Field-induced microwave absorption in Fe3O4 nanoparticles and Fe3O4/polyaniline composites synthesized by different methods

Three kinds of nanoscale powders containing Fe3O4 nanoparticles (NPs) have been studied by ferromagnetic resonance (FMR): (i) Fe3O4 NPs grown and then covered with polyaniline (PANI), (ii) unclad Fe3O4 NPs, and (iii) Fe3O4 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 Fe3O4, the observed resonance fields (Hr) are close to those expected for spheres with negligible magnetocrystalline anisotropy. For all other cases, Hr 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.