Dipolar interactions among magnetic dipoles of iron oxide particles dispersed in mili-size hydrogel beads

Abstract

The recently published Mean Field Interacting Superparamagnet Model (MFISP model), whichintroduces the effective demagnetizing factor , is tested in specimens having a random-likespatial distribution of magnetic nanoparticles, where different hierarchies of clustering are present.These specimens are ferrogel PVA/iron oxide beads synthesized by a one-pot route, havingspheroidal shapes and sizes of about 1 mm, and chain and disk-like arrays (superstructures) ofbeads. Raman analyses indicated that magnetic nanoparticles are composed by a mixture ofmagnetite and maghemite. Beads swell 208 % by hydration in about 40 min.The increase of the ac susceptibility as a function of hydration time closely reflects the effect of bead swelling, in agreement with the expected diminution of dipole-dipole interactions. Measuredsusceptibility is analyzed in terms of the susceptibility of non-interacting particles and the effective demagnetizing factor of the specimen, which depends on swelling. The Specific Absorption Rate of electromagnetic power by the beads grows with the hydration time in agreement with ac susceptibility behavior For long hydration times susceptibility and high field magnetization decrease. This is explained by the occurrence of oxidation of magnetite / maghemite to hematite.Isothermal magnetization experiments are performed on each macrostructure in two perpendicularprincipal directions each. Results are consistently described with the MFISP model by consideringtwo hierarchies of clustering: beads themselves and clusters within the beads.