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Initial results on synthesis, magnetic properties and time stability of iron-iron carbide nanoparticles are presented. It is shown that as-prepared (after sonochemical synthesis in solution) material consists of small iron particles (d<10 nm) embedded in a polymeric matrix. It was found to be superparamagnetic. Consequent thermal annealing of as-prepared material at temperatures up to 1073 K in argon results, first, in the growth of the iron particles up to 150 nm. Second, the obtained powders become ferromagnetic. Third, such powders are air-stable due to the formation of an interfacial iron carbide layer during annealing. The air-stability of the iron-iron carbide powder was tested by time dependence measurements of its resistivity.