Effects of grain boundary width and crystallite size on conductivity and magnetic properties of magnetite nanoparticles

The structural, electrical, and magnetic properties of magnetite nanoparticles, with crystallite sizes 30, 40, and 50 nm, are studied. These crystallite sizes correspond to average particle sizes of 33, 87, and 90 nm, respectively, as determined by TEM. By HRTEM images, it is observed that grain boundary widths decrease as crystallite size increases. Electrical and microstructural properties are correlated based on the theoretical definition of charging energy. Conduction phenomena are investigated as a function of grain boundaries widths, which in turn depend on crystallite size: the calculations suggest that charging energy has a strong dependence on crystallite size. By zero-field-cooling and susceptibility measurements, it is observed that Verwey transition is crystallite size dependent, with values ranging from 85 to 95 K. In addition, a kink at the out-phase susceptibility curves at 35 K, and a strong change in coercivity is associated to a spin-glass transition, which is independent of crystallite size but frequency dependent. The activation energy associated to this transition is calculated to be around 6–7 meV. Finally, magnetic saturation and coercivity are found to be not significantly affected by crystallite size, with saturation values close to fine powders values. A detailed knowledge on the effects of grain boundary width and crystallite size on conductivity and magnetic properties is relevant for optimization of materials that can be used in magnetoresistive devices.