Fast, efficient, and long-lasting magnetisation reversal is a crucial area of research in condensed matter physics with significant technological implications. While large-amplitude precessional motion is a theoretically straightforward approach, it suffers from instabilities at short timescales due to spin-wave excitations arising from the Suhl instability. These instabilities lead to incoherent spin-wave excitations that disrupt magnetic ordering and increase effective dissipation, drastically exceeding that observed in ferromagnetic resonance (FMR). The faster the switching, the more energy is pumped into incoherent spin-wave modes, exacerbating the problem. Adding to the complexity, the interaction between these excitations and the lattice introduces another layer of nonlinearity, the impact of which on large-angle precessional magnetisation switching remains poorly understood. This study addresses this gap by investigating the interplay between spin-wave instabilities and magnetoelastic interactions during magnetic switching, aiming to unveil the underlying mechanisms that govern pattern formation in this regime.

Existing literature extensively explores the challenges of high-speed magnetisation reversal, highlighting the limitations imposed by spin-wave instabilities and their resulting increase in energy dissipation. Studies have characterized the non-linear dynamics using the Landau-Lifshitz-Gilbert equation but emphasize the significant increase in the effective damping constant compared to FMR. The excitation of a broad spectrum of spin waves is identified as the cause of this enhanced damping. The role of magnetoelastic interactions, while recognized as a source of nonlinearity, has not been fully investigated in the context of large-angle precessional switching. Previous research on magnetoelastic interactions has primarily focused on lower amplitude oscillations and has not addressed the emergence of coherent patterns from chaotic dynamics, as observed in this study. This paper builds upon prior work by exploring the previously unknown effects of strong magnetoelastic coupling on large-amplitude magnetisation switching and the resultant pattern formation.

The researchers employed multi-scale pump-probe experiments on a 7.5 µm-thick lutetium- and bismuth-doped yttrium-iron-garnet (Lu:YIG) film. Single infrared pulses from a cavity-dumped free-electron laser were used to resonantly pump optical phonons, dynamically altering the material's structure and inducing a magnetic anisotropy field. This field drives large-amplitude magnetisation dynamics and switching. The subsequent magnetisation dynamics were probed magneto-optically using defocused pulses of wavelength 532 nm and duration 5 ns, providing a temporal resolution of 5 ns and a spatial resolution of approximately 2 µm. Single-shot pump-probe measurements varied the time delay between pump and probe pulses to capture the temporal evolution of the pattern formation. The experiments were conducted at the FELIX free-electron laser facility in the Netherlands. Micromagnetic simulations using the MuMax3 program complemented the experimental data, incorporating the magnetoelastic interaction to model the observed magnetisation dynamics. The simulations used literature values for Lu:YIG physical constants, including exchange constant and saturation magnetisation. A biaxial anisotropy was implemented, and an absorbing boundary region was created to simulate realistic conditions. Strain was introduced in the simulation, mimicking the strain induced by the infrared laser pulse.

The experiments revealed the formation of a distinct four-domain pattern emanating from the center of the irradiated region, characterized by twofold rotational symmetry. This pattern is successfully reproduced by micromagnetic simulations incorporating magnetoelastic interaction, confirming the role of phononic excitation in driving the magnetisation switching. Time-resolved imaging revealed the emergence and outward propagation of triangular domains, accompanied by a surprising periodic ripple pattern with a wavelength (approximately 10 µm) much smaller than the excitation spot size (approximately 300 µm). The ripples decay within approximately 300 ns. The researchers rule out Faraday waves as the cause of the ripple pattern and propose that it originates from the strong coupling between magnons and phonons, leading to magnon-polaron formation. The observed periodicity of the ripples aligns well with the estimated magnon-polaron wavelength based on the sample's ferromagnetic resonance frequency and sound velocities, suggesting that magnon-polarons condense and phase-synchronise, causing complete magnetisation reversal at their maxima. This process is compared to neural entrainment, whereby brainwaves synchronise to external stimuli. Fourier analysis of the ripple patterns shows a peak in the wavenumber space matching the magnon-polaron wavelength.

This study provides compelling evidence for a novel mechanism of magnetisation reversal driven by the self-organisation and phase-synchronisation of magnon-polarons. The emergence of a highly ordered pattern from initially chaotic spin-wave instabilities highlights the profound influence of magnetoelastic coupling at short timescales. The observed magnon-polaron condensation and the analogy to neural entrainment open up exciting possibilities for understanding wave synchronization in different scientific disciplines. The findings challenge conventional understanding of chaotic magnetisation dynamics and suggest new avenues for designing energy-efficient magnetic devices.

This research demonstrates a new mechanism for magnetisation reversal driven by the coherent dynamics of magnon-polarons resulting from the interplay between spin-wave instabilities and magnetoelastic interactions. The observation of a highly ordered pattern originating from initially chaotic dynamics provides important insights into the dynamics of both magnetisation switching and wave synchronisation. Further research exploring sub-nanosecond timescales is needed to fully elucidate the intricate details of magnon-polaron formation and their role in magnetic switching.

The temporal resolution of the experiment is limited to 5 ns by the probe pulse duration, potentially missing crucial details of magnon-polaron formation at sub-nanosecond timescales. The study focuses on a specific material (Lu:YIG), and the generalizability of the findings to other magnetic materials requires further investigation. The simulations used literature values for material parameters, and small variations in these values might influence the simulation results.