  We investigate diamagnetic fluctuations in percolating granular
superconductors. Granular superconductors are known to have a rich phase
diagram including normal, superconducting and spin glass phases. Focusing on
the normal-superconducting and the normal-spin glass transition at low
temperatures, we study he diamagnetic susceptibility $\chi^{(1)}$ and the mean
square fluctuations of the total magnetic moment $\chi^{(2)}$ of large
clusters. Our work is based on a random Josephson network model that we analyze
with the powerful methods of renormalized field theory. We investigate the
structural properties of the Feynman diagrams contributing to the
renormalization of $\chi^{(1)}$ and $\chi^{(2)}$. This allows us to determine
the critical behavior of $\chi^{(1)}$ and $\chi^{(2)}$ to arbitrary order in
perturbation theory.
