Low dimensional Fe3O4 and Fe1-xO: an ab initio approach

Tesis doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física Aplicada. Fecha de lectura: 15-05-201

Biphase ordering at Fe oxides

Oral presentation given at the 13th European Conference on Surface Crystallography and Dynamics, held in Donostia-San Sebastián, Spain, on June 19-21th, 2017

Charge order at magnetite Fe3O4(0 0 1): surface and verwey phase transitions

At ambient conditions, the Fe3O4(0 0 1) surface shows a (√2 x √2)R45° reconstruction that has been proposed as the surface analog of the bulk phase below the Verwey transition temperature, TV. The reconstruction disappears at a high temperature, TS, through a second order transition. We calculate the temperature evolution of the surface electronic structure based on a reduced bulk unit cell of P2/m symmetry that contains the main features of the bulk charge distribution. We demonstrate that the insulating surface gap arises from the large demand of charge of the surface O, at difference with that of the bulk. Furthermore, it is coupled to a significant restructuration that inhibits the formation of trimerons at the surface. An alternative bipolaronic charge distribution emerges below TS, introducing a competition between surface and bulk charge orders below TV.This work has been financed by the Spanish Ministry of Economy and Competitiveness under contracts MAT2009-14578-C03-03 and MAT2012-38045-C04-04. IB acknowledges financial support from the JAE program of the CSIC

Electronic phase transitions in ultrathin magnetite films

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.This work has been financed by the Spanish Ministry of Science under contract MAT2012-38045-C04-04. IB acknowledges financial support from the JAE program of the CSIC

Structure and magnetism of the (2×2) -FeO(111) surface

Based on ab initio calculations, we determine the features and relative stability of different models proposed to describe the (2×2)-FeO(111) reconstruction. Our results suggest that both wurtzite and spinel-like environments are possible, and their coexistence explains phenomena of biphase ordering. The surface phase diagram reflects a competition of charge and magnetic compensation effects, and reveals the important influence of the substrate on the final surface structure. Though antiferromagnetic couplings are dominant, frustration and the delicate balance of surface and bulk exchange interactions lead to a net surface magnetization that accounts for the large measured values

Electronic structure and polaronic charge distributions of Fe vacancy clusters in Fe1-x O

We perform a detailed study of the electronic structure of Fe1-xO at moderate values of x. Our results evidence that the Fe vacancies introduce significant local modifications of the structural, electronic, and magnetic features, which serve to explain the origin of the measured dependencies of the physical properties on x. The final properties are determined by a complex interplay of the charge demand from O, the magnetic interactions, and the charge order at the Fe sublattice. Furthermore, polaronic distributions of charge resembling those at magnetite, Fe3O4, emerge for the most stable defect structures. This defines a unique scenario to understand the nature of the short-range correlations in Fe3O4, and unveils their intimate connection to the long-range charge order developed below the Verwey transition temperature.This work has been financed by the Spanish Ministry of Economy and Competitiveness under Contract MAT2012-38045-C04-04. I.B. acknowledges financial support from the JAE program of the CSIC

Electronic phase transitions in thin magnetite films

Conference paper presented at the 14th edition of Trends in Nanotechnology International Conference (TNT2013), which took place in the Hotel Silken Al-Andalus Palace (Seville, Spain).Fe oxides are versatile materials present in a large number of applications from disparate fields, and in particular in emergent nanotechnologies related to environmental protection, bio-medicine or spintronics. Among them, magnetite (Fe3O4) occupies a relevant position. It is a half-metallic ferrimagnet with high magnetic moment and moderate conductivity, that undergoes a complex metal-insulator transition (the so-called Verwey transition) at low temperatures involving a structural transformation from the inverse spinel structure to a monoclinic symmetry, and the emergence of charge- and orbital- ordered patterns [1]. This transition is ultimately governed by electron-phonon couplings in the presence of strong electron correlations [2]. Interestingly, it has been shown that similar metal-insulator transitions can be induced by application of external electric fields in nanostructures, under hydrostatic pressure or strain, and at surfaces, eventhough much remains to be understood about the detailed microscopic characteristics of these induced transitions [3,4]. A large effort is currentl devoted to the investigation of magnetite thin films. Magnetite-oxide heterostructures are currently under scrutiny for the design of novel devices that exploit magnetism, electron correlation effects and interface phenomena: spin valves controlled by charge-orbital order [5] or non-volatile resistance switching driven by ferroelastic strain [6] have been proposed. All these phenomena rely on the control of the properties of magnetite in thin film form. However, the effect of reduced dimensionality on the metal-insulator transition, and on the relative stability with respect to other Fe oxide phases with different stoichiometry, magnetism and conductivity is not clear. Here we will address the study of thin magnetite films based on first-principles calculations. These calculations provide a unique tool to disentangle interface and bulk effects, and allow to simulate non- equilibrium conditions difficult to achieve in the experiments. We will focus on the effect of boundaries (both with the vacuum and with the substrate) and film thickness on the electronic and magnetic properties of different films. We will show how the emergence of charge and orbital order is intimately related to the symmetry of the film, and how it affects to magnetism and conductivity. Our results indicate the universality of the surface electronic gap, and its consequences on the existence of a threshold thickness to recover a Verwey-lik transition. Finally, we will also provide a detailed description of the magnetic order and how it is much less affected by the thickness of the films. These results support the ability to exploit electronic transitions involving magnetic order in novel devices based on ultrathin magnetite films

Competing charge orders in magnetite ultrathin films

Paper presented at the 13th International Ceramics Congress (2014), held in Montecatini Terme (Italy) during 8-13th JuneMagnetite (Fe3O4) is a half-metallic ferrimagnet with high critical temperature and large magnetic moment. The strong electron correlations lead to a rich scenario of phase transitions at low temperatures where charge order plays a crucial role: the Verwey transition at 120 K, with the opening of an insulating gap accompanied by a structural distortion, or the development of a ferroelectric polarization at 40 K. Modification of the Verwey transition has been pursued in thin films, under pressure or strain, or introducing defects, always causing a decrease of the Verwey temperature. However, the existence of an insulating gap at different Fe3O4(001) terminations at ambient conditions could translate the rich Verwey physics to higher temperatures. By means of ab-initio calculations, we will demonstrate that low dimensionality introduces a competition between surface and bulk charge orders, while preserving the magnetic properties. This is accompanied by a structural distortion involving the surface and subsurface layers, that ultimately manifests in a phase diagram distinct from the bulk one. We will also evidence the existence of a threshold thickness for the emergence of the two Fe valence states required for charge order and half-metallicity

Effect of dimensional constraints on the conduction and magnetic properties of magnetite

Conference paper presented at European Congress and Exhibition on Advanced Materials and Processes (EUROMAT 2013), which took place in Sevilla (Spain) during 8-13th September 2013

Building up a phase diagram for low dimensional Fe oxides

Poster presented at European Congress and Exhibition on Advanced Materials and Processes (EUROMAT 2013), which took place in Sevilla (Spain) during 8-13th September 2013