Distortion and Reconstruction Phenomena in Perovskite Manganite Thin Films

Perovskite manganites are known for their rich physics, which already in single crystals is determined by a complex interplay between magnetism, Jahn--Teller distortions, strong manganese multivalency, and the partly covalent character of the manganese--oxygen bonds. When grown as thin films, various emergent phenomena at interfaces could prove helpful for the development of novel oxide-based electronics. There is however a missing link between bulk and film properties, which can to some degree be ascribed to the fact that bulk and film samples often simply require different preparation methods and experimental probes. It should however be possible to achieve a better overall understanding with the help of theoretical methods describing the local MnO$_6$ cluster. Correspondingly, in this thesis we study the relation between the local electronic and structural ground state that also classifies bulk manganites, and the occurrence of emergent distortion and reconstruction phenomena in thin-film manganites. X-ray spectroscopic methods were used in conjunction with multiplet theory, where we combined several experimental probes to determine the properties of the manganese ions and examine valence reconstruction mechanisms, in doped and undoped LaMnO$_3$ thin films. Previous results with electron microscopy hint at the occurrence of electronic reconstruction phenomena in LaMnO$_3$ grown on SrTiO$_3$, which might produce a high-mobility two-dimensional electron gas similar to the one seen for LaAlO$_3$ on SrTiO$_3$. In this work, resonant x-ray reflectometry was used to obtain valence profiles with high precision. Films of thicknesses $\sim{}5$ and $12\;\text{nm}$ show strong indications for valence reconstruction phenomena of that kind, though no clear proof of electronic reconstruction can be brought forward. Bulk orthorhombic phases of La$_{1-x}$Sr$_x$MnO$_3$ variants with small Sr doping $x$ are known to be characterized by cooperative Jahn--Teller distortions that break the Mn$^{3+}$ $3d$ degeneracy, where the onset of long-range orbital order is controlled by the doping level and thermal fluctuations. We show that in a thin film, orbital order can also be established by the substrate, whereby a very small strain can lead to a strong orbital polarization with sizable $3d$ level splittings. Previous studies of bulk and film La$_{1-x}$Sr$_x$MnO$_3$ samples rendered partly conflicting results on the magnetic properties, where the reported Mn moments cover almost the full range between $0$ and $4\;\mu_\text{B}$. Here we establish a high--spin local moment around $3.7\;\mu_\text{B}$ for the Mn$^{3+}$ ion, which is largely immune to crystal field and doping and thus suggests dead layer scenarios explaining the small degrees of magnetization in some of the reported thin film systems.