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.

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, La_0.67Ca_0.33MnO₃ and La_0.67Sr_0.33MnO₃, and one with ferromagnetic and charge ordered phase coexistence, La_0.325Pr_0.300Ca_0.375MnO₃. 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 La_0.325Pr_0.300Ca_0.375MnO₃ 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.

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)

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.

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.

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.

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.

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.

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.