Probing electronic, magnetic and structural heterogeneity in advanced materials and Nanostructures with x-ray imaging, scattering and spectroscopic techniques.

Abstract

In this dissertation, we have used a combination of synchrotron-based x-ray spectroscopic, scattering and imaging techniques to investigate the electronic, magnetic and structural properties of materials and material systems which exhibit natural as well as engineered nanoscale structural distortions. In order to investigate the interplay between the above-mentioned degrees of freedom with spatial and depth resolution, we have utilized non-destructive techniques, such as x-ray absorption spectroscopy (XAS), polarization-dependent photoemission electron microscopy (PEEM), nanoscale scanning x-ray diffraction microscopy (nano-SXDM) and standing-wave x-ray photoemission spectroscopy (SW-XPS). The results were compared to several types of state-of-the-art first-principles theoretical calculations. In the first part of the dissertation, we have investigated the nanoscale magneto-elastic structure of the Fe3Ga magnetic alloy, which was recently reported to exhibit non-volume conserving magnetostriction. As the result of our combined PEEM and nano-SXDM study, we have discovered the structural basis for this phenomenon – periodic long-wavelength (~269 nm) elastic domain walls, with domains (regions of zero-strain) existing as narrow transition regions. Atto-scale elastic gradients and self-strain across the elastic domain walls were quantitatively measured and imaged by nano-SXDM. Our measurements revealed that the gradients inside the elastic walls are accommodated by gradually increasing/decreasing inter-planar spacing resembling a longitudinal wave. Our element-specific polarization-dependent PEEM measurements revealed that the magnetic structure of the crystal modulates with similar periodicity (~255 nm), and the resulting magneto-elastic coupling produces a ‘giant’ field-induced bulk deformation, which is equal to the measured self-strain of the elastic domain wall. In the second part of this dissertation, we utilized a combination of soft x-ray standing-wave photoemission spectroscopy (SW-XPS), hard x-ray photoemission spectroscopy (HAXPES) and scanning transmission electron microscopy (STEM) to probe the depth-dependent and single-unit-cell resolved electronic structure of isovalent manganite superlattices (Eu0.7Sr0.3MnO3/La0.7Sr0.3MnO3)15 wherein the electronic and magnetic properties are intentionally modulated with depth via engineered O octahedral rotations and A-site displacements. Standing-wave-excited spectroscopy of the Mn 2p and O 1s core-levels confirmed the isovalent nature of the Mn ions in the superlattice and revealed significant depth-dependent variations in the local chemical and electronic environment around the O atoms, consistent with the state-of-the-art theoretical calculations. Furthermore, it was shown that a surface relaxation and orbital reconstruction in the several top Eu0.7Sr0.3MnO3 atomic layers produces substantial changes in the observed electronic structure, which, according to the first-principles theoretical calculations, occur due to the establishment of orbital stripe order in the top unit cell. In summary, we have used synchrotron-based x-ray spectroscopic and microscopic techniques, in conjunction with high-resolution electron microscopy, to study the electronic, magnetic and structural properties of advanced functional materials exhibiting strong nanoscale heterogeneity. We discovered a strong coupling between the nanoscale structural and magnetic properties in the non-conventional magnetostrictive Fe3Ga single crystal. Our results suggest that this coupling provides the fundamental basis for the non-conventional magnetostriction phenomenon in this material. We have also discovered that the electronic properties of the Eu0.7Sr0.3MnO3/La0.7Sr0.3MnO3 superlattices can be epitaxially tuned via engineered A-site cation displacement, which is a result of the strong interfacial coupling between the Eu0.7Sr0.3MnO3 and La0.7Sr0.3MnO3 layers. This suggests a new way of tailoring and spatially-confining electronic and ferroic behavior in complex oxide heterostructures and creating novel ordered surface-reconstruction effects.