The magnetic structure of NiFe$_{2}$O$_{4}$ nanoparticles

The Mössbauer studies of the magnetic structure of nickel ferrite (NiFe$_{2}$O$_{4}$) nanoparticles (NPs) are presented. It is shown that the redistribution of ions over nonequivalent crystallographic positions occurs upon a decrease in the size of NiFe$_{2}$O$_{4}$ crystallites to nanovalues, thus leading to the structural transformation of NiFe$_{2}$O$_{4}$ nanoparticles from inverse spinel to mixed spinel. It has been established that the magnetic moments of Fe ions in the volume of NiFe$_{2}$O$_{4}$ nanoparticles are ferromagnetically (collinearly) ordered, being noncollinear (or oblique) in the near-surface layer probably due to the frustration of moments in the near-surface layer of nanoparticles.