Size Effects in the Verwey Transition of Nanometer-Thick Micrometer-Wide Magnetite Crystals

We have monitored the Verwey transition in micrometer-wide, manometer-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 for bulk crystals, indicating a similar strength of electron-phonon interaction. The evolution of the Raman spectra upon cooling suggests that for islands thicker than 20 urn, structural changes appear first at temperatures starting at 150 K while 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.

Automated summary

The study reveals distinct evolutions of the Verwey transition on magnetite islands of different thicknesses, with islands thicker than 20nm showing differences in charge order compared to their thinner counterparts.