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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The deliberate control over the spatial arrangement of nanostructures is the desired goal for many applications such as, for example, in data storage, plasmonics or sensor arrays. Here we present a novel method to assist the self-assembly process of magnetic nanoparticles. The method makes use of nanostructured aluminum templates obtained after anodization of aluminum discs and the subsequent growth and removal of the newly formed alumina layer, resulting in a regular honeycomb-type array of hexagonally shaped valleys. The iron oxide nanoparticles, 20 nm in diameter, are spin-coated onto the surface of honeycomb nanostructured Al templates. Depending on the size, each hexagon site can host up to 30 nanoparticles. These nanoparticles form clusters of different arrangements within the valleys, such as collars, chains and hexagonally closed islands. Ultimately, it is possible to isolate individual nanoparticles. The strengths of the magnetic interaction between particles in a cluster are probed using the memory effect known from the coupled state in superspin glass systems.

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