We have studied the film thickness dependence of the remanent magnetization (deltaMplots), the magnetic viscosity, and the activation volumes of Fe thin films sputtered on commercial nanochannel alumina of 20 nm average pore diameter. The films have the form of a contiguous interconnected network. A critical film thickness of&fgr;∼20nm defines two regions with very different magnetic behavior. For this critical value of&fgr;we have found a maximum in the remanent coercivity, a minimum in the interparticle interaction parameter&agr;,and a rapid increase in the magnetic viscosity, the irreversible susceptibility and the activation volume. Our results are interpreted in terms of the pinning effects caused by nonmagnetic inclusions. The functional relationship between the remanent coercivity and the activation volume is similar to the one found in long cylinders that reverse magnetization by a curling mechanism. ©1997 American Institute of Physics.