Magnonics is a budding research field in nanomagnetism and nanoscience that addresses the use of spin waves (magnons) to transmit, store, and process information. The rapid advancements of this field during last one decade in terms of upsurge in research papers, review articles, citations, proposals of devices as well as introduction of new sub-topics prompted us to present the first roadmap on magnonics. This is a collection of 22 sections written by leading experts in this field who review and discuss the current status besides presenting their vision of future perspectives. Today, the principal challenges in applied magnonics are the excitation of sub-100 nm wavelength magnons, their manipulation on the nanoscale and the creation of sub-micrometre devices using low-Gilbert damping magnetic materials and its interconnections to standard electronics. To this end, magnonics offers lower energy consumption, easier integrability and compatibility with CMOS structure, reprogrammability, shorter wavelength, smaller device features, anisotropic properties, negative group velocity, non-reciprocity and efficient tunability by various external stimuli to name a few. Hence, despite being a young research field, magnonics has come a long way since its early inception. This roadmap asserts a milestone for future emerging research directions in magnonics, and hopefully, it will inspire a series of exciting new articles on the same topic in the coming years.

Ferromagnetic resonance (FMR) was measured for Cu/permalloy (Py) (20, 30, 40 Å)/Cu ${(d}_{\mathrm{Cu}})/\mathrm{Pt}$ (0, 50 Å) films with various $d_{\mathrm{Cu}}$ to clarify the effect of spin diffusion driven by the precession of magnetization on Gilbert damping. The peak-to-peak linewidth $\Delta H_{\mathrm{pp}}$ of the FMR spectra for Cu/Py/Cu/Pt films was very large at $d_{\mathrm{Cu}}=0\mathrm{}\mathrm{\AA },$ and decreased remarkably at $d_{\mathrm{Cu}}=30\mathrm{}\mathrm{\AA }.$ Above $d_{\mathrm{Cu}}=30\mathrm{}\mathrm{\AA },$ it decreased gradually with increasing $d_{\mathrm{Cu}}$ in the anomalously wide range of $d_{\mathrm{Cu}}.$ The out-of-plane angular dependence of the FMR of Cu/Py(30 Å)/Cu ${(d}_{\mathrm{Cu}})/\mathrm{Pt}$ (0, 50 Å) films was measured and analyzed using a Landau-Lifshitz-Gilbert equation that took into account the local variation of the effective demagnetizing field. The Gilbert damping coefficient G obtained from the analysis for Cu/Py/Cu/Pt films was about twice as large as that for Cu/Py/Cu films even at $d_{\mathrm{Cu}}=100\mathrm{}\mathrm{\AA }$ and decreased gradually as $d_{\mathrm{Cu}}$ increased. At $d_{\mathrm{Cu}}=2000\mathrm{}–3000\mathrm{\AA },$ G for Cu/Py/Cu/Pt and Cu/Py/Cu films has the same value. We discussed the influence of spin diffusion driven by the precession of magnetization in FMR on G using a previously proposed model. The calculated G vs $d_{\mathrm{Cu}}$ fitted well to the experimental one, and the other features of the experimental results are well explained by the model.

The out-of-plane angular dependence of ferromagnetic resonance (FMR) was measured for NM/80NiFe(Py)/NM (NM=Cu, Ta, Pd and Pt) films with various Py, Cu and Ta thicknesses fabricated by magnetron sputtering. The out-of-plane angular dependences of FMR resonance field and linewidth were analyzed using Landau-Lifshitz-Gilbert equation taking account of broadening of linewidth due to magnetic inhomogeneities in a film. Magnetic inhomogeneities were assumed to be the fluctuation of magnitude and direction of the effective demagnetization field which contains both demagnetization and perpendicular anisotropy field for a film. The calculations of the angular variations of linewidth agreed with the experimental ones quantitatively. The fluctuations of magnitude and direction of the effective demagnetization field, which are represented as Δ(4πM _eff.) and Δθ _H , respectively, increased with decreasing Py thickness for all NM/Py/NM films. Δθ _H increased as the thicknesses of the buffer layers increased for Cu/Py(40 Å)/Cu films and was almost constant with increasing buffer layer thickness for Ta/Py(40 Å)/Ta films. Only in the case of NM=Pd and Pt films, the Gilbert damping parameter, which is the speed of decay of magnetization precession, was enhanced significantly as compared with that for the bulk sample and was dependent on Py thickness.

Spin precession with frequencies up to 280 GHz is observed in ${\mathrm{Mn}}_{3-\delta }\mathrm{Ga}$ alloy films with a perpendicular magnetic anisotropy constant $K_u\sim 15\mathrm{M}\mathrm{erg}/{\mathrm{cm}}^3$. The damping constant $\alpha$, characterizing macroscopic spin relaxation and being a key factor in spin-transfer-torque systems, is not larger than 0.008 (0.015) for the $\delta =1.46$ (0.88) film. Those are about one-tenth of $\alpha$ values for known materials with large $K_u$. First-principles calculations well describe both low $\alpha$ and large $K_u$ for these alloys.

Gilbert damping for the epitaxial ${\text{Co}}_2\text{FeAl}$ Heusler alloy films was investigated. Gilbert damping constant for the films was evaluated by analyzing the data of ferromagnetic resonance measured at the frequency of 2–20 GHz. Gilbert damping constant for the film without annealing was rather large, while it decreased remarkably with postannealing. Gilbert damping constant for the film annealed at $600°\text{C}$ was $\simeq 0.001$ . These behavior of Gilbert damping constant can be well explained by the fact that the density of states calculated from first principles decreases with increasing the degree of $B2$ order.