  We compute the AC susceptibility of a weakly dipolar-interacting monodisperse
assembly of magnetic nanoclusters with oriented anisotropy. For this purpose we
first compute the relaxation rate in a longitudinal magnetic field of a single
nanomagnet taking account of both dipolar interactions in the case of dilute
assemblies and surface anisotropy. We then study the behavior of the real and
imaginary components of the AC susceptibility as functions of temperature,
frequency, surface anisotropy and inter-particle interactions. We find that the
surface anisotropy induces an upward shift of the temperature at the maximum of
the AC susceptibility components and that its effects may be tuned so as to
screen out the effects of interactions. The phenomenological Vogel-Fulcher law
for the effect of dipolar interaction on the relaxation rate is revisited
within our formalism and a semi-analytical expression is given for the
effective temperature is given in terms of inter alia the applied field,
surface anisotropy and dipolar interaction.
