Magnetostrição como mecanismo do efeito magnetodielétrico em La2CoMnO6

Abstract

The compound La2CoMnO6 has regained academic and industrial interest due to the manifestation of a magnetodielectric effect at temperatures close to room temperature. Despite recent efforts by the scientific community, the physical mechanisms governing this effect are still not fully understood. In this thesis, the microscopic origin of the magnetodielectric response of this double-perovskite oxide was investigated, with emphasis on the role played by the magnetoelastic coupling observed in the material. The central objective of this work was to establish the connection between magnetic-field-induced structural deformations and the reorganization of the electronic levels of the transition metals in the compound, in order to elucidate how this chain of effects manifests itself in the magnetodielectric effect. Through X-ray diffraction experiments under applied magnetic field and capacitive dilatometry, a giant and anisotropic negative magnetostriction was observed. This magnetoelastic response altered structural parameters that are relevant to orbital hybridization between the transition metals and neighboring oxygen ions, such as the (Co/Mn)–O bond lengths and the Co–O–Mn bond angles. X-ray absorption experiments showed that the magnetic-field-induced changes in the electronic structure of Mn are negligible; whereas for Co significant variations were observed in metal–oxygen hybridization and in the degree of charge transfer with the ligand. Density functional theory calculations indicated that lattice-contraction effects induced by the applied magnetic field affect the spatial distribution of the Co-related electronic density in the vicinity of the Fermi level, impacting polar regions associated with the dielectric response of the material. Therefore, it was possible to construct a plausible scenario to explain a possible pathway for the magnetodielectric effect in the studied compound, demonstrating that, in this material, there is a strong correlation among the elastic, magnetic, and electronic properties.