Observing and controlling solid state quantum systems is an area of intense research in quantum science today. Such systems offer the natural advantage of being bound into a solid device, eliminating the need for laser cooling and trapping of atoms in free space. These solid state "atoms" can interface directly with photonic channels designed to efficiently couple into larger networks of interacting quantum systems. With all of the tools of semiconductor fabrication technology available, the idea of scalable, chip-based quantum networks is a tantalizing prospect. / Physics

Les circuits logiques "Rapid Single-Flux-Quantum" (RSFQ) à base de jonctions Josephson supraconductrices sont utilisés pour générer, traiter et transmettre des impulsions ultra-courtes dont l'aire quantifiée est celle du quantum de flux magnétique h/2e et correspond à 2.07 mV.ps. De tels circuits sont utilisés pour traiter le signal à très haute fréquence avec des fréquences d'horloge dans la gamme 10-120 GHz et une puissance consommée environ 100 à 1000 fois plus faible (incluant le coût énergétique du refroidissement à 4,2 K) que celle des meilleurs circuits semi-conducteurs équivalents. La logique RSFQ est une alternative intéressante pour les super-ordinateurs et offre des performances inégalées pour traiter les signaux micro-ondes à la volée. Une fois les signaux numérisés et traités à température cryogénique, le défi principal est de transférer à température ambiante les signaux numériques de faible tension (dans la gamme 200-1000µV) à des débits de 1 à 10 Gbps par voie, tout en limitant la charge thermique sur le système de réfrigération cryogénique, afin de construire un système performant à très haut débit numérique. Une solution à ce verrou est de transmettre les signaux par un système d'émission-réception sans fil avec la bande passante suffisante. Ce travail examine différents systèmes de transmission sans fil à courte distance, correspondant à la configuration physique entre les étages à températures cryogénique et ambiante, pour des taux de transmission de quelques Gbps. Il s'est construit sur quatre points cruciaux à résoudre :• le choix et l'étude du codage numérique approprié pour être utilisé comme support de transmission en bande de base des signaux sans utiliser de modulation analogique, comme les codages Polar Return-to-Zero et Manchester ;• l'étude et la sélection d'antennes ultra large bande avec une attention particulière portée sur les antennes Vivaldi antipodales et les antennes monopôles pour satisfaire aux contraintes cryogéniques ;• l'étude du taux d'erreur du système de transmission. Deux méthodes ont été développées pour récupérer les signaux numériques et minimiser le taux d'erreur ;• la comparaison entre simulations et mesures afin d'évaluer la performance du système global. / Rapid Single-Flux-Quantum (RSFQ) logic circuits based on superconducting Josephson junctions are using to generate, process and transmit very short quantized pulses whose area is the quantum of magnetic flux h/2e and corresponds to 2.07 mV.ps. Such circuits are used to process signals at very high speed with clock frequencies in the 10-120 GHz range and a power consumption about 100 to 1000 times lower that their best available semiconductor counterparts (including the cost of cooling down to 4,2K). RSFQ logic is an interesting alternative for supercomputers and offers unsurpassed performances for processing microwave signals on the fly. Once digital signals are processed at cryogenic temperature the key challenge is to transfer at room temperature the low-voltage output digital signals (about 200-1000µV) at high rates of about 1-10Gbps per channel, by limiting the thermal burden on the cryogenic system, in order to build high performance high throughput systems.A solution is to transmit the signals with a wireless emitting-receiving antenna set with a suitable bandwidth. This work examines several wireless baseband transmission systems in a short distance configuration, associated to the distance between the cryogenic and room temperature stages, for data rates in the range of a few Gbps. It elaborates on four crucial issues :• the choice and study of the proper line codes to be used for baseband transmission of digital signals without the need for analogue modulations, such as Polar Return-to-Zero and Manchester encodings ;• the study and selection of ultra-wide bandwidth antennas with a focus on small size Antipodal Vivaldi Antennas and monopole antennas to meet cryogenic constraints ;• the study of the Bit Error Rate (BER) of the transmitting system. Two methods were developed to recover the digital output signals and minimize the BER.• the comparison between simulations and measurements to assess the performance of the overall system.

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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.

Magnetorheological fluids (MRFs) are non-colloidal stable suspensions of polarizable mesoscale soft magnetic particles, usually metallic Fe-particles, in a carrier liquid such as oil or water; the solidity of which can be tuned by varying the applied magnetic field strength. Magnetorheological fluids are agile candidates for impact mitigation due to their tunable “solidity”, quick and complete reversibility of physical states, durability and reusability in comparison to their mechanical counterparts. The highly desirable property of an MRF is its yield strength and hence the conventional MRFs are Fe-based. However, uncoated Fe-particles suffer from poor chemical and thermo-oxidative stabilities, poor sedimentation stability and redispersibilities necessitating the coatings / additives; which always lead to compromised performance when used in MRFs. An alternative (to Fe) magnetic filler phase is the use of magnetic oxide particles. Soft magnetic spinel ferrites and garnets (though with moderate yield strength in an MRF) with their excellent chemical, thermo-oxidative and sedimentation stabilities, ready-redispersibility, less stringent synthesis and preservation conditions, lower cost, need no stabilizers and additives make them potential contenders for use in MRFs which can provide reliable MR performance. As the microstructure and magnetic nature of particles have direct influence on the MR property, the effects of these were studied by preparing MRFs with magnetic oxide particles of different sizes and shapes. These MRFs were simple bi-phasic as magnetic oxide particles were dispersed in versatile carrier fluid (silicone oil) without any additives; where the magnetic fill fraction was decided based on off state viscosity and the wettability criteria. As the MRFs in a device can undergo different stress / strain conditions of varying amplitudes and frequencies during their service, such a response was studied in laboratory using magnetorheometer via different modes of operation which mimic the service conditions. By varying the applied magnetic field strength and applied shear conditions, the performance of MRFs was evaluated and correlated to the physical and magnetic properties of the particles. Such a study provides a basis for the choice of magnetic phase in MRFs and their required concentration in the base fluid to provide highest efficiency. The dynamic yield strengths (field dependent yield stress) of MRFs extracted from steady shear measurements showed that the yield strength was strongly dependent on the saturation magnetization as well as on the microstructure of the particles used in MRF. The yield strength scaled with the saturation magnetization, magnetic fill fraction and applied magnetic field strength due to stronger magnetic column formation. The stability of MRFs (via the absence of wall slip) was found to depend predominantly on the microstructure of magnetic particles in the fluid such that MRFs containing structured particles showed the absence of wall slip while the MRFs containing irregular shaped powder particles showed poor stability via the occurrence of wall slip. The steady shear tests highlight the importance of using particles of definite shape with superior magnetic properties at a certain magnetic fill fraction for an efficient and reliable MR performance. The MRFs subjected to different oscillatory shear conditions showed that sturdier structures form in-field (exhibited via high gain modulus or low loss factor) when the particles have certain shapes (and size distribution) which result in high surface contact and are highly magnetic. Hence, the MRF containing Fe3O4 micro-octahedrons with high magnetization and large surface area for contact with other octahedron showed the large value of gain modulus and low loss factor compared to all other MRF samples. Poly-dispersity in spheres was found to be advantageous over monodisperse spherical magnetic particles due to void-bridging effects that strengthen the magnetic structuration. The irregular shaped particles based MRFs showed lower gain (higher loss factor) due to weak structuration. Anomalously high loss factor observed for rod shaped LZFP particles based MRF at medium strains and low field strengths is attributed to the rotation hindrance and low density of particles. The polydisperse particles based MRFs showed need for higher applied field strengths to decrease the loss and irregular particles based MRFs showed noisy response. The magnetosweep results showed that shape anisotropic particles based MRFs respond faster to applied field manifested as a faster decrease in loss factor with field. With magnetorheological parameters showing high dependence on the physical and magnetic nature of particles, oscillatory shear tests can serve as a means to select and assess the suitability of these particles for magnetorheological fluid for specific applications. The time dependent magneto-mechanical behaviour such as creep-recovery in MRFs showed that the strain recovery was dependent on the microstructure and magnetic nature of the particles such that fluids containing structured particles with high saturation magnetization showed higher recovery (due to better in-field structuration) compared to the irregular shaped and lower magnetization particles based MRF counterparts. The endurance of the MRFs (sustenance of strength of the MRF) under sustained stress conditions were estimated by a novel method which showed that MRFs containing ‘structured’ particles with high saturation magnetization showed high creep strength. In case of spherical particles based MRFs, the polydispersity of particles was found to aid in better column strength due to void-filling. The high surface contact between rod-shaped particles in the fluid resulted in good creep-strength among all MRFs. Among all the particles, the octahedron shaped Fe3O4 particles with large surface contact coupled with high saturation magnetization makes the Fe3O4 micro-octahedron particles based MRF the best amongst all the MRFs studied in this work. In case of irregular shaped particles based MRFs, the creep strength lagged behind the yield strength suggesting that such MRFs are not suitable for applications which demand sustained strength over prolonged action of stresses. Thus, the present work highlights the importance of considering the physical and magnetic properties of magnetic particles while selecting them for application specific MRFs where high endurance is sought. The stress relaxation behaviour of MRFs showed an overall high strength (via relaxation moduli) for MRFs containing particles with definite shape and high magnetization values (increased structure strength). However, the rod shaped particles based MRF did not witness increased strain limit with increased field strength, probably due to the mass flow in fluid due to higher inter-particle interaction than the interaction with applied field. The observation of increase in critical strain with increase in field for MRFs containing irregular shaped particles is only due to the higher number of particles resulting in overall increase in viscosity with field. Among all the MRFs, octahedron Fe3O4 particles with superior magnetic properties and large surface contact between facets showed highest critical strain for flow, which is in corroboration with other magnetorheological studies discussed so far. The creep-recovery and stress relaxation behaviours of MRFs are rarely studied, yet very important when selecting an MRF for an application which seeks high retention of MR strength over prolonged action of stress or strains. A comparison of particle shapes used in the MRFs suggests that although both octahedron shaped and rod shaped particles make high surface contact during structuration, the former is better due to lack of rotation hindrance, thus useful for preparing quickly responding MRFs. The inadequacies in th e conventio nal FOMs are address ed by a new FOM which is based o n a wholistic approach formulated consideri ng all relev ant physical and magnetic paramete rs of the particles. Also, the individ ual terms of this FOM help in selecting a particular MRF for a specific application. The FOM is as follows: λ – sedimentation constant (time taken by the MRF to sediment to about 1/eth of its total volume) With the MRFs containing octahedron shape d Fe3O4 pa rticles showing the highest FOM followed by s pheres (mod erate value ) which are succeeded by irregular powder samples based MRFs, the FO M observed in all MRF cases follow the same trend as observed by results from different magnetorheologi cal studies. Hence, the highest F4 (or FAB) observed for Fe3O4 octahedron particles based MR F in comparison to a ll other MR Fs (including Fe-based) is justified by the o Mbserved large yield strength, creep-resistance, low density and ready-redispersibilities, validating the FOM. The entire thesis is organized as follows. Chapter 1 details the motivation for the present research work, introduction to the material of interest (Magnetorheological fluid) with overview of different areas of potential applications, important properties of MRF, the current status of MRF, the challenges / issues needed to be addressed followed by choice of alternate materials for addressal of these drawbacks faced by conventional (Fe based) MRFs. Chapter 2 explains the synthesis of magnetic-oxide particles of different sizes and shapes by following different synthesis techniques. This is followed by the structural, microstructural and magnetic properties characterizations carried out by employing different, standard characterization techniques. The procedure for preparation of MRFs from the synthesized magnetic oxide particles is discussed. The basis of carrier fluid selection and magnetic particle concentration in MRF is explained. Chapter 3 gives a background to magnetorheology, in terms of the instrumentation (magnetorheometer), the relation between the magnetorheological parameters and the instrumental parameters (conversion factors), the different operating modes and the relevance of characterization modes in terms of practical applications, the procedure of different characterizations and the standard response behavior of MRFs to the characterizations. Chapter 4 is comprehensive characterization of all the MRFs subjected to steady shear conditions at various applied fields. The detailed analyses in terms of MR response are given with respect to the structure, microstructure, magnetic nature, and magnetic fill fraction of the magnetic particle in the fluid. Chapter 5 is extensive study of all the MRFs subjected to dynamical shear conditions at various applied fields. The magnetorheological responses of MRFs under different dynamical conditions (amplitude sweep, frequency sweep and magnetosweep) are analyzed in regard to role of microstructure, magnetic nature and magnetic fill fraction of the magnetic particle in the fluid. Chapter 6 explains the creep-recovery response of MRFs for the best magnetic fill fraction, decided from the steady and dynamical shear responses for all concentrations of MRFs. The recovered strain is analyzed with respect to a range of applied field strength and stress values. The creep strength determined from this study is correlated to the microstructure and magnetic nature of particles constituting the MRFs. Chapter 7 elaborates the stress relaxation behaviour of MRFs for the best magnetic fill fraction, decided from the steady and dynamical shear responses for all concentrations of MRFs. The stress relaxation (plateau values) moduli for the MRFs extracted at various applied field strength and strain values are analyzed to estimate the critical stress for flow in MRFs. This relationship between the critical stress that an MRF can withstand and the microstructure and magnetic nature of the particles in the fluid are investigated. Chapter 8 is about the study of sedimentation stability (and the redispersibility) of magnetic oxide particles based MRFs and the comparison of these properties with Fe- based MRFs. The role of mass-density and microstructure of particles in the fluid on sedimentation rate is briefly explained. Chapter 9 compares the important outcome of all the magnetorheological characterizations for all the studied MRFs in terms of extent and speed of response, the sedimentation stability and eases of redispersibility, and relates the observations to the physical and magnetic properties of the magnetic particles. The method of developing a new figure of merit based on a wholistic approach for assessing the efficiency and reliability of MRF is discussed which overcomes the shortcomings of conventional figures of merit. Chapter 10 summarizes the important findings of research work and highlights the validity of the new figure of merit in assessing ‘reliability and performance’ of MRFs.

Increased complexity of novel magnetic materials in the last decade has placed high demands on the manufacturing process as well as on the characterization. One of the possibilities for characterization of magnetic samples is to exploit the magneto-optical effects. The presented work uses the magneto-optical Kerr effect as a major characterization technique to probe the magnetic properties of samples. We have developed a mathematical model describing the effect of the magnetization on the polarized light and present an apparatus capable of measuring the response given by the light-matter interaction. The experimental results show the performance of the apparatus on the various magnetic systems including meta-stable iron layers, Stoner-Wohlfarth particles and magnetic vortices. The scanning vectorial Kerr magnetometer allowed us to probe the vector of magnetization with diffraction limited resolution below 500 nm.

<p dir="ltr">This work is the first report of optically addressable spin qubits in a semi-1D material, Boron Nitride Nanotubes (BNNTs). We perform the characterization of these spin defects and utilize their properties to do omnidirectional magnetic field sensing. We transfer these BNNTs with spin defects onto an AFM cantilever and perform scanning probe magnetometry of a 2D Nickel pattern on a gold waveguide. </p>

No / Geophysical survey techniques are a highly visible part of the scientific toolkit that is now used by archaeologists. In this paper, the history of the use of geophysical techniques in archaeology will be discussed, as will significant research themes associated with the most widely used prospecting devices. It is apparent that while the use of geophysical techniques is at an all-time high, there are many key areas where prospecting is rapidly developing. Some of the advances relate to fundamental aspects of the techniques, while others dictate how we undertake survey in the future. There is a movement away from pre-gridded survey areas towards real-time GPS for navigation. This allows greater integration, or fusion, of disparate data sources using visualization techniques derived from associated disciplines. The analysis of landscapes has become a major component of the application of new technology and there are many challenges to be tackled, including how to analyse and interpret significant archaeology within large-scale, data-rich, multi-technique investigations. The reflective nature of the review acknowledges the important role of Archaeometry in the development of archaeological geophysics.

In the beginning of the twenty-first century, educational and commercial institutions have driven the demand for cheap and efficient data storage. The storage medium known as magnetic recording media has remained the mainstay for most computer systems due to its large storage capacity per dollar. With the recording media's ever-increasing storage density has come reductions in the magnetic grain size per bit. At the recording bit's density threshold, the magnetic grains become more susceptible to thermal activation, which can render the storage medium unusable. An accurate characterisation of the recording layer's sub-granular structure is essential for understanding the magnetic and thermal mechanisms of high-density recording media. Small-Angle Neutron Scattering (SANS) studies have been performed to investigate the magnetic and physical properties of longitudinal and perpendicular recording grains. The SANS studies of longitudinal magnetic recording media have probed the recording layer's magnetic grain size at a sub-nanometer resolution. In conjunction with these studies, SQUID magnetometry was used to characterise the recording grain's bulk magnetism. Measurements showed that the recording grain was composed of a ferromagnetic hard core (Co-enriched) and a weakly magnetic shell (Cr-enriched). These results provided important information on the grain's magnetic anisotropy, which determines the recording media's magnetic stability. The polarised SANS studies were used to characterise the recording layer's physical granular structure. It was shown that the physical grain size was comparable to its magnetic counterpart. These physical measurements provided insight into the recording grain's chemical composition. The magnetic properties of perpendicular magnetic recording media were studied using SANS and VSM measurements. The neutron scattering studies revealed that the recording grain was composed of a hard ferromagnetic centre enriched with cobalt. The VSM studies showed that the magnetic recording grains exhibited a large perpendicular magnetic anisotropy. These combined studies provided information on the recording grain's ferromagnetic composition and magnetic stability. The polarised SANS measurements showed the physical grain size to be slightly smaller than its magnetic counterpart. This size difference was attributed to the non-magnetic grain boundary composed of SiO2. The boundary thickness determined the degree of inter-granular exchange coupling. Further polarised studies investigated the recording layers switching behaviour, which revealed more information on the grain's magnetic stability.

The objective of this study is to test whether human intentions to sustain or cease movements in right and left hands can be decoded reliably from spatially filtered single trial magneto-encephalographic (MEG) signals. This study was performed using motor execution and motor imagery movements to achieve a potential high performance Brain-Computer interface (BCI). Seven healthy volunteers, naïve to BCI technology, participated in this study. Signals were recorded from 275-channel MEG and synthetic aperture magnetometry (SAM) was employed as the spatial filter. The four-class classification for natural movement intentions was performed offline; Genetic Algorithm based Mahalanobis Linear Distance (GA-MLD) and direct-decision tree classifier (DTC) techniques were adopted for the classification through 10-fold cross-validation. Through SAM imaging, strong and distinct event related desynchronisation (ERD) associated with sustaining, and event related synchronisation (ERS) patterns associated with ceasing of hand movements were observed in the beta band (15 - 30 Hz). The right and left hand ERD/ERS patterns were observed on the contralateral hemispheres for motor execution and motor imagery sessions. Virtual channels were selected from these cortical areas of high activity to correspond with the motor tasks as per the paradigm of the study. Through a statistical comparison between SAM-filtered virtual channels from single trial MEG signals and basic MEG sensors, it was found that SAM-filtered virtual channels significantly increased the classification accuracy for motor execution (GA-MLD: 96.51 ± 2.43 %) as well as motor imagery sessions (GA-MLD: 89.69 ± 3.34%). Thus, multiple movement intentions can be reliably detected from SAM-based spatially-filtered single trial MEG signals. MEG signals associated with natural motor behavior may be utilized for a reliable high-performance brain-computer interface (BCI) and may reduce long-term training compared with conventional BCI methods using rhythm control. This may prove tremendously helpful for patients suffering from various movement disorders to improve their quality of life.