Estudo dos efeitos de campo cristalino e estreitamento da estrutura fina em espectros de ressonância paramagnética eletrônica de Eu2+ diluído em Ca1-XEuXB6
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Data
2017-04-24
Autores
Silva, Maicon Carlone [UNESP]
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Universidade Estadual Paulista (Unesp)
Resumo
Compostos do tipo Ca1-xRxB6 (onde R representa o elemento de terra rara) tem sido estudados durantes os últimos anos principalmente no que se refere as suas características magnéticas e de transporte, porem elas ainda são controversas. Em particular a técnica de Ressonância Magnética tem sido usada para estudar compostos de Ca1-xEuxB6 (EPR) para diversas concentrações de Eu, o qual sugere a existência de uma transição metal isolante quando mudamos a concentração de Eu. Neste trabalho deveremos analisar teoricamente os espectros experimentais deste composto, em função da concentração deste elemento de terra rara. Os espectros de EPR foram simulados usando o modelo de Plefka para diversos valores de concentração x em Ca1-xEuxB6 monocristalino, isto é, 7%, 10%, 15%, 30%, 60% e 100%, visando elucidar a evolução das fases isolantes e semimetálicas, além do colapso do espectro com o aumento da concentração da impureza magnética. Tendo em vista que formas de linha simétricas são características de matrizes isolantes e assimétricas de matrizes metálicas, focaremos o nosso estudo na assimetria dos espectros de EPR. Para procurar pelo conjunto de parâmetros que melhor ajusta o espectro teórico ao experimental utilizamos o método de Simulated Annealing (SA), onde minimizamos a diferencia média quadrática entre os espectros. Ao longo deste trabalho foram reproduzidos as principais características dos espectros experimentais. Também obtivemos parâmetros importantes como o de Campo Cristalino, valor de g, parâmetros de troca da interação Eu-Eu e o parâmetro de assimetria do espectro de absorção. Conseguimos mostrar que ao aumentar a concentração o espectro evolui de uma forma Lorentziana para Dysoniana, mostrando a existência da transição metal-isolante.
Compounds of the type Ca1-xRxB6 (where R represents the element of rare earth) have been studied during the last years mainly as regards their magnetic and transport characteristics, but they are still controversial. In particular the Magnetic Resonance technique has been used to study compounds of Ca1-xEuxB6 (EPR) for several concentrations of Eu, which suggests the existence of an insulating metal transition when we change the concentration of Eu. In this work we will theoretically analyze the spectra Experimental results of this compound, depending on the concentration of this element of rare earth. In the present case, the EPR fine structure spectra were simulated using the Plefka model for various values of concentration (x) in monocrystalline Ca1-xEuxB6, that is, 7%, 10%, 15%, 30%, 60% and 100%, aiming to elucidate the evolution of the insulating and semi-metallic phases, besides the collapse of the spectrum with the increase of the concentration of magnetic impurities. Considering that symmetrical line shapes are characteristic of insulating and asymmetric matrices of metal matrices, we will focus our study on the asymmetry of RPE spectra. To find for the set of parameters that best adjusts the theoretical spectrum to the experimental, we used the Simulated Annealing (SA) method, where we minimize the mean square difference between the spectra. Throughout this work the main characteristics of the experimental spectra were reproduced. We also obtained important parameters such as the Crystalline Field, g value, Eu-Eu interaction exchange parameters and the asymmetry parameter of the absorption spectrum. We show that by increasing the concentration the spectrum evolves from a Lorentzian to Dysonian form, showing the existence of the metal-insulating transition.
Compounds of the type Ca1-xRxB6 (where R represents the element of rare earth) have been studied during the last years mainly as regards their magnetic and transport characteristics, but they are still controversial. In particular the Magnetic Resonance technique has been used to study compounds of Ca1-xEuxB6 (EPR) for several concentrations of Eu, which suggests the existence of an insulating metal transition when we change the concentration of Eu. In this work we will theoretically analyze the spectra Experimental results of this compound, depending on the concentration of this element of rare earth. In the present case, the EPR fine structure spectra were simulated using the Plefka model for various values of concentration (x) in monocrystalline Ca1-xEuxB6, that is, 7%, 10%, 15%, 30%, 60% and 100%, aiming to elucidate the evolution of the insulating and semi-metallic phases, besides the collapse of the spectrum with the increase of the concentration of magnetic impurities. Considering that symmetrical line shapes are characteristic of insulating and asymmetric matrices of metal matrices, we will focus our study on the asymmetry of RPE spectra. To find for the set of parameters that best adjusts the theoretical spectrum to the experimental, we used the Simulated Annealing (SA) method, where we minimize the mean square difference between the spectra. Throughout this work the main characteristics of the experimental spectra were reproduced. We also obtained important parameters such as the Crystalline Field, g value, Eu-Eu interaction exchange parameters and the asymmetry parameter of the absorption spectrum. We show that by increasing the concentration the spectrum evolves from a Lorentzian to Dysonian form, showing the existence of the metal-insulating transition.
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Palavras-chave
Ressonância magnética, Estreitamento da estrutura fina, Transição metal-isolante, Magnetic resonance, Narrowing of the fine structure, Metal-insulating transition