Ferromagnetic materials are widely utilized for various fields such as information communication technology, electronic devices, motors, generators, and so on. These various functionalities of magnetic devices are governed by nano-scale spin dynamics. Therefore, it is essentially important to understand the physics of nano-scale spin dynamics and to control it for the advanced magnetic devices. For example, the recording density of hard-disk drives can be significantly increased by controlling the spin dynamics of microwave frequency range, i.e., microwave-assisted magnetic recording technology. The high-performance permanent magnets, which are indispensable for traction motors of electric vehicles, can be developed by controlling the nano-scale spin dynamics at the grain surfaces.