title: Correlation of superparamagnetic relaxation with magnetic dipole interaction in capped iron-oxide nanoparticles.
abstract: Six nanometer sized iron-oxide nanoparticles capped with an organic surfactant and/or silica shell of various thicknesses have been synthesized by a microemulsion method to enable controllable contributions of interparticle magnetic dipole interaction via tunable interparticle distances. Bare particles with direct surface contact were used as a reference to distinguish between interparticle interaction and surface effects by use of Mössbauer spectroscopy. Superparamagnetic relaxation behaviour was analyzed by SQUID-magnetometry techniques, showing a decrease of the blocking temperature with decreasing interparticle interaction energies kBT0 obtained by AC susceptibility. A many-state relaxation model enabled us to describe experimental Mössbauer spectra, leading to an effective anisotropy constant Keff ≈ 45 kJm(-3) in case of weakly interacting particles, consistent with results from ferromagnetic resonance. Our unique multi-technique approach, spanning a huge regime of characteristic time windows from about 10 s to 5 ns, provides a concise picture of the correlation of superparamagnetic relaxation with interparticle magnetic dipole interaction.
authors.affiliation: Faculty of Physics and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, Lotharstr. 1, D-47048 Duisburg, Germany.
authors.forename: J
authors.initials: J
authors.lastname: Landers
authors.forename: F
authors.initials: F
authors.lastname: Stromberg
authors.forename: M
authors.initials: M
authors.lastname: Darbandi
authors.forename: C
authors.initials: C
authors.lastname: Schöppner
authors.forename: W
authors.initials: W
authors.lastname: Keune
authors.forename: H
authors.initials: H
authors.lastname: Wende