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Ítem Acceso Abierto Barkhausen-type noise in the resistance of antiferromagnetic Cr thin films(IOP Publishing, 2012-00-00) Tosi, L.; Osquiguil, E.; Kaul, E.E.; Balseiro, C.A.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe present an experimental study of the changes generated on the electrical resistance R(T) of epitaxial Cr thin films by the transformation of quantized spin density wave domains as the temperature is changed. A characteristic resistance noise appears only within the same temperature region where a cooling-warming cycle in R(T) displays hysteretic behavior. We propose an analysis based on an analogy with the Barkhausen noise seen in ferromagnets. There fluctuations in the magnetization M(H) occur when the magnetic field H is swept. By mapping and , where Ψ0 corresponds to the order parameter of the spin density wave, we generalize the Preisach model in terms of a random distribution of resistive hysterons to explain our results. These hysterons are related to distributions of quantized spin density wave domains with different sizes, local energies and number of nodes.Ítem Acceso Abierto Comparison of a Shastry-Sutherland lattice stability in Ce2Pd2Sn as a function of field and doping(IOP Publishing, 2011-00-00) Sereni, J.G.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasA comparative study of the stability of a Shastry-Sutherland lattice (SSL) in Ce2Pd2Sn under different physical conditions is presented. Applied magnetic field suppresses the SSL of stoichiometric Ce2Pd2Sn in a magnetic critical point at Tcr (B) = 4.2 K and Bcr = 0.13 T, whereas 25% of Ni does it at Tcr (N i) = 3.4 K. Electronic concentration variation, driven by the increase of Pd (holes) concentration in Ce2Pd2+y In1−y , decreases the magnetic transition down to TM = 2.8 K in the limit of the alloy solubility, i.e. y = 0.4. The existence of a M (B) plateau in SSL predicted by theory and the crossing of those isotherms, previously observed in the model compound SrCu2(BO3)2 are analyzed.Ítem Acceso Abierto Competition between pinning produced by extrinsic random point disorder and superconducting thermal fluctuations in oxygen-deficient GdBa2Cu3O x coated conductors(IOP Publishing, 2019-11-13) Haberkorn, N.; Guimpel, J.; Suárez, S.; Lee, Jae-Hun; Lee, Hunju; Moon, S. H.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe report on the influence of random point defects introduced by 3 MeV proton irradiation on the vortex dynamics of 1.3 μm thick GdBa2Cu3Ox coated conductors. Thin films with different oxygen stoichiometry (6.7 < × < 7) were irradiated with 3 MeV proton (p) using a fluence of 2 × 1016 p cm−2. We find a direct correlation between the changes in Tc produced by oxygen content and damage by irradiation on the resulting vortex dynamics in the films. The analysis of the critical current densities Jc at low temperatures indicates that although irradiation produces smooth magnetic field dependences, the self-field values decrease systematically as Tc reduces. Moreover, the analysis of the relaxation of the persistent currents shows that the characteristic glassy exponent μ systematically decreases from 1.7 to 0.66 as Tc decreases from 93 to 55 K.Ítem Acceso Abierto Dependence of vortex phase transitions in mesoscopic Bi2Sr2CaCuO8 superconductor at tilted magnetic fields(IOP Publishing, 2009-00-00) Dolz, M.I.; Pastoriza, H.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasA micron sized single crystal of the superconductor Bi2Sr2CaCuO8 was studied using silicon mechanical micro-oscillators at various tilt angles of the dc magnetic field with respect to the c axis of the sample. Different phases of the vortex matter were detected by measuring changes in the value and sign of the oscillator resonant frequency variation with temperature. We could explain the change in the sign of this variation at high temperatures as the transition from the 2D liquid of decoupled pancakes to a reversible 3D vortex lattice. The data indicates that this transition only depends on the magnetic field perpendicular to the superconducting layers while the dissipation involved in this process depends on the component parallel to them.Ítem Acceso Abierto Effect of mixed pinning landscapes produced by 6MeV oxygen irradiation on the resulting critical current densities Jc in 1.3µm thick GdBa2Cu3O7-d coated conductors grown by co-evaporation(Elsevier, 2017-11-15) Haberkorn, N.; Suárez, S.; Pérez, P.D.; Troiani, H.; Granell, P.; Golmar, F.; Lee, J.-H.; Moon, S.H.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe report the influence of crystalline defects introduced by 6 MeV 16O3+ irradiation on the critical current densities Jc and flux creep rates in 1.3 µm thick GdBa2Cu3O7-δ coated conductor produced by co-evaporation. Pristine films with pinning produced mainly by random nanoparticles with diameter close to 50 nm were irradiated with doses between 2 × 1013 cm−2 and 4 × 1014 cm−2. The irradiations were performed with the ion beam perpendicular to the surface of the samples. The Jc and the flux creep rates were analyzed for two magnetic field configurations: magnetic field applied parallel (H║c) and at 45° (H║45°) to the c-axis. The results show that at temperatures below 40 K the in-field Jc dependences can be significantly improved by irradiation. For doses of 1 × 1014 cm−2 the Jc values at μ0H = 5 T are doubled without affecting significantly the Jc at small fields. Analyzing the flux creep rates as function of the temperature in both magnetic field configurations, it can be observed that the irradiation suppresses the peak associated with double-kink relaxation and increases the flux creep rates at intermediate and high temperatures. Under 0.5 T, the flux relaxation for H‖c and HÍtem Acceso Abierto Effect of Ni doping on vortex pinning in CaK(Fe1-xNix)4As4 single crystals(American Physical Society, 2019-08-00) Haberkorn, N.; Xu, M.; Meier, W. R.; Schmidt, J.; Bud’ko, S. L.; Canfield, P. C.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe study the correlation between chemical composition and vortex dynamics in Ni-doped CaK(Fe1−xNix)4As4 (x=0, 0.015, 0.025, 0.03, and 0.05) single crystals by performing measurements of the critical current densities Jc and the flux creep rates S. The magnetic relaxation of all the crystals is well described by the collective creep theory. The samples display a glassy exponent μ within the predictions for vortex bundles in a weak pinning scenario and relatively small characteristic pinning energy (U0<100K). The undoped crystals display modest Jc values at low temperatures and high magnetic fields applied along the c axis. Jc(T) dependences at high fields display an unusual peak. The enhancement in Jc(T) matches with an increase in U0 and the appearance of a second peak in the magnetization. As Ni doping increases, whereas there is a monotonic decrease in Tc there is a nonmonotonic change in Jc. Initially Jc increases, reaching a maximum value for x=0.015, and then Jc decreases for x≥0.025. This change in Jc(x) is coincident with the onset of antiferromagnetic order. The magnetic field dependence of Jc(H) also manifests a change in behavior between these x values. The analysis of the vortex dynamics for small and intermediate magnetic fields shows a gradual evolution in the glassy exponent μ with Ni content, x. This implies that there is no appreciable change in the mechanism that determines the vortex relaxation.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 metamagnetic transition in the heavy Fermion system CeTiGe(IOP Publishing, 2010-00-00) Deppe, M.; Caroca-Canales, N.; Sereni, J.G.; Geibel, C.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasA recent study of CeTiGe identified this compound as a paramagnetic heavy Fermion system where the full J = 5/2 multiplet is involved in the formation of the ground state. Here we present a preliminary investigation of the dc-magnetization Mdc(H) and of the magnetoresistance ρ(H) of polycrystalline CeTiGe samples in applied magnetic fields up to μ0H = 14 T. The results reveal a pronounced metamagnetic transition at a critical field μ0Hc ≈ 13.5 T at low temperatures, with a step like increase in Mdc(H) of at least 0.6 μB/Ce. The metamagnetic transition leads to a strong decrease in ρ(H). A clear hysteresis in Mdc(H) and ρ(H) indicate that in CeTiGe these metamagnetic features correspond to a true thermodynamic, first order type transition in contrast to the critical behavior observed in the canonical system CeRu2Si2. Measurements at higher temperatures showed a continuous suppression of the metamagnetic transition with increasing T, which vanishes at T ~ 30 K.Ítem Acceso Abierto Evidence for mean free path fluctuation induced pinning in YBa2Cu3O7 and YBa2Cu4O8 films(American Physical Society, 1994-03-21) Griessen, R.; Wen Hai-Hu; Van Dalen, A.J.J.; Dam, B.; Rector, J.; Schnack, H.G.; Libbrecht, S.; Osquiguil, E.; Bruynseraede, Y.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasThe critical current jc and the pinning energy Uc have been determined for three types of yttrium-based superconducting films from current js and dynamic relaxation Q=d lnjs/d ln(dB/dt) data by means of the generalized inversion scheme. For B<2 T and T<80 K the temperature dependence of jc and Uc for all films is found to be in excellent agreement with a model of single vortices pinned by randomly distributed weak pinning centers via spatial fluctuations of the charge carrier mean free path. Pinning due to spatial fluctuations of Tc is not observed.Ítem Acceso Abierto Experimental and numerical study of transverse flux shaking in MgB2 superconductors(IOP Publishing, 2009-00-00) Luzuriaga, J.; Badía-Majós, A.; Nieva, G.; Giordano, J.L.; Lopez, C.; Serquis, A.; Serrano, G.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasMagnetization measurements in the mixed state of thick strips of carbon nanotube doped MgB2 in crossed fields configurations are reported, together with numerical simulations performed with a geometry equivalent to the sample shape. The samples were subjected to magnetic field components along mutually perpendicular directions, an oscillatory field in one direction and a remanent magnetization in the perpendicular direction. The magnetic response along the oscillatory field and the magnetic relaxation perpendicular to it are observed and simulated using the critical state theory. A remarkable quantitative agreement between the experiment and the theory was obtained.Ítem Acceso Abierto First-order phase transition of the vortex lattice in twinned YBa2Cu3O7 single crystals in tilted magnetic fields(American Physical Society, 2000-05-01) Maiorov, B.; Nieva, G.; Osquiguil, E.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe present an exhaustive analysis of transport measurements performed in twinned YBa2Cu3O7 single crystals which establishes that the vortex solid-liquid transition is first order when the magnetic field H is applied at an angle θ away from the direction of the twin planes. We show that the resistive transitions are hysteretic and the V−I curves are nonlinear, displaying a characteristic S shape at the melting line Hm(T), which scales as ɛ(θ)Hm(T,θ), where ɛ(θ) is the anisotropy factor. These features are gradually lost when the critical point H*(θ) is approached. Above H*(θ) the V−I characteristics show a linear response in the experimentally accessible V−I window, and the transition becomes reversible. Finally we show that the first-order phase transition takes place between a highly correlated vortex liquid in the field direction and a solid state of unknown symmetry. As a consequence, our data support the scenario for a vortex-line melting in twinned YBa2Cu3O7 crystals in contrast to a vortex sublimation as recently suggested for untwinned La2−xSrxCuO4,YBa2Cu3O7 and Bi2Sr2CaCu2O8 [T. Sasagawa et al., Phys. Rev. Lett. 80, 4297 (1998)].Ítem Acceso Abierto Influence of random point defects introduced by proton irradiation on the flux creep rates and magnetic field dependence of the critical current density Jc of co-evaporated GdBa2Cu3O7−δ coated conductors(IOP Publishing, 2015-10-28) Haberkorn, N.; Jeehoon, Kim; Suárez, S; Lee, Jae-Hun; Moon, S.H.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe report the influence of random point defects introduced by 3 MeV proton irradiation (doses of 0.5 × 1016, 1 × 1016, 2 × 1016 and 6 × 1016 cm−2) on the vortex dynamics of co-evaporated 1.3 μm thick, GdBa2Cu3O7−δ coated conductors. Our results indicate that the inclusion of additional random point defects reduces the low field and enhances the in-field critical current densities Jc. The main in-field Jc enhancement takes place below 40 K, which is in agreement with the expectations for pinning by random point defects. In addition, our data show a slight though clear increase in flux creep rates as a function of irradiation fluence. Maley analysis indicates that this increment can be associated with a reduction in the exponent μ characterizing the glassy behavior.Ítem Acceso Abierto Interplay between pinning energy and vortex interaction in YBa2Cu3O7-delta with oriented twin boundaries in tilted magnetic fields: Bitter decoration and tilt-modulus measurements(American Physical Society, 2000-08-01) Herbsommer, J.A.; Nieva, G.; Luzuriaga, J.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe have performed Bitter decoration and ac susceptibility measurements in single-crystal YBa2Cu3O7−δ with oriented twin boundaries. The twin boundaries (TB’s) pin vortices over approximately 65% of the sample. The pinned areas are unevenly distributed and some relatively large TB-free regions are present. The Bitter decorations were performed in a 52 Oe dc magnetic field rotated off the c axis so that the plane defined by the field direction and the c axis is perpendicular to the TB’s. Several decorations were performed in the same sample. Additional dynamical information was obtained from ac susceptibility measurements. Results show that for small tilts the vortices remain locked to the c-axis direction, for angles greater than 12° they form a staircase pattern, and in this case pinning by the twin boundaries remains effective up to 75°. We observe vortex chains in twin-free zones of the sample for tilted fields at 65° and 40°. Due to our particular experimental arrangement, in the twinned regions the interplay of the potentials giving rise to the chains and the pinning potential produces a structure with a disordered square symmetry. The data allow us to estimate the dominant energy of the vortex system for some inclinations of the applied magnetic field.Ítem Acceso Abierto Low temperature specific heat of Th-Gd spin glass(Elsevier, 1979-03-00) Sereni, J.G.; Huber, T.E.; Luengo, C.A.The specific heat measurements of T-Gd alloys, with Gd concentration ranging from 0.15% to 10.8% and temperatures between 0.4 and 8 K, are reported. The characteristic spin glass specific heat enhancement and its maximum was found linearly proportional to Gd concentration. At low temperatures a T2 dependence of the specific heat was observed and the entropy associated with the magnetic ordering was the 70% of the expected value for the spin S=72.Ítem Acceso Abierto Low temperature thermopower and magnetoresistance of Sc-rich CeSc1-xTixGe(Elsevier, 2018-05-01) Encina, S.; Pedrazzini, P.; Sereni, J.G.; Geibel, C.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasIn CeSc 1-xTixGe, Ti-alloying reduces the record-high antiferromagnetic (AFM) ordering temperature found in CeScGe at T = 46 KN and induces ferromagnetism for x ≥ 0.5. In this work we focus on the AFM side, i.e. Sc-rich samples, and study their thermopower S T( ) and magnetoresistance ρ H T( , ). The measured S T( ) is small in comparison with the thermopower of other Ce-systems and shows some features that are compatible with a weak hybridization between the f4 and band states. This is a further hint pointing to the local character of magnetism in this alloy. Magnetic fields up to 16 T have a minor effect on the electrical resistivity of stoichiometric CeScGe. On the other hand, for x = 0.65, we find that fields above 4 T suppress the hump in ρ T( ). Furthermore, the 4.2 K magnetoresistance displays a strong decrease in the same field range, also in coincidence with magnetization results from the literature. Our results indicate that ρ T H( , ) is a proper tool to assess the H T− phase diagram of this system.Ítem Acceso Abierto Magnetic relaxation induced by transverse flux shaking in MgB2 superconductors(IOP Publishing, 2009-00-00) Luzuriaga, J.; Badía-Majós, A.; Nieva, G.; Giordano, J.L.; Lopez, C.; Serquis, A.; Serrano, G.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe report on measurements and numerical simulations of the behavior of MgB2 superconductors when magnetic field components are applied along mutually perpendicular directions. By closely matching the geometry in simulations and measurements, full quantitative agreement is found. The critical state theory and a single phenomenological law, i.e. the field dependence of the critical current density Jc(B), are sufficient for a full quantitative description of the measurements. These were performed in thick strips of carbon nanotube doped MgB2 samples. Magnetization was measured in two orthogonal directions using a SQUID magnetometer. Magnetic relaxation effects induced by the application of an oscillatory perpendicular field were observed and simulated numerically. The measurements confirm the numerical predictions, that two relaxation regimes appear, depending on the amplitude of the applied magnetic field. The overall agreement constitutes a convincing validation of the critical state model and the numerical procedures used.Ítem Acceso Abierto Magnetization collapse in polycrystalline YBCO under transport current cycles(IOP Publishing, 2006-00-00) Giordano, J.L.; Luzuriaga, J.; Badía-Majós, A.; Nieva, G.; Ruiz-Tagle, I. A.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe report measurements of the hysteretic magnetization of YBCO under superimposed transport current cycles, together with numerical simulations of magnetization and current density profiles in the corresponding parallel configuration. Field cooled (FC) and zero-field-cooled (ZFC) experiments were carried out on polycrystalline YBa2Cu3O7−x cylinders, with both the applied magnetic field and transport current in the axial direction, and the current cycled several times, around and above the dissipative threshold. As in previously reported multicomponent field configuration experiments, the magnetization is seen to collapse to a more stable state both in FC and ZFC, because of the interplay between the shielding and transport currents. The results of our numerical simulations are in good qualitative agreement with the measurements, and the competition between shielding and transport due to vortex-pinning interactions and equilibrium magnetization effects are shown to play an important role in the range of our experiments.Ítem Acceso Abierto Observation of a gel of quantum vortices in a superconductor at very low magnetic fields(American Physical Society, 2020-00-00) Llorens, J.B.; Embon, L.; Correa, A.; González, J.D.; Herrera, E.; Guillamón, I.; Luccas, R.F.; Azpeitia, J.; Mompeán, F.J.; García-Hernández, M.; Munuera, C.; Sánchez, J.A.; Fasano, Y.; Milošević, M.V.; Suderow, H.; Anahory, Y.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasA gel consists of a network of particles or molecules formed for example using the sol-gel process, by which a solution transforms into a porous solid. Particles or molecules in a gel are mainly organized on a scaffold that makes up a porous system. Quantized vortices in type-II superconductors mostly form spatially homogeneous ordered or amorphous solids. Here we present high-resolution imaging of the vortex lattice displaying dense vortex clusters separated by sparse or entirely vortex-free regions in β−Bi2Pd superconductor. We find that the intervortex distance diverges upon decreasing the magnetic field and that vortex lattice images follow a multifractal behavior. These properties, characteristic of gels, establish the presence of a novel vortex distribution, distinctly different from the well-studied disordered and glassy phases observed in high-temperature and conventional superconductors. The observed behavior is caused by a scaffold of one-dimensional structural defects with enhanced stress close to the defects. The vortex gel might often occur in type-II superconductors at low magnetic fields. Such vortex distributions should allow to considerably simplify control over vortex positions and manipulation of quantum vortex states.Ítem Acceso Abierto Observation of a nonlocal conductivity in the mixed state of YBa2Cu3O7-d: Experimental evidence for a vortex line liquid(American Physical Society, 1994-02-21) Safar, H.; Gammel, P.L.; Huse, D.A.; Majumdar, S.N.; Schneemeyer, L.F.; Bishop, D.J.; Lopez, D.; Nieva, G.L.; de la Cruz, F.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe report on transport measurements in the mixed state of YBa2Cu3O7 single crystals using the dc flux transformer geometry. In the vortex fluid regime we have observed the onset of a nonlocal conductivity. At lower temperatures, close to but above the phase transition temperature the vortices become effectively three dimensional in character. This is in sharp contrast to recent flux transformer experiments on Bi2Sr2CaCu2O which have failed to see a nonlocal conductivity, confirming the two-dimensional character at high temperatures of the vortices in that system. These observations suggest that in a finite temperature interval, the vortex state in YBa2Cu3O7 is a line liquid.Ítem Acceso Abierto Observation of quantum creep in oxygen deficient YBa2Cu3O7−δ at fields up to 7 T(Elsevier, 1994-12-00) van Dalen, A.J.J.; Griessen, R.; Libbrecht, S.; Osquiguil, E.; Bruynseraede, Y.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasSuperconducting current densities js and dynamic relaxation rates Q ≡ d ln js/d ln (dBe/dt) in an oxygen deficient YBa2Cu3O7−δ epitaxial film were measured as a function of temperature (1.5K < T < Tirr) in magnetic fields up to 7 T by means of high sensitivity capacitance torquemeters. Below 5 K, the relaxation rate remains high and does not extrapolate to Q = 0 at T = 0K. This is associated to the quantum tunneling of vortices through the energy barrier separating two adjacent vortex configurations. In the case of ohmic dissipation the quantum creep rate is proportional to the normal state resistivity ϱn(0) at T = 0 K. In order to check this theoretical prediction we measured Q as a function of the oxygen content of the sample. The quantum creep rate increases systematically with increasing ϱn(0). However, Q remains finite even when ϱn = 0.