Rozprawy doktorskie na temat „SUPERCONDUCTING NANOSTRUCTURE”

In the last three decades the scienti c community has been attracted by the possibility of controlling quantum system, for example for quantum computing or quantum simulators. Initially this idea was exploited only in microscopic quantum system as atoms and molecules. However these sys-tems present diffi culties on the large scale application due to the extreme laboratory condition that they need, for example ultra low temperature of the order of 1 microKelvin. On the other hand, great progress has been made with superconducting nanodevices, that can be more easily scaled. One of the most important problems that arise in quantum control is decoherence. We will study quantum control for coherent superconducting nanodevices. This devices are aff ected by a Broad Band Colored and Structured (BBCS) noise, which is qualitatively di fferent to what encountered in atomic physics, since it is chatacterized by a strong non-Markovian low-frequency component with a characteristic power spectrum S (f) proportional to 1/f. In this thesis we will present a roundup of physical situations, inolving both undriven and externally driven open quantum systems, which need to be analyzed in the perspective of quantum control. Promising applications to superconducting nanodevices, as the implementation of "Lambda" systems, possibly allowing control of microwawe photons, are discussed in detail. The thesis is structured as follows. Chapter 1 is an overview of the theoretical background of quantum control and quantum computation. In chapter 2 an archetypical problem for driven quantum systems, namely the Rabi problem, is studied in the presence of BBCS low-frequency noise which is not accounted for in standard Master Equation treatments based on the Markovian assumption. In chapter 3 a protocol named STImulated Raman Adiabatic Passage (STIRAP) is studied in the presence of BBCS noise, in view of its implementation in a class of superconducting nanode-vices named Cooper Pair Box. This is done in chapter where Design and control requirements to achieve large e fficiency are discussed, and a new figure of merit is introduced to characterize the tradeoff between effi cient coupling of the control and noise. Actually selection rules due to charge-parity simmetry make impossible operate STIRAP in these device in the regime of maximum protection from noise. Therefore in chapter 5 we propose a new implementation of STIRAP with superconductive device that allows us to circumvent selection rules, based on three-photon coherent processes and suitable crafted pulses compensating the Stark shifts. In chapter 6 we will study the problem of the tunneling of a quantum particle with strongly coupled environment in a bistable pontential. Finally in chapter 7 the study of the motion of a chiral quasiparticle in graphene a ffected by white noise will be presented.

The Cold-Electron Bolometer (CEB) is a sensitive detector of millimeter-wave radiation, in which tunnel junctions are used as temperature sensors of a nanoscale normal metal strip absorber. The absorber is fed by an antenna via two Superconductor-Insulator-Normal metal (SIN) tunnel junctions, fabricated at both ends of the absorber. Incoming photons excite electrons, heating the whole electron system. The incoming RF power is determined by measuring the tunneling current through the SIN junctions. Since electrons at highest energy levels escape the absorber through the tunnel junctions, it causes cooling of the absorber. This electron cooling provides electro-thermal feedback that makes the saturation power of a CEB well above that of other types of millimeter-wave receivers. The key features of CEB detectors are high sensitivity, large dynamic range, fast response, easy integration in arrays on planar substrates, and simple readout. The high dynamic range allows the detector to operate under relatively high background levels. In this thesis, we present the development and successful operation of CEB, focusing on the fabrication technology and different implementations of the CEB for efficient detection of electromagnetic signals. We present the CEB detector integrated across a unilateral finline deposited on a planar substrate. We have measured the finline-integrated CEB performance at 280-315 mK using a calibrated black-body source mounted inside the cryostat. The results have demonstrated strong response to the incoming RF power and reasonable sensitivity. We also present CEB devices fabricated with advanced technologies and integrated in log-periodic, double-dipole and cross-slot antennas. The measured CEB performance satisfied the requirements of the balloon-borne experiment BOOMERANG and could be considered for future balloon-borne and ground-based instruments. In this thesis we also investigated a planar phase switch integrated in a back-to-back finline for modulating the polarization of weak electromagnetic signals. We examine the switching characteristics and demonstrate that the switching speed of the device is well above the speed required for phase modulation in astronomical instruments. We also investigated the combination of a detector and a superconducting phase switch for modulating the polarization of electromagnetic radiation.

Nous avons etudie l'action a l'echelle mesoscopique de la presence d'un supraconducteur sur la conductance d'un circuit de metal normal. Apres une discussion de differentes theories concernant ce sujet, nous presentons des mesures a tres basse temperature (20 mk) mettant en evidence l'action non locale de la supraconductivite sur la conductance metallique. Nous montrons que la conductance du metal normal est alors fortement dependante de l'energie des electrons, l'energie caracteristique etant l'energie de thouless. Une experience d'interference effectuee dans la configuration aharonov-bohm met en evidence la portee de la coherence quantique de paires d'electrons dans le metal normal. Nous effectuons une comparaison detaillee avec la theorie des fonctions de green quasiclassiques. Cette comparaison met en evidence le role important joue par les parties exterieures de l'echantillon qui constituent les reservoirs. Nous presentons une technique originale de fabrication d'echantillons mesoscopiques hybrides de niobium et de cuivre. De plus, afin de pouvoir controler la formation des barrieres tunnel, nous avons developpe une vanne permettant de maitriser l'entree, dans un enceinte a ultra-vide, d'oxygene pur a partir de l'air. Nous decrivons un programme ecrit en langage c++, qui permet de calculer la conductance d'un circuit hybride quelconque compose de metal normal et de supraconducteur. Dans le cas ou deux supraconducteurs sont presents a des tensions differentes, l'effet josephson alternatif module la densite d'etats dans le metal normal. Nous presentons une experience, en cours de developpement, visant a mesurer les effets de ces variations de la densite d'etats sur le transport.

Au cours de cette thèse, on étudie les systèmes différents : graphène (une monocouche de carbone), graphène fonctionnalisé et les nanofils de Bismuth en induisant la supraconductivité par l’effet proximité. On montre que l’effet proximité fonctionne comme un probe sensible pour les effets des interactions, de couplage spin-orbite, etc.La structure de band de graphène a une relation dispersion linéaire au niveau de Fermi, et le band de conductance et le band de valence est lié aux six points dans l’espace réciproque, appelé le point Dirac. Autour du point Dirac, graphène occupe d’une densité d’état faible (par rapport aux métaux). Alors le niveau de Fermi dans graphene est modulable. On fabrique les jonctions S/Graphene/S avec les contacts de matériaux différents (Al, ReW, Nb). En comparant avec la théorie, on a complété le diagramme du produit R_N I_c (R_N la résistance d’état normal, I_c le courant critique dans une jonction) vs l’énergie Thouless E_Th (une énergie caractéristique intervient dans la jonction SNS longue et dépend la partie normale). Une réduction de R_N I_c globale de la jonction courte à la jonction longue, surtout dans la limite de la jonction longue, la réduction est 10 fois plus grande que celle de la jonction courte. On l’explique par une réduction d’une énergie Thouless effective à cause l’interface S/G imparfaite. Une suppression du supercourant près du point Dirac dans les jonctions longues est considérée comme une signature de la réflexion Andreev spéculaire sur les « puddles » dans le graphène. Aussi, l’injection des paires de Cooper dans les états de bord de l’effet Hall quantique du graphène est étudiée dans cette thèse.L’interaction du couplage spin-orbite et l’effet proximité peut produire les physiques très intéressantes comme le supra de triplet, jonction π, et récemment la formation des Fermions majoranas. Motivé par ces possibilités d’explorer les nouvelles physiques, on a essayé d’induire le couplage spin-orbite dans graphène dans lequel ce couplage est initialement très faible. En greffant les molécules de Pt-porphyrines, qui tiennent un atome de Pt au milieu, on espère que le couplage spin-orbite fort dans l’atome de Pt peut « diffuser » dans le graphène. Au lieu d’avoir vu le couplage spin-orbite, on a plutôt découvert un magnétisme qui dépend la grille dans le graphène induit par les molécules. Plusieurs échantillons avec les contacts normaux ou supraconducteurs sont mesurés avant et après mettant les molécules. Un transfert de charge dans deux sens (électron ou trou) est observé à la température ambiante. Il est lié à l’alignement des niveaux de Fermi des molécules et le graphène. A basse température (~70mK), les hystérésis dans la magnétorésistance (MR) et une asymétrie en B_(//) et B_⊥ impliquent un magnétisme dans graphène. Plus spectaculaire, une asymétrie en la dépendance de la grille du supercurrent est détectée. Bismuth est un élément très lourd et un matériau avec le couplage Rashba spin-orbite fort. On a connecté les nanofils de Bismuth avec tungstène (H_c∼12T) électrodes par FIB (Focused Ions Beam) et induit l’effet proximité dans les fils. Les résultats les plus étonnants sont : (1) le supercourant se tient au champ magnétique jusqu’à 11 Tesla. (2) Il y a des oscillations dans le courant critique en fonction du champ avec une période de centaine gauss qui ressemble à celui d’une structure de SQUID (composé des deux jonctions Josephson en parallèles. (3) Sur ces oscillations, nous trouvons aussi une modulation quasi-périodique lente de quelque milles gauss. Pour expliquer tous ces phénomènes, nous proposons qu’il y a quelques canaux étroites balistiques 1D se forment aux bords des certaines surfaces qui se tiennent au champ jusqu’à 11T et se construisent une interférence entre eux. L’effet Zeeman cause une modulation de phase entre les quasi-particules dans une paire d’Andreev qui module donc le supercourant en échelle de quelques milles gauss
In this thesis we investigated graphene and Bi nanowire systems by inducing superconducting proximity effect in them. Typically the samples are realized in the form of S/N/S junction. The special properties of these systems are revealed by observing some unusual proximity effect in them. The interplay of the superconducting proximity effect and other effects (spin-orbit coupling, Zeeman effect, quantum Hall effect, impurities, etc...) at the mesoscopic scale gives rise to new physics. Some of our main results are listed below.GrapheneWe succeeded to induce superconducting proximity effect in the very long junction limit, thus completing the diagram of the superconducting proximity effect in graphene. Since by changing the gate voltage, one changes the carrier density in graphene and eventually the transport characteristic quantities (l_e, E_Th etc...). We could scan a whole range of Thouless energy. We present a diagram of eR_N I_c vs Thouless energy compared to theoretical prediction. The Thouless energy dependence of the eR_N I_c products varies from the long junction limit to the short junction limit. The discrepancy (mainly due to the imperfect S/G interface) between theory and experiment is also limit dependent: in the short junction limit, the eR_N I_c products are smaller than the theoretical prediction (with a perfect interface) by a factor of about 3-4; in the long junction limit, however, the disagreement is increased to about 100. We show that the factor deduced from the junctions in different limits is length dependent. This can be explained by the effect of finite transmission at the S/G interface in both the critical current I_c and the induced mini-gap in the graphene. In another hand, a suppression of supercurrent near the Dirac point is observed in long junctions which is attributed to the indication of the specular Andreev reflection upon the puddles in graphene. Also the injection of the Cooper pairs into the QHE edge states is investigated in this thesis.Graphene grafted with Pt-porphyrinsBy grafting the Pt-porphyrins onto graphene, we observed a charge transfer between molecules and the graphene both for electrons and holes. One of the important consequences of the charge transfer is that when the molecules are ionized, a collective magnetic order can be formed by the long range RKKY interaction: the magnetic moments interact via the carrier in graphene. This effect is detected by a hysteretic magnetoresistance of the graphene in a perpendicular field and the asymmetric magnetoresistance in parallel field. Even more striking, the observation of a unipolar supercurrent in S/G/S junction implies that this magnetism induced by porphyrins is gate dependent. The theoretical calculations by Uchoa et al. using the Anderson model indeed find that the gate voltage should tune the impurities in graphene between non-magnetic state and magnetic state.Bi nanowireThe observation of a SQUID-like oscillations persisting up to 10 T and thousands Gauss range modulation in I_c hints to a complex physic in the W-Bi nanowire-W junctions. The results are consistent with a SQUID structure consisting of 2 edges channels which could have an I_c oscillation with period defined by the area between the two edges, typically the size of the nanowire. The origin of the edge states formation is attributed to the strong spin-orbit coupling in Bi that leads to the quantum spin Hall (QSH) state. The thousands Gauss range modulation is the consequence of the interplay between the Zeeman effect and the proximity effect. The phase accumulation in an Andreev quasiparticle pair is Δϕ=g_eff⋅μ_B⋅B_(//) (ℏv_F/L) which is of the order of few thousands Gauss. In one particular sample, a full modulation of the critical current with about 1 T range is observed. This is similar to the proximity effect in S/F/S junctions which suggests a 0-π junction transition

ZnO based materials, such as Zn1-xTMxO (TM = Mn, Co, Cu) nanopowders, were synthesised by a Sol gel route to investigate their properties in three fields: catalysis, optics and magnetism. These materials were characterised by complementary techniques such as X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS) and UV-Vis Spectroscopy. The fine structure and electronic properties of these nanomaterials were studied by X-ray Absorption Spectroscopy (XAS) and Electron Paramagnetic Resonance (EPR). These techniques give site, element and chemical specific measurements which allow a better understanding of the interplay and role of each element in the functionality of the system. The catalytic performance of undoped and Cu-doped ZnO nanosystems were tested with respect to the Methanol Steam Reforming (MSR) reaction. Contrary to what is generally accepted in literature, the results obtained in this study demonstrate that ZnO also plays a prominent role in this catalytic process. The structure–activity relationship of ZnO and copper-doped ZnO catalysts described in this work give an insight into the effective function of each component which is vital to enable the rational design of improved catalysts. The luminescence properties of the doped Zn1-xTMxO nanopowders were investigated with X-ray Excited Optical Luminescence (XEOL) techniques: these experiments provided a better understanding of the relationship between the electronic structure of the systems and their properties. Results showed how it is possible to manipulate the luminescence of ZnO grown by Sol gel by modifying synthesis conditions – i.e. the annealing temperature and the nature and concentration of the transition metal ion. Finally, preliminary results were presented on the materials' magnetic properties, obtained by SQUID (Superconducting Quantum Interference Devices) magnetometry, where the coexistence of different contributions has been detected. Even though further characterisation is still needed, this study is a step towards the determination of the nature of magnetic interactions in such systems, of which there has been considerable debate in the scientific community.
Materiali a base di ZnO, in particolare nano-polveri di Zn1-xTMxO (TM = Mn, Co, Cu), sono stati sintetizzati via Sol gel per studiarne le proprietà in tre diversi campi applicativi quali la catalisi, l’ottica ed il magnetismo. Tali materiali sono stati caratterizzati utilizzando diverse tecniche, complementari tra loro, quali X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS) e UV-Vis Spectroscopy. X-ray Absorption Spectroscopy (XAS) ed Electron Paramagnetic Resonance (EPR) vengono invece impiegate per studiare le proprietà elettroniche e di struttura fine delle nano-polveri. Tali caratterizzazioni si sono dimostrate fondamentali per la comprensione delle proprietà del sistema ed, in particolare, per cercare di identificare le interazioni sussistenti tra struttura, composizione, morfologia dei materiali e la loro capacità di espletare una determinata funzionalità. Nano-polveri di ZnO tal quali e drogate con ioni rame vengono testate come catalizzatori nella reazione di Steam Reforming del metanolo. I risultati ottenuti in questo studio dimostrano il ruolo attivo dell’ossido di zinco nel processo catalitico, contrariamente a quanto solitamente accettato in letteratura. La relazione sussistente tra struttura-attività nei catalizzatori a base di ZnO permette di ottenere informazioni circa l’effettiva funzione di ogni componente, aspetto di estrema importanza per la progettazione razionale di catalizzatori con elevate performance. Le proprietà di luminescenza dei sistemi drogati Zn1-xTMxO vengono studiate mediante spettroscopia X-ray Excited Optical Luminescence (XEOL); tali esperimenti forniscono una migliore comprensione del rapporto che sussiste tra la struttura elettronica dei sistemi in esame e le loro proprietà di emissione. I risultati mostrano come sia possibile modulare la luminescenza di ZnO prodotto via Sol gel modificando le condizioni di sintesi – i.e. temperatura di trattamento, natura e concentrazione del metallo di transizione utilizzato come drogante. Infine, risultati preliminari sulle proprietà magnetiche dei materiali ottenuti mediante SQUID magnetometer (Superconducting Quantum Interference Devices) hanno rivelato la coesistenza di diversi contributi magnetici. Nonostante ulteriori caratterizzazioni siano sicuramente necessarie, questo studio si è rivelato un passo avanti verso una comprensione della natura delle interazioni magnetiche in tali sistemi, da tempo causa di vivace dibattito nella comunità scientifica.

This study is about fabrication and characterization of nanostructures towards its application as single photon detector. Since this type of nanostructure can detect infrared radiation up to 2000nm, special care needs to be taken for selection of its dimensions as well as substrate and superconducting material for the growth of thin film. Quantum optical technologies have many applications in the area of quantum teleportation, quantum computation (QC), quantum key distribution (QKD). These applications rely significantly on the performance of a single photon detector. Photo multiplier tube (PMT) is one of the initially invented photon detectors. Si based avalanche photodiode (APD) has better performance metrices in comparison to PMT. However, both these technologies have limitation at telecom wavelengths. Although InGaAs based photodetector shows better detection efficiency at infrared, it has its own shortcomings due to high dark count rate, relatively large time jitter, low count rate and after-pulsing effect. Superconducting nanowire based detectors have shown a lot of potential in recent years as single photon detector at the wavelength of interest for its application in the area of quantum optical technologies due to its better performance metrices. The thesis starts with the study of superconducting properties of materials in case of two dimensional structures as well as one dimensional structure. The superconducting nanowire single photon detector (SNSPD) device fabrication incorporates five main steps, i.e. (1) Device operation principle and modeling. (2) Customized low temperature experimental setup for transport property measurement as well as optical characterization of the sample. (3) Thin film fabrication and optimization using suitable superconducting material and its transport property measurement. (4) Nanostructure fabrication and optimization of its process parameters and its transport property. vi (5) Optical characterization of the nanowires in meander shape using readout electronics in microwave frequency range. This thesis mainly focuses on first, second, third and fourth points as given above. Discussion on the device operation, modeling involves fundamental physics which help understanding the behavior of the device. The modeling of the device along with other components of the readout electronics has helped simulating the output response of the optical setup which can be verified experimentally in future. The overall fabrication steps of nanowires in meander shape have also been discussed along with design of contact pads in coplanar waveguide shape. The optical mask design for its use in fabrication of contact pads to save machine time of EBL is also discussed. It is obvious that detection and measurement of low level light signal is very difficult. When it comes to detecting single photon and that too in the infrared range, the difficulty level enhances due to lower photon energy. Detecting such weak signal with high detection efficiency requires low operating temperature in the range of mK to few Kelvin depending on the material used for the fabrication of detector. It is found that the detection efficiency of SNSPDs increases and the dark count diminishes significantly with decrease in operating temperature. An experimental setup down to 1.8K is designed using liquid helium flow cryostat along with probes for electrical as well as optical characterizations. Economical consumption of liquid helium, sound temperature stability, and efficient optical coupling, easy and user friendly samples changing option without breaking the vacuum, or warming up the cryostat were some of the important requirements taken into considerations while designing the cryostat. Sample holder with necessary arrangements for precise alignment of laser light with the active area of device to enhance optical coupling efficiency is designed for the optical probe. The INVAR alloy is used for sample holder to ensure that the alignment is not disturbed at low temperature. Single mode fibres due to its high transmission rate, minimum attenuation and least distortion have been used to shine light on the samples. Sample holder with 20-pin LCC socket and matching chip carrier provides convenient and fast sample mounting in electrical insert. Mu-metal is employed to cover the sample space in both the inserts to attenuate any electromagnetic interference. Further, temperature stability with the passage of time is also monitored. It is found that variation in temperature is less than 10mK at lowest operating temperature. Apart from the above, another important advantage of the system is its very low liquid helium loss rate (~100ml/hr) with all inserts which allows uninterrupted measurements for several days without refilling of liquid helium. Further, the thesis includes fabrication of thin films, patterning of nanowire structure and transport studies both on thin films and nanostructures. The niobium nitride (NbN) films were deposited using reactive magnetron sputtering. Argon to nitrogen ratio played a crucial role in the synthesis of high quality superconducting NbN. Critical temperatures (Tc) of about 15.5K have been measured for films with a thickness of about 10nm. Zero-Field-Cooled (ZFC) magnetization measurements were carried out to optimize the superconducting properties in ultra thin NbN films. The transport behavior down to 2K was studied using conventional resistance vs. temperature and current-voltage characteristics. Fabrication of NbN superconducting nanowires based on focused ion beam milling (FIB) and electron beam lithography (EBL) is presented. The detailed study of fabrication of nanowires with optimized process parameters using EBL is presented. Effect of gallium contamination on superconducting properties is discussed. Superconducting transition temperature as well as the transition width of nanowires do not show any significant impact of processing steps of standard EBL route, there is a significant degradation of superconducting properties for nanowires prepared using FIB. Gallium ion implantation across the superconducting channel may be considered as the root cause for this. The effect of gallium implantation may be the reason of technological limitations in designing fascinating single photon detector architectures, nevertheless provides some interesting manifestation of intrinsic low dimensional superconducting properties.

In this thesis, superconducting nanostructures for quantum detection of electromagnetic radiation are studied. In this regard, electrodynamics of topological excitations in 1D superconducting nanowires and 2D superconducting nanostrips is investigated. Topological excitations in superconducting nanowires and nanostrips lead to crucial deviation from the bulk properties. In 1D superconductors, topological excitations are phase slippages of the order parameter in which the magnitude of the order parameter locally drops to zero and the phase jumps by integer multiple of 2\pi. We investigate the effect of high-frequency field on 1D superconducting nanowires and derive the complex conductivity. Our study reveals that the rate of the quantum phase slips (QPSs) is exponentially enhanced under high-frequency irradiation. Based on this finding, we propose an energy-resolving terahertz radiation detector using superconducting nanowires. In superconducting nanostrips, topological fluctuations are the magnetic vortices. The motion of magnetic vortices result in dissipative processes that limit the efficiency of devices using superconducting nanostrips. It will be shown that in a multi-layer structure, the potential barrier for vortices to penetrate inside the structure is elevated. This results in significant reduction in dissipative process. In superconducting nanowire single photon detectors (SNSPDs), vortex motion results in dark counts and reduction of the critical current which results in low efficiency in these detectors. Based on this finding, we show that a multi-layer SNSPD is capable of approaching characteristics of an ideal single photon detector in terms of the dark count and quantum efficiency. It is shown that in a multi-layer SNSPD the photon coupling efficiency is dramatically enhanced due to the increase in the optical path of the incident photon.

PhD (Physics)
Department of Physics
Understanding how and why superconductivity (SC) occurs in a given material has been very challenging for physicists for more than a hundred years, notwithstanding the major milestones, such as the London theory, the Landau-Ginzburg theory, and the BCS theory. The extreme challenge to predict the occurrence of SC is symbolized by the long string of unanticipated but breathtaking advances, i.e., the unexpected discoveries of cuprates and Fe-pnictides being the dramatic modern examples. Because of their incompatibility, the nucleation of SC near a ferromagnet is di cult and has never been realized except for the case that another superconductor provides proximity-boosted Cooper pairs. This perceived necessity to start with another superconductor is engrained in the exten- sive study of the proximity e ect in superconductor/ferromagnet (S/F) powder sample, where all the structures involve a superconductor with either stable or metastable struc- ture. Compounding the di culty, it is also generally recognized that SC with substantial Tc is favourable in low dimensionality because of strong quantum uctuation. In this thesis, we report a serendipitous nding of SC that emerges under the most implausible circumstances in low eld microwave absorption measurement. This new revelation may lead to unconventional avenues to explore novel SC for applications in superconducting spintronics. By means of a varienty of techniques, including EPR, SEM, FTIR, PPMS/VSM and XRD, nanonickel incorporated YBCO in di erent weighting factors have been studied. With its complex chemical structure and magnetic properties, Ni-YBCO is far from well understood and the magentic behavior of the system under di erent conditions is investi- gated. From the dilute mixture of nanonickel particles, it is found that groups of normal Josephson junctions (JJs) and JJs due to YBCO-nickel-YBCO interparticle weaklinks form as nickel is ferromagnetic. We experimentally show, for the rst time multiple phase reversals in the non-resonant microwave absorption (NRMA) spectra from Ni-YBCO pos- sibly, due to the formation of JJs. We also show that these multiple phase reversals then vii depend on microwave power and temperature. We argue that microwave power induced coherence among some groups of JJs and breaking of some of the weaker JJs can then lead to the disappearance of multiple phase reversals at higher microwave power levels. Further, we also report a role of pair breaking e ects that shall give a linear eld de- pendence of the derivative microwave absorption signal, which is essentially the NRMA signal. This pair-breaking e ect dominates at temperatures closer to Tc as expected thermodynamically. The presence of two peaks in the system, results in high permeability ferromagnet which acts as a magnetic short circuit for magnetic ux density and creates low reluctance path. A transition from normal to anomalous does not occur in this work, because of the possibility of junction in the sample. As predictable at the region around the origin where the weaklinks are supposed to be very strong for a very low doping or low nanonickel addition ( 0.5 % wt), not much e ect was observed. However, when the nanonickel addition is increased to 2 % and 3% we see a signi cant change in the magnetization and the associated hysteresis, indicating ux pinning.
NRF

Arrays of ultra high-density (1.2 Tera/in2) vertical ferromagnetic cobalt nanowires with aligned shape and crystal magnetic-anisotropy axes are fabricated by electro-deposition in nanoporous polymer templates. The nanoporous films are derived from self-assembling PS-b-PMMA diblock-copolymers, which can provide lateral structure dimensions on the order of 10 nm, small enough to make cobalt nanowires in magnetic single-domain regime. An optimized fabrication procedure is developed for arrays of cobalt nanowires with enhanced perpendicular-to-plane magnetic anisotropy, by combining effects of shape anisotropy with perpendicularly oriented uniaxial magneto-crystalline anisotropy of hcp Co. Special conditions of electrolyte pH (pH>5.1) are required to obtain the desired c-axis crystal orientation, and pulse electrodeposition conditions are found to improve the overall perpendicular magnetic anisotropy, resulting in array-coercivities as large as 2.7 kOe at 300 K. X-ray diffraction and SQUID magnetometry measurements are used to characterize their structural and magnetic properties. A numerical model for the magnetization dynamics of an array of single-domain particles with vertically-aligned anisotropy axes is presented, which includes finite temperature effects, and magnetostatic interactions in a mean-field approximation. The model can qualitatively describe most features of the magnetization behavior, such as the temperature dependence of array-coercivity, "shearing" of the magnetization curves and the reduction of array-coercivity due to array demagnetization effects. Qualitative agreement is also obtained with the observed slow-relaxation magnetization decay of the cobalt nanowire arrays. To investigate particle non-uniformity effects, the model was further extended to include a distribution in energy barriers and particle volumes. The last part is concerned with the experimental investigation of the collective behavior of arrays of superconducting lead (Pb) nanowires with diameters smaller than the coherence length, which are coupled to each other by Andreev reflection at the S-N point contact interfaces with an underlying normal metal film. The system is characterized by magnetization and electrical transport measurements and is found to behave like an effective-medium type II superconductor with vortex pinning.

In this thesis novel, high-end superconducting and spintronic devices have been fabricated and characterized. In summary, the proposed work has been focused on the realization of nanowires, and more generally nanostructures, using the Electron Beam Lithography. Such a technology offers a powerful solution for nanofabrication able to conjugate spatial resolution, operation flexibility, and costs. Two main research fields has been explored: superconductive nanowires for advanced optical detection and nanostructures for magneto-resistance based devices.