Observation of the Verwey Transition in Fe 3 O 4 Nanocrystals
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Observation of the Verwey Transition in Fe3O4 Nanocrystals Gil Markovich1, Tcipi Fried1, Pankaj Poddar1, Amos Sharoni2, David Katz2, Tommer Wizansky2, and Oded Millo2 1 School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel 2 Racah Institute of Physics and the Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel ABSTRACT The electronic properties of arrays and isolated magnetite nanocrystals were studied using tunneling spectroscopy. Macroscopic tunnel junctions were used to study stacked arrays of the nanocrystals. The temperature dependent resistance measurements showed an abrupt increase of the resistance around 100 K, attributed to the Verwey metal-insulator transition, while the current-voltage characteristics exhibit a sharp transition from an insulator gap to a peak in the density of states near the Fermi energy. This conductance peak was sensitive to in-plane magnetic field showing large magnetoresistance. The tunneling spectra obtained on isolated particles using a Scanning Tunneling Microscope exhibit a gap-like structure below the transition temperature that gradually disappeared with increasing temperature, ending with a small peak structure around zero bias. INTRODUCTION The Verwey metal-insulator transition observed in magnetite (Fe3O4) has continuously attracted interest since its discovery more than 60 years ago[1]. Magnetite is a relatively good conductor at room temperature and on cooling below 120K its conductivity sharply drops by two orders of magnitude. It was described as a first-order metal-insulator transition accompanied by a structural phase transition where the cubic symmetry of the Fe3O4 crystal is broken by a small lattice distortion [2]. While the exact nature of the transition is still under controversy, it is understood that the driving force for this phenomenon is the strong electron-electron and electron-lattice interactions in the system. Special attention has been focused on the roles played by long-range and short-range charge ordering in driving the Verwey transition. The former is believed to exist below the transition temperature, TV, and the latter sustains well above it. The long-range order manifests itself by opening a gap in the electronic density of states (DOS) around the Fermi level (EF). This gap was detected by photoemission [3,4], optical [5], and tunneling [6] spectroscopies. The effect of short-range ordering on the DOS is not as clear. However, recent photoemission experiments [3,4] suggest that a reduced gap in the DOS, associated with a short-range ordering, still exists well above the transition temperature. In view of the importance of long-range charge ordering in determining the electronic properties below Tv, an intriguing question arises: Could the Verwey transition be observed in nanoscale magnetite crystals of diameter measuring several unit cells? Recently, Poddar et al. reported on the observation of a sharp Verwey transition in arrays of Fe3O4 (magnetite) nanoparticles of average si
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