Thibaut Devolder

We present a mechanism for self-induced Floquet states involving acoustic and optical modes in synthetic antiferromagnets. By driving optical modes off-resonantly with radio frequency fields in the canted antiferromagnetic state, limit cycles arising from the predator-prey dynamics of the acoustic and optical mode populations can appear. The cyclic growth and decay of these mode populations induce a time-periodic modulation of the canted state, which subsequently generates Floquet states. These states appear as a rich frequency comb in the power spectrum of magnetization oscillations.

Forecasting complex, chaotic signals is a central challenge across science and technology, with implications ranging from secure communications to climate modeling. Here we demonstrate that magnons, the collective spin excitations in magnetically ordered materials, can serve as an efficient physical reservoir for predicting such dynamics. Using a magnetic vortex-state microdisk as a magnon-scattering reservoir, we show that intrinsic nonlinear interactions transform a simple microwave input into a high-dimensional spectral output suitable for time-series predictions. Trained on the Mackey-Glass benchmark, which generates a cyclic yet aperiodic time series, the system achieves accurate and reliable predictions that rival other state-of-the-art physical reservoirs. We further identify key design principles: spectral resolution governs the tradeoff between dimensionality and accuracy, while combining multiple device geometries systematically improves performance. These results establish magnonics as a promising platform for unconventional computing, offering a path toward scalable and CMOS-compatible hardware for real-time prediction tasks.

Driving condensed matter systems with periodic electromagnetic fields can result in exotic states not found in equilibrium. Termed Floquet engineering, such periodic driving applied to electronic systems can induce topological band structures and control spin interactions. In this study, we present a class of Floquet states in a magnetic vortex that arise from nonlinear interactions between the vortex core and microwave magnons. Floquet bands emerge through the periodic oscillation of the core, which can be initiated by either driving the core directly or pumping azimuthal magnon modes. For the latter, the azimuthal modes induce core gyration through nonlinear interactions, which in turn renormalizes the magnon band structure. This represents a self-induced mechanism for Floquet band engineering and opens avenues to study and control nonlinear magnon dynamics.

CoFeB alloy is a ferromagnetic material that is often considered to produce synthetic antiferromagnets (SAF) used in spintronic devices. SAF host spin waves with high tunability, making their coupling with surface acoustic waves (SAWs) a promising avenue for next-generation communication devices. This coupling offers potential advantages such as SAW tunability and non-reciprocal operation in the multi-gigahertz range. Epitaxial lithium niobate (LN) thin films on sapphire substrates have emerged as a promising solution for achieving high SAW phase velocities. The combination of LN thin films with SAF, for instance based on CoFeB-based multilayers, hence offers new possibilities for engineering acoustic wave propagation. As the elastic properties of CoFeB depend on the composition of the alloy, their determination is a requisite to evaluate SAW dispersion. We investigate elastic wave dispersion in Co₄₀Fe₄₀B₂₀-based multilayers on LN/ZX-sapphire, ZX-LN, and SiO₂/Si substrates, combining surface Brillouin light spectroscopy and finite element computations. Surface-guided phonons ranging from 9 to 18 GHz are observed, for a wavelength of about 300 nm, and the elastic constants of Co₄₀Fe₄₀B₂₀ are estimated from them.

We report experiments using magnetic resonance force microscopy to investigate the dynamics of magnetic vortices in submicrometer CoFeB disks grown on a piezoelectric substrate. We compare the vortex gyration excited inductively by microwave magnetic fields and through magnetoelastic coupling via surface acoustic waves (SAWs). Based on the device geometry and modeling performed, we find that the dominant mechanism for SAW-driven excitation is through lattice rotations, which generate an effective field localized at the vortex core. This contrasts with the magnetoelastic strains typically assumed in similar experiments. Moreover, we demonstrate that this magnetorotation torque can be tuned by applying a perpendicular magnetic field. Unlike spin wave excitations, nontrivial spin textures, such as vortices, effectively couple to magnetorotation torques, making them valuable probes for studying such phenomena.

We study the symmetry of the coupling between surface acoustic waves (SAWs) and ferromagnetic resonance in a thin magnetic film of CoFeB deposited on top of a piezoelectric Z-cut LiNbO3 substrate. We vary the orientation of the applied magnetic field with respect to the wavevector of the SAW. Experiments indicate an unexpected twofold symmetry of the absorption of the SAW energy by the magnetic film. We discuss whether this symmetry can arise from the magnetoelastic torque of the longitudinal strain and the magnetic susceptibility of ferromagnetic resonance. We find that one origin of the twofold symmetry can be the weak in-plane uniaxial anisotropy present within the magnetic film. This phenomenon adds to the previously identified other source of twofold symmetry but shall persist for ultrathin films when the dipolar interactions cease to contribute to the anisotropy of the slope of the spin wave dispersion relation.