In this thesis a variety of topics concerning 2D materials that have been separated from bulk layered crystals are discussed. Throughout the thesis, single and few layers of graphene, fluorinated graphene, MoS2 and WS2 are used. Two new methods of freely suspending 2D materials are presented as well as a method of removing the background from optical images. This aids contrast measurements for the determination of the number of layers. Fluorinated graphene is found to be sensitive to beta radiation; the resistance of fluorinated graphene transistors is shown to decrease upon exposure to the radiation. This happens due to the carbon-fluorine bond breaking. The sp3 hybridised structure of the fluorinated graphene is reduced back into the sp2 hybridised structure of pristine graphene. The superconducting properties of molybdenum-rhenium are characterised. It is shown to have a transition temperature of 7.5 K. It is also discovered that the material has a resistance to hydrofluoric acid; the acid etches nearly all other superconducting materials. This makes MoRe a possible candidate to explore superconductivity in conjunction with high mobility suspended graphene. To see if the material is compatible with graphene, a supported Josephson junction is fabricated. A proximity induced super current is sustained through the junction up to biases of ∼ 200 nA. The temperature dependence of the conductivity is measured for both suspended MoS2 and WS2 on a hexagonal boron nitride substrate. The dominant hopping mechanism that contributes to the conductivity at low temperatures is found to be Mott variable range hopping, with the characteristic T−1/3 dependence. The hopping transport is due to impurities that are intrinsic to the crystals, this is confirmed by comparing the results with those of supported devices on SiO2.

We have studied magnetic properties of the intermetallic NdPd5Al2 com- pound by means of specific heat and magnetization measurements and neutron scattering. The compound crystallizes in the tetragonal I4/mmm space group with lattice parameters a =4.147 ˚A and c =14.865 ˚A, orders antiferromagnet- ically below TN =1.3 K and presents large magnetocrystalline anisotropy due to the crystal-field effects. The obtained magnetic phase diagram is charac- terized by two distinct magnetically ordered phases similarly to structurally re- lated tetragonal RTX5 and R2TX8 compounds. The zero-field antiferromagnetic phase is characterized by the propagation vector k = (1 2 00) and antiferromag- netic coupling of Nd moments along the teragonal c-axis with the amplitude of magnetic moments of 2.22 µB/Nd as was revealed by neutron diffraction. The transition from the paramagnetic to magnetically ordered in zero field is the first-order phase transition. The CF excitations in NdPd5Al2 were detected by means of INS at 3.0 meV, 7.4 meV, 8.6 meV and 17.1 meV. We further compare our findings about CF in NdPd5Al2 obtained from INS, susceptibility analysis and first-principles calculations and confront them with the experimental mag- netization and magnetic specific heat data. Our results will be also discussed with respect to related...

[pt] Neste trabalho foram preparadas multi-camadas supercondutor(SC)/ ferromagneto(FM) Nb/Co via pulverização catódica (Magnetron Sputtering). O principal objetivo é estudar o efeito de diferentes espessuras da camada ferromagnética (Co) nas propriedades supercondutoras do Nb. Era esperado que, após tratamentos térmicos, as camadas de Co formassem um plano de nanopartículas magnéticas ordenadas, cujo efeito deve ser muito diferente das nanopartículas aleatoriamente orientadas e camadas magnéticas continuas. As microestruturas foram investigadas por Difração de Raios-X em baixos ângulos (LAXRD), Microscopia de Força Atômica (AFM) e Microscopia Eletrônica de Transmissão (TEM). Propriedades magnéticas e de transporte tem sido estudadas com o Sistema de Medição de Propriedade Físicas (PPMS), da empresa Quantum Design. As medidas magnéticas e de transporte mostram que, com o aumento da espessura das camadas de Co, a temperatura de transição supercondutora (Tc) aumenta significativamente para as amostras como preparadas. Foi relatado na literatura que quando a espessura das camadas magnéticas da ordem de alguns nanômetros, a Tc aumenta e diminui periodicamente com o aumento da espessura das camadas magnéticas. No entanto, nesta pesquisa, a espessura das camadas magnéticas é de dezenas de nanômetros, sendo muito maior do que este alcance e portanto, não pode ser explicado baseando-se no mesmo modelo. Propusemos que a rugosidade da interface entre as camadas de Co e Nb desempenha um papel importante para este comportamento. Os resultados de AFM e XRD mostram que a rugosidade máxima da interface é da ordem de 7 a 10 nm, o que é comparável à espessura de camadas de Co (de 5 a 20 nm). Introduzimos um parâmetro R igual a d, onde R é a rugosidade da interface e d é a espessura da camada magnética, para discutir o efeito da interface sobre as propriedades supercondutoras da nossa amostra. Quando delta maior que 1, a camada magnética pode ser considerada uma forma não-continua e somente quando delta menor que 1, as camadas magnéticas continuas podem ser formadas. Com base em observações de topografia de interfaces na nano-escala , podemos compreender que primeiro a rugosidade aumenta a área da interface, resultando em um efeito de proximidade mais forte, além de aumentar o efeito do campo de dispersão na Tc. Este efeito depende não somente da rugosidade, mas também da espessura da camada magnética. Verificou-se que o parâmetro determina o efeito das camadas magnéticas. As diferentes propriedades magnéticas abaixo da Tc para diferentes amostras também pode ser explicada por este modelo. Após o tratamento térmico, a Tc das amostras diminuiu e as propriedades magnéticas também se tornam piores do que as amostras como preparadas. Os resultados de TEM mostram que as camadas de Co estam interconectadas e depois do recozimento não há indícios de interdifusão entre as camadas Nb e Co. Mais medidas são necessárias para verificar se as camadas magnéticas podem induzir vórtices espontâneos, assim como para explicar a diferença entre as amostras com nanopartículas magnéticas ordenadas comparadas com aquelas orientadas aleatoriamente. / [en] In this work we prepared Superconductor(SC)/ferromagnet(FM) Nb/Co multi-layers with magnetron-sputtering. The main purpose of this work is to study the effect of different shape of ferromagnetic layers on the superconducting properties of Nb. We expected that after annealing the Co layers can form in-plane ordered magnetic nanoparticles and the effect of ordered magnetic nanoparticles should be very different from randomly oriented nanoparticles and continues magnetic layers. The microstructures have been investigated by means of Low Angle X-ray Diffraction (LAXRD), Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM). Magnetic and transport properties have been studied with Physical Property Measurement System (PPMS) from Quantum Design. The magnetic and transport measurements show that with increase of the thickness of Co layers the superconducting transition temperature (Tc) signifficantly increases for the as-prepared samples. It was reported in the literature that when the thickness of the magnetic layers is in the range of several nanometers, Tc increases and decreases periodically with the increase of the thickness of the magnetic layers. In our samples, however, the thickness of the magnetic layers (several tens nanometers) is much larger than that range and therefore, cannot be explained within the same model. We proposed that the roughness of the interface between Co and Nb layers plays an important role for this behavior. The AFM and LAXRD results show that the maxim roughness of the interface is in the range of 7 until 10 nm, which is comparable to the thickness of Co layers (5 until 20 nm). We introduced one parameter R equal d, where R is the roughness of the interface and d is the thickness of the magnetic layer, to discuss the effect of the interface on the superconducting properties of our sample. When delta more 1, the magnetic layer may be in a non-continues form and only when delta less1 continues magnetic layers can be formed. Based upon nano-scale observations of interfaces topography we can understand that the roughness first increases the area of the interface, which gives stronger proximity effect and, second, enhances the effect of the stray eld on Tc. This effect depends not only the roughness but also the thickness of the magnetic layer. It was found out that the parameter determines the effect of the magnetic layers. The different magnetic properties below Tc for different samples can also be explained by this model. After annealing, Tc of the samples decreased and magnetic properties also became worse than the as-prepared samples. The TEM results show that the Co layers is interconnected and after annealing there is no indication of interdiffusion between Nb and Co layers. More measurements are needed to see if the magnetic layers can induce spontaneous vortices and what the difference is between samples with ordered and randomly oriented magnetic nano-particles.

La découverte récente de supraconductivité à relativement haute température (Tc,max=56K) dans les pnictures de Fer soulève des questions fondamentales sur l’origine et la nature de la supraconductivité : en particulier, la présence d’une phase antiferromagnétique à proximité de celle-ci dans leur diagramme de phase, comme dans d’autres supraconducteurs non conventionnels pose la question du lien entre magnétisme et supraconductivité.Nous nous sommes intéressés à la nature de l’état normal ainsi que des phases antiferromagnétique et supraconductrice d’un point de vue local grâce à la Résonance Magnétique Nucléaire (RMN) dans les pnictures de Fer. Nous avons pour cela étudié des pnictures de Fer de même composé parent BaFe2As2 pour des substitutions Co en site Fer de nature hétérovalente et réalisant un dopage électron ou bien Ru en site Fer de nature isovalente. L’état normal de ces matériaux présente des différences notables avec l’état normal des cuprates supraconducteurs : le désordre introduit par les substitutions au niveau intraplan est faible et on constate l’absence d’une phase de PseudoGap pour la susceptibilité de spin. Les diagrammes de phase sont similaires pour le Co et le Ru mais nos mesures montrent que la nature des phases antiferromagnétique et supraconductrice est en réalité qualitativement différente à l’échelle locale pour les deux types de substitution. Pour la substitution au Co réalisant un dopage électron, les phases électroniques sont homogènes et nous démontrons en particulier qu’à certains dopages, un ordre antiferromagnétique incommensurable coexiste avec la supraconductivité jusqu’à une échelle atomique, suggérant une nature itinérante du magnétisme et un état supraconducteur possédant une symétrie non conventionnelle. Pour la substitution isovalente au Ru, les phases électroniques sont inhomogènes à une échelle étonnamment faible, de l’ordre du nanomètre, mettant en jeu une coexistence entre magnétisme et supraconductivité très distribuée spatialement. Ce travail illustre la possibilité d’engendrer une phase supraconductrice non conventionnelle en déstabilisant une phase antiferromagnétique au moyen de mécanismes agissant soit dans l’espace réciproque (dopage électron), soit dans l’espace réel (substitution isovalente) et donnant lieu par ailleurs à une coexistence de ces phases de nature très différente dans les deux cas. / The recent discovery of superconductivity at a rather high temperature in the iron pnictides (Tc,max=56K) has revived some fundamental questions about the existence and the nature of the superconducting phase : in particular, the existence of an antiferromagnetic phase that is in vicinity of the superconducting phase in their phase diagram, as in other unconventional superconductors, raises questions about the link between magnetism and superconductivity. In this thesis, we studied the normal state as well as the antiferromagnetic and superconducting phases of the iron pnictides on a local scale with Nuclear Magnetic Resonance (NMR). Starting from the same parent compound BaFe2As2, we studied heterovalent Co substitution in Fe site realizing an electron doping and isovalent Ru substitution in Fe site. The normal state is shown to display important qualitative differences with the normal state of cuprates superconductors: disorder induced substitutions in electronically active layers is weak and we show the absence of a PseudoGap phase from spin susceptibility measurements. Whereas the phase diagram is similar for Co and Ru substitutions, we show that the nature of the antiferromagnetic and the superconducting phases is qualitatively different on a local scale in the two cases. For Co substitution leading to electron doping, the electronic phases are homogeneous and we demonstrate in particular the homogeneous coexistence of antiferromagnetism and superconductivity down to an atomic scale for some compositions: this suggests a magnetism of itinerant nature and an unconventional superconducting order parameter for the superconducting phase. For the isovalent Ru substitution, the electronic phases are inhomogeneous at a scale surprisingly low of the order of the nanometer scale, leading to a coexistence that is very distributed spatially. This works shows the possibility to induce an unconventional superconducting phase by the weakening of an antiferromagnetic phase made possible with very different means : either in reciprocal space with electron doping or in real space with isovalent substitution. Moreover, this is shown to lead to different kinds of coexistence between these phases in the two cases.

Neste trabalho será apresentado um estudo sobre a investigação de supercondutividade em carbetos, que cristalizam numa estrutura hexagonal, pertencente ao grupo espacial P63/mmc, que são isomorfos com o composto Cr2AlC, também chamadas de fases Nowotny. A representação estequiométrica mais comum destas fases é M2AX, onde M são metais de transição, A são elementos do grupo IIIA e IVA da tabela periódica e X pode ser C ou N. Serão mostrados estudos referentes aos sistemas: Ti-In-C, Ti-In-N, Nb-Sn-C, Nb-In-C, Ti-Ge-C e Nb-Ge-C. Os resultados referentes a todos os sistemas analisados neste trabalho revelam que as fases do sistema M2AX são supercondutoras, cuja temperatura crítica depende basicamente do elemento não metálico. Os resultados que serão apresentados neste trabalho, sugerem que a supercondutividade neste sistema ocorre na camada onde estão quimicamente ligados os átomos MX. Este fato também sugere que o sistema seja anisotrópico e quase bidimensional, tal como sugerido por cálculos de estrutura de bandas. Outro fato surpreendente é a existência de supercondutividade de uma nova fase de estequiometria Nb2GeC, que cristaliza na mesma estrutura protótipo Cr2AlC. A análise cuidadosa do sistema Nb-Ge-C, demonstra que outra fase intersticial também é supercondutora, com temperatura de transição relativamente alta (Tc ~ 15,3 K), cuja estequiometria é Nb5Ge3Cx, Entretanto, a estrutura protótipo é diferente das fases Nowotny Cr2AlC e cristalizam no protótipo Mn5Si3 que também é considerada uma fase Nowotny. Não somente a introdução de carbono no sistema protótipo Mn5Si3 apresenta supercondutividade, como também à introdução de boro, como átomo intersticial, é capaz de gerar supercondutividade neste composto. Finalmente, os resultados apresentados neste trabalho demonstram claramente a existência de uma nova classe de materiais supercondutores de estrutura hexagonal, que cristalizam numa estrutura protótipo Cr2AlC e uma possível nova família de intersticiais que cristalizam numa estrutura protótipo Mn5Si3. / In this work a study about the investigation of carbide superconductors is shown. These compounds are reported to be hexagonal with space group P63/mmc which are isomorphs to the Cr2AlC compound so called Nowotny phases. These materials have common chemical formula M2AX where M is a transition metal, A an A-group element, and X is C and/or N. In this work the Ti-In-C, Ti-In-N, Nb-Sn-C, Nb-In-C, Ti-Ge-C and Nb-Ge-C systems are investigated. The results concerning all systems show that M2AX phase are superconductors and the superconducting critical temperature depends on the transition metal element. The results suggest that the superconductivity occurs in the layers where MX atoms are chemically bonded. This fact suggests anisotropy in these systems which are in agreement with band structure calculations. It will be also shown the discovery of a new phase belonging to the M2AX family with Nb2GeC composition which crystallizes in the Cr2AlC prototype. Careful analyses of the Nb-Ge-C system also show the discovery of a new superconductor of Nb5Ge3Cx composition as an interstitial carbide compound. The temperature dependence for both electrical resistivity and diamagnetism data demonstrated bulk type II superconductivity below 15.3 K. These phases crystallize in the Mn5Si3 prototype which are also called Nowotny phases. On the other hand the boron substitution is also able to induce superconducting behavior. Finally, the results reported in this work show clearly a new superconducting family with hexagonal structure, which crystallizes in the Cr2AlC prototype and a possible new interstitial family which crystallizes in the Mn5Si3 prototype.

The most-discussed pairing symmetry in Sr₂RuO₄ is chiral p-wave, pₓ ± p[sub]y, whose degeneracy is protected by the lattice symmetry. When the lattice symmetry is lowered by the application of a symmetry-breaking field, the degeneracy can be lifted, potentially leading to a splitting of the superconducting transition. To lift the degeneracy, the symmetry breaking field used in this study is uniaxial stress. Uniaxial stress generated by a piezo-electric actuator can continuously tune the electronic structure and in situ lower the tetragonal symmetry in Sr₂RuO₄. Previous studies of magnetic susceptibility and resistivity under uniaxial stress have revealed that there is a strong peak in T[sub]c when the stress is applied along the a-axis of Sr₂RuO₄. In addition, it has been proposed that the peak in T[sub]c coincides with a van Hove singularity in the band structure, and measurements of Hc₂ at the maximum T[sub]c indicate the possibility of an even parity condensate for Sr₂RuO₄ at the peak in Tc. In this thesis, the heat capacity approach is used to study the thermodynamic behavior of Sr₂RuO₄ under uniaxial stress applied along the crystallographic a-axis of Sr₂RuO₄. The first thermodynamic evidence for the peak in T[sub]c is obtained, proving that is a bulk property. However, the experimental data show no clear evidence for splitting of the superconducting transition; only one phase transition can be identified within the experimental resolution. The results impose strong constraints on the existence of a second phase transition, i.e. the size of the second heat capacity jump would be small or the second T[sub]c would have to be very close to the first transition. In addition to these results, I will present heat capacity data from the normal state of Sr₂RuO₄. The experimental results indicate that there is an enhancement of specific heat at the peak in T[sub]c, consistent with the existence of the van Hove singularity. The possibility of even parity superconductivity at the maximum T[sub]c has also been investigated. However, the heat capacity measurements are shown to be relatively insensitive to such a change, so it has not been possible to obtain strong and unambiguous evidence for whether it takes place or not.

O presente trabalho trata do estudo sistemático acerca de supercondutividade em soluções sólidas de monoboretos formados por nióbio e zircônio. Amostras policristalinas de Nb1-xZrxB, com 0 ≤ x ≤ 0,2, de boretos ricos em nióbio Nb1-zBz e ricos em boro NbB1+δ foram preparadas através de um forno a arco elétrico sobre uma base de Cu refrigerada e sob atmosfera de Argônio de alta pureza, ao misturar os elementos Nb em lâminas, flocos de B e pedaços de Zr metálico, com alta pureza. Adicionalmente, algumas amostras foram introduzidas em uma ampola de quartzo com pressão parcial de Argônio e tratadas termicamente à temperatura de 1100 °C durante 150 h. As caracterizações das propriedades físicas dessas amostras foram conduzidas através de medidas de difração de raios-X, dependência com a temperatura e campos magnéticos da magnetização dc M(T, H), resistência elétrica R(T, H) e calor específico Cp(T, H). Também foram obtidas micrografias de algumas amostras, ao utilizar microscopia eletrônica de varredura (MEV). Pequenas perdas de massa foram observadas em todas as amostras como fundidas, ocasionando a formação de soluções sólidas do tipo Nb-B (Nbss) e/ou Nb1-yZry como fases secundárias, apresentando pequenas frações volumétricas. O limite de solubilidade de Zr na matriz NbB é sugerido estar próximo à concentração de 10 %at. Zr. Com exceção do composto rico em boro NbB1,2, todas amostras, como fundidas ou tratadas termicamente, apresentaram propriedades supercondutoras, sendo que o composto estequiométrico NbB exibe uma temperatura crítica supercondutora Tc ~ 9 K, que por sua vez é muito similar àquela conhecida para o nióbio elementar, com Tc ~ 9,2 K. Algumas inconsistências surgem caso a fase ortorrômbica Nb1-xZrxB seja considerada supercondutora, apesar de ser confirmada como a fase majoritária nos padrões de difração de raios-X dessas amostras. Tal fato é sugerido pela enorme diferença entre as frações Meissner nas curvas de susceptibilidade magnética χ(T) de amostras volumétricas e pulverizadas, baixos valores do salto no calor específico em comparação com o previsto pela teoria BCS com acoplamento fraco e a ausência de supercondutividade em um espécime (NbB1,2) no qual a fase NbB é preponderante. Deste modo, os resultados aqui discutidos sugerem que o composto NbB não é supercondutor, em oposição ao que fora reportado previamente, e que as propriedades supercondutoras da solução sólida Nb1-xZrxB são governadas pela presença de fases secundárias supercondutoras do tipo Nbss/Nb1-yZry. / This work is presented as a systematically study regarding superconductivity in niobium-zirconium monoborides and their solid solutions. Polycrystalline samples of Nb1-xZrxB, with 0 ≤ x ≤ 0.2, niobium rich Nb1-zBz and boron rich NbB1+δ borides were arc-melted on a water-cooled Cu hearth under high purity Argon atmosphere, by mixing high purity Nb foils, B flakes and metallic Zr pieces. Some arc-melted samples were placed in a quartz ampoule with Argon partial pressure and heat treated at 1100 °C during 150 h. The samples were characterized by means of X-ray diffraction XRD, temperature and magnetic field dependence of dc magnetization M(T, H), electrical resistance R(T, H) and heat capacity Cp(T, H). Micrographs of few samples were also obtained by scanning electron microscopy (SEM). Small weight losses were observed in all as-cast samples, leading to the formation of Nb-B (or Nbss) and/or Nb1-yZry solid solutions as secondary phases, all of them with low volume fractions. The solubility limit of Zr within Nb1-xZrxB lattice was found to be close to 10 at. % Zr. Except the boron-rich compound NbB1.2, every sample, as-cast or heat treated, exhibited superconducting properties below Tc ~ 9 K, a temperature very close to that known for pure, superconducting niobium of Tc ~ 9.2 K. Some internal discrepancies within the obtained data arise when the orthorhombic phase Nb1-xZrxB is assumed to be superconducting, despite it being the majority phase in the XRD patterns of the samples. This is suggested by a huge difference between the Meissner fraction in magnetic susceptibility χ(T) curves of bulk and powdered samples, small values of specific heat jumps in comparison with the expected within the weak-coupling BCS theory and lack of superconductivity in a specimen (NbB1.2) in which the main phase is NbB. By combining all the experimental results obtained in over two dozen samples studied, we argue that the NbB compound is not a superconducting material, as opposed to what has been reported previously. Also, the superconducting properties found in Nb1-xZrxB alloys are then associated with the occurrence of extra phases Nbss and/or Nb1-yZry which are present in all superconducting samples.

Since the first isolation of graphene in 2004 interest in superconductivity and the superconducting proximity effect in monolayer or few-layer crystals has grown rapidly. This thesis describes studies of both the proximity effect in single and fewlayer graphene flakes, as well as the superconducting transition in few unit cell chalcogenide flakes. Optical and atomic force microscopy and Raman spectroscopy have been used to characterise the quality and number of molecular layers present in these flakes. Graphene structures with superconducting Al electrodes have been realised by micromechanical cleavage techniques on Si/SiO2 substrates. Devices show good normal state transport characteristics, efficient back-gating of the longitudinal resistivity, and low contact resistances. Several trials have been made to investigate proximity-induced critical currents in devices with junction lengths in the range 250-750 nm. Unfortunately, no sign of proximity supercurrents was observed in any of these devices. Nevertheless the same devices have been used to carefully characterise proximity doping, (due to the deposited electrode), and weak localisation/anti-localisation contributions to the conductivity in them. In addition this work has been extended to investigations of the superconducting transition in few unit-cell dichalcogenide flakes. Four-terminal devices have been realised by micromechanical cleavage from a 2H-NbSe2 single crystal onto Si/SiO2 substrates followed by the deposition of Cr/Au contacts. While very thin NbSe2 flakes do not appear to conduct, slightly thicker flakes are superconducting with an onset ܶ௖ that is only slightly depressed from the bulk value (7.2K). The resistance typically shows a small, sharp, high temperature transition followed by one or more broader transitions, which end in a wide tail to zero resistance at low temperatures. These multiple transitions appear to be related to disorder in the layer stacking rather than lateral inhomogeneity. The behaviour of several flakes has been characterised as a function of temperature, applied field and back-gate voltage. The resistance and transition temperatures are found to depend weakly on the gate voltage. Results have been analysed in terms of available theories for these phenomena.

Orientadores: Kleber Roberto Pirota, Pascoal José Giglio Pagliuso / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-09-02T00:56:18Z (GMT). No. of bitstreams: 1 Moura_KarolineOliveira_D.pdf: 3189094 bytes, checksum: 2e5963be1eac4cd985c6e6bea434dc34 (MD5) Previous issue date: 2017 / Resumo: Materiais com dimensões nanométricas podem ter alteradas drasticamente suas propriedades físicas se comparados com a versão volumétrica. Nesses sistemas, as propriedades físicas mostram efeitos de dimensionalidade reduzida, coordenação reduzida de átomos na superfície e presença de alguns efeitos quânticos que dominam em baixas dimensões. Também, mudanças podem surgir quando as dimensões do sistema passam a ser comparáveis com o comprimento característico envolvido com o fenômeno físico estudado. Deste modo, a nanotecnologia proporciona a oportunidade de adaptar novos materiais com características melhoradas ou totalmente novas. Junto com o interesse prático, o crescimento de nanomateriais é também de grande interesse científico fundamental. Neste trabalho, apresentamos os detalhes de um novo método de crescimento de nanofios, desenvolvido no decorrer desta tese, e sua aplicação no estudo de três sistemas com propriedades magnéticas determinadas por diferentes tipos de interação entre elétrons: o intermetálico GdIn3, onde a interação RKKY entre os elétrons "4" f é responsável pelo ordenamento magnético de longo alcance; o metamagneto itinerante Fe3Ga4, onde as flutuações de densidade de spin são responsáveis pelo surgimento de transições entre estados magneticamente ordenados; e o supercondutor elementar Ga, que apresenta valores diferentes de temperatura crítica para suas diversas fases polimórficas. A nova rota de crescimento proposta, nomeada nanonucleação por fluxo metálico, consiste na junção de dois métodos conhecidos: a anodização do alumínio, para obtenção de moldes de alumina nanoporosos; e o método de fluxo metálico, utilizado para o crescimento de monocristais volumétricos de materiais intermetálicos. Em um primeiro passo, medidas de magnetização para o sistema GdIn3 indicam que a interação entre os spins dos elétrons 4f do Gd3+ e os spins dos elétrons de condução não permanece a mesma com a redução da dimensionalidade (nanofios com diâmetro "d ~ 200 nm" ), em acordo com dados de ressonância de spin eletrônico. Por outro lado, o diagrama de fases magnéticas H-T do composto Fe3Ga4, construído a partir de medidas de magnetização, calor específico e resistividade elétrica, muda consideravelmente quando este se apresenta na forma de nanofio ("d ~ 250 nm" ). Finalmente, para o sistema Ga, a condição geométrica particular (nanofios com "d ~ 140 nm" ) favorece a nucleação da fase "? - Ga" com propriedades de um supercondutor tipo II de acoplamento fraco. Os resultados obtidos têm mostrado a grande efetividade do método desenvolvido para produzir sistemas de baixa dimensionalidade com propriedades físicas não previstas. Além disso, a nova rota proposta abre a possibilidade de crescer o nanofio e seu volumétrico correspondente em uma única etapa / Abstract: Materials with nanometric dimensions can have their physical properties dramatically altered compared to the bulk version. In these systems, the physical properties present reduced dimensionality effects, decreased atoms coordination at the surface and presence of some quantum effects that dominate in low dimensions. Changes may also arise when the system dimensions become comparable with the characteristic length involved in the physical phenomenon investigated. Therefore, nanotechnology provides the opportunity to adapt new materials with improved or totally new features. Along with the practical interest, the growth of nanomaterials is also of great scientific interest. In this work, we present the details of a new method of nanowire growth, developed during this thesis, and its application in the study of three systems with magnetic properties determined by different types of interaction between electrons: the intermetallic GdIn3, where the RKKY interaction between the 4f electrons is responsible for the long-range magnetic order; the itinerant metamagnet Fe3Ga4, where the spin density fluctuations are responsible for the apparition of transitions between magnetically ordered states; and the elemental superconductor Ga, which presents different critical temperature values for its various polymorphic phases. The new proposed growth route, named metallic-flux nanonucleation, consists in the junction of two known already methods: the aluminum anodization, to obtain nanoporous alumina templates; and the metallic flux method, used for bulk single crystals growth of intermetallic materials. In a first step, magnetization measurements for the GdIn3 system indicate that the interaction between the Gd3+ 4f electron spins and the conduction electron spins does not remain the same with the dimensionality reduction (nanowires with diameter "d ~ 200 nm" ), according to electron spin resonance data. On the other hand, the H-T magnetic phase diagram of the Fe3Ga4 compound, constructed from magnetization, specific heat and electrical resistivity measurements, considerably changes for nanowires case ("d ~ 250 nm" ). Finally, for the Ga system, the special geometric condition (nanowires with "d ~ 140 nm" ) favors the nucleation of the "? - Ga" phase with properties of a weak coupling type-II superconductor. The obtained results have shown the great effectiveness of the developed method for producing low dimensionality systems with unexpected physical properties. In addition, the new proposed route opens the possibility of growing both the nanowire and its bulk corresponding in a single step / Doutorado / Física / Doutora em Ciências / 148758/2012-6 / CNPQ

Thesis. 1978. Ph.D.--Massachusetts Institute of Technology. Dept. of Physics. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Vita. / Includes bibliographical references. / by William J. Gallagher. / Ph.D.