Electronic phase transitions in ultrathin magnetite films

Abstract

Magnetite (Fe3O4) shows singular electronic and magnetic properties, resulting from complex electron-electron and electron-phonon interactions that involve the interplay of charge, orbital and spin degrees of freedom. The Verwey transition is a manifestation of these interactions, with a puzzling connection between the low temperature charge ordered state and the dynamic charge fluctuations still present above the transition temperature. Here we explore how these rich physical phenomena are affected by thin film geometries, particularly focusing on the ultimate size limit defined by thicknesses below the minimum bulk unit cell. On one hand, we address the influence of extended defects, such as surfaces or antiphase domains, on the novel features exhibited by thin films. On the other, we try to isolate the effect of the reduced thickness on the electronic and magnetic properties. We will show that a distinct phase diagram and novel charge distributions emerge under reduced dimensions, while holding the local high magnetic moments. Altogether, thin film geometries offer unique possibilities to understand the complex interplay of short- and long-range orders in the Verwey transition. Furthermore, they arise as interesting candidates for the exploitation of the rich physics of magnetite in devices that demand nanoscale geometries, additionally offering novel functionalities based on their distinct properties with respect to the bulk form.

Electronic structure and polaronic charge distributions of Fe vacancy clusters inFe1−xO

“…The d(Fe-O) for the octahedral Fe sites do not differ substantially from the case corresponding to three isolated V Fe , but there is an important shortening of the bond lengths corresponding to Fe A . The values are close to those found at the tetrahedral sublattice in magnetite [28]. Oppositely, important differences can be found in the d(Fe-Fe) depending on the arrangement of the V Fe .…”

“…Analysis of the orbital character of these states indicates that they are formed by t 2g orbitals lying along the CL. The combined presence of all these features is a signature of the short-range charge correlation units in magnetite [28], the trimerons, formed by groups of three neighboring Fe B 3+ -Fe B 2+ -Fe B 3+ sites with shortened interatomic distances and polaronic charge sharing [42,43]. We will discuss these aspects in more detail in the last section.…”

“…As mentioned in the Introduction, creation of an Fe vacancy is expected to induce a change of valence on two Fe cations, which should act as Fe 3+ . We label them Fe B 3+ by analogy to magnetite: in Fe 3 O 4 , Fe cations order in octahedral (B) and tetrahedral (A) sublattices, and at low temperatures a charge disproportionation at sublattice B leads to Fe atoms acting either with valence 3+ (Q B = 6.37) or 2+ (Q B = 6.64) [28,41]. Here, the Fe B 3+ sites are among the first neighbors of the Fe vacancy, and belong to the same FM layer [see Fig.…”

“…Then, we describe unit cells with three V Fe , corresponding to a composition Fe 0.906 O, comparing two different situations: disperse V Fe and 4:1 clusters. In the last section we explore the similitudes between the charge distributions of Fe 1−x O and Fe 3 O 4 based on our previous calculations of magnetite [28].…”

Surface Observation and Magnetism of Oil-Extracted Botryococcus braunii Residues before and after Carbonization

“…Fe 3 O 4 is known for adopting the cubic inverse spinel crystal structure, in which Fe 2+ and Fe 3+ cations locate on two ferromagnetic Fe sublattices octahedral and tetrahedral. The Verwey transition is a process involving changes in the structural, electronic, and magnetism [40]. In this measurement, the phase transition temperature at 210 K is close to neither the Verwey transition temperature of Fe 3 O 4 nor the Néel temperature of FeO (298 K) [41].…”