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

Superparamagnetic and ferromagnetic γ-Fe2O3 nanoparticles of different sizes (dXRD = 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 (Ms) of the nanoparticles increased from 21–74 emu g−1 as a function of particle size up to 13 nm, above which the Ms approached the values reported for bulk γ-Fe2O3. The magnetic properties (Ms, 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.