Nanoclusters and nanolines: the effect of molybdenum oxide substrate stoichiometry on iron self-assembly

The growth of Fe nanostructures on the stoichiometric MoO₂/Mo(110) and oxygen-rich MoO₂+x /Mo(110) surfaces has been studied using low-temperature scanning tunnelling microscopy (STM) and density functional theory calculations. STM results indicate that at low coverage Fe nucleates on the MoO₂/Mo(110) surface, forming small, well-ordered nanoclusters of uniform size, each consisting of five Fe atoms. These five-atom clusters can agglomerate into larger nanostructures reflecting the substrate geometry, but they retain their individual character within the structure. Linear Fe nanocluster arrays are formed on the MoO₂/Mo(110) surface at room temperature when the surface coverage is greater than 0.6 monolayers. These nanocluster arrays follow the direction of the oxide rows of the strained MoO₂/Mo(110) surface. Slightly altering the preparation procedure of MoO₂/Mo(110) leads to the presence of oxygen adatoms on this surface. Fe deposition onto the oxygen-rich MoO₂+x /Mo(110) surface results in elongated nanostructures that reach up to 24 nm in length. These nanolines have a zigzag shape and are likely composed of partially oxidised Fe formed upon reaction with the oxygen-rich surface

Electrically-driven phase transition in magnetite nanostructures

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.Comment: 17 pages, 4 figure