Structural and magnetic properties of sonoelectrocrystallized magnetite nanoparticles

The effect of ultrasound power on the morphology, structure and magnetic properties of magnetite nanoparticles synthesized from iron electrodes by the electro-oxidation method was investigated. Samples made in aqueous solution in the absence or presence of an organic stabilizer (thiourea, tetramethylammonium chloride, sodium butanoate or beta-cyclodextrine) were characterized by x-ray diffraction, transmission and scanning electron microscopy, magnetometry and Mossbauer spectrometry. The iron is almost all in the form of 20-85 nm particles of slightly nonstoichiometric Fe3-delta O4, with delta approximate to 0.10. Formation of a paramagnetic secondary phase in the presence of sodium butanoate or beta-cyclodextrine is supressed by ultrasound. Specific magnetization of the magnetite nanoparticles ranges from 19 to 90 A m(2) kg(-1) at room temperature, and it increases with particle size in each series. The particles show no sign of superparamagnetism, and the anhysteretic and practically temperature-independent magnetization curves are associated with a stable magnetic vortex state throughout the size range. The spin structure of the particles and the use of magnetization measurements to detect magnetite in unknown mixtures are discussed.