The concept of space-time crystals (STC), i.e., translational symmetry breaking in time and space, was recently proposed and experimentally demonstrated for quantum systems. Here, we transfer this concept to magnons and experimentally demonstrate a driven STC at room temperature. The STC is realized by strong homogeneous microwave pumping of a micron-sized permalloy (Py) stripe and is directly imaged by scanning transmission x-ray microscopy (STXM). For a fundamental understanding of the formation of the STC, micromagnetic simulations are carefully adapted to model the experimental findings. Beyond the mere generation of a STC, we observe the formation of a magnonic band structure due to back folding of modes at the STC’s Brillouin zone boundaries. We show interactions of magnons with the STC that appear as lattice scattering, which results in the generation of ultrashort spin waves (SW) down to 100-nm wavelengths that cannot be described by classical dispersion relations for linear SW excitation. We expect that room-temperature STCs will be useful to investigate nonlinear wave physics, as they can be easily generated and manipulated to control their spatial and temporal band structures.

Magnonic quasicrystals exceed the possibilities of spin-wave (SW) manipulation offered by regular magnonic crystals, because of their more complex SW spectra with fractal characteristics. Here, we report the direct x-ray microscopic observation of propagating SWs in a magnonic quasicrystal, consisting of dipolar coupled permalloy nanowires arranged in a one-dimensional Fibonacci sequence. SWs from the first and second band as well as evanescent waves from the band gap between them are imaged. Moreover, additional mini band gaps in the spectrum are demonstrated, directly indicating an influence of the quasiperiodicity of the system. Finally, the localization of SW modes within the Fibonacci crystal is shown. The experimental results are interpreted using numerical calculations and we deduce a simple model to estimate the frequency position of the magnonic gaps in quasiperiodic structures. The demonstrated features of SW spectra in one-dimensional magnonic quasicrystals allow utilizing this class of metamaterials for magnonics and make them an ideal basis for future applications.

The magnetization dynamics in $\mathrm{W}/{\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$, ${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}/\mathrm{Pt}$ and $\mathrm{W}/{\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}/\mathrm{Pt}$ multilayers are investigated using spin-orbit-torque ferromagnetic resonance (SOT FMR) technique. An analytical model based on magnetization dynamics due to SOT is used to fit the heavy-metal (HM) thickness dependence of the symmetric and antisymmetric components of the SOT FMR signal. The analysis results in a determination of the properties of the HM layers, such as the spin Hall angle and the spin diffusion length. Spin Hall angles of $-0.36$ and 0.09 are found in the $\mathrm{W}/{\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$ and ${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}/\mathrm{Pt}$ bilayers, respectively, which add up in the case of a $\mathrm{W}/{\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}/\mathrm{Pt}$ trilayer. More importantly, we determine the effective interfacial spin-orbit fields at both the $\mathrm{W}/{\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$ and ${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}/\mathrm{Pt}$ interfaces, which are shown to cancel the Oersted field for particular thicknesses of the heavy-metal layers, leading to pure spin-current-induced dynamics and indicating the possibility of more efficient magnetization switching.

Regularly arranged magnetic out-of-plane patterns in continuous and flat films are promising for applications in data storage technology (bit patterned media) or transport of individual magnetic particles. Whereas topographic magnetic structures are fabricated by standard lithographical techniques, the fabrication of regularly arranged artificial domains in topographically flat films is difficult, since the free energy minimization determines the existence, shape, and regularity of domains. Here we show that keV He(+) ion bombardment of Au/Co/Au layer systems through a colloidal mask of hexagonally arranged spherical polystyrene beads enables magnetic patterning of regularly arranged cylindrical magnetic monodomains with out-of-plane magnetization embedded in a ferromagnetic matrix with easy-plane anisotropy. This colloidal domain lithography creates artificial domains via periodic lateral anisotropy variations induced by periodic defect density modulations. Magnetization reversal of the layer system observed by magnetic force microscopy shows individual disc switching indicating monodomain states.

We show that it is possible to engineer magnetic multidomain configurations without domain walls in a prototypical rare-earth–transition-metal ferrimagnet using keV ${\mathrm{He}}^{+}$ ion bombardment. We additionally show that these patterns display a particularly stable magnetic configuration due to a deep minimum in the energy of the system caused by flux closure and a corresponding reduction of the magnetostatic energy without an increase in energy by exchange and anisotropy terms across the walls. This occurs because light-ion bombardment affects an element’s relative contribution to the properties of the ferrimagnet differently. Therefore, it is possible to control the relative contribution from each magnetic subsystem. The selection of material and the use of light-ion bombardment allow us to engineer domain patterns in continuous magnetic films, which open a way to fabricate them in a much smaller scale than currently possible. Our Letter emphasizes that the right criterion to determine the presence or absence of a domain wall is whether there is a rotation of the spin for each sublattice and that changes of the direction of effective magnetization alone do not constitute an appropriate criterion.

Magnonic crystals are systems that can be used to design and tune the dynamic properties of magnetization. Here, we focus on one-dimensional Fibonacci magnonic quasicrystals. We confirm the existence of collective spin waves propagating through the structure as well as dispersionless modes; the reprogammability of the resonance frequencies, dependent on the magnetization order; and dynamic spin-wave interactions. With the fundamental understanding of these properties, we lay a foundation for the scalable and advanced design of spin-wave band structures for spintronic, microwave, and magnonic applications.