Size effects in the Verwey transition of nanometer-thick micron-wide magnetite crystals

TL;DR

This study investigates how the size and thickness of magnetite nanocrystals influence the Verwey transition, revealing that thinner islands exhibit different charge ordering behavior and transition temperatures compared to thicker ones.

Contribution

It provides new insights into size-dependent effects on the Verwey transition in magnetite nanocrystals using Raman spectroscopy.

Findings

Islands >20 nm thick show bulk-like Verwey transition behavior.

Ultrathin islands (<20 nm) display altered charge order and transition characteristics.

Structural changes begin at ~150 K for thicker islands, with the transition at ~115 K.

Abstract

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 a very different Raman spectra indicating that while a transition takes place, the charge order of the ultrathin islands differs markedly from their thicker counterparts