Spin excitations of magnetic thin films are the founding element for magnetic devices in general. While spin dynamics have been extensively studied in bulk materials, the behaviour in mesoscopic films is less known due to experimental limitations. Here, we employ resonant inelastic X-ray scattering to investigate the spectrum of spin excitations in mesoscopic Fe films, from bulk-like films down to three unit cells. In bulk samples, we find isotropic, dispersive ferromagnons consistent with previous neutron scattering results for bulk single crystals. As the thickness is reduced, these ferromagnetic spin excitations renormalize 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.

磁性薄膜的自旋激发通常是磁性器件的基础元素。虽然自旋动力学已在散装材料中得到广泛研究，但由于实验限制，介观薄膜中的行为鲜为人知。在这里，我们采用共振非弹性 X 射线散射来研究介观 Fe 薄膜中的自旋激发光谱，从类块薄膜到三个晶胞。在大块样品中，我们发现各向同性、色散铁磁子与之前大块单晶的中子散射结果一致。随着厚度的减小，这些铁磁自旋激发重新归一化为沿面外方向的较低能量，同时保持它们在面内方向上的分散。这种厚度依赖性由简单的海森堡模型计算捕获，该模型计算通过 Fe 键的损失解释了面外方向的限制。我们的研究结果强调了介观缩放对自旋动力学的影响，并将厚度确定为微调和控制磁性的旋钮。