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We report about a combined structural and magnetometric characterization of self-assembled magnetic nanoparticle arrays. Monodisperse iron oxide nanoparticles with a diameter of 20 nm were synthesized by thermal decomposition. The nanoparticle suspension was spin-coated on Si substrates to achieve self-organized arrays of particles and subsequently annealed at various conditions. The samples were characterized by x-ray diffraction, and bright and dark field high resolution transmission electron microscopy. The structural analysis is compared to magnetization measurements obtained by superconducting quantum interference device magnetometry. We can identify either multi-phase Fe(x)O/γ-Fe(2)O(3) or multi-phase Fe(x)O/Fe(3)O(4) nanoparticles. The Fe(x)O/γ-Fe(2)O(3) system shows a pronounced exchange bias effect which explains the peculiar magnetization data found for this system.

We show that by capping Co nanoparticles with small amounts of Pt strong changes of the magnetic properties can be induced. The Co nanoparticles have a mean diameter of 2.7 nm.From magnetometry measurements we find that for zero and for small amounts of Pt (nominal thickness t Pt < 0.7 nm) the nanoparticles behave superparamagnetic like. With increasing t Pt the blocking temperature is enhanced from 16 up to 108 K. Capping with Pd yields comparable results. However, for values t Pt > 1 nm a strongly coupled state is encountered resembling a ferromagnet with a T c ∼ 400 K.Presently many efforts are undertaken to enhance the thermal stability of magnetic nanoparticles (NPs) for e.g. magnetic data storage media.1-3 Various strategies have been proposed of how to achieve very high anisotropies and hence to 'beat the superparamagnetic limit'. 1,4 One route is to use Co/Pt or Fe/Pt multilayers or FePt and CoPt L1 0 phases. 5,6Furthermore such NPs are expected to show also a perpendicular magnetic anisotropy (PMA) with respect to the plane of the recording medium to enable perpendicular recording. 7 From thin film studies it is known that Co/Pt multilayers show PMA with relatively large anisotropy values due to interfacial hybridization via the orbital moment of the Co surface a) Electronic

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