As the oldest known magnetic material, magnetite (Fe3O4) has fascinated mankind for millennia. As the first oxide in which a relationship between electrical conductivity and fluctuating/localized electronic order was shown1, magnetite represents a model system for understanding correlated oxides in general. Nevertheless, the exact mechanism of the insulator–metal, or Verwey, transition has long remained inaccessible2,3,4,5,6,7,8. Recently, three-Fe-site lattice distortions called trimerons were identified as the characteristic building blocks of the low-temperature insulating electronically ordered phase9. Here we investigate the Verwey transition with pump–probe X-ray diffraction and optical reflectivity techniques, and show how trimerons become mobile across the insulator–metal transition. We find this to be a two-step process. After an initial 300 fs destruction of individual trimerons, phase separation occurs on a 1.5±0.2 ps timescale to yield residual insulating and metallic regions. This work establishes the speed limit for switching in future oxide electronics10.
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Research at Stanford was supported through the Stanford Institute for Materials and Energy Sciences (SIMES) under contract DE-AC02-76SF00515 and the LCLS by the US Department of Energy, Office of Basic Energy Sciences. Portions of this research were carried out on the SXR Instrument at the LCLS, a division of SLAC National Accelerator Laboratory and an Office of Science user facility operated by Stanford University for the US Department of Energy. The SXR Instrument is funded by a consortium whose membership includes the LCLS, Stanford University through the Stanford Institute for Materials Energy Sciences (SIMES), Lawrence Berkeley National Laboratory (LBNL, contract number DE-AC02-05CH11231), University of Hamburg through the BMBF priority programme FSP 301, and the Center for Free Electron Laser Science (CFEL). The stay of M.S.G. at SLAC was made possible by support from FOM/NWO and the Helmholtz Virtual Institute Dynamic Pathways in Multidimensional Landscapes. The Cologne team was supported by the DFG through SFB 608 and by the BMBF (contract 05K10PK2). The University of Hamburg team was supported by the SFB 925 ‘Light induced dynamics and control of correlated quantum systems’. The Elettra-Sincrotrone Trieste and University of Trieste team was supported by European Union Seventh Framework Programme [FP7/2007–2013] under grant agreement number 280555 and by the Italian Ministry of University and Research under grant numbers: FIRB-RBAP045JF2 and FIRB-RBAP06AWK3. D.F., F.N., M.E. and F.P. thank F. Cilento and F. Randi for technical support.
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Nature Communications (2016)