Structural and magnetic transformation of monodispersed iron oxide particles in a reducing atmosphere

Abstract

Uniform metal iron ellipsoidal particles of around 200 nm in length were obtained by reduction and passivation of alumina-coated ␣ -Fe 2 O 3 ͑hematite͒ particles under different conditions of temperature and hydrogen flow rate. The monodispersed hematite particles were prepared by the controlled hydrolysis of ferric sulfate and further coated with a homogeneous thin layer of Al 2 O 3 by careful selection of the experimental conditions, mainly pH and aluminum salt concentration. The reduction mechanism of ␣ -Fe 2 O 3 into ␣-Fe was followed by x-ray and electron diffraction, and also by the measurements of the irreversible magnetic susceptibility. The transformation was found to be topotactic with the ͓001͔ direction of hematite particles, which lies along the long axis of the particles, becoming the ͓111͔ direction of magnetite and finally the ͓111͔ direction of metal iron. Temperature and hydrogen flow rate during the reduction have been found to be important parameters, which determine not only the degree of reduction but also the crystallite size of the final particles. Magnetic characterization of the samples shows that the only parameters affected by the crystallite size are the saturation magnetization and magnetic time-dependence effect, i.e., activation volume.

Synthesis and characterization of Fe3O4 nanoparticles with perspectives in biomedical applications

“…Studies on the reduction in magnetization in iron oxide nanoparticles (γ-Fe 2 O 3 , Fe 3 O 4 ) had reported that reduction is because of the existence of noncollinear spins at the surface of the nanoparticles 5,23 . Varanda et al 24 described a linear correlation between Ms and nanoparticles size suggesting that defects at the nanoparticle surface can influence the magnetic properties 5,24 . The surface curvature of particles was higher for nanoparticles which stimulated disordered crystal orientation on the surface resulting in significantly decreased Ms…”

Plasma-chemical Synthesis of Fe3O4 Nanoparticles for Doping of High-Temperature Superconductors

“…Furthermore, the produced nanoparticles have less value of M s than that of the particles with large volume. Authors of [28] found a linear relationship between parameter M s and sizes of the nanoparticles; they explained it by the highly developed surface. According to the studies of the specific surface, there is a clusters layer of ~ 2 nm in size on each nanoparticle, which partially prevents dipole-dipole interaction between particles, despite their strong agglomeration.…”

Magnetic and Multifunctional Magnetic Nanoparticles in Nanomedicine: Challenges and Trends in Synthesis and Surface Engineering for Diagnostic and Therapy Applications

“…Also, in magnetite fine particles, Varanda et al have reported a linear correlation between saturation magnetization and particle size, suggesting that defects at the particle surface can influence the magnetic properties. The surface curvature of the NP was much larger for smaller particle size, which encouraged disordered crystal orientation on the surface and thus resulted in significantly decreased Ms in smaller NPs (Varanda et al, 2002b). In this context, advancement in the use of magnetic particles for biomedical applications depends on the new synthetic methods with better control of the size distribution, magnetic properties and the particle surface characteristics.…”