[Paper Review] Parametric Excitation and Instabilities of Spin Waves driven by Surface Acoustic Waves

Moritz Geilen, Roman Verba+11 more|arXiv (Cornell University)|Jan 11, 2022

Magnetic properties of thin films4 citations

TL;DR

This study experimentally demonstrates parametric excitation and instabilities of spin waves in CoFeB thin films driven by surface acoustic waves (SAWs), leveraging magneto-elastic coupling. Using micro-focused Brillouin light scattering and micromagnetic simulations, it identifies dual instability mechanisms—acoustic pumping via four-magnon scattering and enhanced three-magnon splitting—enabling efficient, nonlinear magnon generation with thresholds as low as 64% lower than conventional methods, paving the way for compact, energy-efficient magnonic logic devices.

ABSTRACT

The parametric excitation of spin waves by coherent surface acoustic waves is demonstrated experimentally in metallic magnetic thin film structures. The involved magnon modes are analyzed with micro-focused Brillouin light scattering spectroscopy and complementary micromagnetic simulations combined with analytical modelling are used to determine the origin of the spin-wave instabilities. Depending on the experimental conditions, we observe spin-wave instabilities originating from different phonon-magnon and magnon-magnon scattering processes. Our results demonstrate that an efficient excitation of high amplitude, strongly nonlinear magnons in metallic ferromagnets is possible by surface acoustic waves, which opens novel ways to create micro-scaled nonlinear magnonic systems for logic and data processing that can profit from the high excitation efficiency of phonons using piezoelectricity.

Motivation & Objective

• To experimentally demonstrate parametric excitation of spin waves in metallic ferromagnetic thin films using coherent surface acoustic waves (SAWs).
• To identify and characterize the underlying scattering mechanisms responsible for spin-wave instabilities in the presence of SAW-driven magneto-elastic coupling.
• To determine the threshold conditions for nonlinear magnon excitation and understand the interplay between phonon-magnon and magnon-magnon scattering processes.
• To explore the feasibility of using SAWs as a highly efficient, voltage-controlled excitation source for nonlinear magnonic systems in compact, energy-efficient devices.

Proposed method

• Micro-focused Brillouin light scattering (μBLS) was used to map spin-wave modes in frequency and momentum space with high spatial and spectral resolution.
• Micromagnetic simulations were performed to model the dynamic magnetization response and validate the observed instability mechanisms.
• Analytical modeling based on rate equations and three-magnon scattering theory was applied to calculate threshold amplitudes for parametric excitation processes.
• The SAW amplitude was tuned via an interdigital transducer (IDT) to probe the onset of nonlinear instabilities in the spin-wave spectrum.
• Thresholds for acoustic pumping and three-magnon instability were calculated using coupling parameters derived from the spin-wave dispersion and demagnetization fields.
• A combined analysis of μBLS data, simulations, and analytical models enabled identification of dominant instability channels, including frequency-non-degenerate mode splitting.

Experimental results

Research questions

• RQ1 What are the dominant nonlinear scattering mechanisms responsible for spin-wave instabilities when driven by surface acoustic waves in metallic ferromagnets?
• RQ2 How does the SAW-induced magneto-elastic field lead to parametric excitation of magnons, and what are the threshold conditions for such instabilities?
• RQ3 To what extent do four-magnon and three-magnon scattering processes contribute to the observed nonlinear spin-wave growth?
• RQ4 Can the wavevector and frequency spectrum of the excited magnons be controlled via the SAW's non-zero wavevector and frequency?
• RQ5 How does the interplay between direct SAW-to-magnon coupling and subsequent magnon-magnon scattering influence the overall excitation efficiency and threshold behavior?

Key findings

• The experimentally observed spin-wave instability is a combination of acoustic pumping via four-magnon scattering and a SAW-driven first-order parametric process enhanced by three-magnon scattering.
• The minimal threshold for acoustic pumping (a_th,min^SAW-SW) occurs for secondary magnon modes with wavevectors k₁ = (8.1, -0.5) rad/μm and k₂ = (1.2, 0.5) rad/μm, and frequencies of 4.2 GHz and 2.1 GHz, respectively.
• The threshold for the three-magnon instability (a_th,min^3-magnon) is approximately 64% higher than that of the acoustic pumping process, indicating that acoustic pumping dominates the instability onset.
• The wavevector of the secondary modes differs from micromagnetic simulation predictions, likely due to small discrepancies in demagnetization field calculations.
• The frequency-non-degenerate splitting of secondary modes due to the SAW’s nonzero wavevector enables tunable control over the magnon spectrum, a key advantage for device applications.
• The results confirm that SAW-driven magneto-elastic coupling is strong enough to support complex nonlinear magnon-phonon phenomena, enabling efficient, voltage-controlled excitation of high-amplitude, strongly nonlinear magnons in metallic ferromagnets.