In order to probe the impurity spin magnetization of a thin Fe doped (7 at.%) Au film, which is near the magnetic percolation limit, we measured the polarized neutron reflectivity of the film. The observed magnetic-field dependence of the magnetization indicates that Fe atoms are partially dispersed homogeneously into the Au matrix and partially aggregated as clusters with typical size around 1 nm. Our results imply that polarized neutron reflectivity can be used as a sensitive tool to probe the magnetic properties of dilute magnetic alloy films even in the absence of clustering. ; system details: machine converted author identifier PE to PID, February 2012