Ferromagnetism (FM) and superconductivity (SC) are traditionally thought of as competing states of matter, since the opposite-spin electron pairing mechanism required for conventional SC is rendered impossible by FM spin alignment. However, recently discovered heavy-fermion compounds UGe2 and URhGe are examples where SC and FM are cooperative, and rather than antagonistic the presence of FM is actually necessary for the occurrence of the SC phase. A cooperative state of FM and SC is a topic of interest because it presents a possible solution to one of the two main problems with present superconductors: technology inhibiting limits on the highest temperature and highest magnetic field to which the SC phase can exist. Although both UGe2 and URhGe cease to be superconductors before even reaching 1 K, unlike the various `high temperature' superconductors currently known that easily surpass 100 K, it is their magnetic properties that are interesting, the inherent FM ordering allowing them to exceed conventional limits on the maximum magnetic field that SC can withstand. For example, URhGe remains superconducting above 35 T and the upper limit is so high that it is still experimentally undetermined. How exactly the FM SC phase arises in these compounds is as yet unknown. The necessary opposite-spin pairing mechanism is theoretically provided by magnetic fluctuations in an easily polarizable system right on the edge of a magnetic phase transition, and indeed SC emerges in UGe2 and URhGe around a first-order quantum critical point (QCP) where the temperature of the transition to an FM phase is reduced to absolute zero, by application of pressure in the case of UGe2 and by application of a magnetic field for URhGe. The aim of the research detailed in this thesis is to probe the FM phase transition and the associated QCP related to the emergence of SC in these compounds, to gather more information about the precise nature of the phases either side of the transition and exactly what changes occur in the system crossing the QCP. Specifically, the main objective is to characterise the magnetic fluctuations at the phase boundary and determine whether, by current FM SC theory, these fluctuations could be responsible for SC or if instead other, modified, unconventional theories are required to explain the unconventional electron pairing. The probes of choice for this PhD were Hall effect and magnetoresistance measurements of UGe2, and capacitive torque magnetometry and simultaneous magnetoresistance measurements of URhGe. The main result of the UGe2 project is an observed order-of-magnitude change in the Hall coefficient crossing the FM transition as a function of temperature and a dramatic change, similar in magnitude but also accompanied by a sign reversal, crossing the QCP as a function of pressure. Furthermore, the sign reversal at the critical pressure persists up to roughly 12 K, far beyond the 7 K critical end point of the phase transition, suggesting that in fact three different phases converge at the QCP where fluctuations between them presumably lead to the emergence of SC. Further investigation of the Fermi surface, either by deeper analysis of the Hall effect results or by other experimental methods, will be required to complete the main objective and determine exactly what the differences are between these newly identified phases. The main result of the URhGe project is actually the successful development of the capacitive torque magnetometry technique itself and the proof of operation for simultaneous measurement of all the individual components of both the magnetization and differential susceptibility tensors in a high magnetic field, which is currently not possible by any other technique. Completing the main objective was hampered by the extremely high susceptibility components encountered in the vicinity of the QCP, which in itself could be considered evidence for the theoretical relationship between strong FM fluctuations and the emergence of SC in URhGe. A number of results incidental to the main aim of the URhGe project are also summarised in this thesis, including the characterisation of quantum oscillations frequencies not previously reported in scientific literature and a variety of subtle features in resistivity measurements, which could, in conjunction with evidence from the susceptibility measurements, suggest the presence of another superconducting state such as surface or domain wall SC.

<p>With the advances in MEMS technology, the studies of the properties of magnetic crystals have reached the microscopic level. Critical information such as the magnetization and susceptibility of a magnetic sample can be obtained using a microtorque magnetometer, such as ones incorporating piezoresistive or capacitive detection that have been fabricated and tested by earlier research groups. This type of magnetic information is useful in the study of superconductivity, for example. The microtorque magnetometer designed and fabricated in this thesis has the potential of being used in this field of study.</p> <p>This thesis describes the design, fabrication and testing of a capacitive microtorque magnetometer. By using ANSYS, a computer modelling program, an ideal model of the rotating microtorque magnetometer was devised. Fabrication involved testing a variety of procedures before establishing the successful and efficient method of building the microtorque magnetometer. A fifth order resonant mode was successfully detected during the testing stage. A method of studying the desired resonant mode has been devised and explained in the later chapters of this thesis.</p> / Master of Applied Science (MASc)

Graphite is the strongest diamagnet of all known materials to date. Recent studies of the thickness dependence of the resistance of graphite have demonstrated the heterogeneity of the charge distribution in bulk graphite and prompted the study of its magnetic properties. The studies of the thickness dependence of the magnetic susceptibility of graphite, done in this work, showed that two-dimensional interfaces between the crystalline (Bernal or rhombohedral stacking order) blocks in graphite make a dominant contribution to its diamagnetic susceptibility. Previously proposed models of diamagnetism in graphite are not suitable for explaining its magnetic properties, and therefore new concepts should be considered. Additionally, the studies of the transport and magnetic properties of graphite and multilayer graphene indicated the existence of superconductivity at the interfaces in well-ordered graphite. The possibility of creating permanent circulating currents around artificial holes in highly oriented graphite was studied by highly sensitive magnetization measurements. The obtained results provide hints for the possible existence of superconducting regions inside the bulk highly ordered graphite. In the present thesis, a further thickness dependent phenomenon on the depinning line (DL) of the flux line lattice of the high-Tc superconductor Bi2Sr2CaCu2O8+δ was studied. This geometrical effect shifts to notably lower temperatures in micrometer ring, compared with bulk crystals and thin flakes. The shift is related to a decrease in the overall pinning potential as a result of size effects, caused by: a) the thickness of the sample being smaller than the pinning correlation length, and b) the increase in the effective London penetration depth of the vortices (Pearl vortices). The large shift of the DL to lower temperatures may significantly influence the suitability of such elements for device applications in microstrip antennas and THz emitters.

Conselho Nacional de Desenvolvimento Científico e Tecnológico / By studying the magnetic behavior of materials a very important parameter to be analyzed are the constants of anisotropy. They can be associated with an effective anisotropy energy or to a more specific one as, for example, the magnetocrystalline. During the thin films deposition by magnetron sputtering, an uniaxial effective anisotropy can be induced due to the presence of a magnetic field over the sputtering guns and the relative motion of the substrate. However, several additional factors may control the anisotropy, including the film thickness, composition and substrate. In this work, two set of Co samples were grown by magnetron sputtering, with thickness ranging from 50 Å to 500 Å. The difference between the set of samples is the substrate, glass or Si(111). In order to study the magnetic anisotropy in these samples a torque magnetometer was developed. The study has shown that for both set of samples the uniaxial anisotropy, induced in the growth procedure, is the main one for the thicker samples. Above some thickness the samples present just uniaxial anisotropy, 230 Å and 400 Å for samples grown over glass and Si, respectively. Below these thicknesses, for both set of samples, the interaction with the substrate promotes the appearing of a biaxial and, less intense, triaxial anisotropies. The origin of these anisotropy terms are, probably, from magnetoelastic interaction with substrate and magnetocrystalline. Also, it was made an investigation of the surface s morphology and crystalline structures of the studied samples. / Ao se estudar o comportamento magnético dos materiais um parâmetro de grande importância a ser analisado são as constantes de anisotropia. Elas podem ser associadas à energia de anisotropia efetiva ou a anisotropias mais específicas como, por exemplo, a magnetocristalina. Na deposição de filmes finos pela técnica de "magnetron sputtering" pode-se induzir nas amostras uma anisotropia efetiva uniaxial durante a deposição, em virtude da existência do campo magnético nos canhões e do movimento do substrato sobre esses. Contudo, existem diversos outros fatores que influenciam a anisotropia, entre eles a espessura do filme, sua composição, bem como o tipo de substrato utilizado. Nesse trabalho dois conjuntos de filmes finos de Co, com espessuras de 50 à 500 Å, foram crescidos por essa técnica. Cada conjunto se diferencia pelo tipo de substrato utilizado [vidro ou Si(111)]. Para analisar a anisotropia das amostras foi construido e automatizado um magnetômetro de torque. O estudo mostrou que para os dois conjuntos a anisotropia uniaxial, induzida durante a confecção dos filmes, é predominante nas amostras de maior espessuras. Para o caso do vidro observa-se anisotropia unicamente uniaxial a partir de aproximadamente 230 Å e para o Silício em torno de 400 Å. Abaixo dessas espessuras, em ambos os casos, a interação com o substrato provocou o surgimento de um termo anisotrópico biaxial e, de forma menos intensa, de um termo triaxial. As anisotropias associadas a esses termos, e que se manifestam de forma diferente em cada caso em virtude da forma como cada substrato interage com o Co, provavelmente são de origem magnetoelástica ou magnetocristalina. Nesse estudo também foi feita uma investigação a respeito da estrutura morfológica da superfície e da estrutura cristalográfica dos filmes.