  Magnetic nanoparticles such as FePt in the L10-phase are the bedrock of our
current data storage technology. As the grains become smaller to keep up with
technological demands, the superparamagnetic limit calls for materials with
higher magneto-crystalline anisotropy. This in turn reduces the magnetic
exchange length to just a few nanometers enabling magnetic structures to be
induced within the nanoparticles. Here we describe the existence of spin-wave
solitons, dynamic localized bound states of spin-wave excitations, in FePt
nanoparticles. We show with time-resolved X-ray diffraction and micromagnetic
modeling that spin-wave solitons of sub-10 nm sizes form out of the
demagnetized state following femtosecond laser excitation. The measured soliton
spin-precession frequency of 0.1 THz positions this system as a platform to
develop miniature devices capable of filling the THz gap.
