Abstract

Magnetic nanoparticles exhibit size- and shape-dependent magneticproperties[1,2]and have attracted great interest for a variety of ap-plications, including catalysis[3,4], magneto-optics[5,6]or magneticseparation of enantiomers[7]. The synthesis and application of nano-particles consisting of various magnetic materials have been reported,including ferrites (in particular spinel-type ferrites, e.g. Fe3O4,CoFe2O4), metals (e.g. Co, Fe) and alloys (e.g. FePt) while the synthesisof defined nanosized particles of the hexaferrites has remained scarce[8–11]. Hexaferrites possess a hexagonal crystal structure with closepacking of oxygen ion layers. Trivalent metal cations are located ininterstitial sites of the structure while the heavy ions (e.g. Ba2+) entersubstitutionally the oxygen layers. Several types are known dependingon their chemical and crystalline structure. Hexagonal ferrites of the M-type such as BaFe12O19, are of enormous technical importance, e.g. aspermanent magnets due to their low cost, hard magnetic properties andstability in air[12,13]. Bulk BaFe12O19exhibits a saturation magneti-zationMsof 72 Am2/kg at room temperature[12]together with a re-latively high magnetocrystalline anisotropy (i.e. anisotropic constant33 × 104J/m3) [14]and a Curie temperatureTcof 740 K[12].

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