Effect of Magnetic Dipole Interaction on Magnetic Properties of Fe Nanoparticle Assemblies

Fe/Au nanoparticle (NPs) assemblies with various Fe NPs densities were prepared by simultaneous agglomeration of Fe NPs and Au NPs in solution. The density of the Fe NPs was widely controlled below 43% by modifying the mixture ratio of both NP solutions. Only dipole interaction effectively induced magnetic interaction between Fe NPs in this system. With the increase in the volume fractions of Fe NPs from 0.5% to 43%, the collective behavior of magnetic moments of Fe NPs was induced by strong magnetic dipole interactions. As a result, the tendency of soft magnetic properties was found. The diameter analysis revealed that the magnetic dipole interaction energy depending on close-packed assembly consisting of only Fe NPs was 13 times that of the magnetic dipole interaction energy between two Fe NPs. The maximum magnetic dipole field was estimated to be approximately 3-5 × 105 A/m for around 8 nm of the assembly of Fe NPs. This value is much larger than that of the anisotropic field of Fe NPs. Therefore, the magnetic properties in this system were dominated by magnetic dipole interactions.