The existence of backscattering-immune spin-wave modes is demonstrated in magnetic thin films of nanoscale thickness. Our results reveal that chiral magnetostatic surface waves (CMSSWs), which ...propagate perpendicular to the magnetization direction in an in-plane magnetized thin film, are robust against backscattering from surface defects. CMSSWs are protected against various types of surface inhomogeneities and defects as long as their frequency lies inside the gap of the volume modes. Our explanation is independent of the topology of the modes and predicts that this robustness is a consequence of symmetry breaking of the dynamic magnetic fields of CMSSWs due to the off-diagonal part of the dipolar interaction tensor, which is present both for long- (dipole-dominated) and short-wavelength (exchange-dominated) spin waves. Micromagnetic simulations confirm the robust character of the CMSSWs. Our results open a new direction in designing highly efficient magnonic logic elements and devices employing CMSSWs in nanoscale thin films.
We present a combined numerical, theoretical, and experimental study on stimulated three-magnon splitting in a magnetic disk in the vortex state. Our micromagnetic simulations and ...Brillouin-light-scattering results confirm that three-magnon splitting can be triggered even below threshold by exciting one of the secondary modes by magnons propagating in a waveguide next to the disk. The experiments show that stimulation is possible over an extended range of excitation powers and a wide range of frequencies around the eigenfrequencies of the secondary modes. Rate-equation calculations predict an instantaneous response to stimulation and the possibility to prematurely trigger three-magnon splitting even above threshold in a sustainable manner. These predictions are confirmed experimentally using time-resolved Brillouin-light-scattering measurements and are in a good qualitative agreement with the theoretical results. We believe that the controllable mechanism of stimulated three-magnon splitting could provide a possibility to utilize magnon-based nonlinear networks as hardware for neuromorphic computing.
We present the generation of whispering gallery magnons with unprecedented high wave vectors via nonlinear 3-magnon scattering in a μm-sized magnetic Ni_{81}Fe_{19} disc which is in the vortex state. ...These modes exhibit a strong localization at the perimeter of the disc and practically zero amplitude in an extended area around the vortex core. They originate from the splitting of the fundamental radial magnon modes, which can be resonantly excited in a vortex texture by an out-of-plane microwave field. We shed light on the basics of this nonlinear scattering mechanism from an experimental and theoretical point of view. Using Brillouin light scattering microscopy, we investigated the frequency and power dependence of the 3-magnon splitting. The spatially resolved mode profiles give evidence for the localization at the boundaries of the disc and allow for a direct determination of the modes wave number.
Spin waves are investigated in yttrium iron garnet waveguides with a thickness of 39 nm and widths ranging down to 50 nm, i.e., with an aspect ratio thickness over width approaching unity, using ...Brillouin light scattering spectroscopy. The experimental results are verified by a semianalytical theory and micromagnetic simulations. A critical width is found, below which the exchange interaction suppresses the dipolar pinning phenomenon. This changes the quantization criterion for the spin-wave eigenmodes and results in a pronounced modification of the spin-wave characteristics. The presented semianalytical theory allows for the calculation of spin-wave mode profiles and dispersion relations in nanostructures.
Spin-Hall oscillators (SHO) are promising sources of spin-wave signals for magnonics applications, and can serve as building blocks for magnonic logic in ultralow power computation devices. Thin ...magnetic layers used as "free" layers in SHO are in contact with heavy metals having large spin-orbital interaction, and, therefore, could be subject to the spin-Hall effect (SHE) and the interfacial Dzyaloshinskii-Moriya interaction (i-DMI), which may lead to the nonreciprocity of the excited spin waves and other unusual effects. Here, we analytically and micromagnetically study magnetization dynamics excited in an SHO with oblique magnetization when the SHE and i-DMI act simultaneously. Our key results are: (i) excitation of nonreciprocal spin-waves propagating perpendicularly to the in-plane projection of the static magnetization; (ii) skyrmions generation by pure spin-current; (iii) excitation of a new spin-wave mode with a spiral spatial profile originating from a gyrotropic rotation of a dynamical skyrmion. These results demonstrate that SHOs can be used as generators of magnetic skyrmions and different types of propagating spin-waves for magnetic data storage and signal processing applications.
A two-dimensional analytical model for the description of the excitation of nonreciprocal spin waves by spin current in spin Hall oscillators in the presence of the interfacial Dzyaloshinskii-Moriya ...interaction (i-DMI) is developed. The theory allows one to calculate the threshold current for the excitation of spin waves, as well as the frequencies and spatial profiles of the excited spin-wave modes. It is found that the frequency of the excited spin waves exhibits a quadratic redshift with the i-DMI strength. At the same time, in the range of small and moderate values of the i-DMI constant, the averaged wave number of the excited spin waves is almost independent of the i-DMI, which results in a rather weak dependence on the i-DMI of the threshold current of the spin-wave excitation. The obtained analytical results are confirmed by the results of micromagnetic simulations.
Static magnetization configurations of thin soft ferromagnetic films and nanodots, coupled to a hard antidot matrix with out-of-plane magnetization, are studied by micromagnetic simulations and ...analytical calculations. When the antidot matrix produces sufficient stray fields, having radial symmetry, these nanostructures support the formation of topologically nontrivial magnetic configurations-vortices and skyrmions in nanodots and films, respectively. It is demonstrated that the studied nanostructure reveals an additional degree of freedom-the helicity of the vortex or skyrmion-which can be tuned on demand by a variation of the material parameters and geometry. The variation of helicity γ is not abrupt. In addition to Neel-like (radial) vortices and skyrmions (γ=0,π), it is possible to achieve unconventional configurations with an intermediate helicity γ≠0,±π/2,π, which transform to common Bloch-like configurations (γ=±π/2) in the limit of negligible stray fields from the matrix. We present an analytical model, which allows us to calculate the stability region of pure Neel-like states, outside which unconventional magnetization states with intermediate helicity are realized.
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