Magnetite (Fe3O4), an archetypal transition-metal oxide, has been used for thousands of years, from lodestones in primitive compasses1 to a candidate material for magnetoelectronic devices2. In 1939, Verwey3 found that bulk magnetite undergoes a transition at TV≈120 K from a high-temperature ‘bad metal’ conducting phase to a low-temperature insulating phase. He suggested4 that high-temperature conduction is through the fluctuating and correlated valences of the octahedral iron atoms, and that the transition is the onset of charge ordering on cooling. The Verwey transition mechanism and the question of charge ordering remain highly controversial5,6,7,8,9,10,11. Here, we show that magnetite nanocrystals and single-crystal thin films exhibit an electrically driven phase transition below the Verwey temperature. The signature of this transition is the onset of sharp conductance switching in high electric fields, hysteretic in voltage. We demonstrate that this transition is not due to local heating, but instead is due to the breakdown of the correlated insulating state when driven out of equilibrium by electrical bias. We anticipate that further studies of this newly observed transition and its low-temperature conducting phase will shed light on how charge ordering and vibrational degrees of freedom determine the ground state of this important compound.

Higher environmental standards have made the removal of arsenic from water an important problem for environmental engineering. Iron oxide is a particularly interesting sorbent to consider for this application. Its magnetic properties allow relatively routine dispersal and recovery of the adsorbent into and from groundwater or industrial processing facilities; in addition, iron oxide has strong and specific interactions with both As(III) and As(V). Finally, this material can be produced with nanoscale dimensions, which enhance both its capacity and removal. The objective of this study is to evaluate the potential arsenic adsorption by nanoscale iron oxides, specifically magnetite (Fe3O4) nanoparticles. We focus on the effect of Fe3O4 particle size on the adsorption and desorption behavior of As(III) and As(V). The results show that the nanoparticle size has a dramatic effect on the adsorption and desorption of arsenic. As particle size is decreased from 300 to 12 nm the adsorption capacities for both As(III) and As(V) increase nearly 200 times. Interestingly, such an increase is more than expected from simple considerations of surface area and suggests that nanoscale iron oxide materials sorb arsenic through different means than bulk systems. The desorption process, however, exhibits some hysteresis with the effect becoming more pronounced with small nanoparticles. This hysteresis most likely results from a higher arsenic affinity for Fe3O4 nanoparticles. This work suggests that Fe3O4 nanocrystals and magnetic separations offer a promising method for arsenic removal. r 2006 NIMS and Elsevier Ltd. All rights reserved.

Magnetic separations at very low magnetic field gradients (<100 tesla per meter) can now be applied to diverse problems, such as point-of-use water purification and the simultaneous separation of complex mixtures. High–surface area and monodisperse magnetite (Fe3O4) nanocrystals (NCs) were shown to respond to low fields in a size-dependent fashion. The particles apparently do not act independently in the separation but rather reversibly aggregate through the resulting high-field gradients present at their surfaces. Using the high specific surface area of Fe3O4 NCs that were 12 nanometers in diameter, we reduced the mass of waste associated with arsenic removal from water by orders of magnitude. Additionally, the size dependence of magnetic separation permitted mixtures of 4- and 12-nanometer–size

Numerous studies have examined arsenic adsorption on varying adsorbents including iron oxides, aluminum hydroxides, alumina, and carbon as a means of arsenic removal in drinking water treatments. The objectives of this study were to evaluate the effect of magnetite particle size on the adsorption and desorption behavior of arsenite and arsenate, and to investigate the competitive adsorption between natural organic matter (NOM) and arsenic. Increases in adsorption maximum capacities for arsenite and arsenate were observed with decreasing magnetite particle size. Arsenic desorption is hysteretic, more so with the smaller nanoparticles. Such desorption hysteresis might result from a higher arsenic affinity for magnetite nanoparticles. In the presence of NOM, substantial decrease in arsenic sorption to magnetite nanoparticles was observed. It would be beneficial to thoroughly investigate adsorption and desorption of arsenic on magnetite nanoparticles for further practical purposes.

Iron oxide (Fe3O4, magnetite) nanocrystals of 6 to 30 nm with narrow size distributions (σ = 5–10%) were prepared by the pyrolysis of iron carboxylate salts.