Off-campus UNL users: To download campus access dissertations, please use the following link to log into our proxy server with your NU ID and password. When you are done browsing please remember to return to this page and log out.
Non-UNL users: Please talk to your librarian about requesting this dissertation through interlibrary loan.
Growth and Characterization of Hexagonal Rare-Earth Ferrites (h-RFeO3; R = Sc, Lu, Yb)
In bulk, rare-earth ferrites exist in orthorhombic phase which is a centrosymmetric, hence non-ferroelectric, structure. However, ferrites of rare-earths can be stabilized in hexagonal phase, which is a non-centrosymmetric and ferroelectric structure with space group P63cm, using epitaxial strains from substrates of appropriate surface symmetry. In this work, single-crystalline thin films of multiferroic hexagonal rare-earth ferrites (h-RFeO 3; R = Sc, Lu, Yb) have been grown using pulsed laser deposition method, and their structural and magnetic properties are investigated. Lattice distortions that accompany the transition from high temperature paraelectric hexagonal phase to low temperature ferroelectric hexagonal phase can be broken down into specific phonon modes: Γ2-,K 1 and K3. These distortions comprise tilting and rotation of FeO5 trigonal-bipyramids and buckling of planes containing rare-earth ion with the result of uneven displacement of charges and a net polarization. Effect of these distortions on exchange interactions between Fe3+ ions at the center of FeO5 are investigated at low temperatures. In addition, effects of biaxial strain on improper ferroelectricity of h-LuFeO3 are studied using restrained thermal expansion method. Strains were induced in h-LuFeO3 thin film using a 390 nm pump laser and then the structure was measured using an x-ray pulse. The variations in structure of h-LuFeO3 were studied as a function of strain. The results suggest that a compressive biaxial strain significantly enhances the K3 structural distortion (the order parameter of the improper ferroelectricity). The effect is larger at higher temperatures. The compressive biaxial strain and the enhanced K3 structural distortion together cause an increase in the electric polarization and a reduction in the canting of the weak ferromagnetic moments in h-LuFeO3, according to our first principles calculations. These findings are important for understanding the strain effect as well as the coupling between the lattice and the improper multiferroicity in h-LuFeO3. Based on these understandings, we applied chemical strain on hexagonal rare-earth ferrites using Sc as the R site, which has a significantly smaller atomic radius compared with Lu and Yb. The results show a dramatic increase of ordering temperature, which agrees with the predicted strain effect on the magnetism.
Condensed matter physics
Sinha, Kishan Kumar, "Growth and Characterization of Hexagonal Rare-Earth Ferrites (h-RFeO3; R = Sc, Lu, Yb)" (2018). ETD collection for University of Nebraska - Lincoln. AAI10793763.