Franz Vilsmeier

Inverse design—specifying a desired functionality and letting a computational algorithm search for a structure that meets it—has emerged as a powerful paradigm for magnonic device engineering. In this article, we survey the rapidly growing field of inverse-design magnonics, organising it along two axes: the design variables (topology, material parameters, and magnetic field landscape) and the algorithmic toolbox (gradient-free, gradient-based, and neural-network-based methods) together with the differentiable micromagnetic solvers that enable them. We then identify open frontiers that we consider most promising for the next phase of the field: sensitivity analysis and robust design to bridge the gap between simulation and experiment; input shaping and transducer optimisation; the incorporation of nonlinear spin-wave effects as an explicit design resource; spatially structured amplification; self-adapting media and machine-learning-based design; and the long-term vision of a universal, reconfigurable magnonic platform. We argue that magnonics and artificial intelligence (AI) are converging from two directions—machine-learning tools for designing magnonic devices, and magnonic devices as hardware for neuromorphic computation—and propose the term AI magnonics to describe this emerging paradigm.

Antiferromagnets offer a promising pathway toward robust and ultrafast spintronic devices due to their intrinsically zero net magnetic moment and exchangeenhanced spin dynamics. Here, we demonstrate a spin-orbit torque (SOT)-driven oscillator based on a nanoconstriction patterned from a synthetic antiferromagnet (SAF). Spin rectification measurements reveal SOT-driven excitations of both optical and acoustic modes, along with additional resonances that emerge above a threshold DC current near the spin-flop transition. These resonances originate from self-sustained oscillations whose chirality is determined by the polarity of the driving DC current, and which can injection-lock to the detection frequency. Macrospin and micromagnetic simulations corroborate the existence of these chiral self-oscillations and predict chaotic dynamics near the spin-flop regime, experimentally indicated as the sudden onset of multiple oscillation modes above a DC threshold current.

We experimentally investigate frequency-selective spin wave (SW) transmission in a micrometer-scale, ring-shaped magnonic resonator integrated with a linear yttrium iron garnet stripe. Using super-Nyquist-sampling magneto-optical Kerr effect (SNS-MOKE) microscopyand micro-focused Brillouin light scattering (μ-BLS), we probe SW dynamics in the dipolar regime under in-plane magnetization. Spatially resolved measurements reveal a sharp transmission peak at 3.92 GHz for an external field of 74 mT, demonstrating strong frequency selectivity. This behavior arises from interference and scattering among multiple SW modes in the ring, shaped by anisotropic dispersion, geometric confinement, and local magnetic field inhomogeneities. Caustic-like propagation further limits outcoupling due to fixed group velocity directions. Fourier analysis reveals discrete wavevector components consistent with quantized ring eigenmodes. μ-BLS measurements at 70 mT show a shift of the transmission peak, demonstrating tunability via the external magnetic field.

Spin waves, the fundamental excitations in magnetic materials, are promising candidates for realizing low-dissipation information processing in spintronics. The ability to visualize and manipulate coherent spin-wave transport is crucial for the development of spin wave-based devices. We use a recently discovered method utilizing nitrogen vacancy (NV) centers, point defects in the diamond lattice, to measure spin waves in thin film magnetic insulators by detecting their magnetic stray field. We experimentally demonstrate enhanced contrast in the detected wavefront amplitudes by imaging spin waves underneath a reference stripline and phenomenologically model the results. By extracting the spin wave dispersion and comparing NV center based spin wave measurements to spin wave imaging conducted through the well-established time-resolved magneto-optical Kerr effect, we discuss the advantages and limitations of employing NV centers as spin wave sensors.

As one of the fundamental magnonic devices, magnonic splitter devices have been proposed and spin wave propagation in the devices has been studied numerically and experimentally. In the present work, we fabricated a T-shaped magnonic splitter with three 6 μm-wide arms from a 100 nm-thick yttrium iron garnet film. Using time-resolved magneto-optic Kerr microscopy, we observed spin waves splitting into both, the vertical and the horizontal direction at the junction. Analyzing the results, we found that the spin wave width modes are converted into each other during the splitting process and, consequently, concluded that the width mode conversion efficiency depends on the ratio of wavelength to the vertical branch's width and the anisotropic distribution of the spin wave amplitude at the T-junction.

Spin-wave (SW) propagation close to the hybridization-induced transmission stop band is investigated within a trapezoid-shaped 200 nm thick yttrium iron garnet film using time-resolved magneto-optic Kerr effect microscopy and broadband spin-wave spectroscopy, supported by micromagnetic simulations. The gradual reduction of the effective field within the structure leads to local variations of the SW dispersion relation and results in a SW hybridization at a fixed position in the trapezoid where the propagation vanishes, since the SW group velocity approaches zero. By tuning external field or frequency, spatial control of the spatial stop band position and spin-wave propagation is demonstrated and utilized to gain transmission control over several microstrip lines.

In the context of wave propagation, caustics are usually defined as the envelope of a finite-extent wavefront; folds and cusps in a caustic result in enhanced wave amplitudes. Here, we tackle a related phenomenon, namely, the existence of well-defined beams originating solely from the geometric properties of the corresponding dispersion relation. This directional emission, termed caustic beam, is enabled by a stationary group velocity direction, and has been observed first in the case of phonons. We propose an overview of this “focusing” effect in the context of spin waves excited in soft, thin ferromagnetic films. Based on an analytical dispersion relation, we provide tools for a systematic survey of caustic spin wave beams. Our theoretical approach is validated by time-resolved microscopy experiments using the magneto-optical Kerr effect. Then, we identify two cases of particular interest both from fundamental and applicative perspectives. Indeed, both of them enable broadband excitations (in terms of wave vectors) to result in narrow-band beams of low divergence.