Papers by Author: Patricia Lázpita

Abstract: The characteristics of neutron diffraction applied to the study of Ferromagnetic Shape Memory Alloys are revised. Main studies refer to crystal structure, preferential site occupancy, variant reorientation and magnetic moment distribution in the alloys

Abstract: A Mn-enriched Ni46.4Mn32.8Sn20.8 Heusler alloy has been prepared by the induction melting and casting method. The samples used were annealed at 900°C during 72 h with a subsequent water quench. The magnetostructural transformation (MST) in the vicinity of Tm=230K and Ta=250K has been found, and characterized by DSC, magnetization and resistivity measurements. Changes in the crystal structure due to the MST were observed using neutron diffraction at different temperatures. The crystal structure of martensitic phase shows a good fits to the 4-layered orthorhombic martensite with a Pmma space group. The linear shift of transformation temperatures was found in the entire high magnetic field range up 14T to be dTm/dB0 = -1.50 K/T and dTa/dB0 = -1.26 K/T, whereas in the low field range we found an increase of both characteristic temperatures. The entropy changes at the MST were evaluated using the Clausius-Clapeyron relationship. The transformation-induced magnetoresistance has been measured.

Abstract: Composites of Ni–Mn–Ga particles in a polyurethane matrix can be made by mixing the particles with the polymer, and allowing them to cure under a magnetic field to texture the composites. These composites show large hysteresis and mechanical losses, when subjected to a cyclic stress, that were far larger than the matrix polymer ones. The additional losses are attributed to the motion of twin boundaries in the filler particles and provide a way for obtaining mechanical energy absorption in a wide frequency range. By means of X-ray and neutron diffraction we present evidence that confirms that twins are present in the particles and that they do move when mechanically loading the composite