The sonochemical irradiation of Fe(CO)(5) solution in Decalin has been carried out with and without an external magnetic field. Direct TEM measurements reveal that the sample obtained without a magnetic field consists of spongelike particles with a mean size of about 25 nm, whereas the sample synthesized in a 7 kG magnetic field consists of highly acicular particles of 50 nm in length and 5 nm in width. Our finding sheds light on the process of particle nucleation during sonication, which cannot be a diffusion-assisted growth because of the very small time scale. We conclude, therefore, that particles are forced to form an acicular entity by direct magnetic interactions. The amorphous nature of the as-prepared substance was verified by X-ray diffraction, selected area electron diffraction, and differential scanning calorimetry. The magnetic moment vs temperature measurements and Mossbauer spectroscopy reveal a large shift of the blocking temperature of about 70 K toward higher temperatures for the sample obtained in magnetic field. We attribute the observed shift to the significant enhancement of the particle shape magnetic anisotropy.