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Summary Magnetic fluids show heating effects in ac-magnetic fields which may be useful for application in magnetic hyperthermia proposed as a tumour therapy. Enhancement of the specific loss power (SLP) of magnetic ac-losses would allow a reduction of the tissue load with magnetic material (iron oxide). The several types of loss processes depend strongly on the mean particle size [1] and the size distribution width [2]. To influence and improve the mean size as well as size distribution new approaches in preparation are promising where nucleation and growth of the particles can be influenced independently or where a further growing is possible on small given particles without further nucleation. First experiments on iron oxide powders by a cyclic method based on “conventional” precipitation from Fe-salt solution have shown the feasibility of cyclic growth in an aqueous system. An increasing mean particle size (in the range from about 10 to 30 nm) with increasing number of cycles (up to four) is confirmed by XRD. Magnetic parameters of the saturation hysteresis loop and magnetic hysteresis losses calculated from minor loops are shown. The coercivity increases with the particle size, whereas the hysteresis losses at small applied fields (11 kA/m) reveal lower values again if the particles are magnetically too hard.

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