We investigate the effect of digestion time and alkali addition rate on the size and magnetic properties of precipitated magnetite nanoparticles. It is observed that the time required to complete the growth process for magnetite nanocrystals is very short (approximately 300 s), compared to long digestion times (20-190 min) required for MnO and CdSe nanocrystals. The rapid growth of magnetite nanoparticles suggests that Oswald ripening is insignificant during the precipitation stage, due to the low solubility of the oxides and the domination of a solid-state reaction where high electron mobility between Fe2+ and Fe3+ ions drives a local cubic close-packed ordering. During the growth stage (0-300 s), the increase in the particle size is nominal (6.7-8.2 nm). The effect of alkali addition rate on particle size reveals that the nanocrystal size decreases with increasing alkali addition rate. The particle size decreases from 11 to 6.8 nm as the alkali addition rate is increased from 1 to 80 mL/s. During the size decrease, the lattice parameter decreases from 0.838 to 0.835 nm, which is attributed to an increase in the amount of Fe3+ atoms at the surface due to oxidation. As the alkali addition rate increases, the solution reaches supersaturation state rapidly leading to the formation of large number of initial nuclei at the nucleation stage, resulting in large number of particles with smaller size. When alkali addition rate is increased from 1 to 80 mL/s, the saturation magnetization of the particles decreases from 60 to 46 emu/g due to the reduced particle size.

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We study the effects of surfactant monolayer coating on the reduction of Fe 3 O 4 nanoparticles under vacuum thermal annealing. Oleic acid coated and uncoated Fe 3 O 4 nanoparticles were synthesized by a simple coprecipitation technique. In the temperature range of 300-700 °C, the particle size and lattice constant of uncoated Fe 3 O 4 nanoparticles increased from 9 to 18 nm and from 8.357 to 8.446 Å, respectively. On further heating (above 700 °C), Fe 3 O 4 decomposed into γ-Fe 2 O 3 and FeO phases. In the range of 800-1000 °C, the FeO phase was predominant, and its size grew significantly from 30 to 44 nm. Conversion of oleic acid coated Fe 3 O 4 phase to metallic R-Fe commenced at 500 °C and continued up to 800 °C. After vacuum annealing at 800 °C, the magnetic behavior of the sample changed from ferrimagnetic to ferromagnetic. The activation energies for the phase transitions of uncoated and oleic acid coated nanoparticles were estimated to be 30.304 and 17.349 kJ/mol, respectively. Thermogravimetric analysis (TGA) coupled with mass spectrometry revealed that, for coated nanoparticles, effluents such as H 2 , CO, and CO 2 from oleic acid facilitate the reduction of Fe 3 O 4 into R-Fe and FeO during vacuum thermal annealing. The interaction between the headgroup of the oleic acid and the oxygen in Fe 3 O 4 is expected to lead to weakened bonding, which could result in a lower activation energy for the reduction of the surfactant-coated nanoparticles. This is a plausible reason for the precipitatation of R-Fe at lower temperature (at 500 °C) in the surfactant-coated system.

We report a simple method for producing magnetic nanoparticles with enhanced maghemite (γ-Fe2O3) to haematite (α-Fe2O3) phase transition temperature. By controlling the properties of the solvent media, we have been able to tune the particle size from 2.3 to 6.5 nm. Nanoparticles with higher transition temperatures have been achieved by using NaOH as an alkali during the co-precipitation. The maghemite to haematite phase transition was complete at a temperature below 600 °C for nanoparticles prepared using ammonia, whereas the phase transition was not complete until 750 °C for the samples prepared with NaOH. The increase in average particle size after heat treatment at 600 °C is attributed to coalescence of particles by solid state diffusion, where the system reduces its free energy by reducing the surface area. The final particle diameters of the haematite after the heat treatment were 35.4, 31.38 and 26.85 nm respectively for nanoparticles of initial diameters 5, 7 and 9.8 nm. Our studies on the effect of the initial particle size on the transition temperature show that the transition temperature decreases with decreasing particle size due to the reduced activation energy of the system.

In this paper, we report the variations in the crystal structure, average particle size, and magnetic properties of ZnFe2O4 nanoparticles on thermal annealing, using in situ high temperature x-ray diffraction (XRD). Fine powder of ZnFe2O4 nanoparticles with an average particle size of 9.3nm, prepared through coprecipitation technique, has been used in these studies. The powder is heated from room temperature to 1000°C, under vacuum in steps of 100°C and the XRD pattern is recorded in situ. A sudden drop in the lattice parameter from 8.478to8.468Å is observed at 800°C, above which it increases with increasing temperature. After annealing at 1000°C, the lattice parameter reduces from 8.441to8.399Å and the magnetization value increases from 5to62emu∕g, suggesting the possibility of a conversion of the cubic structured ZnFe2O4 from normal to inverse spinel structure due to canting of ions between the tetrahedral and octahedral interstitial sites. During annealing, the Zn2+ ions move from tetrahedral site to octahedral site whereas Fe3+ ions redistribute within the octahedral and tetrahedral sites in order to reduce the strain. The increase in the average particle size from 9to27nm, after the thermal annealing at 1000°C, can be attributed to coalescence phenomenon, which starts at 600°C. The estimated value of the activation energy of ZnFe2O4 nanoparticles during the growth is 18.207kJ∕mol.