Magnetic properties of superparamagnetic γ-Fe2O3 nanoparticles prepared by coprecipitation technique

γ-Fe2O3 nanoparticles have been synthesized by a chemical coprecipitation technique using the typical pipette drop method and the novel piezoelectric nozzle method. Nanoparticles made by the piezoelectric nozzle method show smaller size and very narrow size distribution compared to the nanoparticles produced by the pipette drop method. The superconducting quantum interference device (SQUID) measurements show superparamagnetism of nanoparticles and reveal that the anisotropy constants of the nanoparticles made by the pipette drop method and the piezoelectric nozzle method are 2.2×106 and 9.0×106 erg/cm3, respectively. By measuring the magnetic relaxation of the magnetization at 5 K, we also obtained magnetic viscosity of γ-Fe2O3 nanoparticles.