Focused Ion Beam Technique Steers Spin Waves Via Lattice Distortions

Researchers are increasingly investigating spin waves as potential information carriers for next-generation wave-based computing, necessitating the creation of meticulously designed magnetic landscapes. Felix Naunheimer, Johannes Greil, and Valentin Ahrens, working at the TUM School of Computation, Information and Technology, Technical University of Munich, in collaboration with Levente Maucha, Ádám Papp, György Csaba from the Faculty of Information Technology and Bionics, P azm any P eter Catholic University, and Markus Becherer from TUM, have now elucidated the fundamental mechanisms behind spin-wave routing achieved through focused ion beam (FIB) patterning. Their work demonstrates that FIB-induced steering of spin waves in yttrium iron garnet arises from magnetoelastic effects caused by lattice dislocations, providing a crucial step towards realising functional, programmable magnonic devices and establishing a physical basis for FIB-engineered graded-index landscapes.

Scientists are establishing a physical basis for engineering spin waves, quasiparticles representing collective excitations of a spin system, to carry information in future analogue computing systems. This work details how precisely controlled ion beam irradiation can sculpt landscapes for these waves, enabling compact and efficient signal processing. Researchers linked the ability to steer spin waves with subtle changes in the material’s crystal structure induced by irradiation, resolving a long-standing question about the underlying mechanisms. The team’s findings demonstrate that focused ion beam patterning modifies the spin-wave dispersion, how the wave’s speed varies with its energy, through magnetoelastic effects arising from lattice dislocations created during irradiation. Following focused ion beam irradiation of yttrium iron garnet (YIG) thin films and subsequent wet-chemical etching, the researchers employed a combination of advanced microscopy and simulation techniques to map the relationship between irradiation dose, structural changes, and spin-wave behaviour. Local height profiles were obtained using atomic force microscopy, providing geometric constraints for analysing spin-wave dispersion relations measured with time-resolved magneto-optical Kerr effect microscopy. By extending elastic deformation, plastic deformation, and partial amorphization of the material. This three-phase deformation scenario is supported by simulations using the SRIM software package, which models the interaction of ions with matter, and by micromagnetic simulations incorporating strain tensors derived from the experimental magnetoelastic field. The simulations accurately reproduce the observed non-monotonic dependence of spin-wave wavelength on ion dose, validating the proposed framework. These results pave the way for creating graded-index (GRIN) spin-wave landscapes, analogous to optical lenses, and ultimately, magnetoelastically programmable magnonic devices for specialised computing applications. Fitting dispersion relations with the Kalinikos, Slavin formalism, extended to include an explicit magnetoelastic field term, allowed for the extraction of key parameters related to strain accumulation and relaxation. The evolution of this magnetoelastic field revealed three distinct deformation regimes, elastic, plastic, and partial amorphization, directly explaining the observed non-monotonic dependence of spin-wave wavelength on ion dose. Specifically, the research demonstrates a non-monotonic relationship between ion dose and spin-wave wavelength, with wavelengths initially increasing, then decreasing as the dose is further increased. This behaviour is linked to the three deformation regimes identified, where the material responds differently to the ion beam. SRIM simulations, modelling the irradiation process, reproduced the trends observed in the extracted magnetoelastic field, validating the fitting approach and the proposed deformation scenario. The extracted magnetoelastic field values directly correlate with the degree of strain induced by the ion beam, providing a quantitative measure of the structural changes within the YIG film. Micromagnetic simulations, incorporating strain tensors derived from the experimentally determined magnetoelastic field, successfully reproduced the characteristic non-monotonic wavelength behaviour observed in the trMOKE measurements. These simulations confirm that the observed spin-wave steering is indeed driven by the strain induced by the FIB irradiation. The consistency between experimental measurements, SRIM simulations, and micromagnetic modelling establishes a robust physical basis for engineering graded-index spin-wave landscapes using FIB irradiation. This work provides a pathway towards creating magnetoelastically programmable magnonic devices with precisely controlled spin-wave propagation. A time-resolved magneto-optical Kerr effect (trMOKE) microscopy setup underpinned the investigation of spin-wave steering in yttrium iron garnet (YIG) films following gallium-ion irradiation. A 100nm YIG thin film was deposited onto a 500μm gadolinium gallium garnet (GGG) substrate via radio-frequency sputtering, subsequently undergoing annealing to achieve the desired crystalline structure. Spin-wave excitation was achieved using a 2μm wide titanium/gold microstrip line (MSL) fabricated by electron-beam evaporation, providing a defined pathway for wave propagation. Adjacent to this MSL, 50 × 50 μm2 squares were directly irradiated with gallium ions using a focused ion beam (FIB) system, employing doses ranging from 2 to 60 × 1012 ions/cm2 in 2 × 1012 ions/cm2 increments. To control the depth of ion penetration, the FIB operated at acceleration voltages of 30 keV, 16 keV, and 8 ke.
Spin-wave propagation was studied in a forward-volume spin-wave (FVSW) configuration, where waves excited in the unirradiated region couple into the irradiated areas, experiencing a change in wavenumber. The trMOKE microscopy allowed for analysis of spin-wave propagation across 30 distinct implantation regions, alongside measurements in the pristine film, enabling tracking of dispersion evolution as a function of ion dose. Measurements were performed at frequencies between 2.285GHz and 2.33GHz in 5MHz steps, with an 8 dBm input power and a 250 mT out-of-plane magnetic field. This precise control over excitation parameters facilitated accurate extraction of key parameters through fitting to the Kalinikos, Slavin model, a standard approach for describing spin-wave dispersion. To further validate the observed effects, SRIM Monte Carlo simulations were performed to model the damage profile induced by Ga+ ion irradiation. These simulations calculated energy loss within the YIG lattice, predicting the formation of defects and lattice disorder. The simulations used a 7◦ tilt to minimise channelling effects. The resulting depth-dependent damage profile was then used to construct a local damage density model. Micromagnetic simulations, incorporating strain tensors derived from the experimentally determined magnetoelastic field, were also conducted to reproduce the observed non-monotonic wavelength behaviour, confirming the magnetoelastic origin of the spin-wave steering. A magnetoelastic coupling constant of 3.48×105 J/m3 and a Poisson ratio of 0.29 were used in these simulations, alongside a saturation magnetisation of 130 kA/m, determined from ferromagnetic resonance (FMR) measurements. Scientists have long sought methods to sculpt spin waves, the wave-like propagation of magnetisation, with the precision needed for advanced information processing. The challenge lies in creating the equivalent of optical lenses or integrated circuits, but for magnons, the carriers of these spin waves. Existing techniques often lack the necessary finesse, or rely on bulky, complex fabrication processes. This work offers a significant step forward by demonstrating a surprisingly direct link between focused ion beam damage and the ability to engineer spin-wave behaviour. The revelation that lattice dislocations, created during ion beam irradiation, are the primary mechanism for steering spin waves is crucial. It moves beyond simply observing the effect to understanding why it happens, opening the door to predictable and repeatable device fabrication. While previous studies hinted at the potential of ion irradiation, the underlying physics remained obscure, hindering rational design. This research establishes a clear, experimentally validated framework, bridging the gap between observation and control. However, the technique isn’t without limitations. The induced damage, while controllable, still represents a perturbation to the material’s intrinsic properties. Achieving truly complex magnonic circuits will require precise control over the density and distribution of these dislocations, and further work is needed to minimise unwanted scattering losses. Moreover, the current understanding is largely confined to yttrium iron garnet; extending these principles to other materials remains an open question. Looking ahead, this work will likely spur a new wave of research into materials irradiation as a versatile tool for magnonics. Combining this technique with machine learning algorithms could enable the automated design of increasingly sophisticated spin-wave landscapes. The ultimate goal is a fully programmable magnonic platform, capable of adapting to diverse computational tasks, and this research provides a solid foundation for building it.