Magnetic nanoparticles with enhanced γ-Fe₂O₃to α-Fe₂O₃phase transition temperature

Abstract

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.

Preparation of bismuth substituted yttrium iron garnet powder and thin film by the metal-organic decomposition method

“…3 shows the XRD pattern of sintered powders at 400-900 1C. The XRD pattern of powder sintered at 400 1C is composed of strong b-Bi 2 O 3 peaks [16] with weak g-Fe 2 O 3 peaks [17] but the Y 2 O 3 phase cannot be seen. This suggests that the decomposition of bismuth and iron carboxylate occur at lower temperature than yttrium carboxylate.…”

Stoichiometry Dependence on the Diameter of La2/3Ca1/3MnO3 Manganite Nanoparticles

“…The forth term in (1) represents the influence of the external magnetic field, being h andĥ magnetic field and its direction respectively. For the simulation, three types of bond are considered: Mn 3+e g -OMn 3+e g , Mn 3+e g -O-Mn 4+d 3 and Mn 3+e g -O-Mn 4+d 3 with exchange parameters (J ij ) of J 1 = 4.65 eV, J 2 = 7.7 meV and J 3 = 1.4 meV respectively, according to the study reported by J.C. Riaño-Rojas et al [16]. On the other hand, in this case, single-spin Metropolis Monte Carlo algorithm was used, for obtaining equilibrium thermodynamic properties.…”

“…Magnetic nanoparticles have attracted a great deal of attention since they lead to many size dependent, electrical, magnetic, chemical, and optical properties [1][2][3]. One of the most studied magnetic materials employed in nanotechnology are doped manganites.…”

Preparation and characterization of carbon-encapsulated iron nanoparticles and their catalytic activity in the hydrogenation of levulinic acid

“…In nature, the transformation of Fe 3 O 4 into c-Fe 2 O 3 and a-Fe 2 O 3 is very common; it occurs in the oxidization of Fe 3 O 4 . [27,28] On the other hand, a-Fe 2 O 3 is reduced to c-Fe 2 O 3 , and Fe 3 O 4 when a reducing agent such as hydrogen gas is used during heat treatment [29]. In our system, during the sample annealing process, the reduction process occurs without requiring any additional reducing agent, thus c-Fe 2 O 3 , Fe 3 O 4 , and Fe can be selectively obtained from as-prepared a-Fe 2 O 3 nanoparticles.…”