  Spin excitations of magnetic thin films are the founding element for novel
transport concepts in spintronics, magnonics, and magnetic devices in general.
While spin dynamics have been extensively studied in bulk materials, their
behaviour in mesoscopic films is less known due to experimental limitations.
Here, we employ Resonant Inelastic X-Ray Scattering to investigate the spin
excitation spectrum in mesoscopic Fe films, from bulk-like down to 3 unit cells
thick. In bulk-like samples, we find isotropic, dispersive ferromagnons
consistent with the dispersion observed by neutron scattering in bulk single
crystals. As the thickness is reduced, these ferromagnons survive and evolve
anisotropically: renormalising to lower energies along the out-of-plane
direction while retaining their dispersion in the in-plane direction. This
thickness dependence is captured by simple Heisenberg model calculations
accounting for the confinement in the out-of-plane direction through the loss
of Fe bonds. Our findings highlight the effects of mesoscopic scaling on spin
dynamics and identify thickness as a knob for fine-tuning and controlling
magnetic properties in films.
