Tuning the magnetic properties of Co nanoparticles by Pt capping

We show that by capping Co nanoparticles (NPs) with small amounts of Pt, strong changes of the magnetic properties can be induced. The Co NPs have a mean diameter of 2.7 nm. From magnetometry measurements we find that for zero and for small amounts of Pt (nominal thickness tPt 0.7 nm) the NPs behave superparamagnetic- (SPM)-like. With increasing tPt the blocking temperature is enhanced from 16 up to 108 K. Capping with Pd yields comparable results. However, for values tPt > 1 nm a strongly coupled state is encountered resembling a ferromagnet (FM) with a Tc {~} 400 K.