Nonreciprocal spin-wave propagation in anisotropy-graded iron films prepared by nitrogen implantation

Gradual modification of the magnetic properties in ferromagnetic films has recently been proposed as an effective method to channel and control spin waves for the development of alternative functionalities in magnonic devices. Here, we investigate graded $\mathrm{Fe}$:$\mathrm{N}$ films prepared by low-dose nitrogen implantation of $\mathrm{Fe}$ epitaxial thin films. Combining Brillouin light-scattering measurements and a spin-wave theoretical approach, we show that nitrogen implantation induces a graded profile of both the in-plane and the perpendicular anisotropies along the film thickness. This graduation leads to a significant modification of the spin-wave spatial localization across the thickness and generates a marked frequency asymmetry in the spin-wave dispersion. Moreover, we find that the anisotropy profile, and as a consequence the dispersion relation, can be tuned on changing the implantation dose, opening a way for the potential use of the graded $\mathrm{Fe}$:$\mathrm{N}$ films in magnonic applications.