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Examinando BAJAS TEMPERATURAS por Materia "ANISOTROPIA"
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Ítem Acceso Abierto Anisotropic response of the moving vortex lattice in superconducting Mo(1-x)Gex amorphous films(Elsevier, 2012-03-00) Dolz, M.I.; Shalóm, D.E.; Pastoriza, H.; López, D.O.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe have performed magnetic susceptibility measurements in Mo1−xGex amorphous thin films biased with an electrical current using anisotropic coils. We tested the symmetry of the vortex response changing the relative orientation between the bias current and the susceptibility coils. We found a region in the DC current–temperature phase diagram where the dynamical vortex structures behave anisotropically. In this region the shielding capability of the superconducting currents measured by the susceptibility coils is less effective along the direction of vortex motion compared to the transverse direction. This anisotropic response is found in the same region where the peak effect in the critical current is developed. On rising temperature the isotropic behavior is recovered.Ítem Acceso Abierto Annealing disorder and photoinduced order of oxygen chains in detwinned YBa2Cu3O6.65 single crystals probed by Raman scattering(American Physical Society, 2000-02-01) Fainstein, A.; Maiorov, B.; Guimpel, J.; Nieva, G.; Osquiguil, E.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasRaman scattering in detwinned YBa2Cu3O6.65 single crystals is studied as a function of photoexcitation and annealing. Copper-oxygen chain-related forbidden Raman bands that are known to strongly bleach with illumination at low temperatures, increase their intensity with chain fragmentation induced by annealing at high temperature. This contrasting behavior proves the conjunction of short Cu-O fragments into longer chains on photoexcitation. We interpret the Raman modes as due to vibrations at the end of CuO chain fragments and Cu-O-Cu monomers, and use their evolution with illumination and annealing as anisotropic sensitive markers of oxygen reordering processes. The identification of the “forbidden” Raman bands is discussed in the context of our results and recent literature in the subject. We also present absorption measurements performed on GdBa2Cu3Ox thin films with varying oxygen content. These experiments show that the 2.2-eV absorption and the chain-related Raman peaks have different dependencies with oxygen content and illumination, ruling out an explanation that suggests that the Raman intensity reduction of these modes is due to a photobleaching of intermediate defect states. These results highlight the potentialities of Raman scattering for oxygen dynamics studies and demonstrate the presence of photoinduced oxygen ordering in these high-Tc superconductor compounds.Ítem Acceso Abierto Bose-glass phase in twinned YBa2Cu3O7-δ(American Physical Society, 1998-09-14) Grigera, S.A.; Morré, E.; Osquiguil, E.; Balseiro, G.; Nieva, G.; de la Cruz, F.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasUsing an extensive scaling analysis of the transport properties in twinned YBa2Cu3O7−δ crystals we have experimentally found the predicted change in the universality class of the Bose-glass to liquid transition when the magnetic field is applied at small angles away from the direction of the correlated defects. The new dynamical critical exponent is s′=1.1±0.3.Ítem Acceso Abierto Bulk phase transition in YBa2Cu3O7−δ single crystals(American Physical Society, 1991-06-01) Safar, H.; Pastoriza, H.; Cruz, F. de la; Bishop, D.J.; Schneemeyer, L.F.; Waszczak, J.V.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasPrecise magnetization measurements of YBa2Cu3O7−δ single crystals have found evidence for a bulk phase transition a few degrees below the superconducting critical temperature. The phase transition is indicated by a sudden drop of the lower critical field, Hc1(T), separating a full coherent state at low fields and temperatures, the Meissner state, from a region at higher temperatures where magnetic excitations are induced at H=0.Ítem Acceso Abierto Detection of discretized single-shell penetration in mesoscopic vortex matter(IOP Publishing, 2014-00-00) Dolz, M. I.; Fasano, Y.; Cejas Bolecek, N. R.; Pastoriza, H.; Konczykowski, M.; van der Beek, C. J.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe investigated configurational changes in mesoscopic vortex matter with less than thousand vortices during flux penetration in freestanding 50 μm diameter disks of Bi2Sr2CaCu2O8+δ. High-resolution AC and DC local magnetometry data reveal oscillations in the transmittivity echoed in peaks in the third-harmonics magnetic signal fainting on increasing vortex density. By means of extra experimental evidence and a simple geometrical analysis we show that these features fingerprint the discretized entrance of single-shells of vortices having a shape that mimics the sample edge.Ítem Acceso Abierto Effects of structural microdomains on the vortex correlation length in a-axis oriented EuBa2Cu3O7 thin films(Elsevier, 2006-00-00) Gonzalez, E.M.; Sefrioui, Z.; Maiorov, B.; Osquiguil, E.; Santamaría, J.; Vicent, J. L.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe have grown a-axis oriented EuBa2Cu3O7 thin films on SrTiO3 (001) substrates using a magnetron sputtering technique. In these films the CuO2 planes are aligned parallel to the substrates. The film microstructure shows microdomains. The microdomains are grains with a 90° rotation of the c-axis. We have done transport measurements in applied magnetic fields up to 16 T. We have found a reduced quasi-2D dimensionality of the vortex glass transition probably resulting of vortex coherence being limited by disorder at domain boundaries. The activation energy for vortex motion in the liquid state displays a 1/H0.5 field dependence characteristic of strongly anisotropic superconductor.Ítem Acceso Abierto Electron concentration effects on the Shastry-Sutherland phase stability in Ce2-xPd2+yIn1-z solid solutions(American Physical Society, 2011-02-23) Sereni, J.G.; Giovannini, M.; Gómez Berisso, M.; Saccone, A.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasThe stability of the Shastry-Sutherland (ShaSu) phase as a function of electron concentration is investigated through the field dependence of thermal and magnetic properties of the solid solution Ce2−xPd2+yIn1−z on the antiferromagnetic (AF) branch. In these alloys, the electronic (holes) variation is realized by increasing Pd concentration. The AF transition TM decreases from 3.7 to 3.0 K as Pd concentration increases from y=0.2 to 0.4. By applying magnetic field, the ShaSu phase is suppressed once the field-induced ferromagnetic polarization takes over at a critical field Bcr, which increases with Pd content. A detailed analysis around the critical point reveals a structure in the maximum of the ∂M/∂B derivative, which is related to an incipient step in the magnetization M(B) predicted by the theory for the ShaSu lattice. The crossing of M(B) isotherms, observed in ShaSu prototype compounds, is also analyzed. The effect of In substitution by Pd is interpreted as an increase in the number of “holes” of the conduction band and results in a unique parameter able to describe the variation of the magnetic properties along the studied range of concentration.Ítem Acceso Abierto Evidence for a dimensionality crossover at the disappearance of magnetism in the Kondo lattice alloy CeCo1-xFexSi(American Physical Society, 2014-01-07) Sereni, J.G.; Gomez Berisso, M.; Betancourth, D.; Correa, V.F.; Canales, N.C.; Geibel, C.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasStructural, magnetic, and thermal measurements performed on CeCo1−xFexSi alloys are reported. Three regions can be recognized: (i) Co rich (x≤0.20) with a decreasing long-range antiferromagnetic order which vanishes at finite temperatures, (ii) an intermediate region (0.20Ítem Acceso Abierto Evidence for a ferromagnetic quantum critical point in CePd1-xRhx(Elsevier, 2005-04-30) Sereni, J.G.; Kuchler, R.; Geibel, C.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasIn the alloy CePd 1-xRhx; one observes the transition from a ferromagnetic (F) state at x=0 to a valence fluctuating state at x=1: We are performing a detailed investigation of the disappearance of the F-state in this alloy using AC-susceptibility, specific heat and thermal expansion measurements and present here preliminary results. Our results indicate a continuous decrease of the ferromagnetic ordering temperature TC with increasing x down to TC=0:25 K at x=0:80: This suggests that in this alloy the ferromagnetic order disappears at a quantum critical point placed around xcr≈0:85.Ítem Acceso Abierto Evidence of spin-density-wave to spin-glass transformation in YNd alloys(American Physical Society, 1995-12-01) Trovarelli, O.; Sereni, J.G.; Pureur, P.; Shaf, J.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasLow-temperature ac and dc magnetic susceptibility and specific-heat measurements have been carried out on YNd dilute alloys, where the Nd concentrations are 1.9, 2.5, 4.5, 6.8, and 9.0 at. %. The most concentrated alloys (4.5, 6.8, and 9.0 at. %) present long-range antiferromagnetic spin-density-wave (SDW) order below the critical temperature, but some residual frustration is likely to produce a reentrant behavior at low temperatures, where the magnetic data show strong irreversibility effects and the specific heat indicates the occurrence of an anisotropy induced gap near zero frequency in the density of states of the magnetic excitations. For the less concentrated alloys (1.9 and 2.5 at. %), the interacting magnetic state shows typical features of spin-glass behavior. The evolution to this state is ascribed as being due to impurity disorder and frustration which induces a breaking up of the SDW coherence into small domains.Ítem Acceso Abierto Experimental Method to Measure Anisotropic Transport in 2D Superconductors(Springer Nature, 2004-04-00) Shalóm, D. E.; Pastoriza, H.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasA two-coil kinetic inductance technique for measuring anisotropic response in two-dimensional (2D) superconductors is presented. Serpentine-shaped coils are lithographically patterned directly on top of the sample, separated by isolation layers. The drive and receive coils are positioned in a way that enhances signal from longitudinal currents and reduces transversal ones, maximizing the ratio. Anisotropic transport can be directly measured with this technique, even when anisotropy is induced by the transport current.Ítem Acceso Abierto Flux-lattice melting, anisotropy, and the role of interlayer coupling in Bi-Sr-Ca-Cu-O single crystals(American Physical Society, 1991-10-01) Duran, C.; Yazyi, J.; de la Cruz, F.; Bishop, D.J.; Mitzi, D.B.; Kapitulnik, A.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe have used the high-Q mechanical-oscillator technique to probe the vortex-lattice structure in high-quality Bi-Sr-Ca-Cu-O single crystals over a wide range of magnetic fields (200 Oe to 40 kOe), and relative orientations θ between the magnetic field and the crystalline c^ axis. In addition to the large softening and dissipation peak previously observed and interpreted as due to flux-lattice melting, another distinctly different peak at higher temperatures is seen. The temperatures where the dissipation peaks take place are solely defined by the parallel component of the field cosθ, while the restoring force on the oscillator is due to both field components. We suggest that the two peaks are due to the softening of interplanar coupling at the low-temperature peak, and melting or depinning of the two-dimensional pancake vortices at the higher-temperature peak.Ítem Acceso Abierto Longitudinal Magnetomorphic Effect in Indium Films(American Physical Society, 1967-11-15) de la Cruz, F.; de la Cruz, M.E.; Cotignola, J.M.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasMeasurements of magnetomorphic effects in indium films of thickness between 0.003 and 0.01 cm have been made at 4.2 and 1.5°K. The results were compared with theoretical calculations based on the free-electron model. To make these comparisons, it was necessary to discount the bulk magnetoresistance by means of a modified Kohler's rule suggested by Olsen. From the comparison of the data with the theory, it was possible to deduce the values of the electronic momentum mv and the product of the bulk resistivity with the bulk mean free path ρbl. Data previously obtained by Olsen in wires have been analyzed with the use of measurements of Kohler's rule and of a criterion for the comparison different from the one used by Olsen. The experimental value of ρbl is discussed on the basis of an analysis made by Bate et al., in which the anisotropy of the mean free path is taken into account.Ítem Acceso Abierto Low Temperature magnetic properties of GdCoIn5(Elsevier, 2015-01-15) Betancourth, D.; Facio, J.I.; Pedrazzini, P.; Jesus, C.B.R.; Pagliuso, P.G.; Vildosola, V.; Cornaglia, P.S.; García, D. J.; Correa, V. F.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasA comprehensive experimental and theoretical study of the low temperature properties of GdCoIn5 was performed. Specific heat, thermal expansion, magnetization and electrical resistivity were measured in good quality single crystals down to 4He temperatures. All the experiments show a second-order-like phase transition at 30 K probably associated with the onset of antiferromagnetic order. The magnetic susceptibility shows a pronounced anisotropy below TN with an easy magnetic axis perpendicular to the crystallographic ĉ-axis. Total energy GGA+U calculations indicate a ground state with magnetic moments localized at the Gd ions and allowed a determination of the Gd-Gd magnetic interactions. Band structure calculations of the electron and phonon contributions to the specific heat together with Quantum Monte Carlo calculations of the magnetic contributions show a very good agreement with the experimental data. Comparison between experiment and calculations suggests a significant anharmonic contribution to the specific heat at high temperature (T≳100 K).Ítem Acceso Abierto Magnetization measurement of single La0.67Ca0.33MnO3 nanotubes in perpendicular magnetic fields using a micromechanical torsional oscillator(Elsevier, 2010-03-00) Antonio, D.; Dolz, M.I.; Pastoriza, H.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasUsing a silicon micromechanical resonator as a sensitive magnetometer, the authors have studied both experimentally and theoretically the magnetic behavior of two isolated ferromagnetic nanotubes of perovskite La0.67Cao.33MnO3. The article investigates the specific configuration where a magnetic field H is applied perpendicular to the magnetic easy axis of an isolated nanotube characterized by an uniaxial anisotropy constant K. In this situation, the magnetization M reduces the effective elastic constant Km of the resonator. This softening of the mechanical system is opposed to the hardening effect of M observed in a previous work, where H was applied parallel to the easy axis. Moreover, in this magnetic field configuration two distinct magnetization regimes are manifested, depending on the magnitude of H. For ⪢ 2K/M the magnetization is almost parallel to the applied magnetic field and for ⪡2K/M it is almost parallel to the easy axis of the nanotube. At a certain value of H there is a sharp transition from one regime to the other, accompanied by a peak in the energy dissipation.Ítem Acceso Abierto Magnetostriction reveals orthorhombic distortion in tetragonal Gd compounds(American Physical Society, 2019-04-00) Betancourth, D.; Correa, V. F.; Facio, J.I.; Fernández J.; Vildosola, V.; Lora-Serrano, R.; Cadogan, J.M.; Aligia, A.A.; Cornaglia, P.S.; García D.J.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe report detailed thermal expansion and magnetostriction experiments on GdCoIn5 and GdRh(In1−xCdx)5 (x=0 and 0.025) single-crystal samples that show a sudden change in the dilation at a field B★ for temperatures below the Néel transition temperature TN. We present a first-principles model including crystal-field effects, dipolar and exchange interactions, and the dependence of the latter with lattice distortions in order to fully account for the magnetostriction and magnetic susceptibility data. The mean-field solution of the model shows that a transition between metastable states occurs at the field B★. It also indicates that two degenerate phases should coexist at temperatures below TN, which may explain the lack of observation, in high-resolution x-ray experiments, of an orthorhombic distortion at the Néel transition, even though the experimentally determined magnetic structure breaks the tetragonal symmetry and the magnetoelastic coupling from our model is significant. These conclusions could be extended to other tetragonal Gd-based compounds that present the same phenomenology.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 Photobleaching and reorientational dynamics of dyes in a nematic liquid crystal(American Physical Society, 1999-01-00) Nöllmann, M.; Shalom, D.; Etchegoin, P.; Sereni, J.G.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasThe polarized fluorescence of excited dyes in a prototype nematic liquid crystal is studied as a function of temperature, polarization of the light, and laser wavelength. We show explicitly the coexistence of photobleaching and dye diffusion through the nematic host as two mechanisms influencing the magnitude of the fluorescence signal. In addition, we exhibit clear evidence of the presence of a torque at low input laser powers that twist the dye molecules with respect to the director orientation of the liquid crystal, if the dyes are resonantly excited. The fluorescence emission in this latter case is able to perceive the birefringence of the nematic liquid crystal host, and this is shown as clear oscillations in the polarized fluorescence as a function of temperature. Extensive qualitative comparisons of the experimental results with the mean-field Maier-Saupe theory of the nematic state are presented, and a model is proposed to account for the observations.Ítem Acceso Abierto Role of twin boundaries on the vortex dynamics in YBaCuO single crystal(American Physical Society, 1999-08-00) Pastoriza, H.; Candia, S.; Nieva, G.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasBy means of a novel technique of rotating the applied current we have directly measured the influence of twin boundaries on the vortex motion in a YBa2Cu3O7 single crystal. The results indicate that the effect of twin planes on the vortex dynamics starts to develop below a certain temperature, being responsible for an anisotropic viscosity in the vortex liquid state and a guided motion in the solid state.Ítem Acceso Abierto Softening of the flux line lattice in Ge/Pb multilayers(IOP Publishing, 1992-00-00) Neerinck, D.; Temst, K.; Baert, M.; Osquiguil, E.; Van Haesendonck, C.; Bruynseraede, Y.; Gilabert, A.; Schuller, I.K.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasThe authors measured the critical current density Jc of artificially grown Ge/Pb multilayers as a function of the magnetic field Hperpendicular to applied perpendicular to the layers. In these layered structures Jc(Hperpendicular to ) rapidly drops to a pronounced minimum at a field H*perpendicular to , then increases through a broad maximum and finally decays to zero at Hc2 perpendicular to . The systematic evolution of this minimum as a function of temperature, layer thickness and pinning strength allows us to conclude that this peculiar behavior is due to a thermally driven softening of the flux line lattice which can be related either to a magnetic decoupling or a melting of the vortex structure.