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Superparamagnetic behavior is characterized by a thermally fluctuating vector of magnetization, leading to magnetization curves free of hysteresis: it is a property of isolated ferrite particles with sizes below ca. 10 nm. These particles fulfil the condition Kv < kT with K … energy of unisotropy, v … volume of the particle, kT has the usual meaning. To produce a superparamagnetic macroscopic part it is necessary to avoid the interaction of the particles. This can be achieved by coating the particles with a second non-magnetic phase. This special material can be synthesized using the microwave plasma process. Because of the specific interaction of charged particles with an oscillating electrical field, microwave plasmas excel in relatively low reaction temperatures. The low reaction temperature and the electrical charging of the particles in the plasma reduce the probability of agglomeration. Therefore, it is possible to pass the gas stream with the as produced particles through a second reaction zone, where the particles will be coated with a polymer. The thickness of the coating can be adjusted in the range from 1 to 5 nm. The composition of the ferrite kernel is selected either for maximum susceptibility or maximum frequency of application. They may consist either of maghemite, manganese-, manganese-zinc-, or magnesium iron spinell for maximum susceptibility or cobalt-iron spinelle type material for maximum frequency. Polymer coated ferrite nanoparticles can be consolidated by hot pressing with temperatures around 100 °C. Superparamagnetism is shown by static magnetic measurements and Moessbauer spectrometry. Dynamic measurements of the complex susceptibility show interesting properties up to frequencies in the gigahertz range.

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