Abstract: We have measured the specific heat of GdxEu1−xBa2Cu3O7−δ (0 ≤ × ≤ 1). The data show a λ-type anomaly at high concentrations and a broad Schottky type anomaly at intermediate and low concentrations. We compare this behavior with theoretical predictions for a 2-dimensional Ising system.

Abstract: We report structural, magnetic and transport measurements on La0.6Sr0.4MnO3 thin films grown on MgO and TiSrO3 substrates with thickness varying from 5 to 500 nm. We find that the lattice mismatch between substrates and films affects the morphology and induced-strains of the films. We show that these two different effects strongly influence the ferromagnetic order, the metal-insulator transition, the localization of the current carriers and the magnetoresistance of these materials. (C) 2000 Elsevier Science B.V. All rights reserved.

Abstract: The time dependence of the magnetic and transport properties on La0.6Sr0.4MnO3 films and bulk samples has been studied through magnetization and resistivity measurements. A magnetic after-effect has been observed in all samples. At low temperatures, the low-field magnetization, can be described by the function M(t) = M-c + M-d exp(-t/tau) + S(H, T)ln(t), The resistivity increases logarithmically in the same temperature range. indicating the evolution of the sample to a more disordered state. Above a characteristic temperature, this behaviour is reversed and an increase of the magnetization with time is observed. The relaxation parameters depend on the bulk or films character of the samples. In the latter case, a dependence on the film thickness was found. A direct correlation between the time dependence of the resistivity and magnetization curves in manganite compounds was found. (C) 2001 Elsevier Science B.V. All rights reserved.

Abstract: The ferromagnetic phase of La0.6Sr0.4MnO3 thin films has been investigated through measurements of magnetization loops at different temperatures, zero-field-cooled and field-cooled magnetization curves obtained under different magnetic fields. We have found that the main sources of the “bulk” coercivity in manganite films are the film/substrate interface and film surface. The temperature dependence of the coercivity is described by a “strong domain-wall pinning” model, independently of the thickness and substrate. The magnetization-vs-temperature curves, measured under different magnetic fields (10 Oe<H<2.5 kOe), have been explained in terms of the magnetic hysteresis of the films. We find no evidence of glass states or of the existence of single-domain clusters, as suggested by other authors.

Abstract: We have studied the structure, magnetic, and transport properties of manganite-based multilayers A/B-i, with A=La0.55Sr0.45MnO3 (metallic ferromagnet), and spacers (B-i) of different nature: B-1=SrTiO3 (nonmagnetic insulator), B-2=La0.9Sr0.1MnO3 (insulator ferromagnet), and B-3=La0.67Ca0.33MnO3 (paramagnetic-insulator/ferromagnet-metal). The samples are strongly textured in the direction perpendicular to the sample surface, and present a good interface quality with small roughness and interdiffusion. We have found that in the A/B-1/A trilayers, the ferromagnetic electrodes are ferromagnetically coupled for thin spacer layers and becomes decoupled for spacer thickness larger than 3 nm. Instead, the others multilayers are ferromagnetic for all spacer thicknesses and temperature range. This result was expected for temperatures well below the spacer Curie temperature. We attributed the ferromagnetic behavior of the system, found for temperatures above the ordering temperature of the spacer, to direct exchange coupling through short-range ordered zones in spacer layer. As expected from magnetization results, in fact, no extrinsic magnetoresistance was measured in these systems in the temperature range between 4.2 and 300 K. (C) 2003 American Institute of Physics.