  The study of system of magnetic nano-particle has received increasing
attention recently both because of the novel physical concepts involved and
also because of their vast potential for application. The influence of
background material (the substrate coating) on magnetic properties of such
systems is a relatively open topic and often a full understanding is missing.
In the present work we discuss our experiments and interpretation for two
systems: Ni nanoparticles coated with graphitic carbon and Ni nanoparticles
coated with Au. While the latter system exibits behavior typical of
superparamagnetic particle systems the former shows several puzzling results
such as extremely high blocking temperature ($T_{B}$), very fast relaxation
time well below $T_{B}$, temperature independent field-cooled magnetization and
very small coercivity and remanent magnetization. We interpret these findings
as being a result of weak ferromagnetism, characteristic of the graphitic
carbon. This induces strong magnetic interactions between the Ni particles in
the presence of small magnetic fields. Such systems give rise to a dramatic
difference in blocking temperature between measurements performed at zero field
and those performed at very small magnetic fields.
