Interplay between magnetic and dielectric phenomena at transition metal oxide interfaces

The present work is concerned with the preparation, characterization and analysis of thin lm heterostructures of perovskite oxide materials. Two di erent systems have been analyzed in detail: La0.66Sr0. 33MnO3/SrTiO3 (LSMO/STO) heterostructures have been investigated in order to understand the unusual occurrence of an exchange bias e ect in multilayers of these two oxides [1]. Monocrystalline LSMO single and LSMO/STO bilayers have been grown on STO by both High Oxygen Pressure Sputter Deposition (HSD) and Pulsed Laser Deposition. It was possible to reproduce the Exchange Bias e ect in the samples grown by HSD by reducing the oxygen pressure during the layer growth. In fact, the size of the e ect can be increased by further reduction of the oxygen pressure. The macroscopic sample analysis by X-ray Di raction and Vibrating Sample Magnetometry suggests that the occurence of the Exchange Bias e ect is linked to oxygen de ciencies in the LSMO layer. By combining X-ray Re ectometry, Polarized Neutron Re ectometry and X-ray Resonant magnetic Scattering (XRMS), the magnetic depth pro le of the samples has been determined. By this, a region in LSMO at the interface to STO has been detected, where the magnetic moment is strongly suppressed. By putting together the results of the macroscopic sample analysis and the scattering experiments, an explanation for the occurence of the e ect can be given: It is proposed, that a combination of strain and oxygen de ciencies shifts the LSMO at the interface in the antiferromagnetic phase of the LSMO strain vs. doping phase diagram. This interface region thus couples to the ferromagnetic part of the LSMO causing the Exchange Bias e ect. The second heterostructure system under investigation in this work are bilayers of La0.5Sr0.5MnO3 (LSMO) and BaTiO3 (BTO). A possible dependence of the interface near magnetic structure of La1-xSrxMnO3 having a doping level x close to the ferromagnetic-antiferromagnetic phase diagram boundary on the orientation of the electric polarization in the BTO lm was proposed theoretically [2]. Therefore, here it was the aim to analyze to what extend these predictions can be reproduced experimentally. Epitaxial La0.5Sr0.5MnO3/BaTiO3 bilayers have been grown by Molecular Beam Epitaxy on conductive Nb doped STO substrates. It was found that the magnetic properties of the LSMO thin lm highly depend on the stacking order of the bilayer: the sample with LSMO on top of BTO shows antiferromagnetic, the one with BTO on top of LSMO ferromagnetic behavior. This can be explained by the di erent strain states present in the two con gurations and proves that the doping level x in the La1-xSrxMnO layer is very close to the nominal value of 0.5. The magnetic depth pro le in the samples has been determined by XRMS. The simulations reveal a continuous magnetization in the ferromagnetic sample and a small net magnetization in the antiferromagnetic sample probably is created by canted magnetic moments due to the applied magnetic eld during the experiment. The experiment reveals that this small net moment is suppressed at the interface to air, which can be explained by the reduced dimensionality close to the surface. Therefore, this XRMS experiment reveals that it is possible with this technique to resolve magnetic pro les in samples with very small magnetic moments. So far, no dependence of the magnetic pro le on the application of electric elds has been detected. Besides the possibility that the proposed e ect cannot be reproduced experimentally in general, it is also possible that the electric polarization in BTO is not coherently switchable in the samples prepared in this work. The characterization and improvement of the ferroelectric properties needs to be the focus for future investigations on this system.