Surface and inter-phase analysis of Composite Materials using Electromagnetic Techniques based on SQUID Sensors

In this thesis an electromagnetic characterization and a non-destructive evaluation of new advanced composite materials, Carbon Fiber Reinforced Polymers (CFRP) and Fiber-Glass Aluminium (FGA) laminates, using an eddy-current technique based on HTS dc-SQUID (Superconductive QUantum Interference Device) magnetometer is proposed. The main goal of this thesis is to propose a prototype based on a superconducting sensor, such as SQUID, to guarantee a more accuracy in the quality control at research level of the composite materials employed in the aeronautical applications. A briefly introduction about the superconductivity, a complete description of the SQUID properties and its basic working principles have been reported. Moreover, an overview of the most widely used non destructive technique employed in several industrial and research fields have been described. Particular attention is given to the eddy current testing and the technical improvement obtained using SQUID in NDE. The attention has been focused on two particular application, that are the main topics of this thesis. The first concerns with the investigation of the damage due to impact loading on the composites materials, and the second is the study of the corrosion process on the metallic surface. The electrical and mechanical properties of the tested advanced composite materials, such as Carbon Fiber Reinforced Polymers (CFRPs) and Fiber-glass Aluminium (FGA) laminates are investigated. The experimental results concern the non-destructive evaluation of impact loading on the CFRPs and FGA composites, by means of the electromagnetic techniques; the investigation of the electromechanical effect in the CFRPs using the SQUID based prototype and the AFM analyses; and the study of corrosion activity of the metallic surface using magnetic field measurement

HTS YBCO Resonator Configuration with Coplanar Optimized Flux Concentrator Strongly Coupled to rf SQUID

We developed a novel magnetic coupling module formed of a monolayer superconducting flux concentrator, which is integrated with a coplanar resonator strongly coupled to HTS rf-SQUID. Three types of resonators, including a long stripline resonator between input loop and pick-up loop of the flux concentrator, a complementary split ring resonator (CSRR), and also a spiral shape inside the input loop are explored. The resonance quality factors as well as the coupling to the SQUID of different patterns of these three types of the resonators is evaluated using Finite Element Method (FEM) simulations. Several readout methods to couple the electronic system to the resonators are tested, including inductive (coil) and capacitive (transmission line) couplings, and the optimum readout is reported for each of the resonators. Among the evaluated resonator types, a spiral shape resonator with optimal design showing the highest quality factor (5900) together with the strongest coupling to the SQUID (-0.5 dB) at resonance frequency of 836 MHz, is fabricated using 200 nm thick superconducting YBCO on a 1 mm thick crystalline LaAlO3 substrate. The flux concentrator of the module is optimized by the variation of its linewidths and also its input loop radius to obtain maximum flux transformation efficiency.Comment: 5 page

Non Destructive Surface and Sub-surface Material Analysis using Scanning SQUID Magnetic Microscope

Non Destructive Testing (NDT) based on magnetic technique for the investigation of surface and sub-surface material properties is carried out using a room-temperature sample Scanning Magnetic Microscope. The performances of such instrument are well suited in the field of non destructive evaluation, thanks to the good combination of the spatial resolution and the magnetic field sensitivity of its own superconducting magnetic sensor. The aim of this work is to show the capability and the advantages of the NDT technique based on Superconducting Quantum Interference Device (SQUID) sensors. We start by describing our Scanning SQUID Microscope in terms of its performances, the different non destructive techniques we can apply to perform the measurements, and the efforts we have done to improve its capability to detect weak magnetic field variations. Two main applications are presented. On of this is based on the high magnetic field sensitivity of the SQUID sensor at low frequencies, and it consists to excite the sample with an alternating magnetic field (AC). This technique is applied to detect subsurface flaws in paramagnetic samples, for instance, in multilayer structures of aeronautical interest. The other field of application concerns the capability of the sensor to detect, with high spatial resolution, the direct magnetic field (DC) distribution on ferromagnetic samples, due to their residual magnetization. In this way, we can visualize magnetic domain structures of ferromagnetic particles. This capability is also exploited to evaluate the changing of magnetic field distribution in proximity of crack initialization on structural steels, subjected to fatigue cycles

High-temperature superconducting magnetometers for on-scalp MEG

In the growing field of on-scalp magnetoencephalography (MEG), brain activity is studied by non-invasively mapping the magnetic fields generated by neuronal currents with sensors that are flexibly placed in close proximity to the subject\u27s head. This thesis focuses on high-temperature superconducting magnetometers made from YBa2Cu3Ox-7 (YBCO), which enables a reduction in the sensor-to-room temperature standoff distance from roughly 2 cm (for conventional MEG systems) down to 1 mm. Because of the higher neuromagnetic signal magnitudes available to on-scalp sensors, simulations predict that even a relatively low-sensitivity (higher noise) full-head on-scalp MEG system can extract more information about brain activity than conventional systems.In the first part of this thesis, the development of high critical temperature (high-Tc) superconducting quantum interference device (SQUID) magnetometers for a 7-channel on-scalp MEG system is described. The sensors are single layer magnetometers with a directly coupled pickup loop made on 10 mm 7 10 mm substrates using bicrystal grain boundary Josephson junctions. We found that the kinetic inductance strongly varies with film quality and temperature. Determination of all SQUID parameters by combining measurements and inductance simulations led to excellent agreement between experimental results and theoretical predictions. This allowed us to perform an in-depth magnetometer optimization. The best magnetometers achieve a magnetic field noise level of 44 fT/√Hz at 78 K. Fabricated test SQUIDs provide evidence that noise levels below 30 fT/√Hz are possible for high quality junctions with fairly low critical currents and in combination with the optimized pickup loop design. Different feedback methods for operation in a densely-packed on-scalp MEG system were also investigated. Direct injection of current into the SQUID loop was identified as the best on-chip feedback method with feedback flux crosstalk below 0.5%. By reducing the operation temperature, the noise level can be further reduced, however, the effective area also decreases because of the decreasing kinetic inductance contribution. We present a method that allows for one-time sensor calibration independent of temperature.In the second part, the design, operation, and performance of the constructed 7-channel on-scalp MEG system based on the fabricated magnetometers is presented. With a dense (2 mm edge-to-edge) hexagonal head-aligned array, the system achieves a small sensor-to-head standoff distance of 1-3 mm and dense spatial sampling. The magnetic field noise levels are 50-130 fT/√Hz and the sensor-to-sensor feedback flux crosstalk is below 0.6%. MEG measurements with the system demonstrate the feasibility of the approach and indicate that our on-scalp MEG system allows retrieval of information unavailable to conventional MEG.In the third part, two alternative magnetometer types are studied for the next generation system. The first alternative is magnetometers based on Dayem bridge junctions instead of bicrystal grain boundary junctions. With a magnetometer based on the novel grooved Dayem bridge junctions, a magnetic field noise level of 63 fT/√Hz could be achieved, which shows that Dayem bridge junctions are starting to become a viable option for single layer magnetometers. The second alternative are high-Tc SQUID magnetometers with an inductively coupled flux transformer. The best device with bicrystal grain boundary junctions reaches a magnetic field noise level below 11 fT/√Hz and outperforms the best single layer device for frequencies above 20 Hz.In the last part, the potential of kinetic inductance magnetometers (KIMs) is investigated. We demonstrate the first high-Tc KIMs, which can be operated in fields of 9-28 \ub5T and achieve a noise level of 4 pT/√Hz at 10 kHz

Spinodal nanodecomposition in magnetically doped semiconductors

This review presents the recent progress in computational materials design, experimental realization, and control methods of spinodal nanodecomposition under three- and two-dimensional crystal-growth conditions in spintronic materials, such as magnetically doped semiconductors. The computational description of nanodecomposition, performed by combining first-principles calculations with kinetic Monte Carlo simulations, is discussed together with extensive electron microscopy, synchrotron radiation, scanning probe, and ion beam methods that have been employed to visualize binodal and spinodal nanodecomposition (chemical phase separation) as well as nanoprecipitation (crystallographic phase separation) in a range of semiconductor compounds with a concentration of transition metal (TM) impurities beyond the solubility limit. The role of growth conditions, co-doping by shallow impurities, kinetic barriers, and surface reactions in controlling the aggregation of magnetic cations is highlighted. According to theoretical simulations and experimental results the TM-rich regions appear either in the form of nanodots (the {\em dairiseki} phase) or nanocolumns (the {\em konbu} phase) buried in the host semiconductor. Particular attention is paid to Mn-doped group III arsenides and antimonides, TM-doped group III nitrides, Mn- and Fe-doped Ge, and Cr-doped group II chalcogenides, in which ferromagnetic features persisting up to above room temperature correlate with the presence of nanodecomposition and account for the application-relevant magneto-optical and magnetotransport properties of these compounds. Finally, it is pointed out that spinodal nanodecomposition can be viewed as a new class of bottom-up approach to nanofabrication.Comment: 72 pages, 79 figure

Structural, Electrical and Magnetic Properties of CoFe2O4 and BaTiO3 Layered Nanostructures on Conductive Metal Oxides

Multiferroic materials exhibit simultaneously, magnetic and electric order. In a magnetoelectric composite structure, a coupling is induced via an interfacial elastic interaction between magnetostrictive and piezoelectric materials enabling the control of the magnetisation by applying an electric field and vice versa. However, despite the potential of such coupling, experimental limits of theoretical models were observed. This work sheds some light on these limits by focusing the research on the chemistry of nanocomposite CoFe2O4 and BaTiO3, particularly at the interfaces where the coupling predominates. A comparison of the most common conductive oxides, Nb doped SrTiO3 and SrRuO3, was made for the bottom electrode application. The variation of conductive properties in Nb-SrTiO3 thin films at high temperature has been quantified when artificially strained and 60 nm SrRuO3 film was found to be the best bottom electrode choice for room temperature use. Epitaxial growth of magnetic CoFe2O4 was achieved on various metal oxide substrates despite large lattice mismatches. Crystallographic properties and strain evaluation were investigated and a Stranski-Krastanov growth mechanism, arising from the PLD deposition, was predominant. A notable drop of magnetisation was observed depending on the growth template, particularly on BaTiO3 substrates, the piezoelectric counterpart of the magnetoelectric structures. However, an encouraging magnetoelectric coupling induced by thermal phase transition of BaTiO3 was revealed. For BaTiO3, a control of the growth direction was realised by varying the deposition pressure, and the existence of both 180° and 90° ferroelectric domains was observed for films up to 300 nm in thickness. However, both the ferroelectric and piezoelectric properties were reduced in the thin films due to the clamping effect of the substrate. Finally, highly crystalline multilayers of CoFe2O4 and BaTiO3 were prepared on SrRuO3 buffered SrTiO3 substrates. It was found that the degradation of both magnetic and ferroelectric properties was proportional to the increase in the number of interfaces. A thorough microscopic study revealed interdiffusion and chemical instability occurring between CoFe2O4 and BaTiO3 at the interface. This undesired effect was partially recovered by the insertion of an ultra thin layer of SrTiO3, acting as a barrier layer at every interface. This research shows how interfacial chemistry need to be understood to achieve high magnetoelectric coupling in these types of epitaxial engineered structures

Phase Transitions in Nanoscale Designed Magnetic Thin Films

187 p.El fenómeno de las transiciones de fase termodinámicas (TPT, por sus siglas en inglés) en la materia, asociadas con un cambio abrupto en cierta cantidad física, son de una importancia fundamental, tanto en su comprensión teórica como en aplicaciones tecnológicas. Algunos ejemplos significativos de aplicaciones de las TPT incluyen la transición de fase en materiales superconductores, usados, por ejemplo, en la reducción del consumo energético en equipos de resonancia magnética, o los materiales de cambio de fase, empleados en la refrigeración de ordenadores o en almacenamiento térmico de energía.Los materiales ferromagnéticos (FM) son también un tipo de sistemas en los que pueden observarse TPTs. En estos materiales, un tipo de TPT está determinado por una temperatura especifica, llamada temperatura de Curie TC, por debajo de la cual el sistema exhibe una fase FM ordenada. A temperaturas inferiores a TC, el material presenta dos estados equivalentes en ausencia de campo magnético externo, también separados por otro tipo de TPT. Dichas TPT en materiales FM y sus estados magnéticos asociados son relevantes en aplicaciones tecnológicas ampliamente extendidas, como la refrigeración o las memorias magnéticas.En general, las tecnologías basadas en películas delgadas FM han considerado históricamente interfases abruptas entre capas principalmente. Dichos cambios abruptos inducen y/o amplifican efectos necesarios, por ejemplo, para la lectura y escritura de sus estados magnéticos. Sin embargo, cambiosgraduales en las propiedades físicas de las películas delgadas pueden mejorar su rendimiento, en determinadas circunstancias.En este contexto, es bien sabido que cambios graduales en la interacción de intercambio en películas delgadas FM influyen en el fenómeno de la TPT. En estas películas, pueden coexistir diferentes fases quasi-paramagneticas (PM)/FM. No solo eso, dicha separación de fases puede ser controlada con la temperatura, mediante el diseño del perfil de energía de intercambio en la película. Así, películas delgadas FM con intercambio-gradual, son una herramienta novedosa con potenciales aplicaciones tecnológicas mediante el diseño de dichos perfiles.Paralelamente, el fenómeno de la TPT está asociado con el equilibrio termodinámico, en el que todas las cantidades permanecen constantes en el tiempo. Sin embargo, las transiciones de fase pueden ocurrir en sistemas que se encuentran lejos del equilibrio termodinámico en presencia de una fuerza dependiente del tiempo. De hecho, las llamadas transiciones dinámicas de fase (DPT), son bien conocidas en materiales FM. Sin embargo, su verificación experimental sólo ha sido posible recientemente, mediante experimentos diseñados exprofeso. Comprender las DPTs es crucial en la física del no-equilibrio termodinámico, debido a sus similitudes con respecto a las TPT. Dichas similitudes permitirían usar metodologías originalmente concebidas para las TPT en sistemas dinámicos.En esta tesis, se llevan a cabo una serie de investigaciones relacionadas con la fenomenología de las transiciones de fase en películas delgadas FM diseñadas en la nanoescala. Más concretamente, se investigan aspectos relevantes de los dos fenómenos previamente mencionados, es decir, las transiciones de fase en películas delgadas con interacción de intercambio graduada, y transiciones dinámicas de fase en películas delgadas F