Interparticle interactions in magnetic core/shell nanoarchitectures

Magnetic and M\"ossbauer measurements are reported on single-crystalline monodispersed $\ensuremath{\gamma}{\text{-Fe}}_{2}{\text{O}}_{3}$ nanoparticles of 12.5 nm diameter, coated with multiple silica and mesoporous silica layers of various thicknesses. The high crystallinity and monodispersity of the samples allow resolution of the tetrahedral (A) and octahedral [B] iron sites of the spinel structure of maghemite. The precisely tuned silica coating thickness offers controlled, homogeneous magnetic dilution with the sample magnetic volume fraction ranging from 1 to 0.002, leading to various degrees of interparticle dipole-dipole interaction strength. Analysis of the zero-field-cooled magnetization data shows that dipole-dipole interactions increase the superparamagnetic energy barrier. The relevant magnetic parameters, for both the individual particles $({K}_{\text{eff}})$ and the particle assemblies $({U}_{\text{int}})$, are determined within the framework of weak and strong dipolar-interaction models. At low temperatures, the steepness of the magnetic hyperfine field reduction with increasing temperature in the strongly interacting particle assemblies is examined within the context of the collective magnetic-excitations model.