Influence of Microstructural Defects on the Martensitic Transformation and on the Magnetic Properties of Ni-based Heusler Alloys

The thesis deals with the effect that microstructural defects have on the magnetic properties and martensitic transformation in Ni-Mn-Z (Z = Ga, In, Sn) and Ni-Fe-Ga alloys. For this purpose, along with the standard characterization techniques, two additional specific nuclear techniques have been employed; $^{119}$Sn Mössbauer Spectroscopy and Positron Annihilation Lifetime Spectroscopy, PALS. For the consecution of this thesis one PALS spectrometer has been designed, builtand optimized. For offline emission Mössbuauer experiments, a setup has been also built for measuring ion implanted samples. Finally, a high temperature annealing furnace has been designed and built to carry out the annealing of powdered samples under controllable atmosphere. Regarding the effect that mechanically induced defects have on the properties of Ni-Mn-In alloys, it is shown that long ball milling times induce the disappearance of the long-range atomic order and the subsequent occurrence of the martensitic transformation, which ultimately leads to a frustrated magnetic system. The characterization of the obtained amorphous state reveals a canonical spin glass state ofthe ball-milled Ni$_{50}$Mn$_{34}$In$_{16}$ powders. The subsequent study of the crystallization dynamics reveals that the recovery takes place at two-stage crystallization processes. On heating, an abrupt crystallization process occurs around 500 K leading to a cubic B2 structure, evincing the suitability of ball milling and annealing procedures forreproducing the high-temperature B2 phase at much lower temperatures. On further heating, and concurrently with a B2-L2$_{1}$ atomic ordering, a relaxation process takes place above 700 K, which gives rise to an anomalous two-step thermal expansion. Finally, the magnetocaloric effect is comparatively evaluated in both bulk and ball-milled powders. The relative cooling power linked to the magnetocaloric effect of the obtained nanoparticles makes them suitable for practical applications of magnetic refrigeration at nanoscale.