National Institute Of Materials Physics - Romania

Mossbauer measurements performed on Fe100-xGdx thin films and on Fe80-xGdxB20 both as thin films and ribbons show a dependence of the spins orientation and [H-hyp] versus temperature, Gd content and preparation conditions. Increasing the Gd content, the initial low anisotropy disappears and [H-hyp] decreases. A sharp increase of the anisotopy with temperature in ribbons with low Gd concentration is evidenced.

Magnetic and Mossbauer experiments were performed on Fe100-xGdx thin films and on Fe80-xGdxB20 both in thin films and ribbons for x less-than-or-equal-to 20. The ferrimagnetic order is perturbed by the presence of B and Fe-interactions are.different. For small Gd content, the low anisotropy is in the film plane, while for greater Gd the anisotropy goes out of the plane giving typical features of hysteresis loops and strong decrease of Mossbauer hyperfine fields.

The dissipation mechanism in high-T(c) superconducting ceramics was studied by analyzing the magnetic field dependence of the transport critical current and the form of the current-voltage characteristics. It was found that the actual magnetic field dependence of the transport critical current is significantly slower than that predicted by the weak-link-quenching model. The low-voltage-level current-voltage characteristics were described in terms of thermally activated flux creep at grain boundaries, taking into account collective pinning of intergranular Josephson vortices. For the investigated samples, a low-field, high-temperature mean pinning energy barrier of a few tens of meV was determined.

The operational range of the gas counters used in conversion electron Mossbauer spectroscopy (CEMS) is at the limit between the proportional counter and Geiger Muller domains. Starting from the theory of phenomena in gas detectors under space charge conditions, a general expression for the multiplication factor was obtained. The theoretical results, presented graphically, are discussed in connection with experimental data.

The temperature dependence of the critical current density at high temperatures and in weak applied magnetic field for YBa2Cu3O7-y ceramic samples with a pronounced granular character is analyzed. The experimental results can be explained in terms of thermally activated motion of the intergrain Josephson vortices at grain boundaries, which may be an indication that the actual limiting factor for the critical current density in ceramic samples results from a weak pinning force density for the intergrain vortices rather than from the weak-link quenching.

This work reports the effect of the rapid solidification technique and thermal treatment on the martensitic transformation (MT), magnetic and magnetostrictive properties on the off-stoichiometric Ni49Mn31Ga20 and Ni51Mn28Ga21 ferromagnetic shape memory ribbons. The samples were investigated by X-ray diffraction, differential scanning calorimetry, scanning electron microscopy, magnetic and magnetostrictive measurements. The temperature dependence of the X-ray phases analysis shows the presence of martensite structures, both tetragonal and monoclinic, at room temperature and allowed to study their evolution through MT. The thermal treatment induces changes in the microstructure with implications in MT and Curie temperatures evolution. The competition between the magnetization orientation and twin boundary motion within martensitic variants under magnetic field evidenced in the magnetic-strain curves was discussed and correlated with the magnetic data.

Exchange coupling in a SrFe12O19 - Fe3O4 nanocomposite magnet was explored in this study. The composition, microstructure, local structure and magnetic properties were investigated by XRD, SEM, Mossbauer spectroscopy, and SQUID magnetometry. The magnetoplumbite SrFe12O19 and spinel Fe3O4 structures were verified by X-ray diffraction. The morphology of the composite reveals the characteristics of the two components. The hyperfine parameters allowed the identification of the Wyckoff positions of the iron ions corresponding to the involved phases. The magnetic measurements of the composite, showing a single-phase-like magnetic hysteresis loop, confirmed the exchange coupling between the hard and soft magnetic phases.

NiTi cylindrical materials were obtained by spark plasma sintering at 850??C or 900??C from high purity Ni and Ti powders. These mixtures were previously processed by mechanical alloying for 8 and 15 hours. In order to increase the alloy homogeneity, the NiTi materials were subjected to post aging treatment in protective atmosphere at constant temperature 400??C and cooling in water with ice. All samples were characterized by optical microscopy, differential calorimetry (DSC), X-ray measurements (XRD) and micro/nanoindentation measurements. The results demonstrate that the NiTi materials obtained by this preparation route have good mechanical properties and can serve as a quantitative reference for the microstructure design of shape memory materials for various applications, such as the biomedical ones.

The paper presents the research results in the field of TiNiCu shape memory alloys processed by powder metallurgy techniques. The powders mixtures used to synthesize Ti50Ni30Cu20 alloys were obtained by blended constitutive elemental powders or by mechanical alloying of powder mixtures for 10 and 20 hours, respectively. Finally, a subsequent annealing process was carried out, followed by a rapid cooling in ice water. All experiments were performed under 99.9% argon atmosphere. At room temperature, the obtained materials contained monoclinic and orthorhombic martensite type. The structural and thermal investigation of these materials were discussed in view of the potential development of actuators.