Studies on magnetocaloric and magnetic coupling effects

Doutoramento em FísicaO presente trabalho apresenta novas metodologias desenvolvidas para a análise das propriedades magnéticas e magnetocalóricas de materiais, sustentadas em considerações teóricas a partir de modelos, nomeadamente a teoria de transições de fase de Landau, o modelo de campo médio molecular e a teoria de fenómeno crítico. São propostos novos métodos de escala, permitindo a interpretação de dados de magnetização de materiais numa perspectiva de campo médio molecular ou teoria de fenómeno crítico. É apresentado um método de estimar a magnetização espontânea de um material ferromagnético a partir de relações entropia/magnetização estabelecidas pelo modelo de campo médio molecular. A termodinâmica das transições de fase magnéticas de primeira ordem é estudada usando a teoria de Landau e de campo médio molecular (modelo de Bean-Rodbell), avaliando os efeitos de fenómenos fora de equilíbrio e de condições de mistura de fase em estimativas do efeito magnetocalórico a partir de medidas magnéticas. Efeitos de desordem, interpretados como uma distribuição na interacção magnética entre iões, estabelecem os efeitos de distribuições químicas/estruturais nas propriedades magnéticas e magnetocalóricas de materiais com transições de fase de segunda e de primeira ordem. O uso das metodologias apresentadas na interpretação das propriedades magnéticas de variados materiais ferromagnéticos permitiu obter: 1) uma análise quantitativa da variação de spin por ião Gadolínio devido à transição estrutural do composto Gd5Si2Ge2, 2) a descrição da configuração de cluster magnético de iões Mn na fase ferromagnética em manganites da família La-Sr e La-Ca, 3) a determinação dos expoentes críticos β e δ do Níquel por métodos de escala, 4) a descrição do efeito da pressão nas propriedades magnéticas e magnetocalóricas do composto LaFe11.5Si1.5 através do modelo de Bean-Rodbell, 5) uma estimativa da desordem em manganites ferromagnéticas com transições de segunda e primeira ordem, 6) uma descrição de campo médio das propriedades magnéticas da liga Fe23Cu77, 7) o estudo de efeitos de separação de fase na família de compostos La0.70-xErxSr0.30MnO3 e 8) a determinação realista da variação de entropia magnética na família de compostos de efeito magnetocalórico colossal Mn1-x-yFexCryAs

Estudo de manganites modificadas com iões de terra rara

O presente trabalho baseia-se num estudo das propriedades magnéticas e estruturais de manganites do tipo LaSrMnO3 e LaCaMnO3, modificadas por troca de iões de Lantânio por iões de Érbio ou Európio. Uma das propriedades em estudo mais detalhado é a variação de entropia magnética sob campo magnético aplicado – o efeito magnetocalórico. É desenvolvido um método de aplicação da teoria de transições de fase de Landau no estudo da influência do acoplamento magnetoelástico no efeito magnetocalórico dos materiais em estudo. Para um dado material ferromagnético, o máximo de variação de entropia magnética ocorre perto da sua temperatura de Curie (transição paramagnética-ferromagnética, TC). No caso das composições iniciais em estudo, os valores de TC são ~ 375 K para La0.70Sr0.30MnO3 e ~ 260 K para La0.70Ca0.30MnO3 . Ao substituir os iões de Lantânio por iões de Érbio ou Európio, o TC diminui em ambos os sistemas. Devido à diferença de raios iónicos entre os elementos substituídos, ao introduzir iões de Érbio no sistema La0.70Sr0.30MnO3 ocorre formação de uma segunda fase estruturalmente equivalente a ErMnO3. Este fenómeno tem consequências nas propriedades magnéticas do sistema em estudo. A diminuição de TC com a substituição de iões permite o controle de TC por composição, que tem interesse tecnológico no campo de refrigeração magnética, podendo no caso do sistema La0.70Sr0.30MnO3 ser controlada para refrigeração magnética à temperatura ambiente. Os estudos de microscopia electrónica, difracção de raios-X, magnetização até campo aplicado de 5T permitiu um estudo detalhado das propriedades magnéticas e estruturais dos sistemas em estudo, que, em conjunto com a aplicação da teoria de Landau ao efeito magnetocalórico desenvolvida neste trabalho, permitiu igualmente interpretar a influência do acoplamento magnetoelástico nos resultados obtidos.This work presents a study of magnetic and structural properties of LaSrMnO3 and LaCaMnO3 manganites, where the Lanthanum ion is substituted by Erbium or Europium ions. A detailed study of magnetic entropy variation near the ferromagnetic-paramagnetic phase transition, under an applied magnetic field (magnetocaloric effect) is presented. The Landau theory of phase transitions is applied to the study of the magnetocaloric effect. For any type of ferromagnetic material, the maximum of magnetic entropy variation occurs near its Curie temperature (paramagnetic-ferromagnetic transition, TC). The parent compositions under study have TC values of ~ 375 K for La0.70Sr0.30MnO3 and ~ 260 K for La0.70Ca0.30MnO3. By substituting the Lanthanum ions by either Erbium or Europium, the TC values decrease. Due to the ionic size mismatch, by substituting Erbium ions in the LSMO system there occurs a formation of secondary phase, structurally equivalent to ErMnO3. This phenomenon will alter the magnetic properties of the studied systems. The lowering of TC by substitution allows the tuning of the operating temperature of the magnetic material, which can be approximated to values near room temperature in the case of the LSMO series. The electronic microscopy, X-ray diffraction and magnetization studies (up to 5 Tesla) allowed a detailed study of the structural and magnetic properties, which, in conjunction with the application of the Landau theory of phase transitions, allowed us to also interpret the influence of magnetoelastic coupling on the magnetocaloric properties of these ferromagnetic materials.Mestrado em Ciência e Engenharia de Materiai

Novel magnetic stimulation methodology for low-current implantable medical devices

Recent studies highlight the ability of inductive architectures to deliver therapeutic magnetic stimuli to target tissues and to be embedded into small-scale intracorporeal medical devices. However, to date, current micro-scale biomagnetic devices require very high electric current excitations (usually exceeding 1 A) to ensure the delivery of efficient magnetic flux densities. This is a critical problem as advanced implantable devices demand self-powering, stand-alone and long-term operation. This work provides, for the first time, a novel small-scale magnetic stimulation system that requires up to 50-fold lower electric current excitations than required by relevant biomagnetic technology recently proposed. Computational models were developed to analyse the magnetic stimuli distributions and densities delivered to cellular tissues during in vitro experiments, such that the feasibility of this novel stimulator can be firstly evaluated on cell culture tests. The results demonstrate that this new stimulative technology is able to deliver osteogenic stimuli (0.1-7 mT range) by current excitations in the 0.06-4.3 mA range. Moreover, it allows coil designs with heights lower than 1 mm without significant loss of magnetic stimuli capability. Finally, suitable core diameters and stimulator-stimulator distances allow to define heterogeneity or quasi-homogeneity stimuli distributions. These results support the design of high-sophisticated biomagnetic devices for a wide range of therapeutic applications.This work was funded by the Portuguese Foundation for Science and Technology (FCT), through the grant references SFRH/BPD/117475/2016, SFRH/BD/129340/2017 and IF/01089/2015, and by the European Structural and Investment Funds, through the project reference POCI-01-0145-FEDER-031132 and POCI-01-0145-FEDER-007679. It was also support by the TEMA - Centre for Mechanical Technology & Automation (UID/EMS/00481/2013-FCT and CENTRO-01-0145-FEDER-022083) and CICECO - Aveiro Institute of Materials (UID /CTM /50011/ 2013).in publicatio

COVID-19 symptoms at hospital admission vary with age and sex: results from the ISARIC prospective multinational observational study

Background: The ISARIC prospective multinational observational study is the largest cohort of hospitalized patients with COVID-19. We present relationships of age, sex, and nationality to presenting symptoms. Methods: International, prospective observational study of 60 109 hospitalized symptomatic patients with laboratory-confirmed COVID-19 recruited from 43 countries between 30 January and 3 August 2020. Logistic regression was performed to evaluate relationships of age and sex to published COVID-19 case definitions and the most commonly reported symptoms. Results: ‘Typical’ symptoms of fever (69%), cough (68%) and shortness of breath (66%) were the most commonly reported. 92% of patients experienced at least one of these. Prevalence of typical symptoms was greatest in 30- to 60-year-olds (respectively 80, 79, 69%; at least one 95%). They were reported less frequently in children (≤ 18 years: 69, 48, 23; 85%), older adults (≥ 70 years: 61, 62, 65; 90%), and women (66, 66, 64; 90%; vs. men 71, 70, 67; 93%, each P < 0.001). The most common atypical presentations under 60 years of age were nausea and vomiting and abdominal pain, and over 60 years was confusion. Regression models showed significant differences in symptoms with sex, age and country. Interpretation: This international collaboration has allowed us to report reliable symptom data from the largest cohort of patients admitted to hospital with COVID-19. Adults over 60 and children admitted to hospital with COVID-19 are less likely to present with typical symptoms. Nausea and vomiting are common atypical presentations under 30 years. Confusion is a frequent atypical presentation of COVID-19 in adults over 60 years. Women are less likely to experience typical symptoms than men

Superferromagnetism in mechanically alloyed fcc Fe23Cu77 with bimodal cluster size distribution

International audienceMagnetic measurements, x-ray diffraction and Mössbauer spectroscopy were used to characterize a nanostructured fcc Fe23Cu77 at.% alloy prepared by high-energy ball-milling, addressing in particular the effect of clustering on the nature of the interacting magnetic entities. The interpretation of magnetization measurements leads to the conclusion that grains, whose mean size is ~16 nm, contain two populations of magnetic Fe-rich nanoclusters with a bimodal size distribution. These two sets of clusters contain about 14 and 400 Fe atoms and have magnetic moments of 30 µB and 860 µB, respectively. The inter-cluster ferromagnetic interactions that lead to superferromagnetism with a Curie temperature TC~220 K can be described by a mean field determined by the smaller clusters only, which account for 90% of the magnetization

Black rat Rattus rattus eradication by trapping allows recovery of breeding roseate tern Sterna dougallii and common tern S.hirundo populations on Feno Islet, the Azores, Portugal

Mass trapping successfully achieved elimination of black rat Rattus rattus on Feno islet (1.6 ha), Terceira island (Azores archipelago), thus enabling roseate terns Sterna dougallii and common terns Sterna hirundo to recolonize the islet. Rats were first detected on Feno in 2003, when tern breeding-numbers had decreased dramatically. During 2005 no terns bred on the islet and in 2006 fewer than five common tern pairs attempted to nest. Rat eradication was initiated in September 2006. The last rats were captured in March 2007. Monitoring conducted in September 2007, and May and September 2008 indicated that rats had not recolonized. Common terns quickly resumed breeding on Feno islet but numbers (c.120 pairs in 2009) are still below peak levels (c.240-280 pairs) recorded before rat infestation. Roseate terns on the other hand were slower to return but recovered faster with around 260 pairs in 2009, representing 22% of the Azores population. The success of the black rat eradication shows that surveillance and timely action are fundamental to conserve tern colonies vulnerable to rat predation in the Azores.OGAMP Ordenamento e Gestão de Áreas Marinhas Protegidas (Interreg IIIB-MAC/4.2/A2); MARMAC II - Conhecimento, Promoção e Valorização para o uso sustentável de Áreas Marinhas Protegidas na Macaronésia (Interreg IIIB 05/MAC/4.2/A4); Life Programmes "Conservação das Comunidades e Habitats de Aves Marinhas dos Açores" and "Gestão Integrada das Zonas Costeiras e Marinhas nos Açores" (Contracts B4-3200/95-351 and B4-3200/98-509info:eu-repo/semantics/publishedVersio

Dynamic off-centering of Cr3+^{3+} ions and short-range magneto-electric clusters in in CdCr2_2S4_4

The cubic spinel CdCr$_2$S$_4$ gained recently a vivid interest, given the relevance of relaxor-like dielectric behavior in its paramagnetic phase. By a singular combination of local probe techniques namely Pair Distribution Function and Perturbed Angular Correlation we firmly establish that the Cr ion plays the central key role on this exotic phenomenon, namely through a dynamic off-centering displacement of its coordination sphere. We further show that this off centering of the magnetic Cr-ion gives rise to a peculiar entanglement between the polar and magnetic degrees of freedom, stabilizing, in the paramagnetic phase, short range magnetic clusters, clearly seen in ultra-low field susceptibility measurements. Moreover, the Landau theory is here used to demonstrate that a linear coupling between the magnetic and polar order parameters is sufficient to justify the appearance of magnetic cluster in paramagnetic phase of this compound. These results open insights on the hotly debated magnetic and polar interaction, setting a step forward in the reinterpretation of the coupling of different physical degrees of freedom