BAJAS TEMPERATURAS
URI permanente para esta comunidad
Examinar
Examinando BAJAS TEMPERATURAS por Materia "CAPAS FINAS"
Mostrando 1 - 18 de 18
Resultados por página
Opciones de ordenación
Ítem Acceso Abierto Compositional effects on the electrical properties of extremely disordered molybdenum oxynitrides thin films(Elsevier, 2020-02-15) Hofer, J.A.; Bengio, S.; Rozas, G.; Pérez, P.D.; Sirena, M.; Suárez, S.; Haberkorn, N.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasMolybdenum oxynitride (MoNxOy) thin films were grown by reactive sputtering on Si (100) substrates at room temperature. The partial pressure of Ar was fixed at 90%, and the remaining 10% was adjusted with mixtures N2:O2 (varying from pure N2 to pure O2). The electrical properties of the films depend on the chemical composition. Thin films grown using mixtures up to 2% O2 have γ-Mo2N phase and display superconductivity. The superconducting critical temperature Tc reduces from ∼6.8 K to below 3.0 K as the oxygen increases. On the other hand, the films are mostly amorphous for gas mixtures above 2% O2. The electrical conductivity shows a semiconductor-like behavior well described by variable-range hopping conduction. The analysis of the optical properties reveals that the samples do not have a defined semiconductor bandgap, indicating that the high structural disorder produces electron excitation for a wide range of energies.Ítem Acceso Abierto Development and characterization of shape memory Cu-Zn-Al thin films(Elsevier, 2010-06-15) Haberkorn, N.; Lovey, F. C.; Condó, A. M.; Guimpel, J; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasTernary Cu–Zn–Al alloys show good shape memory properties with narrow hysteresis and a wide range of martensitic transformation temperature (Ms), depending on the alloy composition. Thin films of Cu–Zn–Al with shape memory effect were grown for the first time using a new procedure. First Cu–Al thin films were obtained by DC sputtering on Si (1 0 0) substrates at room temperature, and second, the Cu–Al films were encapsulated and annealed in the presence of a Cu–Zn–Al bulk reference in order to fix a Zn vapour pressure. In this way a controlled amount of Zn is transported from the bulk reference into the film, in such a way that the Ms of the film becomes nearly the same as the bulk reference. The structures and microstructures of the as grown films were analysed by X-ray diffraction and transmission electron microscopy. The martensitic transformation temperature was determined by resistivity measurements.Ítem Acceso Abierto Effects of spin density wave quantization on the electrical transport in epitaxial Cr thin films(American Physical Society, 2012-03-00) Osquiguil, E.; Kaul, E.E.; Tosi, L.; Balseiro, C.A.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe present measurements of the electrical resistivity ρ in epitaxial Cr films of different thicknesses grown on MgO (100) substrates, as a function of temperature T. The ρ(T) curves display hysteretic behavior in a certain temperature range, which is film thickness dependent. The hysteresis are related to the confinement of quantized incommensurate spin density waves (ISDW) in the film thickness. Our important finding is to experimentally show that the temperature Tmid where the ISDW changes from N to N+1 nodes decreases as the film thickness increases. Identifying Tmid with a first-order transition between ISDW states with N and N+1 nodes, and using a Landau approach to the free energy of the ISDW together with Monte Carlo simulations, we show that the system at high temperatures explores all available modes for the ISDW, freezing out in one particular mode at a transition temperature that indeed decreases with film thickness L. The detailed dependence of Tmid(L) seems to depend rather strongly on the boundary conditions at the Cr film interfaces.Ítem Acceso Abierto Electrical transport across nanometric SrTiO3 and BaTiO3 barriers in conducting/insulator/conducting junctions(IOP Publishing, 2018-01-11) Navarro, H.; Sirena, M.; González Sutter, J.; Troiani, H.E.; del Corro, P.G.; Granell, P.; Golmar, F.; Haberkorn, N.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe report the electrical transport properties of conducting/insulator/conducting heterostructures by studying current–voltage IV curves at room temperature. The measurements were obtained on tunnel junctions with different areas (900, 400 and 100 μm2) using a conducting atomic force microscope. Trilayers with GdBa2Cu3O7 (GBCO) as the bottom electrode, SrTiO3 or BaTiO3 (thicknesses between 1.6 and 4 nm) as the insulator barrier, and GBCO or Nb as the top electrode were grown by DC sputtering on (100) SrTiO3 substrates For SrTiO3 and BaTiO3 barriers, asymmetric IV curves at positive and negative polarization can be obtained using electrodes with different work function. In addition, hysteretic IV curves are obtained for BaTiO3 barriers, which can be ascribed to a combined effect of the FE reversal switching polarization and an oxygen vacancy migration. For GBCO/BaTiO3/GBCO heterostructures, the IV curves correspond to that expected for asymmetric interfaces, which indicates that the disorder affects differently the properties at the bottom and top interfaces. Our results show the role of the interface disorder on the electrical transport of conducting/insulator/conduction heterostructures, which is relevant for different applications, going from resistive switching memories (at room temperature) to Josephson junctions (at low temperatures).Ítem Acceso Abierto Hyperuniform vortex patterns at the surface of type-II superconductors(American Physical Society, 2019-10-29) Rumi, G.; Aragón Sánchez, J.; Elías, F.; Cortés Maldonado, R.; Puig, J.; Cejas Bolecek, N.R.; Nieva, G.; Konczykowski, M.; Fasano, Y.; Kolton, A.B.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasA many-particle system must possess long-range interactions in order to be hyperuniform at thermal equilibrium. Hydrodynamic arguments and numerical simulations show, nevertheless, that a three-dimensional elastic-line array with short-ranged repulsive interactions, such as vortex matter in a type-II superconductor, forms at equilibrium a class-II hyperuniform two-dimensional point pattern for any constant-z cross section. In this case, density fluctuations vanish isotropically as ∼qα at small wave vectors q, with α=1. This prediction includes the solid and liquid vortex phases in the ideal clean case and the liquid in presence of weak uncorrelated disorder. We also show that the three-dimensional Bragg glass phase is marginally hyperuniform, while the Bose glass and the liquid phase with correlated disorder are expected to be nonhyperuniform at equilibrium. Furthermore, we compare these predictions with experimental results on the large-wavelength vortex density fluctuations of magnetically decorated vortex structures nucleated in pristine, electron-irradiated, and heavy-ion-irradiated superconducting Bi2Sr2CaCu2O8+δ samples in the mixed state. For most cases, we find hyperuniform two-dimensional point patterns at the superconductor surface with an effective exponent αeff≈1. We interpret these results in terms of a large-scale memory of the high-temperature line-liquid phase retained in the glassy dynamics when field cooling the vortex structures into the solid phase. We also discuss the crossovers expected from the dispersivity of the elastic constants at intermediate length-scales, and the lack of hyperuniformity in the x−y plane for lengths q−1 larger than the sample thickness due to finite-size effects in the z direction. We argue these predictions may be observable and propose further imaging experiments to test them independently.Ítem Acceso Abierto Improving the Josephson energy in high-Tc superconducting junctions for ultra-fast electronics(IOP Publishing, 2020-00-00) Navarro, H.; Sirena, M.; Haberkorn, N.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe report the electrical transport of thin vertically-stacked Josephson tunnel junctions. The devices were fabricated using 16 nm thick GdBa2Cu3O7−δ electrodes and 1–4 nm SrTiO3 as an insulating barrier. The results show Josephson coupling for junctions with SrTiO3 barriers of 1 and 2 nm. Subtracting the residual current in the Fraunhofer patterns, energies of 3.1 mV and 5.7 mV at 12 K are obtained for STO barriers of 1 nm and 2 nm, respectively. The residual current may be related to the contribution of pinholes and thickness fluctuations in the STO barrier. These values are promising for reducing the influence of thermal noise and increasing the frequency operation rate in superconducting devices using high-temperature superconductors.Ítem Acceso Abierto Influence of the oxygen content on the normal-state Hall angle in YBa2Cu3Oxn films(American Physical Society, 1993-03-01) Wuyts, B.; Osquiguil, E.; Maenhoudt, M.; Libbrecht, S.; Gao, Z.X.; Bruynseraede, Y.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe report on measurements of in-plane resistivity ρab(T) and Hall coefficient RH(T) in c-axis-oriented oxygen-deficient YBa2Cu3Oxn thin films. Independent of the changes induced in RH(T) and ρab(T) by the oxygen depletion, a quadratic temperature dependence is observed for the Hall angle θH. The slope of cotθH versus T2 decreases as the oxygen content xn is reduced and saturates in the Tc(xn)=60 K plateau region. The results are discussed in the framework of the two-dimensional Luttinger liquid theory as proposed by Anderson.Ítem Acceso Abierto Interrelation between persistent photoconductivity and oxygen order in GdBa2Cu3Ox thin films(American Physical Society, 1997-08-15) Guimpel, J.; Maiorov, B.; Osquiguil, E.; Nieva, G.; Pardo, F.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe study the metastable states induced by photoexcitation, oxygen disorder, and both effects combined, in superconducting GdBa2Cu3Ox thin films. The states are identified by the temperature dependence of the resistivity. The relaxation dynamics is characterized through the time evolution of the conductivity. The results show that photoexcitation and oxygen disorder behave as noninteracting and do not cancel each other out. However, in both cases the relaxation dynamics is related to oxygen movement.Ítem Acceso Abierto Ion implantation inducing two-way shape memory effect in Cu-Al-Ni thin films(Elsevier, 2019-11-15) Morán M.,Condó A. M. ,Suarez S. ,Soldera F., Haberkorn N.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe report two-way shape memory effect (TWSME) induced by Al ion implantation in 6 μm thick Cu-Al-Ni thin films. The films display an average grain size of 3.7 μm and a martensitic transformation temperature (MS) of ≈ 240 K. The film was irradiated with 2 MeV Al ions with a fluence of 6 × 1015 ion.cm−2 (penetration distance up to ≈ 1.1 μm). After irradiation, the film displays well defined TWSME with a radius of curvature of ≈ 1 mm. The results indicate that the irradiation produces mainly changes in the austenitic order.Ítem Acceso Abierto Josephson coupling in high-Tc superconducting junctions using ultra-thin BaTiO3 barriers(Elsevier, 2020-12-00) Navarro, H.; Sirena, M.; Kim, J.; Haberkorn, N.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe study the electrical transport of vertically-stacked Josephson tunnel junctions using GdBa2Cu3O7−δ electrodes and a BaTiO3 barrier with thicknesses between 1 nm and 3 nm. Current-voltage measurements at low temperatures show a Josephson coupling for junctions with BaTiO3 barriers of 1 nm and 2 nm. Reducing the barrier thickness bellow a critical thickness seems to suppress the ferroelectric nature of the BaTiO3. The Josephson coupling temperature reduces as the barrier thicknesses increases. The Josephson energies at 12 K are of ≈ 1.5 mV and ≈ 7.5 mV for BaTiO3 barriers of 1 nm and 2 nm, respectively. Fraunhofer patterns are consistent with fluctuations in the critical current due to structural inhomogeneities in the barriers. Our results are promising for the development of Josephson junctions using high-Tc electrodes with energy gaps much higher than those usually present in conventional low-temperature superconductors.Ítem Acceso Abierto Magnetic properties of SrTiO3/Pr2/3Ca 1/3MnO3 multilayers(IOP Publishing, 2010-00-00) Niebieskikwiat, D.; Haberkorn, N.; Guitarra, R.; Guimpel, J.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasNanoscale ferromagnetic (FM) clusters embedded within insulating (I) layers of the antiferromagnetic Pr2/3Ca1/3MnO3 (PCMO) are known to improve the magnetoresistance ratio of FM-I magnetoresistive multilayers. Here, we study the magnetic properties of perovskite superlattices comprised of five PCMO layers of variable thickness t separated by 5nm-thick spacers of SrTiO3 (STO). Several multilayers (1.5 nm ≤ t ≤ 8 nm) were grown on (001) STO substrates by magnetron sputtering. We show that the magnetic moment of PCMO due to the FM inclusions presents a maximum for t ~ 3 nm, accompanied by an abrupt increase of coercivity and exchange bias field. The nonmagnetic nature of the neighboring STO layers demonstrates that the observed behavior is not related to any interlayer exchange coupling, but the geometrical matching between layer thickness and FM domain size is the key driver for the enhancement of the FM moment and anisotropy energy. These results open a new door for the optimization of perovskite based spintronic devices.Ítem Acceso Abierto Martensitic transformation in as-grown and annealed near-stoichiometric epitaxial Ni2MnGa thin films(Taylor&Francis, 2015-00-00) Machain, P.; Condó, A.M.; Domenichini, P.; Pozo López, G.; Sirena, M.; Correa, V.F.; Haberkorn, N.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasMagnetic shape memory nanostructures have a great potential in the field of the nanoactuators. The relationship between dimensionality, microstructure and magnetism characterizes the materials performance. Here, we study the martensitic transformation in supported and free-standing epitaxial Ni47Mn24Ga29 films grown by sputtering on (0 0 1) MgO using a stoichiometric Ni2MnGa target. The films have a Curie temperature of ~390 K and a martensitic transition temperature of ~120 K. Similar transition temperatures have been observed in films with thicknesses of 1, 3 and 4 μm. Thicker films (with longer deposition time) present a wider martensitic transformation range that can be associated with small gradients in their chemical concentration due to the high vapour pressure of Mn and Ga. The magnetic anisotropy of the films shows a strong change below the martensitic transformation temperature. No features associated with variant reorientation induced by magnetic field have been observed. Annealed films in the presence of a Ni2MnGa bulk reference change their chemical composition to Ni49Mn26Ga25. The change in the chemical composition increases the martensitic transformation temperature, being closer to the stoichiometric compound, and reduces the transformation hysteresis. In addition, sharper transformations are obtained, which indicate that chemical inhomogeneities and defects are removed. Our results indicate that the properties of Ni–Mn–Ga thin films grown by sputtering can be optimized (fixing the chemical concentration and removing crystalline defects) by the annealing process, which is promising for the development of micromagnetic shape memory devices.Ítem Acceso Abierto Martensitic transformation in free-standing Cu-Al-Ni thin films with micrometric grain size.(IOP Publishing, 2019-07-12) Morán M., Condó A. M., Bengio S., Soldera F., Sirena M. and Haberkorn N.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasThe martensitic transformation of polycrystalline thin films based on shape memory alloys is usually affected by the average grain size and by the thickness. We have carried out a study of the temperature driven martensitic transformation in micrometric grain size Cu-Al-Ni films with 18R structure. Thin films with a thickness of 6 μm were grown by sputtering on highly oriented pyrolytic graphite HOPG (0001) substrate at 873 K. After that, the samples were peeled-off from the substrate and annealed at 1123 K for 30 min. The observed microstructure shows an average grain size of 3.7 (± 0.2) μm. The martensitic start temperature (Ms) for different films ranges from 170 K to 370 K due to small changes in the chemical concentration. The influence of surface oxides and changes in the atomic order produced by post-quench aging treatments is analyzed. The results show that while surface passivation has a weak influence, changes in the atomic order increase Ms without impacting significantly on the hysteresis. Comparison with previous results of nanometric and micrometric grain size samples reveals that the barriers for the transformation are mainly given by plastic deformation at grain boundaries.Ítem Acceso Abierto Recrystallization and martensitic transformation in nanometric grain size Cu-Al-Ni thin films grown by DC sputtering at room temperature(Elsevier, 2018-05-00) Morán; M. J.; Condó; A. M.; Haberkorn; N.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe report the recrystallization of metastable hexagonal and body centered cubic BCC phase in free-standing 6 μm thick Cu-Al-Ni films grown by DC sputtering at room temperature. The results show that the BCC phase recrystallizes to L21 at around 533 K together with precipitation of γ2-phase. Annealing temperatures (533 K–623 K) at short times (0–30 min) produce systematic increment of the fraction of γ2-phase. Films with phase coexistence of hexagonal, γ2 and L21 structure display temperature driven martensitic transformation. This transformation is observed for samples with austenite grain size of around 30 nm. In all cases, the martensitic transformation temperature (MS) notoriously decreases (compared to bulk) and systematically increases with the annealing temperatures and annealing time. In addition, the films display extended transformation and retransformation ranges along with asymmetric hysteresis, which may be associated to restitutive forces due to elastic deformations at martensite–martensite and martensite–austenite interfaces. Annealing temperatures above 623 K produce fast precipitation to equilibrium α- and γ2-phases. The austenitic phase can be recovered after annealing over 1000 K and fast quenching in ice-water.Ítem Acceso Abierto Smooth surfaces in very thin GdBa2Cu3O7δfilms for application in superconducting tunnel junctions(Elsevier, 2015-03-15) Navarro, H.; Sirena, M.; Kim, J.; Haberkorn, N.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasThis paper provides a systematic analysis of the morphology and the superconducting critical temperature obtained in very thin GdBa2Cu3O7−δ films grown on (0 0 1) SrTiO3 substrates by DC sputtering. We find that the use of a very thin SrTiO3 buffer layer (≈2 nm) modify the nucleation of GdBa2Cu3O7−δ on the surface of the substrate reducing the formation of 3 dimensional clusters. Our results demonstrate that 16 nm thick GdBa2Cu3O7−δ films with an average root-mean-square (RMS) smaller than 1 nm and large surface areas (up 10 μm2) free of 3 dimensional topological defects can be obtained. In films thinner than 24 nm the onset (zero resistance) of superconducting transition of the films is reduced, being close to liquid nitrogen. This fact can be associated with stress reducing the orthorhombicity and slightly drop in oxygen stoichiometry.Ítem Acceso Abierto Structures in textured Cu-Al-Ni shape memory thin films grown by sputtering(Elsevier, 2014-10-00) Espinoza Torres, C.; Condó, A.M.; Haberkorn, N.; Zelaya, E.; Schryvers, D.; Guimpel, J.; Lovey, F.C.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasThe structure and texture formation in Cu–Al–Ni thin films of different thicknesses (1 μm to 5 μm) grown by DC magnetron sputtering without any intentional heating of the substrate are reported. The as-grown films present grains with an average size of 20 nm. The films with thickness of 1 μm have a single metastable phase with a hexagonal structure and are textured with planes (0002) parallel to the plane of the films. It was observed that thicker films present phase coexistence between metastable hexagonal and body centered cubic structures with a gradual increment of the body centered cubic phase fraction. The films with thickness of 5 μm are textured with planes (0002) and in the hexagonal structure, whereas in the body centered cubic structure the films are textured with {110} planes parallel to the plane of the films. This fact can be associated with self-heating of the substrate during the growth of the films and with the relative stability of the metastable phases. Free standing films annealed in a second step (1123 K for 1 h) present austenitic phase with L21 structure and sub-micrometric grains textured with {220}L21 planes parallel to the plane of the films. The martensitic transformation temperature was determined from the analysis of resistance against temperature measurements.Ítem Acceso Abierto Synthesis of nanocrystalline delta-MoN by thermal annealing of amorphous thins films grown on (100) Si by reactive DC sputtering at room temperature(Elsevier, 2018-08-30) Haberkorn, N.; Bengio, S.; Troiani, H.; Suárez, S.; Pérez, P. D.; Sirena, M.; Guimpel, J.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe report on the synthesis and characterization of nanocrystalline δ-MoN by crystallization of amorphous thin films grown on (100) Si by reactive sputtering at room temperature. Films with chemical composition MoN were grown using a deposition pressure of 5mTorr with a reactive mixture of Ar/(Ar + N2) = 0.5. The as-grown films display mostly amorphous structure. Nanocrystalline δ-MoN phase is obtained after annealing at temperatures above 600 °C. The superconducting critical temperature Tc depends on film thickness. Thick films (170 nm) annealed at 700 °C for 30 min display a Tc = 11.2 K (close to the one reported for bulk specimens: 13 K), which is gradually suppressed to 7.2 K for 40 nm thick δ-MoN films. Our results provide a simple method to synthesize superconducting nitride thin films on silicon wafers with Tc above the ones observed for conventional superconductors such as Nb.Ítem Acceso Abierto Thickness dependence of the superconducting properties of Y- Mo2N thin films on Si (001) grown by DC sputtering at room temperature(Elsevier, 2018-01-15) Haberkorn, N.; Bengio, S.; Troiani, H.; Suárez, S.; Pérez, P.D.; Granell, P.; Golmar, F.; Sirena, M.; Guimpel, J.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe study the crystalline structure and superconducting properties of γ-Mo2N thin films grown by reactive DC sputtering on AlN buffered Si (001) substrates. The films were grown at room temperature. The microstructure of the films, which was studied by X-ray diffraction and transmission electron microscopy, shows a single-phase with nanometric grains textured along the (200) direction. The films exhibit highly uniform thickness in areas larger than 20 × 20 μm2. The superconducting critical temperature Tc is suppressed from 6.6 K to ≈ 3.0 K when the thickness decreases from 40 nm to 5 nm. The residual-resistivity ratio is slightly smaller than 1 for all the films, which indicates very short electronic mean free path. The films are in the superconducting dirty limit with upper critical field Hc2 (0) ≈ 12 T for films with thickness of 40 nm, and 9 T for films with thickness of 10 nm. In addition, from the critical current densities Jc in the vortex-free state, we estimate a penetration depth λ(0) ≈ (800 ± 50) nm and a thermodynamic critical field Hc (0) = (500 ± 80 Oe).