Antonio, D., P. Julian, P. Mandolesi, and H. Pastoriza. "Amplificador de transimpedancia criogénico para detección capacitiva de MEMS." In Actas de la Escuela Argentina de Microelectrónica, Tecnología y Aplicaciones: Trabajos Breves, 49–51. Vol. 1., 2007.
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Bauer, E., H. Kaldarar, A. Prokofiev, E. Royanian, A. Amato, J. G. Sereni, W. Bramer-Escamilla, and I. Bonalde. "Heavy Fermion Superconductivity and Antiferromagnetic Ordering in CePt3Si without Inversion Symmetry." Journal of the Physical Society of Japan 76, no. 5 (2007): 051009.
Abstract: Heavy fermion superconductivity in absence of inversion symmetry of the crystal structure is basically controlled by a Rashba-like antisymmetric spin orbit coupling which splits the Fermi surface and removes the spin degeneracy of electrons. The Fermi surface splitting originates a mixing of spin-singlet and spin-triplet states in the superconducting condensate. Such constraints are responsible for various uncommon features of the superconducting ground state and are discussed here in view of CePt 3 Si, the first heavy fermion superconductor missing a centre of symmetry. We recall and discuss normal and superconducting properties of CePt 3 Si and relate them to recently developed phenomenological theories.
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Bridoux, G., P. Pedrazzini, F. de la Cruz, and G. Nieva. "Angular field dependence of the Nernst effect in Bi2Sr2CaCu2O8+delta." PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 460 (2007): 841–842.
Abstract: The anomalous Nernst effect in hole-doped cuprates has been interpreted in terms of the existence of a superconducting order parameter well above the transition temperature, T-c(H). In order to improve the understanding of this problem, we have measured the angular field dependence of the Nernst signal in Bi2Sr2CaCu2O8+delta single crystals. The experiment was done tilting the magnetic field with respect to the direction of the temperature gradient, above and below T,(H), with applied fields up to 16 T. We have found that all the curves are scaled on one curve against the reduced field Hcos theta at a fixed temperature. (c) 2007 Elsevier B.V. All rights reserved.
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Curiale, J., R. D. Sánchez, H. E. Troiani, C. A. Ramos, H. Pastoriza, A. G. Leyva, and P. Levy. "Magnetism of manganite nanotubes constituted by assembled nanoparticles." Phys. Rev. B 75 (2007): 224410.
Abstract: We present the study of the magnetic properties of manganite nanotubes with 800 nm of external diameter and 3-5 μm length. The study includes two ferromagnetic compounds, La0.67Ca0.33MnO3 and La0.67Sr0.33MnO3, and one with ferromagnetic and charge ordered phase coexistence, La0.325Pr0.300Ca0.375MnO3. In this case, the walls are formed by grains and have approximately thickness of 150 nm and the most probable grain size is 70 nm. For the ferromagnetic nanotubes smaller values are observed, between 45 and 60 nm for the thickness and 25 nm for the grain size, respectively. In La0.325Pr0.300Ca0.375MnO3 we observe distinctive features comparing with the ferromagnetic nanotubes. After cooling below T$_C$ the samples at $H=0$, the first magnetization curve shows a gradual growth of the magnetization until reach a maximum value, which depends of H. After this, the material presents expected ferromagnetic hysteresis loops. Also, significant differences between the cooling and warming magnetization processes and large linewidth in the ferromagnetic resonances are observed. These experimental facts are explained in terms of the competition between the charge ordered and the ferromagnetic phases, including strong interaction among these last regions. Observations of dipolar interactions between the magnetic grains are evidenced by isothermal remanent magnetization and direct current demagnetization experiments.
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Dolz, M., D. Antonio, and H. Pastoriza. "Measurement of mesoscopic High-$T_c$ superconductors using Si mechanical micro-oscillators." Physica B 398 (2007): 329–332.
Abstract: In a superconducting mesoscopic sample, with dimensions comparable to the London penetration depth, some properties are qualitatively different to those found in the bulk material. These properties include magnetization, vortex dynamics and ordering of the vortex lattice. In order to detect the small signals produced by this kind of samples, new instruments designed for the microscale are needed. In this work we use micromechanical oscillators to study the magnetic properties of a Bi$2$Sr$2$CaCu$_2$O8 + δ disk with a diameter of $13.5$ microns and a thickness of $2.5$ microns. The discussion of our results is based on the existence and contribution of inter and intra layer currents.
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Gómez, M. E., G. Campillo, J. - G. Ramirez, A. Hoffmann, and J. Guimpel. "Detailed magnetic and structural properties of exchange-biased La1-xCaxMnO3." physica status solidi (c) 4, no. 11 (2007): 4181–4187.
Abstract: We measured structural, magnetic and, magneto-transport properties of heterostructures consisting of La1–xCaxMnO3 ferromagnetic (FM) layers (x = 0.33) and antiferromagnetic (AF) layers (x = 0.67). We grew FM/AF superlattices via a high-pressure sputtering technique on (001) oriented SrTiO3 substrates. We systematically varied the thickness of the ferromagnetic layers, while maintaining a fixed thickness of the antiferromagnetic layers. Alternatively, we also varied the thickness of the antiferromagnetic layers, keeping a fixed ferromagnetic layer thickness. The total superlattice thickness was kept approximately constant. XRD analysis confirms the existence of the superlattice structure by the observation of multiple satellite peaks around the (00l) manganite Bragg reflections. We extracted the average lattice parameter of the superlattice and its dependence on the thicknesses of the ferro- and antiferromagnetic layers. We conducted field cooling (FC) and zero field cooling (ZFC), magneto-thermal, and magneto-resistance measurements. The existence of an exchange bias effect at temperatures below the Néel temperature of the AF layer was revealed by magnetization loops measured after FC. We studied the dependence of magnetic parameters on the thicknesses of the ferro- and antiferromagnetic layers. We found that the structural, magnetization, and magneto transport properties as functions of F-layer thickness are all correlated. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
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Guimpel, J., C. Monton, N. Haberkorn, and F. de la Cruz. "Magnetic behaviour of superconductor/ferromagnet metallic and perovskite based superlattices." physica status solidi (c) 4, no. 11 (2007): 4174–4180.
Abstract: We present a study of the magnetic behaviour of superconducting/ferromagnetic superlattices, both for metallic and perovskite based systems. In the first case we find that the response is determined by the ferromagnetic layer regime. If these layers are very thin, no ferromagnetism is observed, but the superconducting layers are decoupled, and the superlattice behaves as a stack of independent superconducting films. For thicker ferromagnetic films, ferromagnetism is established and the effective field resulting from the superposition of stray and applied fields dominates the superconducting response of the superlattice. For the perovskite based superlattices, the physical properties are more affected by interface disorder. In this case we have found that the behaviour can be interpeted as charge carrier transfer between both materials, inducing a change in the magnetic character of the manganite. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
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Haberkorn, N., G. Bridoux, E. Osquiguil, G. Nieva, and J. Guimpel. "Hall effect in a GdBa2Cu3O7-?/La0.75 Sr0.25MnO3 perovskite bilayer." Applied Surface Science 254, no. 1 SPEC. ISS. (2007): 222–224.
Abstract: We present results on the Hall coefficient RH in the normal state for a GdBa2Cu3O7-?/La0.75 Sr0.25MnO3 bilayer and a La0.75Sr0.25MnO3 film grown by dc magnetron sputtering on (1 0 0) SrTiO3. We find that the electric transport on the bilayer can be qualitatively described using a simple parallel layers model. The GdBa2Cu3O7-? layer presents a carrier density approximately equal to that reported for 7 – ? = 6.85 oxygen doping. Also we observe an unexpected presence of two Hall resistivity regimes, effects that may be associated with the internal magnetic field induced on the superconducting layer by the ferromagnetic layer.
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Haberkorn, N., F. Lovey, A. M. Condo, G. Nieva, and J. Guimpel. "Glasslike behavior at the PrBa2Cu3O7/La0.75Sr0.25MnO3 interface." PHYSICAL REVIEW B 75, no. 2 (2007): 024427.
Abstract: We study the crystalline structure and the magnetic properties of two different PrBa2Cu3O7-delta/La0.75Sr0.25MnO3 (PBCO/LSr0.25MO) superlattices grown by dc magnetron sputtering on (100) SrTiO3. The microstructure was studied by high-resolution transmission electron microscopy (HRTEM). Magnetic, magnetotransport, and time dependence measurements indicate a behavior compatible with phase separation at the interface, strain induced by the lattice parameter difference, and the presence of a structural transition in the manganite material. Spin glasslike blocking behavior was found at low temperatures, below 70 K.
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Kaul, E. E., N. Haberkorn, and J. Guimpel. "Crystalline orientation of BiMnO3 thin films grown by rf-sputtering." Applied Surface Science 254, no. 1 (2007): 160–163.
Abstract: BiMnO3 thin films were deposited by means of rf-sputtering onto [1 0 0] oriented SrTiO3 substrates. X-ray diffraction measurements revealed epitaxial growth with [0 0 1](m) orientation in the monoclinic structure representation, equivalent to the [1 1 1](pc) direction of the pseudocubic cationic lattice. This [0 0 1](m) orientation was obtained in a wide deposition parameters range. Preliminary magnetization measurements suggest that these films do not present ferromagnetism below the bulk Curie temperature, T-c = 105 K. (C) 2007 Elsevier B.V. All rights reserved.
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Luccas, R. F., N. Haberkorn, J. Guimpel, G. Bridoux, and G. Nieva. "Growth of GdBa2Cu3O7 thin films on single crystalline substrates of Y1-xPrBa2Cu3O7." PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 460 (2007): 1377–1378.
Abstract: Superconducting GdBa2CU3O7 thin films were grown by dc magnetron sputtering on Y1-xPrxBa2CU3O7 superconducting bulk single crystals. We optimized the in plane lattice matching with the Pr concentration. The samples were characterized by X-ray diffraction and electrical transport experiments. Magnetization hysteresis loops show high values of the critical currents for the films. (c) 2007 Elsevier B.V. All rights reserved.
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Monton, C., F. de la Cruz, and J. Guimpel. "Magnetic behavior of superconductor/ferromagnet superlattices." PHYSICAL REVIEW B 75, no. 6 (2007).
Abstract: A study of the electromagnetic response of superconductor/ferromagnet Nb/Co superlattices is presented. The dc magnetization is measured as a function of Co layer thickness, applied field H-a, and temperature. For very thin Co layers, no ferromagnetism is detected and the observed response is essentially that of a collection of uncoupled Nb layers. When the Co layer is thick enough for ferromagnetism to be established, prior to the field-cooled magnetization measurements, the Co layers are magnetically saturated in the parallel or antiparallel directions to H-a. In this case, the magnetic response of the otherwise uncoupled Nb layers is found to depend on both H-a and the stray fields.
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Monton, C., J. Guimpel, and F. de la Cruz. "Superconducting behaviour of Nb/Co superlattices." Applied Surface Science 254, no. 1 (2007): 375–377.
Abstract: We study the superconducting and magnetic behaviour of Nb/Co superlattices, for superconducting Nb layer thickness of 44 nm and Co layer thickness less than 1 nm. In this limit no ferromagnetism is observed for Co. The superlattice behaves as independent superconducting Nb layers. We obtain the penetration depth and superconducting gap of the material, and analyze the results in terms of microscopic models and theories. (C) 2007 Elsevier B.V. All rights reserved.
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Pasquini, G., D. Luna, and G. Nieva. "Oscillatory dynamics in the vortex matter of twinned and untwinned YBa2Cu3O7 crystals." PHYSICAL REVIEW B 76, no. 21 (2007): 212302.
Abstract: Experiments of oscillatory dynamics in twinned YBa2Cu3O7 (YBCO) crystals in a dc magnetic field tilted with respect to the twin planes indicate that history effects (HEs) in these samples cannot be ascribed to an equilibration process but have its origin in the oscillatory character of the vortex dynamics. However, the role that extended twin boundaries play in the HEs of YBCO crystals has been the subject of controversies until now. In this work, we study the role of correlated defects in the dynamic history effects by performing angular ac susceptibility measurements in twinned and detwinned YBCO crystals. We confirm that in any case, whenever HEs are observed, the temporal symmetry of the shaking ac field plays a major role.
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Sereni, J. G. "Crossover from classical to quantum regime in Ce-lattice systems." Physica B: Condensed Matter 398, no. 2 (2007): 412–415.
Abstract: Different types of magnetic phase diagrams are analyzed with the scope to detect signs of a crossover between the classical (thermal fluctuations) regime and the critical (quantum fluctuations) one. In systems where T-N,T-C is driven down to the mK range by alloying Ce-ligands (x), clear changes in the magnetic phase are observed once T-N,T-C < 2K. On the contrary, when T-N vanishes at finite temperature (driven by pressure or x) the phase boundaries do not reach temperatures lower than approximate to 3 K. In a third type of phase diagrams, characterized by a weak TN(x,p) dependence, signs of phase separation are suspected. Distinctive properties of each mentioned group are recognized and some exemplary cases analyzed in detail. Those properties coincide indicating a crossover temperature slightly above 2 K. (c) 2007 Elsevier B.V. All rights reserved.
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Sereni, J. G. "Peculiar thermal features of Ce-systems around their critical points." JOURNAL OF LOW TEMPERATURE PHYSICS 147, no. 3-4 (2007): 179–197.
Abstract: Some non-predicted thermal behavior observed in various Ce-lattice compounds close to their critical points are presented. From a comparison between phase diagrams driven by doping (x) and pressure, a pre-critical region (x* < x < x(cr)) is identified in the former group. Some systematic behaviors, observed in the specific heat (C-m/T) and the entropy gain, are recognized as characteristic features of that region. Different measured temperature dependencies of C-m/T are compared, detecting that the onset of the non-Fermi-liquid behavior occurs much closer to x* than x(cr). In a detailed analysis of the evolution of the thermal dependence within the pre-critical region, a further change in the T dependence is frequently observed before to reach the critical point. A generalized power law function is proposed to compare different systems, which allows to identify a low temperature C-m/T(T) anomaly that only involves about 0.01 x R ln 2 of entropy.
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Sereni, J. G., N. Caroca-Canales, and C. Geibel. "Ferromagnetic instability at the edge of intermediate valence." JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS 310, no. 2 (2007): E214–E216.
Abstract: Preliminary study of structural, thermal and magnetic properties on ferromagnetic CePt1-yRhy (0.40 <= y <= 1) is presented. Like in the recently studied CePd1-xRhx, this new system undergoes a quantum critical behavior between the ferromagnetic CePt (T-C = 6K) and unstable valence CeRh. Regardless the electronic and atomic size similarities between Pt and Pd atoms, signi. cant differences in their respective phase diagrams are observed around the critical points. While CePd1-xRhx shows an asymptopic T-C(x) decrease indicating a smeared quantum phase transition, in CePt1-yRhyTC(y) vanishes beyond y = 0.80 after a plateau starting at y = 0.60 at T-C approximate to 1.8K. Peculiarities of this critical region are compared with other Ce-systems. (c) 2006 Elsevier B.V. All rights reserved.
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Sereni, J. G., T. Westerkamp, R. Kuchler, N. Caroca-Canales, P. Gegenwart, and C. Geibel. "Ferromagnetic quantum criticality in the alloy CePd1-xRhx." Physical Review B 75, no. 2 (2007): 024432.
Abstract: The CePd1-xRhx alloy exhibits a continuous evolution from ferromagnetism (T-C=6.5 K) at x=0 to a mixed-valence (MV) state at x=1. We have performed a detailed investigation on the suppression of the ferromagnetic (F) phase in this alloy using dc (chi(dc)) and ac (chi(ac)) susceptibility, specific heat (C-m), resistivity (rho), and thermal expansion (beta) techniques. Our results show a continuous decrease of T-C(x) with negative curvature down to T-C=3 K at x(*)=0.65, where a positive curvature takes over. Beyond x(*), a cusp in chi(ac) is traced down to T-C(*)=25 mK at x=0.87, locating the critical concentration between x=0.87 and 0.90. The quantum criticality of this region is recognized by the -log(T/T-0) dependence of C-m/T, which transforms into a T-q (q approximate to 0.5) one at x=0.87. At high temperature, this system shows the onset of valence instability revealed by a deviation from Vegard's law (at x(V)approximate to 0.75) and increasing hybridization effects on high-temperature chi(dc) and rho(T). Coincidentally, a Fermi liquid contribution to the specific heat (gamma) arises from the MV component, which becomes dominant at the CeRh limit. In contrast to antiferromagnetic systems, no C-m/T flattening is observed for x > x(cr) but, rather, the mentioned power-law divergence, which coincides with a change of sign of beta(T). The coexistence of F and MV components and the sudden changes in the T dependencies are discussed in the context of randomly distributed magnetic and Kondo couplings.
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Uemura, Y. J., T. Goko, I. M. Gat-Malureanu, J. P. Carlo, P. L. Russo, A. T. Savici, A. Aczel, G. J. MacDougall, J. A. Rodriguez, G. M. Luke et al. "Phase separation and suppression of critical dynamics at quantum phase transitions of MnSi and (Sr1-xCax)RuO3." Nature Physics 3, no. 1 (2007): 29–35.
Abstract: Quantum phase transitions (QPTs) at zero temperature are generally studied by means of pressure or composition tuning. Volume-integrated probes such as neutron and magnetization measurements, as well as pressure uncertainties in NMR studies using powder specimens, however, have limited the characterization of magnetism and detection of discontinuous changes at QPTs. Overcoming these limitations, we carried out muon spin relaxation measurements that have a unique sensitivity to volume fractions of magnetically ordered and paramagnetic regions, and studied QPTs from itinerant helimagnet or ferromagnet to paramagnet transitions in MnSi (single crystal; varying pressure) and (Sr1-xCax)RuO3 (ceramic specimens; varying x). Our results provide the first clear evidence that both cases are associated with phase separation and suppression of dynamic critical behaviour, reveal slow dynamics of the 'partial order' diffuse spin correlations in MnSi above the critical pressure and suggest the possibility that a majority of QPTs in correlated electron systems involve first-order transitions and/or phase separation.
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