Comportamento termomagnético multifuncional em perovskitas duplas desordenadas em dois sítios

• Ano de defesa: 2023
• Autor(a) principal: Silva Junior, Romualdo Santos
• Orientador(a): Ferreira, Nilson dos Santos
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: por
• Programa de Pós-Graduação: Pós-Graduação em Física
• Palavras-chave em Português: Perovskita dupla; Desordem; Efeito magnetocalórico; Comportamento crítico
• Palavras-chave em Inglês: Double perovskite; Disorder; Magnetocaloric effect; Critical behavior
• Área do conhecimento CNPq: CIENCIAS EXATAS E DA TERRA::FISICA
• Link de acesso: http://ri.ufs.br/jspui/handle/riufs/17261

n this thesis, a detailed systematic study of the structural, magnetic, magnetocaloric properties and critical behavior of double perovskites RSrCoFeO6 (R = Nd and Gd), prepared by the sol-gel and solid-state method, respectively, was carried out. Initially, a structural characterization was performed by X-ray Diffraction (XRD), Synchrotron X-ray Diffraction (SXRD), Rietveld refinement, X-ray Photoelectron Spectroscopy (XPS) and Transmission Electron Microscopy (TEM), revealing that the samples They crystallize in an orthorhombic structure with mixed valence (Nd,Gd)/Sr and Co/Fe ions randomly occupying their respective sites. Then, measurements of DC magnetization as a function of temperature M(T) and field M(H), and AC susceptibility were carried out to unravel the thermomagnetic behavior of the samples, which revealed multiple magnetic transitions originating from short and long magnetic competitions. long range, and, interestingly, interesting magnetic characteristics such as Griffiths phase (GP) and spin glass (SG), related to the presence of disorder and magnetic frustration in these materials. Specific heat measurements confirm the observed magnetic transition temperatures for both samples, revealing a Schottky anomaly behavior at low temperature. Complementarily, the analysis of SXRD as a function of temperature indicates a magnetoelastic effect that coincides with the magnetic transition temperature. The magnetocaloric effect was investigated through analysis of agnetic etropy variation (∆SM) and relative cooling power (RCP), which reveal a power law dependence as a function of the applied field. The magnetic nature was revealed through burp-plot curves and normalized universal curve, proving a typical second-order phase transition (SOPT). Finally, the critical behavior was explored through the critical exponents (β, γ and δ) and scale hypothesis analysis, demonstrating a behavior distinct from any conventional universality class.