Cobalt ferrite, CoFe 2 O 4 , nanoparticles have been obtained from pre-prepared layered double hydroxide carbonate, LDH-CO 3 , by mechanical milling. X-ray powder diffraction shows the only product of the milling for 5 h of the LDH-CO 3 is cobalt ferrite nanoparticles. Longerterm milling induces particle growth characterized by sharpening of the Bragg peaks and an increase of the blocking temperature, while prolonged milling results in the formation of some cobalt metal. Mo ¨ssbauer spectra and temperature dependence of the magnetization of the 5-h milled sample suggest that it consists of nanoparticles of size less than 10 nm with blocking temperature of 200 K, in good agreement with microscopy showing an average size of 6 nm. The magnetic properties exhibit a strong dependence on the particle size as a result of an unusual cation distribution and of surface effect. The saturation magnetization at 5 K and the squareness of the hysteresis loops increase with the average particle size. The difference between zero-field-cooled and field-cooled hysteresis loops is correlated with the presence of randomness of the moments at the surface.

The possibilities of mechanochemical activation as a successful route for direct synthesis of NiWO4 have been studied. A stoichiometric mixture of NiO and WO3 in a 1:1 molar ratio was subjected to intense mechanical treatment in air using a planetary ball mill for different periods of time. The phase and structural transformations were monitored by X-ray diffraction (XRD), infrared spectroscopy (IR), and X-ray photoelectron spectroscopy (XPS). It was found that 7.5 h of milling of the reagents leads to complete crystallization of single nanostructured phase NiWO4 at room temperature.

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