Magnetite shows the famous Verwey phase transition at 120 K, the first reported metal-insulator transition (in the 1940s). But what happens in nanostructures? We analyze highly perfect magnetite crystals with heights of nm’s.

We have monitored the Verwey transition in micrometer-wide, nanometer-thick magnetite islands on epitaxial Ru films on Al2O3(0001) using Raman spectroscopy. The islands have been grown by high-temperature oxygen-assisted molecular beam epitaxy. Below 100 K and for thicknesses above 20 nm, the Raman spectra correspond to those observed in bulk crystals and high-quality thin films for the sub-Verwey magnetite structure. At room temperature, the width of the cubic phase modes is similar to the best reported in bulk crystals, indicating a similar level of electron–phonon interaction. The evolution of the Raman spectra upon cooling suggests that for islands thicker than 20 nm, structural changes appear first at temperatures starting at 150 K and the Verwey transition itself takes place at around 115 K. However, islands thinner than 20 nm show very different Raman spectra, indicating that while a transition takes place, the charge order of the ultrathin islands differs markedly from their thicker counterparts. A. del Campo, S. Ruiz-Gómez, E.M. Trapero, C. Granados-Miralles, A. Quesada, M. Foerster, L. Aballe, J.E. Prieto and J. de la Figuera, "Size Efects in the Verwey Transition of Nanometer-Thick Micrometer-Wide Magnetite Crystals", J. Phys. Chem. C 126, 13755−13761 (2022). https://doi.org/10.1021/acs.jpcc.2c03391