Experimental study of the structural and magnetic properties of γ-Fe₂O₃ nanoparticles

Abstract

Structural and magnetic properties of γ-Fe₂O₃ nanoparticles annealed at 200 and 250°C have been studied by using high resolution transmission electron microscopy (TEM), electron energy loss spectroscopy, x-ray diffraction, neutron diffraction, polarized small angle neutron scattering (SANS) and dc magnetization techniques. Here we have dealt with an important problem of the symmetry of crystal structure of γ-Fe₂O₃ nanoparticles with cation vacancy distribution. Present study also correlates various structural and magnetic results that have been obtained for γ-Fe₂O₃ nanoparticles using various experimental techniques. Analysis of x-ray and neutron diffraction data shows cation vacancies at the octahedral site, consistent with the space group P4332. A log-normal distribution of particle size for both the samples has been found. A good crystallinity has been found from TEM, x-ray diffraction and neutron diffraction studies. Neutron diffraction study at room temperature confirms a ferrimagnetic ordering of tetrahedral and octahedral site moments with a net moment of ∼1.58 and 1.83μ_B per formula unit for the samples annealed at 200 and 250°C, respectively. SANS data analysis is consistent with a mass fractal spatial arrangement of these nanoparticles with a fractal dimension of 1.25. The SANS results are consistent with the same form factor for both the chemical particle size and the magnetic particle size indicating the absence of any observable shell with disordered spins. Superparamagnetic behavior of noninteracting nanoparticles with a log-normal distribution of blocking temperature (with a particle moment of ∼28000μ_B for the sample annealed at 200°C) is found in our dc magnetization study at room temperature.