In the endeavor of exploiting the multifunctional properties of Ni-based Heusler alloys, the miniaturization of the active elements is consolidating as an effective approach to overcome their defective mechanical properties. Although the performance of microparticles, nanoparticles and powdered samples present unique advantages, in such regime, the control of internal strains, local stresses and point defects acquires a key relevance. Defects greatly influence both the magnetic properties and the martensitic transformation of Ni-based Heusler alloys. As the multifunctional properties are directly related with the characteristics of the martensitic transformation, the determination of the influence that defects have on the magnetic properties and on the martensitic transformation acquires a pivotal role for their development of practical devices and applications. This thesis deals with the influence that defects have on the magnetic properties and on the martensitic transformation of Ni- Mn-Z (Z = Ga, Sn, In) and Ni-Fe-Ga alloys. In order to overcome the elusive nature of defects, a nuclear characterization approach has been selected for the consecution of this thesis. By means of positron annihilation lifetime spectroscopy and Mössbauer spectroscopy, the study is conducted at atomic scale level.
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