Lee Sungbae, Fursina Alexandra, Mayo John T, Yavuz Cafer T, Colvin Vicki L, Sofin R G Sumesh, Shvets Igor V, Natelson Douglas
Department of Physics and Astronomy, Rice University, 6100 Main St., Houston, Texas 77005, USA.
Nat Mater. 2008 Feb;7(2):130-3. doi: 10.1038/nmat2084. Epub 2007 Dec 16.
Magnetite (Fe3O4), an archetypal transition-metal oxide, has been used for thousands of years, from lodestones in primitive compasses to a candidate material for magnetoelectronic devices. In 1939, Verwey found that bulk magnetite undergoes a transition at TV approximately 120 K from a high-temperature 'bad metal' conducting phase to a low-temperature insulating phase. He suggested 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 controversial. 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.
磁铁矿（Fe3O4）作为一种典型的过渡金属氧化物，已有数千年的应用历史，从原始指南针中的天然磁石到磁电子器件的候选材料。1939年，韦尔韦发现块状磁铁矿在约120K的温度（TV）下会发生转变，从高温的“不良金属”导电相转变为低温绝缘相。他认为高温导电是通过八面体铁原子的波动和相关价态实现的，并且这种转变是冷却时电荷有序化的开始。韦尔韦转变机制以及电荷有序化问题仍然极具争议。在此，我们表明磁铁矿纳米晶体和单晶薄膜在低于韦尔韦温度时表现出电驱动的相变。这种转变的特征是在高电场中出现尖锐的电导切换，且具有电压滞后现象。我们证明这种转变不是由于局部加热，而是由于电偏压使其偏离平衡态时相关绝缘态的破坏。我们预计，对这种新观察到的转变及其低温导电相的进一步研究将有助于揭示电荷有序化和振动自由度如何决定这种重要化合物的基态。
