Low field ac susceptibility and magnetization curves of milled Sm2Fe17N3powders are measured for isotropic and aligned samples with average particle sizes ranging from 3.0 to 52 &mgr;m. Most of the particles are in a multidomain state before application of a magnetic field. The transverse ac susceptibility (&khgr;⊥ac∼0.08) due to magnetization rotation against the anisotropy field is independent of particle size. The parallel ac susceptibility &khgr;∥acand the susceptibility of isotropic powders are dominated by almost free domain wall movement in multidomain particles for which ⟨&khgr;⟩=1.35. In very low fields (<0.5 mT) &khgr;∥acis small (∼0.1), but it increases toward a constant value of order 1 when the field exceeds a value of order 1 mT which is inversely proportional to the particle size. There is a corresponding increase in loss angle which is attributed to weak pinning of domain walls at surface defects. The volume fraction of single‐domain and multidomain powder are deduced as a function of average particle size both in the unmagnetized and remanent states, from an analysis of high‐field magnetization curves on isotropic powder. For example, about 87% of the volume of powder with an average size of 6.7 &mgr;m is multidomain in the unmagnetized state, but only 43% of the volume is multidomain in the remanent state. ©1996 American Institute of Physics.