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We report a study of the morphological, chemical, structural and magnetic properties of FeB nanoparticles (NPs) grown using an ion cluster source. A large size distribution of NPs with the mean size of dominant population around 10&-14 nm is obtained. The structural characterization confirms the spherical shape and the amorphous nature of the FeB NPs. The evolution of the chemical composition of the NPs in depth is analyzed by means of x-ray photoelectron spectroscopy. A mixture of Fe and B oxides and oxynitrides surrounds a core where metallic Fe is present. According to the magnetic measurements, this core size is close to 3 nm. The majority of NPs are superparamagnetic at room temperature and the blocking temperature was found to be 17.5 K. The outer shell thickness of more than 7 nm formed by oxides and oxynitrides inhibits the magnetic interaction between NPs resulting in an almost negligible coalescence, making these NPs suitable to be used in biomedical applications.