Electrically-driven phase transition in magnetite nanostructures

TL;DR

This paper reports an electrically driven phase transition in magnetite nanostructures below the Verwey temperature, characterized by sharp conductance switching, which is not caused by heating but by breakdown of the insulating state.

Contribution

It demonstrates a new electrically induced phase transition in magnetite nanocrystals and thin films, providing insights into charge ordering and ground state mechanisms.

Findings

Observation of sharp conductance switching under high electric fields

Transition driven by electrical bias, not local heating

Hysteretic behavior in voltage during the transition

Abstract

Magnetite (Fe$_{3}$O$_{4}$), an archetypal transition metal oxide, has been used for thousands of years, from lodestones in primitive compasses[1] to a candidate material for magnetoelectronic devices.[2] In 1939 Verwey[3] found that bulk magnetite undergoes a transition at T$_{V}$ $\approx$ 120 K from a high temperature "bad metal" conducting phase to a low-temperature insulating phase. He suggested[4] that high temperature conduction is via the fluctuating and correlated valences of the octahedral iron atoms, and that the transition is the onset of charge ordering upon cooling. The Verwey transition mechanism and the question of charge ordering remain highly controversial.[5-11] 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.