  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 approx. 400 K
