Superparamagnetic Behavior in Noninteracting NiFe2O4 Nanoparticles Grown in SiO2 Matrix

Abstract

Inverse spinel NiFe2O4 nanoparticles (2.5 nm–25 nm) in SiO2 matrix were prepared by sol–gel method. The phase and particle size of the samples were determined by X-ray diffraction and transmission electron microscope. Magnetic properties and Mössbauer spectra of the samples were studied with different ferrite particle sizes. Superparamagnetic behavior was observed at room temperature when the average particle size was less than 6 nm and also for larger particles at higher temperatures. The Mössbauer spectra changed from two-peaks to six-peaks pattern with the increase in particles size and decrease in temperature. Two sets of sextets in the Mössbauer spectra indicate the presence of iron ion in the tetrahedral as well as in octahedral sites. The Langevin function used to explain paramagnetism is found to describe well the magnetic behavior of these samples consisting of single domain particles in the superparamagnetic state.