Pressão Química e Deformações Plásticas em Ligas Heuslerianas à Base de : Efeitos de Ordem/Desordem Atômica

• Autor(a) principal: Guimarães, Carlos Eduardo Alves
• Data de Publicação: 2018
• Tipo de documento: Tese
• Idioma: por
• Título da fonte: Repositório Institucional da Universidade Federal do Espírito Santo (riUfes)
• Texto Completo: http://repositorio.ufes.br/handle/10/10547
• topic: Condensed matter; Magnetism; Mossbauer Spectroscopy; Matéria condensada; Magnetismo; Mossbauer, Espectroscopia de; Física; Matéria condensada; Magnetismo; Mossbauer, Espectroscopia de; 53

In this work, we prepared (by arc-voltaic melting technique) and studied the policrystalline pseudo-Heusler ????2????????1-???????? alloys, in the 0 = ?? = 1 range, in order to understand the crystalline structures and magnetic orders in the stoichiometric phases (?? = 0 and 1). We also investigated the role of two important effects in the structural, hyperfine and magnetic properties: (i) one of plastic deformation due to the cold-work and the subsequent annealing of the samples; and (ii) one of chemical pressure due to the substitution of ???? by ???? (isoeletronic atoms with different atomic radii). Overall, we produced 100 ???? thick ribbons which we characterized by X-rays diffraction, magnetization (DC and AC) measurements and ???? 57 Mössbauer spectroscopy. On one hand, we determined that the ordered ????2???????? (?? = 0) alloy stabilized in the ??12 phase after annealing from the as-cold-worked ??1-type structure. On the other hand, the ordered ????2???????? (?? = 1) alloy has shown an ??21 pattern from the as-cold-worked ??2-type structure. Their properties are sensitive to the degree of atomic order/disorder. The lamination causes grain refinement, texture, defects and tensions in the crystal lattice, leading the alloys to different magnetic states at room temperature (the ferromagnetic ??2 and the antiferromagnetic ??1). The structurally ordered ribbons behaved as if they possessed two independent magnetic sublattices in a wide range of temperatures, but they coupled in the non-collinear (sperimagnetic) configuration at low temperatures. With the substitution of ???? by ????, we observed a significant reduction of 1,6% in the cell’s volume which resulted in the ??21 crystallization and changes in the following parameters: magnetization, magnetic order temperature and the spin and charge densities in ???? atoms. We determined the reduction of ???? and ???? magnetic moments by first principals calculations that causes the increase of the magnetization up to a concentration of ?? = 0.5 (due to faster reduction of ???? moment). Above a concentration of ?? = 0.5 there is a discrepancy between experimental and theoretical results, which can be explained by the noncollinear configuration of the spins (we performed the collinear state in the calculations).