Magnetic properties of γ-Fe2O3 nanoparticles obtained by vaporization condensation in a solar furnace

Abstract

The magnetic properties of γ-Fe2O3 nanoparticles synthesized by vaporization condensation in a solar image furnace have been studied using both magnetic measurements and Mössbauer spectroscopy. The mean size of the particles turns out to be easily controlled by changing the pressure conditions in the growth chamber. The particles exhibit superparamagnetic behavior at room temperature. Magnetic measurements show the appearance of magnetic hysteresis in the low-temperature range and from the evolution with temperature of the ferromagnetic ratio, MR/MS, we have determined the distribution of the blocking temperatures for the smallest particles that is fitted to a log-normal distribution leading to a mean blocking temperature 〈TB〉=38±15 K. The size distribution of the magnetic unit is also determined from this fitting, as well as from the Mössbauer spectra, obtaining a mean particle volume of about 3.5×105 Å3.

Nanostructured Pure γ‐Fe2O3 via Forced Precipitation in an Organic Solvent

“…First, our simple synthetic procedure has resulted in pure nanoscale mesoporous particles of g-Fe 2 O 3 , while most of the literature-reported work involved stabilizing the particles by dispersion into organic or inorganic matrices, [22±30] or by coating with relatively thick layer of organic additives. [31,32,49] Second, all studies that have been reported on the synthesis of pure g-Fe 2 O 3 involved extra steps such as the preparation of a-Fe 2 O 3 first followed by vaporization condensation to convert it to the g-phase, [38] or the preparation of Fe 3 O 4 first followed by oxidation to gFe 2 O 3 . [8,13] Direct precipitation in aqueous media under specific conditions has also resulted in g-Fe 2 O 3 , but this method is very sensitive to experimental conditions especially the pH of the solution which makes the formation of impurities very likely to occur.…”

Intra- and interparticle magnetism of cobalt-doped iron-oxide nanoparticles encapsulated in a synthetic ferritin cage

“…In the bulk, M S (T ) is described by the Bloch T 3/2 law, M S (T ) = M 0 (1 − BT α ) where α is equal to 3/2 and B is the Bloch constant (∝ 1/J). For the un-doped particles, M S (T ) was well represented by the normal Bloch law (α = 3/2), and provided B = 3.83(3)×10 −5 K −3/2 , consistent with an Fe-oxide nanoparticle 40,41 . Modifications to the Bloch law are obtained when a gap in the spin wave spectrum results from a high crystalline anisotropy 42 , or due to finitesize effects 43,44 .…”

Magnetic nanoparticle temperature estimation

“…31,32 In practice, we have found that the value of b for iron oxide, 2.8ϫ 10 −5 K −3/2 , is accurate and that a can vary slightly between samples when the frequency is higher than the blocking frequency but is generally approximately 1.65 for the commercial iron oxide nanoparticles we used, Feridex ® ͑Bayer AG, Leverkusen, Germany͒. These values were used for all the results presented.…”

“…31,32 The magnetization follows a Bloch's law T 3/2 relationship impacted by several factors including the coating of the nanoparticles, the surface structure, and the chemical composition. 33 The general relationship is of the form…”

“…33͒ rather than the more common value of 3 2 . 31,32 We looked at four nanoparticle samples that had essentially the same value of a for lower frequencies, below 2 kHz, but diverged significantly for frequencies in the 10-15 kHz range. The blocking frequency suggests that the effect might be related to aggregation but we were not able to verify differences between the samples using electron microscopy.…”