Understanding and optmizing the magnetostructural correlation of novel materials

Abstract

Magnetostructural correlation is an important feature in electronic devices, such as sensors, actuators, memory devices, shape-memory materials, and others. Perovskite systems and Heusler alloys are the main candidates for the aforementioned phenomenon owing to their magnetic properties, such as spin crossover, magnetoelastic, and magnetocaloric effects, whose response is dependent on magnetostructural coupling. This study is based in three compounds: i) LaCoO3, whose discussion about the magnetic properties of this compound lasted for decades, mainly due to the origin of the long-range magnetic behavior on nanostructures. From a detailed discussion about the magnetic fashion, we were able to understand that long-range magnetic behavior is obtained from a strong magnetostructural correlation due to the Jahn-Teller effect induced by an increase in the number of vacancies, which leads to similar properties on LaCoO3 nanoparticles; ii) Fe2MnSi1−xGex series, is a promising compound for technological application. The discovery of hexagonal Heusler alloy opened discussion about the stability and properties; thus, the evaluation of the Si substitution based on the Ge effect in the structure was related to the stabilization of the hexagonal phase. The X-ray diffraction analysis allied with magnetic measurements exhibited a transformation in the structure from a cubic (x = 0) to a hexagonal (x = 1). We enhanced the saturation magnetization while changing the symmetry; iii) finally, Fe2MnSi Heusler alloy is a well-known system that can present an ordered (L21) or disordered (DO3, A2 or B2) cubic structure (space group Fm-3m), and it can be minimized with chemical and/or thermal treatments. By achieving an ordered structure (L21), it is possible to enhance the magnetic properties of this system, oncethe position of atoms influences the magnetism behavior. In this study, several thermal treatments were performed on Fe2MnSi, and preliminary results suggest that a treatment at 800 oC for 24 h minimizes the DO3 disorder in the Full Heusler alloy. Although this particular study are not fulled concluded, few conclusions were drawn based on the observation of enhanced saturation magnetization and, consequently, a magnetocaloric response for the sample cooled by inertia. My contribution to this work is the synthesis of Fe2MnSi1−xGex series and Fe2MnSi compounds using an arc-melt furnace and La1−xxCo3+1−xCo4+x O3 bulk samples using the sol-gel method. The X-rays diffraction and the refinement using Rietveld method for all compounds discussed in this study were perfomed by me. I had a fundamental role in the structural analysis of the compounds presented here. The magnetic measurements of the Fe2MnSi1−xGex series were done by me. I have performed the magnetic analysis of the compounds presented in this study.