In part I, a numerical study of the mixed states in a mesoscopic type-II superconducting cylinder is described. Steady-state configurations and transient behavior of the magnetic vortices for various values of the applied magnetic field H are presented. Transitions between different multi-vortex states as H is changed is demonstrated by abrupt changes in vortex configurations and jumps in the B vs H plot. An efficient scheme to determine the equilibrium vortex configuration in a mesoscopic system at any given applied field, not limited to the symmetry of the system, is devised and demonstrated. In part II, a superconducting thin film is subject to a non-uniform magnetic field from a vertical magnetic dipole, consisting of two magnetic monopoles of opposite charges. For a film with constant thickness and with no pins, it has been found that the film carries two pairs of vortex-antivortex in the steady state in the imposed flux range of 2.15 < (Phi)+ < 2.90 (in units of flux quantum) and no vortex at all for (Phi)+ <= 2.15. Transitions from a superconducting state with 3 pairs of vortex-antivortex to one with 2 pairs, where a pair of vortex-antivortex annihilates, have been observed in the pseudo-time sequence. With a perturbation with antidots (holes), vortexantivortex pair has been created for lower magnetic fluxes down to (Phi)+ = 1.3. In the sample of size 16(Xi) x 16(Xi), the attraction force between the vortex and antivortex always dominates over the pinning force, so that they eventually come out of pins, move toward each other, and annihilate each other. The annihilation rate, measured with time taken for the annihilation, is reduced noticeably by the increase of the distance between pins, or the increase in the pin size. A simulation of the magnetic vortex pinning in the sample of size 32(Xi) x 32(Xi) suggests we are likely to achieve pinning of the vortex-antivortex pair with the sample size around this and vortex-antivortex separation of 22(Xi). Using this sample as a template, the maximum density of pinned vortices achievable is calculates to be about 7.6 x 10^14 vortices/m2 for (Xi) =~ 1.6A°.

This dissertation describes experimental studies of how a spatially alternating magnetic field can effectively pin the magnetic flux in a superconducting thin film (Pb 82 Bi18), thereby enhancing the superconductivity. The spatially alternating magnetic field was provided by a periodic array of nano-sized magnetic structures: 300 nm spacing triangular array of cobalt rods with 100 nm diameter and 300 nm height. The superconducting film deposited on top of the magnetic structures, or an embedded Ferromagnet- Superconductor Hybrids (FSH), showed enhanced critical current and critical magnetic field. The embedded FSH also showed the field matching effect, the field compensation effect, and hysteresis. This dissertation also explains how to fabricate and characterize magnetic nano- structures. Electron beam lithography and electroplating method were used to fabricate the magnetic nanostructures. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to characterize the structures of the magnetic rods. Magnetic force microscopy (MFM) was used to study their magnetic properties.

In this work the interactions between neighboring superconducting thin film and ferromagnetic structures, i.e. superconductor-ferromagnet hybrid systems, were studied. A type-II superconducting thin film (Pb82Bi12), was deposited in close proximity to various ferromagnetic structures. These magnetic structures include: (i) alternating iron-brass shims of 275 mu m period, (ii) an array of 4 mu m wide Co stripes with smaller period (9 mu m), (iii) a square array of 50nm diameter, high aspect ratio (5-7) Ni rods with 250nm period. Measurements of critical transport current (IC), resistance (RH(T)) and second critical field (HC2) are reported. A variety of novel effects (enhancement of (IC) and (HC2), matching field effect, field compensation effect, and large hysteresis) are also reported. Using measurements on thin superconducting films atop a Co stripe array with a 9 mu m period, a superconductor-ferromagnet hybrid device (a mechanical superconducting persistent switch) is proposed. In addition, scanning Hall probe microscopy (SHPM) and other imaging techniques were used to characterize the magnetic properties of the systems mentioned. The SHPM was also used to acquire B-H and M-H curves. An additional sharp magnetic needle and electromagnetic coil assembly intended for micromanipulation of small magnetic particles and individual cells was also characterized.

Esta tese visa o estudo das propriedades magnéticas de um sistema formado por um substrato de alumina nanoporosa contendo um arranjo organizado de nanofios de Ni sob o qual é depositado um filme supercondutor de Nb. Os substratos foram obtidos através da técnica de anodização em dois passos, seguida de uma eletrodeposição AC do material ferromagnético. O filme de Nb foi depositado via magnetron sputtering. A resposta magnética deste sistema foi investigado num amplo intervalo de temperaturas através de medidas de susceptibilidade magnética AC, magnetização DC e imagens de magneto-ótica (MOI). Inicialmente foram estudadas as propriedades magnéticas do sistema de nanofios de Ni. Para isto, realizamos medidas de magnetização a temperaturas acima da temperatura de transição supercondutora do Nb, no intervalo de 10 a 300 K, onde o filme supercondutor não contribui a magnetização. Estes resultados mostraram o forte caráter uniaxial da anisotropia efetiva neste tipo de sistemas, assim como também a sua dependência com a temperatura e com os diferentes parâmetros estruturais dos nanofios. Medidas de curvas de inversão magnética de primeira ordem (FORC) evidenciaram a presença predominante de interações magnéticas desmagnetizantes, assim como efeitos não lineares nos campos de interações. Propriedades supercondutoras do filme de Nb foram estudadas a partir de medidas de susceptibilidade AC e curvas de magnetização M(H) para diferentes temperaturas abaixo de Tc. As curvas de histerese magnéticas mostraram claramente anomalias em altos campos, comportamento que é atribuído a efeitos de correspondência (matching) entre a rede de vórtices no filme de Nb e o arranjo poroso com nanofios do substrato. As medidas mostraram que o substrato gera um forte potencial de aprisionamento de vórtices no filme supercondutor, o qual incrementa a sua eficiência a medida que a temperatura é diminuída. A análise das curvas de inversão de primeira ordem realizada na região supercondutora mostraram uma distribuição de eventos mais complexa do que o normalmente observado para um ferromagneto. Por causa disso, realizamos medidas FORC num sistema mais simples formado por um filme de Nb crescido num substrato de Si. No diagrama de níveis se observaram regiões negativas associadas a eventos de queda na magnetização, e regiões positivas fortemente localizadas como consequência da mudança na densidade de vórtices no supercondutor quando o campo é variado. Uma análise através de imageamento magneto-ótico (MOI) mostra que em baixas temperaturas e baixos campos as amostras apresentam um padrão de penetração de fluxo conhecido como avalanches de vórtices, efeito que aparece como consequência de um processo de instabilidade termomagnética em diferentes regiões do filme supercondutor. Esta penetração de fluxo é suave quando a temperatura é próxima à Tc do supercondutor, mas quando a temperatura é diminuída a penetração de fluxo adota um perfil dendrítico aleatório. / In this work we have studied the magnetic properties of a system formed by a Nb thin film deposited on top of an array of ferromagnetic Ni nanowires embedded in a ordered porous alumina membrane as substrate. The substrates were obtained by the two-step anodization technique, followed by an electrodeposition AC of the ferromagnetic material. The Nb film was deposited via magnetron sputtering. The magnetic response of this system was investigated over a wide range of temperatures through AC susceptibility, DC magnetization and magneto-optical images (MOI) measurements. Initially, the magnetic properties of Ni nanowire system were studied. We perform magnetization measurements above the superconducting transition temperature of Nb, in the range of 10 to 300 K where the superconducting film does not contribute to magnetization. These results showed the strong uniaxial character of the effective anisotropy in such systems, as well as its dependence on the temperature and the different structural parameters of the nanowires. First order reversal curves measurements showed the predominant of demagnetizing magnetic interactions, as well as non linear effects in the interaction fields. Superconducting properties of Nb thin film were studied from AC susceptibility and magnetization DC measurements at different temperatures below Tc. Magnetic hysteresis curves clearly showed anomalies in high fields, a behavior that is attributed to matching effects between the network of vortices in the Nb film and the porous arrangement with nanowires in the substrate. These measurements show that the substrate generates a strong vortex potential pinning which increases its efficiency as the temperature is decreased. Because of this, we performed FORC measurements on a simpler system consisting of a Nb film grown on a Si substrate. In the diagram of levels we observed negative regions associated with events of magnetization drop, and positive regions strongly localized as a consequence of the variation in density of vortices in the superconductor when the field is changed. An analysis using magneto-optical imaging (MOI) shows that at low temperatures and low magnetic fields the samples exhibit a flux penetration pattern known as vortex avalanches, an effect that appears as a consequence of a thermomagnetic instability process in different regions of the superconductor. This flux penetration is smooth when the temperature is close to Tc, but when the temperature is decreased the flux penetration adopts a random dendritic profile.

Les systèmes hybrides supraconducteur-ferromagnétique présentent souvent de nouveaux phénomènes physiques, et pourraient également être utiles pour concevoir de nouvelles mémoires non-volatiles et haute densité pour les circuits supraconducteurs. Cette thèse étudie deux types différents d'hybrides SF, chacun suivant une approche possible de dispositif mémoire, en se focalisant sur les aspects fondamentaux. L'un porte sur l'effet de proximité dans des hétérostructures d'oxydes. Dans celles-ci, des corrélations triplet apparaissent, qui sont à la fois supraconductrices et polarisées en spin. Elles permettraient d'utiliser des effets de la spintronique comme la GMR, mais sont également très sensibles aux propriétés d'interface. Nous les avons étudiées dans des tricouches SFS d'oxydes, par des mesures de conductance. Celles-ci montrent des oscillations, en partie liées à ces états triplets. Nous observons également que les effets d'interface affectent les propriétés électroniques du ferromagnetique, en particulier lorsque cette couche est mince. Un autre genre d'interaction se produit par les champs de fuite provenant des structures de domaines. Des propositions théoriques récentes ont suggéré que de petites structures en tourbillon appelées skyrmion peuvent interagir avec la supraconductivité par ce mécanisme. Nous avons étudié ce couplage dans des bicouches, dans lesquelles les propriétés de transport sont dominées par la dynamique des vortex supraconducteurs. Nous avons vu une augmentation du courant critique en présence de skyrmions comme de domaines. Celles-ci créent également un effet Hall inhabituel dans l'état supraconducteur. La plupart de ces propriétés peuvent être expliquées qualitativement par la dynamique et le mouvement guidé des vortex. / Superconductor-ferromagnet hybrid systems often bring about new physics and may as well be useful to design new non-volatile, high-density memory devices for superconducting electronics.In this thesis, we study two different types of SF hybrids, each following a possible approach to memory devices, but focusing on fundamental aspects. One is about the proximity effect in oxide heterostructures. In these, triplet correlations appear, that are both superconducting and spin-polarized. These enable using effects from spintronics like GMR, but are also very dependent on interface properties. We investigated these in SFS oxide trilayers by conductance measurements. These showed oscillations which may, in part, be related to these triplet states. We also observed that interface effects affect the electronic properties of the ferromagnet, especially when that layer is thin. Another type of interaction occurs through stray magnetic fields from the domain structures. Recent theoretical proposals suggested that small swirling spin textures called skyrmions could similarly interact with superconductivity through this mechanism. We investigated such coupling in bilayers, in which the superconducting vortex dynamics dominate the transport properties. We found that the presence of skyrmions and domains alike enhances the critical current. It also leads to an unusual Hall effect in the superconducting state. Most of these properties can be explained qualitatively in terms of vortex pinning and guided motion.

Condensed matter systems undergoing phase transitions rarely allow exact solutions. The presence of disorder renders the situation  even worse but collective Monte Carlo methods and parallel algorithms allow numerical descriptions. This thesis considers classical phase transitions in disordered spin systems in general and in effective models of superfluids with disorder and novel interactions in particular. Quantum phase transitions are considered via a quantum to classical mapping. Central questions are if the presence of defects changes universal properties and what qualitative implications follow for experiments. Common to the cases considered is that the disorder maps out correlated structures. All results are obtained using large-scale Monte Carlo simulations of effective models capturing the relevant degrees of freedom at the transition. Considering a model system for superflow aided by a defect network, we find that the onset properties are significantly altered compared to the $\lambda$-transition in $^{4}$He. This has qualitative implications on expected experimental signatures in a defect supersolid scenario. For the Bose glass to superfluid quantum phase transition in 2D we determine the quantum correlation time by an anisotropic finite size scaling approach. Without a priori assumptions on critical parameters, we find the critical exponent $z=1.8 \pm 0.05$ contradicting the long standing result $z=d$. Using a 3D effective model for multi-band type-1.5 superconductors we find that these systems possibly feature a strong first order vortex-driven phase transition. Despite its short-range nature details of the interaction are shown to play an important role. Phase transitions in disordered spin models exposed to correlated defect structures obtained via rapid quenches of critical loop and spin models are investigated. On long length scales the correlations are shown to decay algebraically. The decay exponents are expressed through known critical exponents of the disorder generating models. For cases where the disorder correlations imply the existence of a new long-range-disorder fixed point we determine the critical exponents of the disordered systems via finite size scaling methods of Monte Carlo data and find good agreement with theoretical expectations. / <p>QC 20150306</p>

No description available.