Unconventional superconductors are those which are not described by the BCS theory, and for which no known theoretical description currently exists. The careful study of the behaviour of superconducting vortices in such systems yields crucial insights into the underlying physics of these exciting materials. This thesis describes a series of magnetometry experiments conducted on three different unconventional superconductors: Sr2RuO4, MgB2 and Bi2Sr2CaCu2O8+; utilising two techniques: Scanning Hall probe microscopy and magneto-optical imaging. An exotic p-wave chiral order parameter is thought to exist in Sr2RuO4 and is expected to produce several identifiable magnetic signatures. A search for these signals via scanning Hall probe measurements failed to detect any such signatures, but did capture a structural transition of the vortex lattice that is consistent with the proposed chiral order parameter. Studies of several samples also suggest that the vortex behaviour is strongly modified with even tiny amounts of disorder, a conclusion that has important consequences for interpretations of vortex patterns in Sr2RuO4. Several recent experiments have reported vortex configurations consistent with a competing short-range repulsion and long-range attraction in the intervortex interaction in MgB2 single crystals. We observe the spontaneous formation of vortex chains and labyrinths in a 160nm MgB2 thin-film that are suggestive of a non-monotonic vortex interaction, but perhaps more indicative of an intermediate-range attraction in harness with short and long-range repulsions. The suitability of seven potential mechanisms of vortex attraction in MgB2 are reviewed in light of the unusually short electronic mean-free path of our sample. Finally, magneto-optical imaging has been used to study the penetration of flux into regular polygon-shaped Bi2Sr2CaCu2O8+ platelets with various geometries. The variation of HP with geometry qualitatively contradicts conventional estimates of demagnetisation factors based on equivalent ellipsoids using inscribed circles. This work has important implications for the estimation of appropriate effective demagnetisation factors in arbitrarily shaped superconducting bodies.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第21554号 / 理博第4461号 / 新制||理||1640(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 石田 憲二, 教授 前野 悦輝, 教授 松田 祐司 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第21555号 / 理博第4462号 / 新制||理||1640(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 前野 悦輝, 教授 石田 憲二, 教授 寺嶋 孝仁 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

京都大学 / 新制・課程博士 / 博士(理学) / 甲第22989号 / 理博第4666号 / 新制||理||1669(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 石田 憲二, 教授 柳瀬 陽一, 教授 前野 悦輝 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18060号 / 理博第3938号 / 新制||理||1567(附属図書館) / 30918 / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 石田 憲二, 教授 前野 悦輝, 教授 松田 祐司 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

京都大学 / 新制・課程博士 / 博士(理学) / 甲第24399号 / 理博第4898号 / 新制||理||1700(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 石田 憲二, 教授 松田 祐司, 教授 柳瀬 陽一 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

Ferromagnétisme et supraconductivité sont habituellement considérés comme deux ordres antagonistes, aussi la découverte de leur coexistence dans URhGe et UGe2 a généré beaucoup d'intérêt. Le mécanisme expliquant un tel état n'a cependant pas encore était totalement élucidé. La supraconductivité dans ces composés serait non conventionnelle: les fluctuations magnétiques pourraient être responsables de l'appariement des électrons en paires de Cooper avec des spins parallèles.<br /><br />Cette thèse porte sur l'étude du composé ferromagnétique supraconducteur URhGe. URhGe devient ferromagnétique en dessous d'une température de Curie de 9,5 Kelvin, avec des moments spontanés alignés selon l'axe c de sa structure orthorhombique. Pour des températures inférieures à 260 mK et des champs plus petits que 2 Tesla, une phase supraconductrice a été observée dès 2001. Au cours de cette thèse une deuxième phase supraconductrice induite sous champ a été mise en évidence dans des monocristaux pour des hauts champs magnétiques appliqués selon l'axe b des cristaux. Cette deuxième poche de supraconductivité enveloppe une transition métamagnétique ayant lieu pour un champ de 12 Tesla. Nous présentons dans ce manuscrit une étude détaillée de cette supraconductivité et de sa relation avec la transition métamagnétique. Nous discutons de l'existence d'un point critique quantique dans le diagramme de phase magnétique et du rôle des fluctuations magnétiques émergeant de ce point critique quantique dans le mécanisme d'appariement des électrons.

URhGe has garnered interest recently as one of the few known ferromagnetic superconductors. The superconductivity in this material appears to arise from magnetic fluctuations rather than phonons, and take a triplet form which is remarkably resistant to field. In this thesis, a number of measurements on the material are presented. Some probe the Fermiology, with strong evidence appearing for a model which as both light open sheets and heavy, small, closed pockets. The open sheets, associated with chains of real-space electron density running along the b axis, dominate the conductivity in most circumstances. Evidence for their existence arises from the general large and non-saturating magnetoresistance, and from the unusual observation of negative temperature coefficient of resistance at high fields. The closed pockets have provided a few Shubnikov-de Haas oscillations, but mostly they remain inferred from the high specific heat γ and their role in the magnetism. In order to better probe the superconductivity, a high precision low noise DC resistance measurement bridge was built using a SQUID. Along with conventional measurements, this provides evidence that the two pockets of superconductivity on the phase diagram are the same phase. The re-entrance an be understood simply as a result of magnetic field being a tuning parameter, but also suppressing bulk superconductivity through orbital limiting. The SQUID bridge allowed the detection of domain wall superconductivity linking up these two pockets. The SQUID bridge was also used to study the highly structured superconducting transition in BiPd. This material lacks inversion symmetry in its crystal structure, so is a good candidate for unusual forms of superconductivity. Here again non-bulk superconductivity is considered the most likely cause for the structure. Unusual and distinctive IV curves have been measured, and a simple model of inhomogeneous conductivity channels with different critical currents is proposed as an explanation.

The work presented in this thesis was broadly focussed on the investigation of the magnetic behaviour of different superconducting materials in the form of bulk (singe crystals and pellets) and thin films (nanomagnetic devices like superconducting spin valves etc). Neutrons and muons were extensively used to probe the structural and magnetic behaviour of these systems at the nanoscale along with bulk characterisation techniques like high-sensitive magnetic property measurements, scanning probe microscopy and magneto-transport measurements etc. The nanoscale interplay of Superconductivity and Ferromagnetism was studied in the thin film structures using a combination of Polarised Neutron Reflectivity (PNR) and Low Energy Muon Spin Rotation (LE-µSR) techniques while bulk Muon Spin Rotation (µSR) technique was used for microscopic magnetic investigation in the bulk materials. In the Fe/Pb heterostructure, evidence of the Proximity Effect was observed in the form of an enhancement of the superconducting penetration depth (λs) with an increase in the ferromagnetic layer thickness (dF) in both the bilayered and the trilayered structures. The existence of an Inverted Magnetic Region was also detected at the Ferromagnet-Superconductor (F/S) interface in the normal state possibly originating from the induced spin polarisation within the Pb layer in the presence of the neighbouring Fe layer(s). The spatial size (height and width) of the Inverted Magnetic Region did not change much while cooling the sample below the superconducting transition temperature(Tc)and it also stayed unaffected by an increase in the Fe layer thickness and by a change of the applied magnetic field. In the superconducting spin valve structure containing Permalloy (Py) as ferromagnetic layer and Nb as the superconducting layer, LE-µSR measurements revealed the evidence of the decay of magnetic flux density (as a function of thickness) within the Nb layer symmetrically from the Py/Nb interfaces towards the centre of the Nb layer in the normal state. The thickness dependent magnetisation decay occurred over two characteristic length scales in the normal state that stayed of similar values in the superconducting state also. In the superconducting state, an additional contribution towards the magnetisation was found in the vicinity of the Py/Nb interfaces possibly originating from the spin polarisation of the singlet Cooper pairs in these areas. The nanoscale magnetic investigation on a highly engineered F/S/F structure (where each of the F blocks made of multiple Co/Pd layers with magnetic moments aligned perpendicular to the plane of these layers and neighbouring magnetic blocks separated by Ru layers giving rise to antiferromagnetic alignment) using LE-µSR showed an antisymmetric thickness dependent magnetic flux density profile with two characteristic length scales. In the superconducting state, the magnetic flux density profile got modified within the superconducting Nb₆₇Ti₃₃ layer near the F/S interfaces in a way similar to that of observed in the case of Py/Nb system, most likely because of the spin polarisation of the superconducting electron pairs. The vortex magnetic phase diagram of Bi₂Sr₂Ca₂Cu₃O10-δ was studied using the Muon Spin Rotation (µSR) technique to explore the effects of vortex lattice melting and rearrangements for vortex transitions and crossover as a function of magnetic field and temperatures. At low magnetic fields, the flux vortices undergo a first order melting transition from a vortex lattice to a vortex liquid state with increasing temperature while another transition also occurred with increasing field at fixed temperature to a vortex glass phase at the lowest temperatures. Evidence of a frozen liquid phase was found in the intermediate field region at low temperature in the form of a lagoon in the superconducting vortex state which is in agreement with earlier observations made in BiSCCO-2212. The magnetic behaviour of the unconventional superconductor Sr₂RuO₄ was investigated using µSR to find the evidence of normal state magnetism and the nature of the vortex state. In the normal state, a weak hysteretic magnetic signal was detected over a wide temperature and field range believed to be supporting the evidence of a chiral order parameter. The nature of the vortex lattice structure was obtained in different parts of the magnetic phase diagram and the evidence of magnetic field driven transition in the lattice structure was detected from a Triangular→Square structure while the vortex lattice stayed Triangular over the entire temperature region below Tc at low fields with a disappearance of pinning at higher temperatures.