The magnetic relaxation processes following the dynamical excitation of the spin system of ferromagnets are investigated by ferromagnetic resonance (FMR) between 1 and $70\mathrm{GHz}$ using epitaxial ${\mathrm{Fe}}_3\mathrm{Si}$ films as a prototype system. Two relaxation channels, i.e., dissipative, isotropic Gilbert damping $G$ as well as anisotropic two-magnon scattering $\Gamma$, are simultaneously identified by frequency and angle dependent FMR and quantitatively analyzed. The scattering rates due to two-magnon scattering at crystallographic defects for spin waves propagating in ⟨100⟩ and ⟨110⟩ directions, $\gamma {\Gamma }_{⟨100⟩}=0.25\left(2\right)\mathrm{GHz}$ and $\gamma {\Gamma }_{⟨110⟩}=0.04\left(2\right)\mathrm{GHz}$, and the Gilbert damping term $G=0.051\left(1\right)\mathrm{GHz}$ are determined. We show that changing the film thickness from $8\text{to}40\mathrm{nm}$ and slightly modifying the Fe concentration influence the relaxation channels. Our results, which reveal the contributions of longitudinal and transverse relaxation processes may be of general importance for the understanding of spin-wave dynamics in magnetic structures.

Ferromagnetic resonance experiments of magnetic nanostructures over a large frequency range from $1\text{to}225\mathrm{GHz}$ are presented. We find unambiguous evidence for a nonlinear frequency dependence of the linewidth. The viscous Gilbert damping and two-magnon scattering are clearly separated. Both angular and frequency dependent measurements give a transverse scattering rate within the magnetic subsystem of the order of ${10}^9{\mathrm{s}}^{-1}$, whereas the longitudinal Gilbert relaxation into the thermal bath is one to two orders of magnitude smaller.

Frequency nonreciprocity of wave phenomena describes the situation where the wave dispersion depends on the sign of the wave vector, i.e., counterpropagating waves exhibit different wavelengths for the same frequency. Such behavior has recently been observed in heavy-metal–ferromagnetic interfaces with Dzyaloshinskii-Moriya coupling, and is also known for coupled magnetic bilayers, where the nonreciprocity is enhanced when the two layers are aligned antiparallel. Besides the conventional uses of spin waves, nonreciprocity adds further functionalities, such as its potential applications in communication technologies and logic operations. In the current paper, we thus examine the spin-wave nonreciprocity induced by dipolar interactions in a coupled bilayer consisting of two ferromagnetic layers separated by a nonmagnetic spacer. We derive an easy-to-use formula to estimate the frequency difference provided by the nonreciprocity, which allows one to choose an optimal system in order to maximize the effect. For small wave numbers, the nonreciprocity scales linearly, while for larger wave vectors the nonreciprocity behaves nonmonotonically, with a well-defined maximum. The study is carried out by means of analytical calculations that are complemented by micromagnetic simulations. Furthermore, we confirm our model by experimental investigation of the spin-wave dispersion in a prototype antiparallel-coupled bilayer system. Since the relative magnetic orientation can be controlled through a bias field, the magnon nonreciprocity can then be turned on and off, which lends an important functionality to the coupled ferromagnetic bilayers.

The field of magnon spintronics is experiencing an increasing interest in the development of solutions for spin-wave-based data transport and processing technologies that are complementary or alternative to modern complementary metal-oxide semiconductor architectures. Nanometer-thin yttrium iron garnet (YIG) films have been the gold standard for insulator-based spintronics to date, but a potential process technology that can deliver perfect, homogeneous large-diameter films is still lacking. We report that liquid phase epitaxy (LPE) enables the deposition of nanometer-thin YIG films with low ferromagnetic resonance losses and consistently high magnetic quality down to a thickness of 20 nm. The obtained epitaxial films are characterized by an ideal stoichiometry and perfect film lattices, which show neither significant compositional strain nor geometric mosaicity, but sharp interfaces. Their magnetostatic and dynamic behavior is similar to that of single crystalline bulk YIG. We found that the in-plane Gilbert damping coefficient ${\alpha }_{\left|\right|}$ is independent of the film thickness and close to $1\times {10}^{-4}$, while the out-of-plane coefficient ${\alpha }_{\perp }$ increases with decreasing thickness. Together with an inhomogeneous peak-to-peak linewidth broadening of $\mathrm{\Delta }{H}_{0\left|\right|}=0.4\mathrm{G}$, these values are among the lowest ever reported for YIG films with a thickness smaller than 40 nm. These results suggest that nanometer-thin LPE films can be used to fabricate nano- and microscaled circuits with the required quality for magnonic devices. The LPE technique is easily scalable to YIG sample diameters of several inches.

Energy loss due to ohmic heating is a major bottleneck limiting down-scaling and speed of nano-electronic devices, and harvesting ohmic heat for signal processing is a major challenge in modern electronics. Here, we demonstrate that thermal gradients arising from ohmic heating can be utilized for excitation of coherent auto-oscillations of magnetization and for generation of tunable microwave signals. The heat-driven dynamics is observed in Y₃Fe₅O₁₂/Pt bilayer nanowires where ohmic heating of the Pt layer results in injection of pure spin current into the Y₃Fe₅O₁₂ layer. This leads to excitation of auto-oscillations of the Y₃Fe₅O₁₂ magnetization and generation of coherent microwave radiation. Our work paves the way towards spin caloritronic devices for microwave and magnonic applications.