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.

The magnetoresistance ͑MR͒ behavior of epitaxial magnetite Fe 3 O 4 grown on low-vicinal ͑small miscut͒ and high-vicinal ͑large miscut͒ MgO substrates is compared. Magnetization measurements on Fe 3 O 4 films on high-vicinal substrates showed reduced magnetic moment as compared with the films grown on low-vicinal MgO, which correlates well with the expected reduction in magnetic moment due to step edge induced additional antiphase boundaries ͑APBs͒ with out-of-plane shift vectors. The MR is significantly higher ͑12.3% at 2 T͒ for a 45 nm Fe 3 O 4 film on high-vicinal substrate than that observed ͑7.2% at 2 T͒ for a film on low-vicinal substrate. A strong anisotropy in the MR is observed in correlation with the direction of atomic step edges. In addition to the increase in MR, the field dependency of the MR is also modified. The observed modification in the magnetotransport behavior of epitaxial Fe 3 O 4 films is attributed to an enhanced spin scattering arising due to the presence of atomic height steps that lead to the formation of a greater density of antiferromagnetically coupled APBs.

In many transition metal oxides the electrical resistance is observed to undergo dramatic changes induced by large biases. In magnetite, Fe3O4, below the Verwey temperature, an electric field driven transition to a state of lower resistance was recently found, with hysteretic current-voltage response. We report the results of pulsed electrical conduction measurements in epitaxial magnetite thin films. We show that while the high-to low-resistance transition is driven by electric field, the hysteresis observed in I − V curves results from Joule heating in the low resistance state. The shape of the hysteresis loop depends on pulse parameters, and reduces to a hysteresis-free "jump" of the current provided thermal relaxation is rapid compared to the time between voltage pulses. A simple relaxation time thermal model is proposed that captures the essentials of the hysteresis mechanism.PACS numbers: 71.30.+h,72.20.Ht Dramatic changes in resistance induced by electric fields, so called resistive switching (RS), have recently attracted much attention due to this phenomenon's potential application in memory devices (resistive random access memory, ReRAM) [1,2]. RS from high-to lowresistance states is driven by application of high voltage, and corresponding up-and-down sweeps of currentvoltage (I-V ) characteristics often show hysteresis, i.e. in sweeps up and down in bias voltage, the current does not retrace itself. Systems exhibiting hysteretic RS include organic compounds [3] and transition-metal oxides such as widely-studied colossal resistance manganites [4], perovskites (e.g. SrTiO 3 [5]), 1D cuprates Sr 2 CuO 3 [6], NiO [7], TiO 2 [8] etc.For some RS systems, while sweeping out a hysteresis loop in I-V with a switch to a low resistance state at high bias, the low resistance state persists down to zero current as voltage approaches zero. This behavior is often the case for RS systems where the switching is based on metallic filament formation at a transition point [5]. However, for some RS systems the low-resistance state persists only in some voltage interval, and the system returns to the high-resistance state before voltage returns to zero. This is the case for some complex oxides [4,9,10] as well as for magnetite nanostructures, which were recently shown to exhibit RS at low temperatures [11,12].Magnetite, Fe 3 O 4 , is an example of strongly correlated material. In equilibrium, bulk magnetite undergoes a structural transition at the Verwey temperature, T V ∼120 K, accompanied by three-order-of-magnitude change in electrical conductivity, i.e. a metal-insulator transition (MIT) [13]. Recently we demonstrated that magnetite nanoparticles and thin films, once in the insulating state below T V , exhibit RS under a sufficiently large voltage bias [11]. By examining RS systematically in different device geometries, the switching was demon-strated to be driven by the applied in-plane electric field. This is in contrast to previously observed transitions in magnetite driven by Joule heating of the samples above T V...

Strain relaxation studies in epitaxial magnetite ͑Fe 3 O 4 ͒ thin films grown on MgO ͑100͒ substrates using high-resolution x-ray diffraction and cross-sectional transmission electron microscopy reveal that the films remain fully coherent up to a thickness of 700 nm. This thickness is much greater than the critical thickness t c for strain relaxation estimated from mismatch strain. Anomalous strain relaxation behavior of Fe 3 O 4 / MgO heteroepitaxy is attributed to the reduction in the effective stress experienced by the film due to the presence of antiphase boundaries ͑APBs͒ that enable the film to maintain coherency with the substrate at large thickness. However, the stress accommodation in the film depends upon the nature and density of the APBs.

For nanoscale electrical characterization and device fabrication it is often desirable to fabricate planar metal electrodes separated by large aspect ratio gaps with interelectrode distances well below 100 nm. We demonstrate a self-aligned process to accomplish this goal using a thin Cr film as a sacrificial etch layer. The resulting gaps can be as small as 10 nm and have aspect ratios exceeding 1000, with excellent interelectrode isolation. Such Ti/Au electrodes are demonstrated on Si substrates and are used to examine a voltage-driven transition in magnetite nanostructures. This shows the utility of this fabrication approach even with relatively reactive substrates.PACS numbers: 81.07. -b,81.16.-c,85.35.-p There is much interest in the electronic characterization of nanoscale materials and the creation of working molecular-based devices [1]. Both goals demand the fabrication of metallic electrodes separated by a distance comparable with the targeted length, i.e. a few nanometers. Much recent progress has been made in nanogap fabrication, and several techniques were proposed, including electromigration [2,3,4], electrodeposition [5,6], mechanically controlled break junctions [7], advanced ebeam lithography methods [8,9,10,11,12], on-wire lithography [13], etc. Interelectrode distances down to 1-2 nm may be achieved [4,6,9] by some of these methods, though without much control of gap aspect ratio.A significant challenge is fabricating two electrodes separated by a nanometer gap running parallel over a macroscopic width (high-aspect-ratio (HAR) nanogaps). HAR nanogap fabrication has been demonstrated based on a selective etching of cleaved GaAs/AlGaAs heterostructures [14,15]. However, this method requires particular substrates and allows only restricted gap geometries. A much simpler technique was proposed recently [16] with potentially no limitations on the width of the gap. Two separate lithographic patterning steps are used to define first and second electrodes, while the interelectrode separation is controlled by the oxidation of an Al sacrificial layer deposited upon the first electrode. The native aluminum oxide layer, Al 2 O 3 , overhangs the underlying metal and serves as a mask during the deposition of the second electrode. This layer must be removed afterwards, but since Al 2 O 3 , corundum, is one of the most chemically inert materials [17], removal by direct chemical etching is very difficult. In a refinement, the authors deposited an additional sacrificial layer of SiO 2 and subsequently used etchant for SiO 2 to remove the SiO 2 and Al/Al 2 O 3 layers [16]. The use of SiO 2 etchant greatly limits the use of this approach in conjunction with conventional silicon electronics. While this method potentially allows fabrication of HAR nanogaps, the reported aspect ratios are less than 10 [16].In this letter we report a highly reproducible and flexible method for nanometer-sized (10-20 nm) gap fabrication with aspect ratios exceeding 1000. Modifying the original method [16] by replacing the Al layer wi...

One-dimensional Ag nanoparticle arrays have been grown on step-bunched vicinal Al 2 O 3 in ultrahigh vacuum using deposition at a glancing angle. The structures grown showed a strong optical anisotropy in the visible region of the spectrum. The optical anisotropy was measured in situ using reflection anisotropy spectroscopy. Relevant optical properties were determined as a function of deposition angle and Ag thickness. A simple phenomenological model was developed to reproduce the features seen in the spectra. With this model it was possible to use the inhomogeneous broadening as a guide to the nanoparticle dispersion.

Strain relaxation studies in epitaxial magnetite, Fe 3 O 4 , thin films grown on MgAl 2 O 4 ͑100͒ substrates are reported. The study shows that the films were relaxed in line with the theoretical model prediction with a critical thickness, t c = 5 nm. Antiphase boundaries ͑APBs͒ are not expected to form in Fe 3 O 4 films grown on MgAl 2 O 4 substrates because both film and substrate have the same crystal symmetry. In contrast, our study reveals the formation of APBs within the Fe 3 O 4 films. Our analysis shows that the APBs in a Fe 3 O 4 / MgAl 2 O 4 heteroepitaxial system are formed by partial dislocations, which accommodate the misfit. This formation mechanism of APBs is fundamentally different from the one found in the Fe 3 O 4 / MgO system, where APBs are formed as a consequence of equivalent nucleation sites on the MgO substrate separated by nontranslational vectors of the Fe 3 O 4 lattice. The mechanism for the formation of antiphase boundaries through partial dislocations should be applicable to a wide range of epitaxial systems having identical symmetries of the film and the substrate and significant lattice mismatch.

We observe a strong crystallographic direction dependence on the low-field magnetoresistance (MR) behavior of the epitaxial Fe 3 O 4 (110) films grown on MgO (110) substrates. The sign of MR is positive when the current and field are parallel to [001], whereas along the [110] direction its sign is negative, similarly to that commonly observed for (100) oriented Fe 3 O 4 films. We relate this effect to the presence of antiphase boundaries (APB) and subsequent reduction in the width of canted spin structure in its vicinity, due to the hard axis behavior of Fe 3 O 4 (110) films along this crystallographic direction. At fields greater than the anisotropy field, usual negative MR behavior related to a reduction in spin scattering at the APBs is observed. An analytical model based on the half-infinite spin chains across the APBs is provided to show that the positive MR is due to the domain walls along APBs. The temperature and film thickness dependency of the APB domain wall magnetoresistance is discussed.