J. Briatico

Structural and magnetic properties of Fex–C1−x nanocomposite thin films

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...

Unusual magneto-transport of YBa2Cu3O7−δ films due to the interplay of anisotropy, random disorder and nanoscale periodic pinning

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?? 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.

Hysteretic magnetic pinning and reversible resistance switching in high-temperature superconductor/ferromagnet multilayers

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.

BiFeO3/YBa2Cu3O7−δ heterostructures for strong ferroelectric modulation of superconductivity

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.

Study and optimization of ion-irradiated high Tc Josephson junctions by Monte Carlo simulations

High Tc Josephson junctions (HTc JJ) made by 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 JJ 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 JJ characteristics. A “universal” behavior has been evidenced, which allows us to propose new strategies to optimize JJ reproducibility.

Improving ion irradiated high Tc Josephson junctions by annealing: The role of vacancy-interstitial annihilation

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.

Imprinting nanoporous alumina patterns into the magneto-transport of oxide superconductors

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.

Joule heating and high frequency nonlinear effects in the surface impedance of high Tc superconductors

Using the dielectric resonator method, we have investigated nonlinearities in the surface impedance Zs=Rs+jXs of YBa2Cu3O7−δ 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−δ thin film as a function of microwave power is due to local heating.

Annealing of ion irradiated high TC Josephson junctions studied by numerical simulations

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.

Improving the IcRn product and the reproducibility of high Tc Josephson junctions made by ion irradiation

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.