Estudos de distorções da rede e correlações electrónicas em manganites utilizando sondas lociais

Doutoramento em FísicaNesta tese apresenta-se um estudo experimental das distorções locais e correlações electrónicas em óxidos magnéticos com magnetoresistência colossal. A técnica de sonda local – Correlação Angular Perturbada – é utilizada em amostras caracterizadas quanto às suas propriedades macroscópicas nomeadamente propriedades estruturais, magnéticas e eléctricas, tendo em vista a obtenção de informação microscópica relevante via gradiente de campo eléctrico e campo magnético hiperfino, focando em particular os seguintes aspectos: -Distorções de rede e agregados de polarões no sistema LaMnO3+∆. É apresentado um estudo dos gradientes de campo eléctrica e campo magnético hiperfino em amostras representativas do sistema LaMnO3+∆, correlacionando estas propriedades locais com a caracterização das propriedades macroscópicas, efectuada nas mesmas amostras. Desta forma, foi possível estudar a natureza microscópica das distorções polarónicas. Foi dada especial atenção ao composto com composição LaMnO3.12 uma vez que este é um sistema padrão de uma manganite ferromagnética-isoladora que apresenta uma transição estrutural romboédrica (R)-ortorrômbica (O) perto da temperatura ambiente. O estudo revelou que agregados de distorções locais sobrevivem até 776 K, na fase de estrutura média mais simétrica (romboédrica), onde, por simetria, os octaedros MnO6 deveriam ser regulares. Estas distorções são semelhantes às observadas no sistema LaMnO3 onde os octaedros MnO6 apresentam uma distorção Jahn-Teller colectiva. Com a diminuição da temperatura observa-se um aumento contínuo destes agregados. Abaixo de uma temperatura crítica estas distorções relaxam acomodando-se numa estrutura com reduzidas distorções Jahn-teller. Verificou-se também que a transição estrutural (macroscópica) pode ser entendida como uma transição de percolação dos ambientes microscópicos. -Coexistência das ordens eléctrica e magnética no sistema Pr1-xCaxMnO3. É apresentado o primeiro estudo de gradiente de campo eléctrico no sistema Pr1-xCaxMnO3. Este estudo foi efectuado numa larga gama de temperaturas permitindo estudar localmente as diversas transições que este sistema apresenta. Em particular, na região do diagrama de fases onde existe ordenamento de carga e orbital (0.32<x<0.90), a componente principal do gradiente de campo eléctrico apresenta uma descontinuidade perto desta transição. Esta anomalia no gradiente de campo eléctrico é relacionada com possiveis distorções da simetria local sugerindo a presença de uma polarização eléctrica espontânea a temperaturas inferiores às de ordenamento de carga.This thesis presents an experimental study on lattice distortions and electronic correlations in colossal magnetoresistive magnetic oxides. The Perturbed Angular Correlation local probe technique is used to study selected manganite systems in order to obtain relevant insight into microscopic phenomena responsible for their macroscopic properties. Complementary structural, magnetic and electric characterization was performed. The work is focused on the following aspects: - Lattice distortions and polaron clusters in LaMnO3+∆ system. A study of the electric field gradient and magnetic hyperfine field was performed in representative samples of the LaMnO3+∆ system, and correlated with macroscopic information obtained in the same samples. Particular attention was given to the LaMnO3.12 sample since this compound is a prototype of a ferromagnetic-insulator manganite, presenting a rhombohedricorthorhombic structural phase transition near room temperature. We found that random distributed polaron clusters survive in the undistorted Rhombohedric phase, up to temperatures as high as 776 K. These distortions are as strong as those observed in the orbital ordered LaMnO3. Lowering temperature, the clusters continuously expand until a microscopic transition takes place. Below the transition, the distortions are accommodated into a weaker JT distorted phase. Additionally, the macroscopic structural phase transition can be viewed as a percolation transition of the microscopic environments. - Coexistence of electric and magnetic order in the Pr1-xCaxMnO3 system. The electrical field gradient (EFG) was studied for several compositions of the Pr1-xCaxMnO3 system. This local probe analysis was complemented with the study of the magnetic and structural properties. This allowed the determination of the electrical field gradient phase diagram for this system. The problematic of the charge/orbital order was also studied. Perturbed Angular Correlation studies were used to infer about atomic-scale distortions in a wide temperature range encompassing the charge/orbital and magnetic ordering transitions. The electrical-field gradient generated by the charge distribution around the probe shows strong anomalies when the system undergoes the charge-order (CO) transition. In particular, the principal component of the EFG presents a sharp discontinuity below the CO transition. The anomaly of EFG below TCO was related with the displacements of the ions causing a distortion of the local symmetry and thus connected with the existence of a local electric polarization

PAC studies of implanted 111 ⁣^{111}\!Ag in single-crystalline ZnO

The local environment of implanted $^{111}\!$Ag ( t$_{1/2}$=7.45d ) in single-crystalline [0001] ZnO was evaluated by means of the perturbed angular correlation (PAC) technique. Following the 60 keV low dose ($1\!\times\!10^{13}$ cm$^{-2}$) $^{111}\!$Ag implantation, the PAC measurements were performed for the as-implanted state and following 30 min in vacuum annealing steps, at temperatures ranging from 200 ºC to 1050 ºC. The results revealed that 42% of the probes are located at defect-free S$\scriptstyle_\textrm{Zn}$ sites ($\nu\scriptstyle_{Q} \sim$ 32 MHz, $\eta=0$) in the as-implanted state and that this fraction did not significantly change with annealing. Moreover, a progressive lattice recovery in the near vicinity of the probes was observed. Different EFGs assigned to point defects were furthermore measured and a general modification of their parameters occurred after 600 ºC. The 900 ºC annealing induced the loss of 30% of the $^{111}\!$Ag atoms, 7% of which were located in heavily damaged regions

Influence of O and C co-implantation on the lattice site of Er in GaN

The lattice location of low-dose implanted Er in GaN, GaN:O, and GaN:C was investigated using the emission channeling technique. The conversion electrons emitted by the probe isotope $^{167m}$Er give direct evidence that the majority (~90%) of Er atoms are located on substitutional Ga sites for all samples. Annealing up to 900 °C does not change these fractions, although it reduces the Er root-mean-square (rms) displacements. The only visible effect of oxygen or carbon doping is a small increase in the rms displacements with respect to the undoped sample

Phase Competitions behind the Giant Magnetic Entropy Variation: Gd5Si2Ge2 and Tb5Si2Ge2 Case Studies

Magnetic materials with strong spin-lattice coupling are a powerful set of candidates for multifunctional applications because of their multiferroic, magnetocaloric (MCE), magnetostrictive and magnetoresistive effects. In these materials there is a strong competition between two states (where a state comprises an atomic and an associated magnetic structure) that leads to the occurrence of phase transitions under subtle variations of external parameters, such as temperature, magnetic field and hydrostatic pressure. In this review a general method combining detailed magnetic measurements/analysis and first principles calculations with the purpose of estimating the phase transition temperature is presented with the help of two examples (Gd5Si2Ge2 and Tb5Si2Ge2). It is demonstrated that such method is an important tool for a deeper understanding of the (de)coupled nature of each phase transition in the materials belonging to the R5(Si,Ge)4 family and most possibly can be applied to other systems. The exotic Griffiths-like phase in the framework of the R5(SixGe1-x)4 compounds is reviewed and its generalization as a requisite for strong phase competitions systems that present large magneto-responsive properties is proposed

Estudo numérico dos fenómenos de transporte em sistemas de multicamadas descontínuas metal/isolador

Dissertação apresentada para obtenção do grau de Mestre em Métodos Computacionais em Ciências e Engenharia, na Faculdade de Engenharia e Faculdade de Ciências da Universidade do Porto, sob a orientação do Prof. Doutor Manuel António Salgueiro da Silv