  The family of superconducting fullerides (NH_3)_xNaK_2C_60 shows an anomalous
correlation between T_c and lattice parameter. To better understand the origin
of this anomaly we have studied a representative x=0.75 compound using SQUID
magnetometry and MuSR spectroscopy. The lower critical field H_c1, measured by
the trapped magnetization method, is less than 1 G, a very small value as
compared with that of other fullerides. Muon spin depolarization in the
superconducting phase shows also quite small local field inhomogeneities, of
the order of those arising from nuclear dipolar fields. On the other hand, the
40 T value for H_c2, as extracted from magnetometry data, is comparable to that
of other fullerides. We show that these observations cannot be rationalized
within the framework of the Ginzburg-Landau theory of superconductivity.
Instead, the anomalous magnetic properties could be interpreted taking into
account the role played by polaronic instabilities in this material.
