Magnetic and ferroelectric materials have both had a very important impact in our society, not only because of the fascinating science behind the two phenomena, but also as a result of their use in many technological applications. The coupling and coexistence of these two order parameters within the same material opens the door to exiting new functional devices. Materials where magnetism and ferroelectricity coexist are known as multiferroic materials. In this thesis, muon spectroscopy and other complementary experimental techniques, including neutron scattering and resonant ultrasound spectroscopy, are used to investigate two di↵erent multiferroics. Muon and total neutron scattering studies have been performed on BiFeO3, one of the most studied multiferroic materials. Muon measurements reveal an anomaly in the temperature region of 200 - 220 K with a sudden and abrupt change in the muon’s precession frequency that corresponds to a process of muon di↵usion throughout the entire sample. The pair distribution function, calculated from total neutron scattering experiments on the compound, suggest that a change in the local structure of the material involving the bismuth-oxygen bond, in the same temperature region as the muon di↵usion sets in, is a strong indicative that there is a link between two in terms of the muon di↵usion being triggered by these local changes. Also, an extensive analysis and characterisation of the magnetic and ferroelectric properties of Ba4Dy0.87Nb10O30, an entirely new tetragonal tungsten bronze magnetoelectric material, is given. Neutron scattering and dielectric measurements are used to show that this material becomes ferroelectric below 470 K. We use muon spectroscopy and magnetic susceptibility measurements to investigate the magnetic properties of the material. Muon measurements under an applied electric field indicate that there is a strong coupling between the magnetism and ferroelectricity in the material. Resonant ultrasound spectroscopy is use to investigate whether the source of this coupling could be related to strain e↵ects. Magnetic neutron scattering measurements show that there is no long range ordering in the material.

Orientador: Keizo Yukimitu / Resumo: O novo sistema vítreo ZBT, 70 ZnO-30B2O3 (mol%) + xTe (wt%), sendo (x= 0, 1, 2, 3, 4, 5, 10, 20, 30, 40, 50) foi sintetizado por meio da técnica fusão resfriamento. As amostras foram tratadas termicamente numa temperatura de 450°C por um período de 3 h. Foi feito o estudo estrutural, térmico e óptico do sistema vítreo a partir das técnicas de Difratometria de Raios-X, Espectroscopias FT-IR e Raman, Análise Térmica Diferencial, Absorção óptica; Índice de refração e Determinação da densidade. O trabalho em questão tem como objetivo sintetizar uma matriz vítrea hospedeira que apresente boas características estruturais, térmicas e ópticas com potencial para aplicações tecnológicas. O índice de refração das amostras vítreas ZBT varia de acordo com a composição do material, a concentração com 0 Te (%wt) apresentou índice de refração de 1,48, enquanto que para concentração 50 Te Te (%wt) o índice de refração é de 1,84. Este aumento no índice de refração potencializa o material vítreo para aplicações ópticas. Tratamentos térmicos acima da Tg foram feitos a fim de estudar o comportamento térmico, em termos de estabilidade térmica e cristalização do vidro. Os vidros apresentaram boa estabilidade térmica, podendo ser aplicados em fibras ópticas. Espectroscopia Raman identificaram ligações O-Te-O; tetraedros ZnO; presença de anéis de boroxol com triângulos BO3 e tetraedros BO4, ligações Te-O- e vibrações de estiramento Te=O em TeO3+1 que indicam presença de oxigênios não ligantes (non-br... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: The new glassy system ZBT, 70 ZnO-30B2O3 (mol%) + xTe (wt%), being (x = 0, 1, 2, 3, 4, 5, 10, 20, 30, 40, 50) was synthesized by melting quentching. The samples were thermally treated at temperature of 450° C for 3 h. Structural, thermal and optical analysis of the vitreous system were carried out by of X-ray Diffraction, FT-IR and Raman Spectroscopy, Differential Thermal Analysis, Optical Absorption, Index of refraction and Determination of density techniques. The work in question aims to synthesize a host glass matrix that presents good structural, thermal and optical characteristics with potential technological applications. The refractive index of the ZBT glass samples varies according to the composition of the material, the concentration with 0 Te (% wt) had a refractive index of 1.48, whereas for the concentration 50 Te Te (% wt) the refractive index is 1.84. This increase in the refractive index enhances the vitreous material for optical applications. Thermal treatments above the Tg were done in order to study the thermal behavior in terms of thermal stability and glass crystallization. The glasses presented good thermal stability and could be applied in optical fibers. Raman spectroscopy has identified O-Te-O bonds; ZnO tetrahedra; presence of boroxol rings with BO3 triangles and BO4 tetrahedra, Te-O- bonds and Te=O stretching vibrations in TeO3+1 indicating the presence of non-bridging NBO's. FT-IR spectra reveal bands corresponding to ZnO, TeO3, TeO6, BO3, BO4. The ... (Complete abstract click electronic access below) / Doutor