Nanostructured CoFe 2 O 4 particles were prepared by a sonochemical approach, first by preparation of the amorphous precursor powders, followed by heat treatment at relatively very low temperatures. The precursor was prepared by sonochemical decomposition of solutions of volatile organic precursors, Fe(CO) 5 and Co(NO)(CO) 3 , in Decalin at 273 K, under an oxygen pressure of 100-150 kPa. The amorphous nature of these particles was confirmed by various techniques, such as scanning and transmission electron microscopy (SEM and TEM), electron microdiffraction, and X-ray diffractograms. Magnetic measurements, Mo ¨ssbauer, and electron paramagnetic resonance (EPR) spectral studies indicated that the as-prepared amorphous particles were superparamagnetic. The Mo ¨ssbauer parameters and the significantly low (45 emu/g) observed saturation of magnetization of the annealed sample, compared to that of the bulk sample (72 emu/g), reflected its nanocrystalline nature.

Nanosized amorphous NiFe 2 O 4 powder was prepared by sonochemical decomposition of solutions of volatile organic precursors, Fe(CO) 5 and Ni(CO) 4 , in decalin at 273 K, under an oxygen pressure of 100-150 kPa. The amorphous nature of these particles was confirmed by various techniques, such as SEM, TEM, electron microdiffraction, and X-ray diffractograms. Magnetic measurements, Mössbauer, and EPR spectral studies indicated that the as-prepared NiFe 2 O 4 ferrite particles were superparamagnetic. The Mössbauer spectrum of the crystallized sample showed a clear sextet pattern, with hyperfine field values of 500 and 508 kOe for A (tetrahedral) and B (octahedral) sublattices, respectively, of the inverse spinel NiFe 2 O 4 . Saturation magnetization of the annealed sample (25 emu/g) was significantly lower than that for the reported multidomain bulk prticles (55 emu/g), reflecting the ultrafine nature of the sample. Thermogravimetric measurements with a permanent magnet gave Curie temperatures of 440°C for amorphous and 560°C for the crystallized forms.

The hyperfine parameters of iron atoms are studied in iron nanocrystallites prepared by different methods: ball milling of iron powder, partial crystallization of Fe-Zr-B-Cu amorphous ribbons, and vacuum evaporation of Fe-B polycrystalline multilayers. Careful analysis of the spectral contribution of the possible impurities and chemical mixing at interfaces reveals that no specific grain boundary contribution can be separated in the Mössbauer spectra when the grain size is in the 2-10 nm range. The results indicate that excluding chemical effects the hyperfine fields of iron atoms at the bcc interfaces are very close to those in the bulk, and Mössbauer spectra of the iron nanocrystallites studied can be understood without supposing a separate grain boundary phase with very distorted structure or highly reduced density.

Thin 57 Fe layers evaporated onto an MgO(100) single-crystal substrate and covered by an evaporated MgO layer were studied by low-temperature conversion electron Mössbauer spectroscopy. The temperature dependence of the spectra indicates superparamagnetic behavior below 8 ML nominal thickness of the Fe layer signaling a cluster-type growth mode. The low-temperature hyperfine fields are consistent with a model that defines two types of metallic Fe atoms: bulklike and interfacial ones. Formation of FeO or (Fe,Mg)O at the interface layer is not observed. The sample with a 4-ML Fe layer when grown over a cleaved MgO substrate shows almost perfect perpendicular magnetization, as locally probed at 15 K by the hyperfine magnetic field, while random magnetization orientation and lower blocking temperature is observed in the case of a polished substrate. The perpendicular anisotropy observed at low temperature is attributed to mechanical stresses arising from the epitaxial relation and the different temperature dilatation of the subsequent layers.