Programa de Pós-graduação em Física

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Navegando Programa de Pós-graduação em Física por Por Orientador "Bufaiçal, Leandro Felix de Sousa"

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    Investigação da influência da estrutura cristalina no efeito exchange bias espontâneo da série La1,5(Sr0,5-xBax)CoMnO6
    (Universidade Federal de Goiás, 2021-03-18) Belluzi, Marlon Boldrin; Bufaiçal, Leandro Felix de Sousa; http://lattes.cnpq.br/1392206843733548; Bufaiçal, Leandro Felix de Sousa; Ferreira, Letície Mendonça; Franco Júnior, Adolfo
    The exchange bias (EB) effect is of great academic interest due to its potential applications in spintronics. Recently there were discovered some material exhibiting the EB effect spontaneously, i.e., even after being cooled in the absence of an external magnetic field. The main focus of this work is to investigate the influence of crystalline and electronic structures on the spontaneous EB effect. Therefore, here we describe the synthesis and structural, electronic and magnetic characterizations of polycrystalline samples of the La1.5(Sr0.5-xBax)CoMnO6 series. The main objective was to investigate the evolution of the magnetization of the system when Sr is gradually substituted by Ba. Polycrystalline samples were synthesized by conventional solid state reaction, and their structural properties were characterized by powder X-ray diffraction and the Rietveld Refinement. The magnetic properties were investigated by means of magnetization as a function of temperature [M(T)] and magnetization as a function of magnetic field [M(H)] measurements. The compounds were formed in a single phase, in the rhombohedral space group 𝑅3̅c. In addition, it was observed that the lattice parameters increase as the Ba concentration increases. The M(T) measurements indicate that all compounds have ferromagnetic (FM) and antiferromagnetic (AFM) ordering, where an increase in the proportion of the AFM phase was observed as the Ba concentration increases. Also, a trend of change is observed in the average spin state of the Co3+ ions, which varies from the low spin (LS) configuration to the high spin (HS) configuration as the Ba concentration increases. The M(H) measurements performed after cooling the samples in zero field showed a robust spontaneous EB effect, with the concentration x = 0.25 having the greatest effect ever observed. M(H) measurements taken after cooling the materials in the presence of an external field showed a qualitative behavior similar to those taken after cooling in zero field. The variations in the exchange bias effect observed throughout the series can be explained by the variations in the AFM phase and in the spin state of Co3+.
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    Propriedades estruturais, eletrônicas e magnéticas dos óxidos La2−xCaxCoIrO6
    (Universidade Federal de Goiás, 2015-02-27) Coutrim, Leandro Tolentino; Bufaiçal, Leandro Felix de Sousa; http://lattes.cnpq.br/1392206843733548
    In this work we describe the synthesis of the novel series La2−xCaxCoIrO6 and its structural, electronic and magnetic characterization. The main objective of this project is to investigate the magnetic evolution of the system when modified due to the chemical substitution of Ca at La site, which can induce changes in the magnetic interactions. The policristaline samples were synthesized by Solid State Reaction process, and studied combining global characterization techniques as powder x-ray diffraction, temperature dependence of magnetic susceptibility and magnetization as a function of applied magnetic field. All compounds crystallize in monoclinic structure, with space group P21/n. Magnetic susceptibility results indicate that the superexchange antiferromagnetic interaction between Co2+ and Ir4+ in La2CoIrO6 lead to ferrimagnetic behavior due to the different magnetic moment of the transition metal ions. For the Ca2+ doped at La3+ site compounds, a magnetic evolution is observed with doping, induced by Co and Ir valence changes. As expected, this system presents a strong correlation between its structural, electronic and magnetic properties, in a way that combining the different results of each compound we can explain its behavior and understand the big picture of this system.
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    Caracterização estrutural e investigação do efeito exchange bias espontâneo na perovskita dupla La1,5Sr0,5CoMn0,5Fe0,5O6
    (Universidade Federal de Goiás, 2021-02-26) Silva, Arthur Garcia; Bufaiçal, Leandro Felix de Sousa; http://lattes.cnpq.br/1392206843733548; Bufaiçal, Leandro Felix de Sousa; Bittar, Eduardo Matzenbacher; Santana, Ricardo Costa de
    In this work we describe the synthesis and characterization of the structural, eletronic and magnetic properties of La1.5Sr0.5CoMn0.5Fe0.5O6 compound. Our main focus was on the investigation of 50% of Mn to Fe substitution in La1.5Sr0.5CoMnO6, with a particular interest in the spontaneous exchange bias effect, a phenomena characterized by the shift in the magnetic hysteresis curves taken after the system bein cooled in zero magnetic field. The polycrystalline sample here investigated was syntesized by conventional solid state reaction technique and investigated by means of X-ray powder diffraction, magnetization as a function of temperature [M(T)] and magnetization as a function of magnetic field [M(H)]. The La1.5Sr0.5CoMn0.5Fe0.5O6 crystal forms in rhombohedral symmetry, space group, in agreement with results observed for similar compounds. The M(T) measurements have revealed a ferromagnetic-like transition at TC ≈ 150 K, and also indicated the presence of competing magnetic phases, which leads to the formation of ferromagnetics clusters embedded in a paramagnetic matrix at high temperature. This is a hallmark of the Griffiths Phase, here occurring in the 150 K < T < 205 K range. The M(H) curves carried after cooling the system in zero field have showed that La1.5Sr0.5CoMn0.5Fe0.5O6 exhibits the spontaneous Exchange Bias effect at low temperatures, while for the measurements carried after cooling the system in the presence of a external magnetic field it is observed a significant enhancement of the shift in the M(H) curve.