Sonochemistry under an applied magnetic field: Determining the shape of a magnetic particle

The sonochemical irradiation of Fe(CO)(5) solution in Decalin has been carr ied out with and without an external magnetic field. Direct TEM measurement s reveal that the sample obtained without a magnetic field consists of spon gelike particles with a mean size of about 25 nm, whereas the sample synthe sized 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 p article nucleation during sonication, which cannot be a diffusion-assisted growth because of the very small time scale. We conclude, therefore, that p articles are forced to form an acicular entity by direct magnetic interacti ons. The amorphous nature of the as-prepared substance was verified by X-ra y diffraction, selected area electron diffraction, and differential scannin g calorimetry. The magnetic moment vs temperature measurements and Mossbaue r 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 particl e shape magnetic anisotropy.