  The dynamic magnetic properties of Ni nanoparticles diluted in an amorphous
SiO2 matrix prepared from a modified sol-gel method have been studied by the
frequency f dependence of the ac magnetic susceptibility \c{hi}(T). For samples
with similar average radii ~ 3-4 nm, an increase of the blocking temperature
from TB ~ 20 to ~ 40 K was observed for Ni concentrations of ~ 1.5 and 5 wt.%,
respectively, assigned to the effects of dipolar interactions. Both the
in-phase \c{hi}'(T) and the out-of-phase \c{hi}"(T) maxima follow the
predictions of the thermally activated N\'eel-Arrhenius model. The effective
magnetic anisotropy constant Keff inferred from \c{hi}"(T) versus f data for
the 1.5 wt.% Ni sample is close to the value of the magnetocrystalline
anisotropy of bulk Ni, suggesting that surface effects are negligible in the
present samples. In addition, the contribution from dipolar interactions to the
total anisotropy energy Ea in specimens with 5 wt.% Ni was found to be
comparable to the intrinsic magnetocrystalline anisotropy barrier.
