Modelização e estudo experimental de óxidos multiferróicos

The main goal of this work was the theoretical and experimental study of multiferroics oxides. This particular approach focused mainly on the ferroelectric material barium titanate. The essence of this theoretical study is the ab initio calculation of hiperfine parameters using the Wien2k software. This software, which uses the selfconsistent $LAPW$ method to solve the Kohn-Sham equations, allowed to determine the electric field gradient and the BaTiO$_3$ system's total energy in its different structural phases, and study its behaviour in function of the different lattice parameters. The modeling of stratified systems of the (XTiO$_3$).(BaTiO$_3$)$_{n-1}$ and (BaZO$_3$).(BaTiO$_3$ )$_{n-1}$, with X=Cd and Hg and Z=Fe, from n=2 to n=6 type, built from the experimental latice parameters (formula) in the tetragonal sage was also made. This simulation can be related to the physical situation of the epitaxial growth of XTiO$_3$ a layer on n BaTiO$_3$ layers. It was also studied the effect of the Cd ion volume dilution in the barium titanate stucture replacing the Cd in a Ba site and the dilution of the Fe ion in a Ti site, in order to compare with hyperfine properties experimental studies in which the probe is diluted in a low concentration. The perturbed angular correlation spectroscopy, PAC, is the experimental technique analyzed in this dissertation. Besides the exposition of the theoretical of the PAC technique, a brief description of the radioactive isotope production process and its implantation in the studied samples was conducted. The measuring system's operating principle and the data analysis procedure were also explained. This nuclear spectroscopy technique was applied in the study of the bulk BaTiO$_3$ measured at different temperatures (10K to 420K, cobring for structural phases) and in a film deposited on a SrTiO$_3$ substrate analyzed at the room temperature. The experiment was conducted at the ISOLDE lab in CERN, and consisted on the implantation of the Cd radioactive isotope in the BaTiO$_3$ where it performed as an atomic level local probe. It was possible to determine the electric field gradient of the material using a six detectors measuring system. The results are discussed and compared with the theoretical modelization and agree in order of magnitude. The annealing of defects was not optimized.