Slow oxidation of magnetite nanoparticles elucidates the limits of the Verwey transition

Abstract

Magnetite (Fe3O4) is of fundamental importance for the Verwey transition near TV = 125 K, below which a complex lattice distortion and electron orders occur. The Verwey transition is suppressed by chemical doping effects giving rise to well-documented first and second-order regimes, but the origin of the order change is unclear. Here, we show that slow oxidation of monodisperse Fe3O4 nanoparticles leads to an intriguing variation of the Verwey transition: an initial drop of TV to a minimum at 70 K after 75 days and a followed recovery to 95 K after 160 days. A physical model based on both doping and doping-gradient effects accounts quantitatively for this evolution between inhomogeneous to homogeneous doping regimes. This work demonstrates that slow oxidation of nanoparticles can give exquisite control and separation of homogeneous and inhomogeneous doping effects on the Verwey transition and offers opportunities for similar insights into complex electronic and magnetic phase transitions in other materials.

Magnetism and the Trimeron Bond

“…Insight into the origin of the first and second order regimes of doped magnetites has recently been provided by a study of slow oxidation of magnetite nanoparticles . This revealed that the Verwey transition is initially suppressed to a minimum value at T V ≈ 80 K, but on further oxidation recovers to a persistent value of T V = 95 K as shown in Figure .…”

“… Variation of the Verwey transition of magnetite nanoparticles with oxidation time in air from ref ( 52 ). Square, circle, triangle, and diamond symbols show T V values from magnetization ( M ), powder X-ray diffraction (XRD), NMR, and heat capacity ( C / T ) measurements, respectively.…”

“…This persistent value is identified as the limit of the first order (homogeneous) doping regime. Material reprinted from ref ( 52 ). Published 2021 by Springer Nature Limited under a Creative Commons license ( ).…”

“…24 − 26 , 28 Developments in nanoparticle chemistry have enabled influences of particle size and oxygen content on the Verwey transiton to be determined in exquisite detail. 34 , 52 High pressure discoveries of new mixed-valent binary iron oxides have broadened insights into magnetite. 56 , 58 − 60…”

“…The origin of the change from first to second order Verwey transitions in doped magnetites has also been revealed by a nanoparticle oxidation study. 52 …”