Field-induced microwave absorption in Fe 3O 4 nanoparticles and Fe 3O 4/polyaniline composites synthesized by different methods

Abstract

Three kinds of nanoscale powders containing Fe 3O 4 nanoparticles (NPs) have been studied by ferromagnetic resonance (FMR): (i) Fe 3O 4 NPs grown and then covered with polyaniline (PANI), (ii) unclad Fe 3O 4 NPs, and (iii) Fe 3O 4 NPs grown “in situ” with the PANI. In every case, there is no low field microwave absorption, rather a single FMR line is observed. However, the half-power widths are of order of 1 kOe presumably due to a distribution of internal fields. For type I particles with a low concentration (below 40%) of Fe 3O 4, the observed resonance fields ( H r) are close to those expected for spheres with negligible magnetocrystalline anisotropy. For all other cases, H r values are significantly lower. Such shortfalls can be roughly understood by invoking dipolar interactions between the grains, stresses frozen in grains during manufacture (method III), as well as anisotropy fields when the specimens are prepared in an aligning field.