Investigamos os fenômenos de reorientação de spins em diversos compostos intermetálicos com a estrutura tipo ThMn IND. 12. Foram estudadas as ligas pseudoternárias (Dy IND. XGd IND. 1-x)Fe IND. 11Ti, (Dy IND. XEr IND. 1-x)Fe IND. 11Ti e (Tb IND. XGd IND. 1-X)Fe IND. 11Ti e o composto ternário DyFe IND. 10Si IND. 2. As técnicas utilizadas neste trabalho foram: suscetibilidade AC em campos baixos, dependência angular da magnetização em amostras orientadas, e espectroscopia Mössbauer. Todas as ligas sofrem uma reorientação de spins do tipo axial-cônico ao abaixar-se a temperatura. Além disto, (Dy IND. XGd IND. 1-x)Fe IND. 11Ti, (Dy IND. XEr IND. 1-x)Fe IND. 11Ti exibem uma reorientação cônico-planar a baixas temperaturas, mas somente para um intervalo limitado de concentrações de Gd ou Er, (Tb IND. XGd IND. 1-X)Fe IND. 11Ti, por outro lado, sofre uma transição axial-planar para x > 0,6, e nenhuma transição para x < 0,6. A partir de um modelo a um íon para as interações de troca e de campo cristalino, calculamos diagramas de fase magnéticos, que apresentaram boa concordância com os dados experimentais. Finalmente, estudamos a reorientação de spins em DyFe IND. 10Si IND. 2 por meio de uma análise detalhada da variação dos parâmetros hiperfinos do ANTPOT. 57 Fe com a temperatura. Dessa forma, determinamos com precisão o intervalo de temperaturas da transição axial-cônica e confinamos a ausência de uma transição cônica-planar neste composto. / We have investigated spin reorientation phenomena in various rare earth-iron intermetallic compounds with the ThMnl2 structure. Pseudoternary (DyxGdl-x)Fe11Ti, (DyEr1-x)Fe11Ti and (TbxGd1-x)Fe11Ti alloys and the ternary compound DyFe10Si2 were studied. The experimental techniques used in this work were: low-field AC susceptibility, oriented-powder magnetization angular dependence, and Mössbauer spectroscopy. All alloys studied undergo an axial-conical spin reorientation on cooling. In addition, (DyxGdl-x)Fe11Ti and(DyEr1-x)Fe11Ti exhibit a conical-planar reorientation at lower temperatures, but on1y for a small Gd or Er concentration range. (TbxGd1-x)Fe11Ti alloys, on the other hand, undergo a single axial-planar transition for x > 0.6, and no reorientation at all for x < 0.6. Theoretical magnetic phase diagrams were calculated with a single-ion model for exchange and crystal-field interactions, showing good agreement with the experimental data. Finally, the DyFe10Si2 reorientation was studied by means of a detailed analysis of the 57Fe hyperfine parameters temperature dependence; in this way the axial-conical transition temperature interval was accurately determined, and the absence of a further conical-planar transition for this compound was confirmed.

Neste trabalho, apresentamos um metodo baseado no formalismo lmto-asa (combinacao linear de orbitais muffin-tin na aproximacao de esfera atomica) e no metodo de recorrencia, que possibilita calculos autoconsistentes de primeiros principios com polarizacao de spin no espaco direto. Para testar o metodo, calculamos a estrutura eletronica dos sistemas FENI IND.5 (ferromagnetico) e FEMN (antiferromagnetico). Os resultados mostraram boa concordancia com os obtidos por outros metodos. A fim de ilustrar o metodo aplicado a um sistema nao-periodico, calculamos a estrutura eletronica e a distribuicao local de momentos magneticos para um sistema constituido de uma impureza substitucional de FE numa matriz CU. Nossos resultados para momento magnetico e densidade de estados local para o sitio de impureza, concordam bem com resultados via metodo kkr-funcao de gren e experimentais, da literatura. O metodo aqui descrito e bastante flexivel e e muito util na obtencao de momentos magneticos locais em sistemas complexos / In this work we present an approach based on the linear muffin-tin orbital (LMTO) formalism in the atomic sphere approximation (ASA) and on the recursion method which allows us to perform first principies calculations in real space. To test the method, we obtain the electronic structure of ferromagnetic FeNi3 e antiferromagnetic FeMn. The results compare well with those obtained by others methods. To illustrate the scheme applied to a non-periodic system, we calculate the electronic structure and local magnetic moments for a substitutional impurity of Fe in a Cu host. Our results for the magnetic moment and local density of states at the Fe site, agree well with KKR-Green functions and experimental results in the literature. The scheme described here is extremely flexible and is very useful to obtain local distribuition of magnetic moments in complex metallic systems.

Since the first discovery of atomically thin sheets of carbon, two-dimensional (2D) materials have captured the interest from scientific community to expand the understanding in fundamental physics and chemistry at low dimensional systems. With extraordinary phenomena only possible at atomically thin limits, there has been high demand to reveal new and unique 2D materials and manipulate their structures and properties. Structural tunability of superatomic solids motivates us to control dimentionality of the materials and construct layered structures which could be exfoliated to 2D materials. The layered crystal [Co6Se8(PEt2phen)6][C60]5 can be used as a template to create a 2D C60-based material with an optical gap in mid-infrared. Re6Se8Cl2 and Mo6S3Br6, are presented as the first examples of covalently linked 2D superatomic solids built from nanoscale building blocks with hierarchical structures and semiconducting properties. We further demonstrate the emergence of hierarchical coherent phonons in a 2D superatomic semiconductor Re6Se8Cl2. Lastly, we explore complex magnetic phases in 2D ferromagnetic semiconductor CrSBr using second harmonic generation and Raman spectroscopy. 2D superatomic semiconductors and 2D magnetic materials provide additional sets of design principles to manipulate structural, electronic, phononic, and magnetic properties at the atomically thin limits. These materials hold promises as model systems to study fundamental physical principles as well as platform for applications with phonon engineering and magnetic optoelectronic devices.

Bibliography: p. 65-66. / ASTIA 110028

Thesis (Ph. D.)--Rutgers University, 2009. / "Graduate Program in Physics and Astronomy." Includes bibliographical references (p. 86-91).

Thesis (Ph. D.)--University of California, San Diego, 2003. / Vita. Includes bibliographical references.

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