The Verwey transition - a topical review

Abstract

This review encompasses the story of the Verwey transition in magnetite over a period of about 90 years, from its discovery up to the present. Despite this long period of thorough investigation, the intricate multi-particle system Fe3O4 with its various magneto-electronic interactions is not completely understood, as yet - although considerable progress has been achieved, especially during the last two decades. It therefore appeared appropriate to subdivide this retrospect into three eras: (I) from the detection of the effect to the Verwey model (1913-1947), being followed by a period of: (II) checking, questioning and modification of Verwey's original concepts (1947-1979). Owing to prevailing under-estimation of the role of crystal preparation and qualitiy control, this period is also characterized by a series of uncertainties and erroneous statements concerning the reaction order (one or two) and type of the transition (multi-stage or single stage). These latter problems, beyond others, could definitely be solved within era (III) (1979 to the present) - in favour of a first-order, single-stage transition near 125 K - on the basis of experimental and theoretical standards established in the course of a most inspiring conference organized in 1979 by Sir Nevill Mott in Cambridge and solely devoted to the present topic. Regarding the experimental field of further research, the remarkable efficiency of magnetic after-effect (MAE) spectroscopy as a sensitive probe for quality control and investigation of low-temperature (4 K<T<Tv) charge transport mechanisms is pointed out. Under theoretical aspects two concepts, going back to Mott and Ihle-Lorenz, presently appear most promising. Mott's view of the Verwey transition, as corresponding to the phase changing of a Wigner glass (T>Tv) into a Wigner crystal (T<Tv), describes most adequately the various low-temperature mechanisms in Fe3O4 in terms of tunnelling and variable range hopping of small polarons. On the other hand, the well-elaborated Ihle-Lorenz model, assuming a superposition of polaron-band and -hopping conductivity, is in better agreement with the high-temperature data (Tv<T<600 K).

Electrical transport properties of Ti-doped Fe2O3(0001) epitaxial films

“…33,34 Such a transition from high-temperature SPH to lowtemperature VRH has also been suggested to occur in other oxides such as Fe 3 O 4 , 44 Ni-doped LaFeO 3 , 45 and Cr-doped rare-earth manganites. 46 F. The effect of growth rate on conductivity…”

Mechanism of the Verwey Transition in Magnetite

“…Verwey suggested explaining it by a metalinsulator transition due to the reduction of electron mobility caused by ordering of Fe 3+ and Fe 2+ ions below T V . In spite of great experimental effort, however, the existence of ionic-like charge ordering at T < T V could not be confirmed, and the origin of the VT in magnetite remains still puzzling [2].Magnetite is a ferrimagnetic spinel with anomalously high critical temperature T c ≃ 860 K. Hence, it is viewed as an ideal candidate for room temperature spintronic applications. It crystalizes in the inverse spinel cubic structure, Fd3m, with two types of Fe sites: the tetrahedral A sites and the octahedral B ones, see Ref.…”

“…Verwey suggested explaining it by a metalinsulator transition due to the reduction of electron mobility caused by ordering of Fe 3+ and Fe 2+ ions below T V . In spite of great experimental effort, however, the existence of ionic-like charge ordering at T < T V could not be confirmed, and the origin of the VT in magnetite remains still puzzling [2].…”

Unconventional magnetization of Fe3O4 thin film grown on amorphous SiO2 substrate

“…[1][2][3][4] These properties make Fe 3 O 4 a promising candidate for spintronic devices. 5,6 Magnetite has a cubic inverse spinel structure that is based on a face-centered cubic (fcc) lattice of oxygen anions.…”

“…Upon cooling, bulk Fe 3 O 4 undergoes a metal-insulator transition termed as the Verwey transition. 1,4 Below 120 K, the hopping action is frozen and consequently the resistivity is increased by two orders of magnitude with a concomitant decrease in the magnetic moment. This transition is generally from a disordered phase to a charge ordered phase of Fe 2+ and Fe 3+ cations.…”