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New surprise in an old material: Nonequilibrium Phase Transition in Magnetite

Magnetite (Fe3O4), also known as lodestone, has been studied for thousands of years, since its first use in primative compasses to current applications in magnetoelectronic devices. A ferrimagnet, magnetite is an example of a strongly correlated material - one that cannot be described with conventional approaches that ignore electron-electron interactions. Magnetite is moderately conducting at room temperature, but upon cooling becomes more resistive. Upon cooling through 120 K (-153 C), magnetite goes through a phase change (the "Verwey transition") to a considerably more insulating state. The nature of this insulating state and the mechanism of the transition have been highly controversial since their discovery in 1939

Over the last two decades, many techniques have been developed to study the electronic properties of materials at the nanometer scale. Until very recently these methods have only been applied to conventional semiconductors and metals, rather than strongly correlated materials like magnetite. At the same time, new chemical and physical methods have been developed for growing both nanoscale and macroscopic single crystals of many interesting materials, including magnetite.

Natelson's group, working with material provided by collaborators, measured the conduction properties of magnetite nanostructures and made a surprising discovery: At low temperatures it is possible to "kick" magnetite out of its insulating state and back into a more conducting state by applying a sufficiently large electric field. This nonequilibrium transition is extremely sharp and hysteretic, and provides a new avenue of study of magnetite and related materials. This is also an example of the kind of new measurements enabled by the development of nanoscale electronic techniques.

S. Lee, A. Fursina, J. T. Mayo, C. T. Yavuz, V. L. Colvin, R. G. S. Sofin, I. V. Shvets, and D. Natelson Electrically-driven phase transition in magnetite nanostructures. Nature Materials, in press (Reprint)

Press Release
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Current-voltage characteristics of a magnetite single-crystal thin film at different temperatures. Below the Verwey transition temperature, there is dramatic switching and hysteresis. In this particular device there are multiple switching thresholds. Understanding the origins of these transitions will give new insights into the nature of the insulating ground state and the nonequilibrium conducting state of this long-investigated material.