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International audience; Radiofrequency (RF) coils fashioned from high-temperature superconductor (HTS) have the potential to increase the sensitivity of the magnetic resonance imaging (MRI) experiment by more than a dozen times compared to conventional copper coils. Progress, however, has been slow due to a series of technological hurdles. In this article, we present the developments that recently led to new perspectives for HTS coil in MRI, and challenges that still need to be solved. First, we recall the motivations for the implementations of HTS coils in MRI by presenting the limits of cooled copper coil technology, such as the anomalous skin effect limiting the decrease of the electric resistance of normal conductors at low temperature. Then, we address the progress made in the development of MRI compatible cryostats. New commercially available low-noise pulsed-tube cryocoolers and new materials removed the need for liquid nitrogen-based systems, allowing the design of cryogen-free and more user-friendly cryostats. Another recent advance was the understanding of how to mitigate the imaging artifacts induced by HTS diamagnetism through field cooling or temperature control of the HTS coil. Furthermore, artifacts can also originate from the RF field coupling between the transmission coil and the HTS reception coil. Here, we present the results of an experiment implementing a decoupling strategy exploiting nonlinearities in the electric response of HTS materials. Finally, we discuss the potential applications of HTS coils in bio-imaging and its prospects for further improvements. These include making the technology more user-friendly, implementing the HTS coils as coil arrays, and proposing solutions for the ongoing issue of decoupling. HTS coil still faces several challenges ahead, but the significant increase in sensitivity it offers lends it the prospect of being ultimately disruptive.

The use of high temperature superconducting (HTS) radio frequency (RF) coils in Magnetic Resonance Imaging (MRI) greatly improves the signal-to-noise ratio (SNR) in many biomedical applications and particularly in micro-MRI. However, a detailed understanding of the electrical behavior of HTS coils is important in order to optimize their performance through MR experiments. This paper presents a simple and versatile cryogen-free cryostat designed to characterize the RF properties of HTS coils prior to their use in MRI. The cryostat can be used at temperatures from 50 K to 300 K, with a control precision of approximately 3 mK at 70 K, and can measure the RF electrical power transmitted to an HTS coil over a range from 1 μW to 10 W. The quality factor and resonance frequency of the tested HTS coil are determined as a function of the temperature and the power it dissipates. This cryostat also permits the dynamic adjustment of the coil resonance frequency via temperature control. Finally, this study demonstrates that the HTS coil takes less than 12 μs to transit from the superconducting to the dissipative state, which is compatible with MRI requirements.

The term tunnel electroresistance (TER) denotes a fast, non-volatile, reversible resistance switching triggered by voltage pulses in ferroelectric tunnel junctions. It is explained by subtle mechanisms connected to the voltage-induced reversal of the ferroelectric polarization. Here we demonstrate that effects functionally indistinguishable from the TER can be produced in a simpler junction scheme—a direct contact between a metal and an oxide—through a different mechanism: a reversible redox reaction that modifies the oxide’s ground-state. This is shown in junctions based on a cuprate superconductor, whose ground-state is sensitive to the oxygen stoichiometry and can be tracked in operando via changes in the conductance spectra. Furthermore, we find that electrochemistry is the governing mechanism even if a ferroelectric is placed between the metal and the oxide. Finally, we extend the concept of electroresistance to the tunnelling of superconducting quasiparticles, for which the switching effects are much stronger than for normal electrons. Besides providing crucial understanding, our results provide a basis for non-volatile Josephson memory devices., The non-volatile switching of tunnel electroresistance in ferroelectric junctions provides the basis for memory and neuromorphic computing devices. Rouco et al. show tunnel electroresistance in superconductor-based junctions that arises from a redox rather than ferroelectric mechanism and is enhanced by superconductivity.

The possibility of controlling the interfacial properties of artificial oxide heterostructures is still attracting researchers in the field of materials engineering. Here, we used surface sensitive techniques and high-resolution transmission electron microscopy to investigate the evolution of the surface spin-polarization and lattice strains across the interfaces between La0.66Sr0.33MnO3 thin films and low-doped manganites as capping layers. We have been able to finely tune the interfacial spin-polarization by changing the capping layer thickness and composition. The spin-polarization was found to be highest at a critical capping thickness that depends on the Sr doping. We explain the non-trivial magnetic profile by the combined effect of two mechanisms. On one hand, the extra carriers supplied by the low-doped manganites that tend to compensate the overdoped interface, favouring locally a ferromagnetic double-exchange coupling. On the other hand, the evolution from a tensile-strained structure of the inner layers to a compressed structure at the surface that changes gradually the orbital occupation and hybridization of the 3d-Mn orbitals, being detrimental for the spin polarization. The finding of an intrinsic spin-polarization at the A-site cation observed in XMCD measurements reveals also the existence of a complex magnetic configuration at the interface, different from the magnetic phases observed at the inner layers., Comment: 17 pages and 3 figures

We studied vortex dynamics in a YBa 2 Cu 3 O 7−δ thin film with two different sources of pinning: intrinsic random defects and an artificial square array of defects created by masked ion irradiation. We study commensurability effects between the vortex lattice and the pinning array as a function of the vortex velocity v and the temperature. We find that at low temperatures the commensurability effects (magneto-resistance drop at the matching fields) are stronger at low velocities, in contrast with the behavior previously observed in low-critical-temperature superconductors.

We use magnetic flux quanta (superconducting vortices) on artificial energy landscapes (pinning arrays) to create a new type of artificial ice. This vortex ice shows unusual temperature effects that offer new possibilities in the study of ice systems. We have investigated the matching of the flux lattice to pinning arrays that present geometrical frustration. The pinning arrays are fabricated on YBCO films using masked O+ ion irradiation. The details of the magneto-resistance imply that the flux lattice organizes into a vortex ice. The absence of history-dependent effects suggests that the vortex ice is highly ordered. Due to the technique used for the artificial energy landscape fabrication, we have the ability to change the pinning array geometry using temperature as a control knob. In particular we can switch the geometrical frustration on and off, which opens the door to performing a new type of annealing absent in other artificial ice systems. * Work supported by the French ANR “MASTHER”, and the Fundacion Barrie (Galicia, Spain)

The penetration of a superconducting current from a superconductor into a half-metallic ferromagnet is usually forbidden. Resonances in the conductance spectra of superconductor/half-metal heterostructures suggest this restriction is lifted by the occurrence of unconventional equal-spin Andreev reflection.

We have investigated the transport properties of an insulating sub-micrometric YBa2Cu3O7-d bridge, patterned on a thin film. As expected for a variable-range-hopping insulator, transport is found non linear. The reduced dimension allows for the observation of an individual fluctuator generating random telegraph noise, which dynamics could be explored as a function of the temperature and transport current. Some recordings clearly exhibit three levels fluctuating resistance, with comparable level separation and correlated dynamics, which likely result from the existence of two states or correlated clustered charge traps.

The field of fluxtronics is based on transmitting and processing information using quanta of magnetic flux in a type-II superconductor. In this study, temperature is used to reversibly tune the energy landscape of flux vortices pinned in a patterned sample of YBa${}_{2}$Cu${}_{3}$O${}_{7}$, providing an avenue to thermally switchable device functionalities. This nanopatterning method could also be applied to other oxide materials, such as ferromagnets or ferroelectrics, to possibly extend the concept of temperature-dependent functionality.

Summary of the XRD analysis of a YBCO thin film and the superlattices [(YBCO) M /(PBCGO) N ] 10 //STO. Sample Λ (nm) M N c (nm) T c (K) AL1319 11.47 3 6 1.1749 60.5 AL1235 11.0 4 5 1.17135 68.9 AL1318 31.0 9 18 1.1740 77.7 AL1481 25.3 15 6 1.1714 89 AL959 Total = 100 (film) 1.1687 40 50 60 70 80 900,010,1 AL1235 AL1318 AL1319 AL1481 STemperature (K) H dc = 10 mTH ac = 0.5 mT Fig. 1 Derived S ( T ) curves from AC susceptibility measurements of the superlattices characterised in Table 2. The S ( T ) curve for the superlattices are visible Fig. 1. As expected, we observe an increase of the relaxation rate with temperature for all the samples, the pinning energy vanishing at T c . Although there is a significant difference between the critical temperature of a superlattice and that of a superconducting thin film, this graph shows that the variation of S ( T ) for the strained films (AL1319 and AL1318) are similar while it looks closer to a single YBCO film [1] for the relaxed films (AL1235 and AL1481), and not correlated to M. It indicates that the main contribution to vortex pinning comes from twin boundaries in spite of the outgrowth density. Note that although there is a large number of interfaces in the superlat-tices, pinning by surface roughness does not dominate the pinning properties for the relaxed films. In addition, previous study [2] has shown that it is possible to obtain NBCO films with a single [110] twin variant as shown by the TEM picture Fig. 2a. Whereas sample LDM596 (with a twinning proportion

We have made high quality c -axis NdBa 2 Cu 3 O 7 thin films on [0 0 1] SrTiO 3 by pulsed laser deposition, with much better crystalline properties than YBa 2 Cu 3 O 7 : smooth surface and low outgrowth density. TEM and 4-circle X-ray diffraction experiments reveal only a single orientation of the twins [1 1 0], as opposed to YBCO where both [1 1 0] and [1 1 0] twin orientations are observed. This anisotropy is consistently observed by critical current measurements and magneto-optical microscopy in the superconducting state. Anisotropy in magnetic field penetration is attributed to the anisotropic twinning rate along the [1 1 0] and [1 1 0] directions which is 0/1 in NdBa 2 Cu 3 O 7 films, whereas it is close to 0.5/0.5 for YBa 2 Cu 3 O 7 deposited on [0 0 1] SrTiO 3 . We present devices requiring such features as vortex mobility or periodic pinning of the vortices.

We have investigated the effect of different types of disorder on the propagation, roughness, and scaling properties of magnetic flux fronts in a type II superconductor. A progression from the usual (Kardar–Parisi–Zhang-type) scaling to multiscaling is observed as the disorder strength is increased. A hierarchy of disorder strengths is established for YBa 2 Cu 3 O 7 − δ thin films. The results cast light on the physical origin of the roughening of flux fronts, and they are of interest for the design and elimination of flux noise in microscopic superconducting thin-film devices.

The problem of an ensemble of repulsive particles on a potential-energy landscape is common to many physical systems and has been studied in multiple artificial playgrounds. However, the latter usually involve fixed energy landscapes, thereby impeding in situ investigations of the particles' collective response to controlled changes in the landscape geometry. Here, we experimentally realize a system in which the geometry of the potential-energy landscape can be switched using temperature as the control knob. This realization is based on a high-temperature superconductor in which we engineer a nanoscale spatial modulation of the superconducting condensate. Depending on the temperature, the flux quanta induced by an applied magnetic field see either a geometrically frustrated energy landscape that favours an ice-like flux ordering, or an unfrustrated landscape that yields a periodic flux distribution. This effect is reflected in a dramatic change in the superconductor's magneto-transport. The thermal switching of the energy landscape geometry opens new opportunities for the study of ordering and reorganization in repulsive particle manifolds.

Iron–carbon nanocomposite thin films with iron concentrations ranging from 12 to 74 at. % were grown by ion-beam sputtering codeposition at different substrate temperatures. The microstructure of the films was characterized by transmission electron microscopy, atomic force microscopy, extended x-ray absorption fine structure, and grazing incidence small-angle x-ray scattering. A granular morphology consisting of body-centered-cubic iron-rich nanoparticles (2–5 nm in diameter and 3–8 nm in height with a relatively sharp size distribution) regularly distributed within a more or less graphitic matrix was obtained. Structural properties as well as magnetic ones were found to depend strongly on composition, substrate temperature, and postdeposition treatments (annealing or Ar+ irradiation). The temperature dependence of the susceptibility exhibited a superparamagnetic response with blocking temperature in the range of 13–180 K. The maximum low-temperature (5 K) coercivity value obtained in this study was 850 O...

We report on aggregation of Co on a smooth amorphous alumina surface, during sputter deposition of Co atoms at room temperature. The atomic structure of the Co grains appears to be size dependent: hep in the larger bean-shaped clusters (greater than 3 nm), and often fee in the smaller, almost spherical clusters. The deposits show relatively narrow distributions of the cluster sizes and of the intercluster distances, which results in a noticeable local order and explains how charge blocking (the so-called Coulomb blockade) could be observed in large sets of such particles by Schelp et al. (1997, Phys. Rev. B, 56, R5747). Electrical percolation occurs at a nominal thickness of about 2 nm.

We report on the structure of cobalt cluster films obtained by aggregation of metal atoms sputter-deposited on amorphous alumina. The cluster layers were encapsulated with a second amorphous alumina layer, also made by sputtering in the same chamber. Electron energy loss spectroscopy (EELS) indicates that encapsulation with sputtered alumina keeps the clusters free from cobalt oxide. Transmission electron microscopy (TEM) exhibits relatively narrow distributions of the cluster sizes and of the inter-cluster distances, which results in a noticeable local order. Size distributions appear quasi-Gaussian, but TEM misses an important number of small particles. Extended X-ray absorption fine structure (EXAFS) data shows that the actual average sizes are smaller, and Monte Carlo simulations suggest that the actual distributions could be bimodal, with a secondary peak in the small-size range.

International audience; We study the general problem of a manifold of interacting elastic lines whose spatial correlations are strongly affected by the competition between random and ordered pinning. This is done through magneto-transport experiments with YBa2Cu3O7−δ thin films that contain a periodic vortex pinning array created via masked ion irradiation, in addition to the native random pinning. The strong field-matching effects we observe suggest the prevalence of periodic pinning, and indicate that at the matching field each vortex line is bound to an artificial pinning site. However, the vortex-glass transition dimensionality—quasi-two dimensional instead of the usual three dimensional—evidences reduced vortex-glass correlations along the vortex line. This is also supported by an unusual angular dependence of the magneto-resistance, which greatly differs from that of Bose-glass systems. A quantitative analysis of the angular magneto-resistance allows us to link this behaviour to the enhancement of the system anisotropy, a collateral effect of the ion irradiation.

We have developed a masked ion irradiation technique to engineer the energy landscape for vortices in oxide superconductors. This approach associates the possibility to design the landscape geometry at the nanoscale with the unique capability to adjust the depth of the energy wells for vortices. This enabled us to unveil the key role of vortex channeling in modulating the amplitude of the field matching effects with the artificial energy landscape, and to make the latter govern flux dynamics over an unusually wide range of temperatures and applied fields for high-temperature superconducting films.

Using heterostructures that combine a large-polarization ferroelectric (BiFeO3) and a high-temperature superconductor (YBa2Cu3O(7-δ)), we demonstrate the modulation of the superconducting condensate at the nanoscale via ferroelectric field effects. Through this mechanism, a nanoscale pattern of normal regions that mimics the ferroelectric domain structure can be created in the superconductor. This yields an energy landscape for magnetic flux quanta and, in turn, couples the local ferroelectric polarization to the local magnetic induction. We show that this form of magnetoelectric coupling, together with the possibility to reversibly design the ferroelectric domain structure, allows the electrostatic manipulation of magnetic flux quanta.

We study a high-TC superconducting (YBa2Cu3O7-d) / ferromagnetic (Co/Pt multilayer) hybrid which exhibits resistance switching driven by the magnetic history: depending on the direction of the external field, a pronounced decrease or increase of the mixed-state resistance is observed as magnetization reversal occurs within the Co/Pt multilayer. We demonstrate that stray magnetic fields cause these effects via i) creation of vortices/antivortices and ii) magnetostatic pinning of vortices that are induced by the external field., Comment: to appear in Physical Review B

We used oxygen ion irradiation to transfer the nanoscale pattern of a porous alumina mask into high-T(C) superconducting thin films. This causes a nanoscale spatial modulation of superconductivity and strongly affects the magneto-transport below T(C), which shows a series of periodic oscillations reminiscent of the Little-Parks effect in superconducting wire networks. This irradiation technique could be extended to other oxide materials in order to induce ordered nanoscale phase segregation.

Co/Al2O3/NiFe tunnel junctions were deposited on misoriented silicon substrates. A thermal treatment activates a step bunching mechanism on the silicon surface leading to a modulated topology with a nanometric lateral period. The bottom magnetic electrode and the barrier follow this topology. Such junctions present good magnetotransport characteristics with a magnetoresistance about 14% at room temperature. Due to the uniaxial anisotropy induced by the topological modulation, the antiparallel configuration of the magnetization is well defined leading to perfect square magnetoresistance cycles at all temperatures. The magnetic behavior of patterned junctions has been investigated by transport measurements and Kerr microscopy. More than inducing a strong uniaxial anisotropy, the modulated topology is shown to strongly influence the switching mechanism of the magnetization.

International audience; Using the dielectric resonator method, we have investigated nonlinearities in the surface impedance Zs=Rs+ jXs of YBa2Cu3O7− thin films at 10 GHz as a function of the incident microwave power level and temperature. The use of a rutile dielectric resonator allows us to measure the precise temperature of the films. We conclusively show that the usually observed increase in the surface resistance of YBa2Cu3O7− thin film as a function of microwave power is due to local heating

We report a transport study down to 2 K and in high magnetic fields up to 20 T of a highly $c$-axis-oriented epitaxial thin film of electron-doped ``infinite-layer'' cuprate superconductor ${\text{Sr}}_{1\ensuremath{-}x}{\text{La}}_{x}{\text{CuO}}_{2}$ with ${T}_{c}$ of 26 K. A 16 T magnetic field perpendicular to the $ab$ plane of the film completely suppresses superconductivity, even at the lowest temperature. Perpendicular magnetic fields shift the resistive transition to lower temperatures, without significant broadening, in a way similar to that seen in conventional superconductors. The upper critical field ${H}_{c2\ensuremath{\perp}}(T)$ exhibits an almost linear temperature dependence. It is shown that electron-doped infinite-layer cuprate presents three-dimensional superconductivity due to a weaker anisotropy $(\ensuremath{\gamma}=15)$ and smaller ${H}_{c2\ensuremath{\perp}}(T)$ values than the ones of hole-doped cuprates.

Using the dielectric resonator method, we have investigated nonlinearities in surface impedance Zs = Rs + jXs of YBa2Cu3O7-delta thin films at 10 GHz as a function of the incident microwave power level and temperature. The use of a rutile dielectric resonator allows us to measure the precise temperature of the films. We conclusively show that the usually observed increase of the surface resistance of YBa2Cu3O7-delta thin film as function of microwave power is due to local heating.

Applied Superconductivity Conference 2006, Seattle, WA, AUG 27-SEP 01, 2006; International audience; In this paper, we present several steps in the development of an HTS Giaever transformer, based on a SIS heterostructure, where both superconducting thin film should present low pinning properties and be insulated with a thin insulating layer. High Resolution AC Susceptibility measurements on YBCO//STO and NBCO//STO thin films show that the most important pinning centers are twin boundary intersections (TBI), and that their elimination with proper growth conditions allows to improve vortex mobility. When the base layer outgrowth density exceeds 10(5)/cm(2), pinholes through the insulating layer (PBCO/STO) short-circuit the electrodes. Therefore, ion beam milling at grazing incidence has been performed to reduce the height of the outgrowths otherwise protruding between 0.5 and 1.5 mu m above the surface of the film. We could reduce their density by 10 and barrier leakage by 1000 with only 10 nm of insulator thickness. Several Giaever devices were fabricated with this ion milling process and by adjusting the regrowth temperature. The influence of this new technology was investigated: AES analysis indicates a preferential erosion of copper ions, a damaged surface layer of 5 nm is deduced from RBS analysis.

We report on the effects of small miscuts of the (001) SrTiO 3 (STO) substrate surface, on the microstructure of epitaxial films of YBa 2 Cu 3 O 7 - δ and NdBa 2 Cu 3 O 7 - γ (NBCO). The most noticeable result is that, by growing NBCO films on miscut STO substrates with the steps intentionally oriented along a direction within 15° of [110], one reliably obtains NBCO films with a single [110] twinning variant (the [110] is completely eliminated), which is a precious characteristic for all applications involving flux flow.

Vortex pinning in high-TC superconductors is usually dominated by intersection of twin boundaries (Physica C, 372–376 (2002) 634). We have grown NdBa2Cu3O7−δ thin films by pulsed laser deposition on (1 0 0) SrTiO3 substrates, which present only one [1 1 0] twinning variant (the [1 1 0] being completely eliminated), thereby drastically reducing the density of twin boundary intersections. High-resolution AC susceptometry reveals several peaks in χ′′(T) curve, each one presumably associated with a specific pinning mechanism. For different samples, we correlate χ(T) data with microstructure to characterise vortex pinning sources.

We describe the growth, structural, and functional characterization of BiFeO3/YBa2Cu3O7−δ ferroelectric/superconductor heterostructures. High-structural-quality bilayers are obtained, which display good ferroelectric and superconducting properties. We demonstrate that an unusually strong field-effect modulation of the YBa2Cu3O7−δ superconducting critical temperature can be produced upon ferroelectric switching of the BiFeO3 overlayer, and we show that this effect is non-volatile and reversible.

The transport properties of ultra-thin SrTiO(3) (STO) layers grown over YBa(2)Cu(3)O(7) electrodes were studied by conductive atomic force microscopy at the nano-scale. A very good control of the barrier thickness was achieved during the deposition process. A phenomenological approach was used to obtain critical parameters regarding the structural and electrical properties of the system. The STO layers present an energy barrier of 0.9 eV and an attenuation length of 0.23 nm, indicating very good insulating properties for the development of high-quality Josephson junctions.

A phenomenological approach was used to obtain critical information about the structure and electrical properties of ultra thin Ba0.05Sr0.95TiO3 (BSTO) layers over Nb electrodes. The method allows, in a simple way, to study and to optimize the growth of the barrier in order to improve the performance and application of Josephson junctions. A very good control of the layer thickness with a low roughness was achieved during the deposition process. The BSTO layers present an energy barrier of 0.6 eV and an attenuation length of 0.4 nm, indicating its good insulating properties for the development of Josephson junctions with improved performance. Fil: Sirena, Martin. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Area de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche); Argentina. Comisión Nacional de Energía Atómica. Gerencia del Area de Energía Nuclear. Instituto Balseiro; Argentina. Universidad Nacional de Cuyo; Argentina Fil: Aviles Felix, Luis Steven. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Area de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche); Argentina. Comisión Nacional de Energía Atómica. Gerencia del Area de Energía Nuclear. Instituto Balseiro; Argentina. Universidad Nacional de Cuyo; Argentina Fil: Carvacho Vera, G. A. . Universidad de Concepción; Chile Fil: Navarro Fernández, Henry Luciano. Universidad de Concepción; Chile Fil: Steren, Laura Beatriz. Comisión Nacional de Energía Atómica. Gerencia del Area de Investigaciones y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Constituyentes); Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Bernard, R. . Universite Paris Sud; Francia. Centre National de la Recherche Scientifique; Francia Fil: Briático, J. . Universite Paris Sud; Francia. Centre National de la Recherche Scientifique; Francia Fil: Bergeal, N. . Centre National de la Recherche Scientifique; Francia Fil: Lesueur, J. . Centre National de la Recherche Scientifique; Francia Fil: Faini, G.. Centre National de la Recherche Scientifique; Francia

International audience; In order to probe the vortex shear viscosity in the vortex liquid phase, we have introduced two types of vortex-confining structures in optimally doped Bi2Sr2CaCu2O8 single crystals. First, walls of strong vortex pinning separated by weakly pinning channels are fashioned by heavy ion irradiation through 25 um-thick Ni masks. Second, a low density of homogeneously distributed amorphous columnar defects is known to impose a polycrystalline structure to the vortex lattice. Resistivity measurements show that the inclusion of confining structures impede vortex flow in the liquid. The resistivity is remarkably well described by the Halperin-Nelson theory for the viscosity due to free two-dimensional vortex lattice dislocations.

Recently, annealing of ion irradiated high Tc Josephson iunctions (JJs) has been studied experimentally in the perspective of improving their reproducibility. Here we present numerical simulations based on random walk and Monte Carlo calculations of the evolution of JJ characteristics such as the transition temperature Tc′ and its spread ΔTc′, and compare them with experimental results on junctions irradiated with 100 and 150 keV oxygen ions, and annealed at low temperatures (below 80 °C). We have successfully used a vacancy-interstitial annihilation mechanism to describe the evolution of the Tc′ and the homogeneity of a JJ array, analyzing the evolution of the defects density mean value and its distribution width. The annealing first increases the spread in Tc′ for short annealing times due to the stochastic nature of the process, but then tends to reduce it for longer times, which is interesting for technological applications.

We have studied the annealing effects on the transport properties of High Tc Josephson Junctions (JJ) made by ion irradiation. Several JJ were measured for different annealing times and the experimental data were compared to numerical simulations. We have successfully used a vacancy-interstitial annihilation mechanism to describe the evolution of the JJ coupling temperature (TJ) and the homogeneity of a JJ array, related to the evolution of the defects density mean value and its distribution width. For sufficient long annealing times (t > 600 min), ΔTJ was significatively reduced. This result appears to be very encouraging for future applications where the spread in JJ characteristics has to be as low as possible.

A simple model has been proposed to explain the spread in the characteristics of high Tc Josephson junctions made by ion irradiation, assuming that the source of dispersion is the slit’s size variation. Accordingly, increasing ion energy should lead to a significant reduction of inhomogeneities. Test samples have been fabricated using two different beam energies. As predicted, the spread in critical current decreases upon increasing energy. Moreover, since the actual width of the barrier is reduced in this case, the IcRn product increases significantly. These results seem promising for future technological applications.

The authors have studied the annealing effect in the transport properties of high Tc Josephson junctions (JJs) made by ion irradiation. Low temperature annealing (80°C) increases the JJ coupling temperature (TJ) and the IcRn product, where Ic is the critical current and Rn the normal resistance. They have found that the spread in JJ characteristics can be reduced by sufficient long annealing times, increasing the reproducibility of ion irradiated Josephson junctions. The characteristic annealing time and the evolution of the spread in the JJ characteristics can be explained by a vacancy-interstitial annihilation process rather than by an oxygen diffusion one.

High Tc Josephson nanoJunctions (HTc JnJ) made by ion irradiation have remarkable properties for technological applications. However, the spread in their electrical characteristics increases with the ion dose. We present a simple model to explain the JnJ inhomogeneities, which accounts quantitatively for experimental data. The spread in the slit's width of the irradiation mask is the limiting factor.Monte Carlo simulations have been performed using different irradiation conditions to study their influence on the spread of the JnJ charcateristics. A "universal" behavior has been evidenced, which allows to propose new strategies to optimize JnJ reproducibility., Comment: 14 pages, 6 Figures. accepted in Journal of Applied Physics

Strong correlated manganites are still under intense research owing to their complex phase diagrams in terms of Sr-doping and their sensitivity to intrinsic and extrinsic structural deformations. Here, we performed X-ray absorption spectroscopy measurements of manganite bilayers to explore the effects that a local Sr-doping gradient produce on the charge and antiferromagnetic anisotropies. In order to gradually tune the Sr-doping level along the axis perpendicular to the samples we have grown a series of bilayers with different thicknesses of low-doped manganites (from 0 nm to 6 nm) deposited over a La0.7Sr0.3MnO3 metallic layer. This strategy permitted us to resolve with high accuracy the thickness region where the charge and spin anisotropies vary and the critical thickness tc over which the out of plane orbital asymmetry does not have any further modifications. We found that the antiferromagnetic spin axis points preferentially out of the sample plane regardless the capping layer thickness. However, it tilts partially into the sample plane far from this critical thickness, owing to the combined contributions of the external structural strain and electron doping. Furthermore, we found that the doping level of the capping layer strongly affects the critical thickness, giving clear evidence of the influence exerted by the electron doping on the orbital and magnetic configurations. These anisotropic changes induce subtle modifications on the domain reorientation of La0.7Sr0.3MnO3, as evidenced from the magnetic hysteresis cycles., Authors thanks the financial support of FONCYT PICT 2014-1047, CONICET PIP 112-201501-00213, MINCYT and the European Commission through the Horizon H2020 funding by H2020-MSCA-RISE-2016 – Project No. 734187 – SPICOLOST.

International audience; We have investigated the angular behavior of the tunnel magnetoresistance ͑TMR͒ in transition-metal-based junctions using the low-field susceptibility of the crossed magnetic configuration. The noncollinear arrangement , stabilized by combining step anisotropy and interfacial exchange-bias coupling, is shown to be of a particular interest for an accurate analysis of the angular dependence of the TMR. We show that the intrinsic tunnel processes are reflected on a linear behavior of the conductivity giving a more complex form for the resistance, as expected by the model of Slonczewski. The more intuitive ''high-field'' saturating regime deviates the hard layer from its nominal pinning direction and consequently is shown to be less adapted for the experimental study of the intrinsic angular response of the TMR.