Speed limit of the insulator–metal transition in magnetite

The insulator-to-metal transition occurring in magnetite is known as the Verwey transition, and its precise mechanism has recently come under renewed attention. Using pump–probe X-ray diffraction and optical reflectivity techniques, the dynamics of excitations known as trimerons are now examined, revealing the switching limits of this ubiquitous oxide material. As the oldest known magnetic material, magnetite (Fe 3 O 4 ) has fascinated mankind for millennia. As the first oxide in which a relationship between electrical conductivity and fluctuating/localized electronic order was shown 1 , 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 inaccessible 2 , 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 phase 9 . 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 electronics 10 .