Susceptibility and magnetization processes in Sm2Fe17N3 powders

Abstract

Low field ac susceptibility and magnetization curves of milled Sm2Fe17N3 powders are measured for isotropic and aligned samples with average particle sizes ranging from 3.0 to 52 μm. Most of the particles are in a multidomain state before application of a magnetic field. The transverse ac susceptibility (χ⊥ac∼0.08) due to magnetization rotation against the anisotropy field is independent of particle size. The parallel ac susceptibility χ∥ac and the susceptibility of isotropic powders are dominated by almost free domain wall movement in multidomain particles for which 〈χ〉=1.35. In very low fields (<0.5 mT) χ∥ac is 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 μm is multidomain in the unmagnetized state, but only 43% of the volume is multidomain in the remanent state.