Judite Fernandes

Magnetism and charge ordering in Fe 3 O 2 BO 3 studied by Fe 57 Mössbauer spectroscopy

The homometallic Fe ludwigite Fe 3O2BO3, is an oxyborate with a chain structure of oxygen octahedra occupied by Fe 2+ and Fe3+ with four different crystallographic sites. We report Fe Mössbauer spectroscopic studies on the magnetic spin arrangements and on the charge ordering in Fe 3O2BO3, between 4 and 450 K. From the temperature dependence of the isomer shifts and quadrupole interactions, together with the information from previous x-ray and transport data, we are able to discuss the structural arrangement of Fe 2+ and Fe3+ and the dynamics of the electronic configurations. Below TN1=112 K the Mössbauer spectra show the onset of antiferromagnetic order only for a fraction of the Fe ions, the complementary fraction remaining paramagnetic down to 74 K. Below 74 K all the sample becomes magnetically ordered, and an increase and a canting of the magnetic hyperfine field for each Fe ion is observed. This canting is related to the presence of weak ferromagnetism, as proposed from magnetization measurements. Below ,50 K antiferromagnetic order is found. From Mössbauer spectra taken under applied external magnetic fields, there is strong evidence for magnetic short-range order above TN1 involving ferromagnetic clusters. The observed magnetic transitions appear to be clearly related to charge ordering.

Contributions of hydrogeophysics to the hydrogeological conceptual model of the Albufeira-Ribeira de Quarteira coastal aquifer in Algarve, Portugal

Geoelectrical and electromagnetic (time and frequency domains) hydrogeophysical methods were applied and jointly interpreted together with auxiliary information such as regional piezometric maps, borehole lithological logs and offshore data. The objective was to retrieve the structure and geometry of the Albufeira-Ribeira de Quarteira coastal aquifer system (Algarve, Portugal) and to upgrade the current hydrogeological conceptual model. The results allowed for the detection of the freshwater–saltwater interface along the coastline and identification of the water-bearing layers and aquitards and their hydraulic relationships. An explanation for the location of the inter- and subtidal fresh groundwater discharge is also presented and a new modeling unit is proposed for groundwater flow modeling. Limitations of the used hydrogeophysical methods are indicated and recommendations are made for follow-up studies.RésuméLes méthodes hydrogéophysiques de type géoélectrique et électromagnétique (domaines temporel et spatial) ont été appliquées et conjointement interprétées en utilisant comme information auxiliaire des surfaces de piézométrie régionale, des logs lithologiques de forage et des données de la plate-forme littorale. L’objectif était d’obtenir la structure et la géométrie du système aquifère côtier Albufeira-Ribeira de Quarteira (Algarve, Portugal) et de réviser l’actuel modèle conceptuel hydrogéologique. Les résultats ont permis de détecter l’interface eau douce-eau salée le long du littoral, de différencier niveaux aquifères et aquitards et d’établir leurs relations hydrauliques. Une explication de la localisation des décharges d’eau souterraine douce dans la zone inter- et subtidale est avancée et une nouvelle unité de modélisation de l’écoulement d’eau souterraine est proposée. Les limitations des méthodes hydrogéophysiques utilisées sont indiquées et des recommandations sont faites pour les études à venir.ResumenSe aplicaron métodos hidrogeofísicos geoeléctricos y electromagnéticos (en los dominios del tiempo y frecuencia) y se interpretaron en forma conjunta con información auxiliar, tales como mapas piezométricos regionales, registros litológicos de pozos y datos offshore. El objetivo fue recuperar la estructura y geometría del sistema acuífero costero Albufeira-Ribeira de Quarteira (Algarve, Portugal) y actualizar el actual modelo hidrogeológico conceptual. Los resultados permitieron la detección de una interfaz agua dulce-agua salada a lo largo de la línea costera y la identificación de las capas, acuíferos y acuitardos y sus relaciones hidráulicas. Se presenta también una explicación para la ubicación de la descarga de agua dulce subterránea inter y submareal y se propone una nueva unidad de modelado para el modelación de flujo de agua subterránea. Se indican las limitaciones de los métodos hidrogeofísicos utilizados y se hacen recomendaciones para los estudios de seguimiento.摘要应用了地电和电磁(时间和频率范围)水文地球物理方法并跟根据诸如区域测压图、钻孔岩性记录和近海资料等辅助信息对此进行了解译。目标就是获取(葡萄牙阿尔加维)Albufeira-Ribeira de Quarteira沿海含水层的结构和几何形状,对目前的水文地质概念模型进行升级。依赖获得的结果能检测到沿海岸线的淡水-咸水界面,识别出含水层和隔水层及其水力关系。还论述了交互的和潮下的地下淡水补给位置,为地下水流模拟提出了新的模拟单元。指出了所用水文地球物理方法的局限性,为后续研究提出了建议。ResumoOs métodos hidrogeofísicos geoelétricos e eletromagnéticos (domínios de tempo e de frequência) foram aplicados e conjuntamente interpretados utilizando como informação auxiliar mapas piezométricos regionais, logs litológicos de sondagens e dados da plataforma costeira. O objectivo foi obter a estrutura e a geometria do sistema aquífero costeiro Albufeira-Ribeira de Quarteira (Algarve, Portugal) e actualizar o modelo hidrogeológico conceptual. Os resultados permitiram detectar a interface água doce-água salina ao longo da cost, identificar as camadas aquíferas e aquitardas e definir as suas relações hidráulicas. Uma explicação para a localização das descargas de água doce subterrânea inter- e subtidais é avançada e uma nova unidade de modelação do fluxo de água subterrânea é proposta. As limitações dos métodos hidrogeofísicos utilizados foram indicadas e recomendações foram feitas para futuros estudos.

Input of geophysics to understand hydrogeology towards the assessment of geoenvironmental conditions in Beira city, Mozambique

As the fourth largest city in Mozambique, and located right beside the Indian Ocean, Beira faces sanitary and environmental problems that are a strong concern to the local and national authorities. One of the major problems that Beira city has to deal with is the existence of undrinkable groundwater in many dug-wells that supply a large amount of the city population. The cooperation project among Direcção Nacional de Geologia of Mozambique, the Laboratório Nacional de Energia e Geologia of Portugal and the Instituto de Apoio ao Desenvolvimento Português that took place from 2007 to 2011 has as one of its main goals the production of the Geological and Geoenvironmental Maps of Beira at scale 1:50,000, intending to create a tool to support the implementation of governmental policies regarding sustainable groundwater and natural resources exploitations, land management and environmental protection. These goals were achieved through the identification, characterization and mapping of the different geological units and the evaluation of the city environmental problems regarding water and soil contamination in urban areas, involving the use of several methodologies that were subsequently integrated and jointly interpreted. These comprised remote sensing techniques, hydrogeology analysis and evaluation and the use of geochemistry and electrical geophysical methods. This work emphasizes the input of geophysical and hydrogeological methods to assess the geoenvironmental conditions of Beira, namely those related with the prediction of occurrence of saline and brackish water in the shallow dug-wells that supply a large amount of Beira populations, especially in the crowded suburbs. Several geophysical surveys using electrical and electromagnetic methods were conducted and interpreted regarding the evaluation of the several hypotheses that were stood for this feature. Results are related with hydrogeological conditions and organic and inorganic hydrochemical analysis. Considering these facts, this study was an important input for mapping areas with saline and brackish water imprisoned in the subsurface geological formations, using a wider set of methodologies that were integrated in these results.

Non-linear conduction due to depinning of charge order domains in Fe3O2BO3

The oxyborate Fe3O2BO3 presents a charge density wave (CDW) transition close to room temperature. As we show here, this is associated with a well defined anomaly in the specific heat. Below this transition, when applying in a single crystal of Fe3O2BO3 a DC voltage above a temperature dependent threshold, a high current is liberated in this material. We study the conduction in single crystals of Fe3O2BO3 with voltage applied parallel and perpendicular to the crystallographic c axis direction. The observed currents are attributed to the depinning of charge ordered domains above a threshold voltage V T2 that gives rise to a collective conduction due to coherent domains. Compliance limited DC data shows that above a lower threshold voltage depinning is smooth and follows a power law scaling. Similar depinning with power law scaling is also revealed in the AC conductivity.