gradually, whereas above it the BLS signal rises abruptly, signalling that the spin‑Hall torque has fully compensated the intrinsic Gilbert damping and the system enters a self‑oscillatory regime. In this regime an additional higher‑frequency mode appears in the centre of the wire, roughly 0.4 GHz above the original fundamental mode. This extra mode becomes visible for currents > 3 mA and coexists with the amplified fundamental and edge modes at the highest currents studied (4 mA).

To elucidate the underlying physics, the authors perform micromagnetic simulations based on the Landau‑Lifshitz‑Gilbert equation augmented with a spin‑Hall torque term. The simulations reproduce the experimental red‑shifts, intensity enhancement, and the emergence of the extra mode. They also reveal that the spatial profiles of the modes evolve with current: the edge mode spreads toward the centre, while the fundamental mode broadens, reflecting nonlinear mode coupling. The appearance of the new mode is attributed to a combination of nonlinear frequency pulling, effective field modulation by the spin current, and mode hybridisation.

The work demonstrates that SHE‑driven spin currents can efficiently compensate damping in a nanoscopic ferromagnet, leading to selective amplification of distinct spin‑wave modes and the onset of nonlinear dynamics above a well‑defined current threshold. These findings are directly relevant for the design of spin‑Hall nano‑oscillators, magnonic amplifiers, and other spin‑tronic devices that rely on low‑power, current‑controlled generation of coherent magnons. Moreover, the study showcases the power of micro‑focused BLS combined with quantitative micromagnetic modelling as a comprehensive toolbox for probing and engineering spin‑wave phenomena at the nanoscale.