Insight into microstructural and magnetic properties of flame-made γ-Fe2O3 nanoparticles

Abstract

Superparamagnetic and ferromagnetic γ-Fe₂O₃ nanoparticles of different sizes (d_XRD = 6–53 nm) were synthesized via a rapid, one-step flame spray pyrolysis (FSP) technique. Insightful information on the structural characteristics of γ-Fe₂O₃ nanoparticles were obtained by combining extensively various characterisation techniques such as XRD, TEM, EDS, AFM, FTIR, and SQuID magnetometry. The morphology of γ-Fe₂O₃ crystals transformed gradually from near-spherical shapes to 2-D hexagonal/octagonal platelet structures, as a function of increasing particle size. Along with the morphological transformtaion, the synthesized particles evolved from disordered cubic phase to fully ordered tetragonal γ-Fe₂O₃. Saturation magnetisation (M_s) of the nanoparticles increased from 21–74 emu g⁻¹ as a function of particle size up to 13 nm, above which the M_s approached the values reported for bulk γ-Fe₂O₃. The magnetic properties (M_s, exchange bias and coercivity) correlated strongly with the microstructure and the degree of cation vacancy ordering of the nanoparticles. The study provided an insightful view on the use of the flame aerosol technique to design magnetic nanoparticles with closely controlled physicochemical and magnetic properties.