Tracking the Verwey Transition in Single Magnetite Nanocrystals by Variable-Temperature Scanning Tunneling Microscopy

Abstract

Variable-temperature scanning tunneling spectroscopy revealed a sharp Verwey transition in individual ∼10 nm magnetite nanocrystals prepared by the coprecipitation technique and embedded in the surface of a gold film. The transition was observed as a significant change in the electronic structure around the Fermi level, with an apparent band gap of ∼140-250 meV appearing below the transition temperature and a pseudogap of ∼75 ± 10 meV appearing above it. The transition temperature was invariably observed around 101 ± 2 K for different nanocrystals, as opposed to 123 K typically reported for stoichiometric bulk crystals. This suggests that the lowering of the transition temperature is an intrinsic finite size effect, probably due to the presence of the surface.

Magnetic Properties of Nanocomposites

“…It should be remarked that this material does not show any trace of the Verwey transition. As a matter of fact, this transition is a rather elusive effect in systems of magnetite nanoparticles: for instance, the recent literature indicates a strong reduction [28][29][30][31] or even a complete disappearance [32,33] of the Verwey transition temperature T V in small particles. Moreover, the particle shape plays an important role in the Verwey transition, at least in ultrasmall nanoparticles [34].…”

Band Gap in Magnetite above Verwey Temperature Induced by Symmetry Breaking

“…21 Another STS investigation on magnetite nanocrystals shows that a band gap of 140–250 meV is present below T V and of 75 ± 10 meV above T V . 22 However, one may question that both PES and STS are surface sensitive techniques and the results may depend on the surface considered. 23 Schrupp et al 14 probed deeper bulk layers by using higher energy photons and provided evidence for the existence of strongly bound small polarons, which were proposed to be responsible for the conductivity above T V .…”

Reverse‐Engineering Strain in Nanocrystallites by Tracking Trimerons

“…The decrease from the bulk T V of ≈123 to 20 K and 60 K for the 40 and 50 nm nanorods, respectively, is consistent with previous nanoparticle results, where T V decreases with nanoparticle size. [ 22,34–36 ] Because the average crystallite sizes of the 40 and 50 nm nanorods are very similar, our results indicate that the transition temperature is affected by crystallite strain not size, with T V decreasing with an increase in the isotropic compressive strain (−0.53 ± 0.03% vs −0.21 ± 0.05%). Thus, the disappearance of the Verwey transition in very small (<6 nm) Fe 3 O 4 nanoparticles [ 22 ] is caused by increased isotropic strain from surface lattice distortion.…”