Chemical Pressure and Plastic Deformations in Heuslerian à-Based Alloys: Order / Atomic Disorder Effects.

Name: Carlos Eduardo Alves Guimarães
Type: PhD thesis
Publication date: 11/10/2018

Namesort descending Role
Edson Passamani Caetano Advisor *

Examining board:

Namesort descending Role
Dalber Ruben Sanchez Candela External Examiner *
Edson Passamani Caetano Advisor *
Elisa Maria Baggio-Saitovitch External Examiner *
Thiago Eduardo Pedreira Bueno Internal Examiner *
Wanderlã Luis Scopel Internal Examiner *

Summary: In this work, we prepared (by arc-voltaic melting technique) and studied the policrystalline pseudo-Heusler 𝐹𝑒2𝑀𝑛𝐺𝑎1−𝑥𝐴𝑙𝑥 alloys, in the 0 ≤ 𝑥 ≤ 1 range, in order to understand the crystalline structures and magnetic orders in the stoichiometric phases (𝑥 = 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 𝐺𝑎 by 𝐴𝑙 (isoeletronic atoms with different atomic radii). Overall, we produced 100 𝜇𝑚 thick ribbons which we characterized by X-rays diffraction, magnetization (DC and AC) measurements and 𝐹𝑒 57 Mössbauer spectroscopy. On one hand, we determined that the ordered 𝐹𝑒2𝑀𝑛𝐺𝑎 (𝑥 = 0) alloy stabilized in the 𝐿12 phase after annealing from the as-cold-worked 𝐴1-type structure.
On the other hand, the ordered 𝐹𝑒2𝑀𝑛𝐴𝑙 (𝑥 = 1) alloy has shown an 𝐿21 pattern from the as-cold-worked 𝐴2-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 𝐴2 and the antiferromagnetic 𝐴1). 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 𝐺𝑎 by 𝐴𝑙, we observed a significant reduction of 1,6% in the cell’s volume which resulted in the 𝐿21 crystallization and changes in the following parameters: magnetization, magnetic order temperature and the spin and charge densities in 𝐹𝑒 atoms. We determined the
reduction of 𝐹𝑒 and 𝑀𝑛 magnetic moments by first principals calculations that causes the increase of the magnetization up to a concentration of 𝑥 = 0.5 (due to faster reduction of 𝐹𝑒 moment). Above a concentration of 𝑥 = 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).

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