Amorphous nanoparticles of Fe 80− x Cr x B 20 (5⩽ x⩽25) embedded in an alumina matrix have been studied. The samples have been prepared by the cosputtering technique and by using a mixed target consisting of amorphous ribbons and alumina. Several area ratios have been used leading to different average diameters of particles. The samples have been characterized by means of an electron microprobe and transmission electron microscopy with ETX analysis. Thermal variation of the magnetic properties have been determined mainly from zero-field-cooled magnetization, magnetization versus applied field, AC susceptibility and Mössbauer spectroscopy measurements. The average diameters of the particles obtained range between 1.9 and 5.3 nm. The same kind of random packing of atoms occurs in particles as in bulk (ribbons) though the preparation process and the size scale are different. The hyperfine parameters deduced from Mössbauer spectroscopy are similar to those obtained from the ribbons, but show a size dependence which is only appreciable for the smallest particles. The magnetic structure and the magnetic properties are similar to those of the corresponding ribbons except for x above 20–25 where a canted spin structure probably appears, favoured by the magnetic defects existing in the particle surface. The dynamical properties of particles are more intricate. Interparticle interactions are present, but the model [Dormann et al., Adv. Chem. Phys. 98 (1997) 283] accounting for the dipolar interactions is not sufficient for explaining the properties. It is suggested that, due to the magnetic disorder coming from the Cr substitution, the effective anisotropy acting on the reversal process of the magnetic moments of particles is temperature dependent. A good agreement is obtained for the variation of the blocking temperature with the measuring time.