Maciej Krawczyk

We investigate spin-wave transport in hybrid two-dimensional magnonic crystals comprising a low-damping yttrium iron garnet (YIG) film coupled to a periodic array of CoFeB nanodisks. Using propagating spin-wave spectroscopy, super-Nyquist magneto-optical Kerr effect microscopy, and micromagnetic simulations, we show that these systems host pronounced, tunable bandgaps that do not originate from conventional Bragg scattering, i.e., from standing-wave quantization restricted to a single axis by translational invariance along the propagation direction. Instead, the gaps arise from hybridization between the fundamental magnonic-crystal mode and in-plane transverse standing modes of the nanodisk array. Their spectral position and width are controlled by the array geometry and by the magnetic state of the nanodisks, including their vortex configuration, which sets both the static and dynamic dipolar coupling to the film. At larger lattice periods, additional gaps emerge from hybridization with modes quantized both transverse and parallel to the propagation direction, reflecting two-dimensional dispersion folding. Our results establish mode hybridization as a versatile mechanism for engineering spin-wave band structures beyond the constraints of conventional Bragg scattering and provide a pathway toward reconfigurable magnonic devices based on dipolar-coupled hybrid architectures.

Recent studies on spin-wave propagation in ferromagnetic waveguides have highlighted the role of nonreciprocity resulting from the chiral nature of dipolar interactions in curved elements. However, the impact of spin-wave mode type on nonreciprocity remains unexplored. Using micromagnetic simulations supported by analytical modeling, we systematically analyzed the propagation of edge, fundamental, and width-quantized spin-wave modes in a ferromagnetic nanowire with a crescent-shaped cross section. Our results show that the strength and sign of nonreciprocity depend on the mode type, as well as on the curvature magnitude of the nanowire’s top and bottom surfaces and the strength of the external magnetic field. Interestingly, changing the mode type, for instance, induced by altering the curvature or magnetic field, results in a significant change in the dispersion relation asymmetry. This effect underscores the important role of spin-wave profiles in nonreciprocity, deepens our fundamental understanding of spin-wave dynamics in curved geometries, and paves the way for designing magnonic waveguides with tailored properties.

Three-wave scattering is a fascinating phenomenon with many applications in various technologies. Reducing the system symmetry greatly affects three-wave scattering, which, in this case, goes beyond the simple momentum conservation law. In this study, we examine three-magnon scattering at the edge of a thin ferromagnetic film, when a bulk spin wave interacts with an edge-localized propagating spin wave upon the reflection. This creates new bulk spin waves at mixed frequencies by means of three-magnon confluence or stimulated splitting processes. Using our developed analytical theory, which has been confirmed by full micromagnetic simulations, we demonstrate that the amplitude of the wave generated in the stimulated splitting process is several times larger than that generated in the confluence process. This is primarily due to the lower group velocity of the wave formed in the splitting process than in confluence. Furthermore, the intensity of inelastically scattered waves exhibits a pronounced dependence on the incidence angle and frequency of the edge spin wave that goes beyond existing qualitative models. We show that the observed behaviors can only be explained by taking into account that the scattered waves are created by several elementary three-magnon processes involving the incident and reflected waves. The complex nature of the scattered wave creation results in a strong sensitivity of its amplitude to the phase accumulation of spin waves upon reflection.

Magnetic media with artificial periodic modulation—magnonic crystals (MCs)—enable tunable spin-wave dynamics and band structure engineering. Nanoscaling enhances these capabilities, making magnonic nanocrystals promising for both fundamental studies and applications. Here, we report on the design, fabrication, and characterization of one-dimensional YIG MCs with nanoholes (d≈150 nm) spaced a≈1 μm apart. Microfocused Brillouin light scattering and propagating spin-wave spectroscopy, supported by TetraX and MuMax3 simulations, reveal spin-wave transmission over 5 μm in the Damon–Eshbach configuration and the formation of pronounced bandgaps with rejection levels up to 26 dB. Detailed analysis of the spin-wave dispersion uncovered complex mode interactions, including two prominent anticrossings at 3.1 and 18.7 rad/μm, between which the spin-wave energy is predominantly carried by the n=2 mode, enabling efficient transmission. The results advance the development of functional MCs and open pathways toward 2D magnonic nanoarrays and magnonic RF nanodevices.

We demonstrate a novel approach to controlling and stabilizing magnetic skyrmions in ultrathin multilayer nanostructures through spatially engineered magnetostatic fields generated by ferromagnetic nanorings. Using analytical modeling and micromagnetic simulations, we show that the stray fields from a Co/Pd ferromagnetic ring with out-of-plane magnetic anisotropy significantly enhance the Néel-type skyrmion stability in an Ir/Co/Pt nanodot, even stabilizing the skyrmion in the absence of Dzyaloshinskii-Moriya interactions. We demonstrate precise control over the skyrmion size and stability. We observe a multistability phenomenon, where the skyrmion can be stabilized at two or more distinct equilibrium diameters depending on the ring's magnetization orientation. These stable states exhibit energy barriers that substantially exceed thermal fluctuations at room temperature, suggesting potential applications in robust multibit memory storage. Furthermore, we demonstrate that a skyrmion can be switched between two metastable states using a suitably designed nanosecond magnetic field pulse. Our findings pave the way for advanced spintronic nanodevices.

Materials with perpendicular magnetic anisotropy (PMA) and antisymmetric exchange interactions are widely explored in spintronics but are of limited use in magnonics due to high damping. We present a hybrid magnonic crystal composed of a chain of circular nanodots with strong PMA and Dzyaloshinskii–Moriya interaction (DMI), positioned above a spin-wave waveguide made of permalloy. Due to the dipolar coupling between the subsystems, a strongly bound hybrid magnetization texture is formed, with two stable magnetization states in the nanodots: a single-domain state and an egg-shaped skyrmion state, allowing reprogramming of the system properties. Numerical results show complex spin-wave spectra with several key features for magnonics: programmable Bragg and non-Bragg bandgaps correlated with magnon–magnon couplings, the flat bands and bound states for the skyrmion state, and exclusively waveguide-dominated modes for the single-domain state. With these properties, the proposed hybrid magnonic crystal has different functionalities that overcome the damping limitations of materials with PMA and DMI and open up potential applications in spin-wave filtering, spin-wave generation, quantum magnonics, and analog magnonics, in particular in the realization of magnonic neural networks.