References of "Weber, Mads Christof 112016524C"
Bookmark and Share    
Full Text
Weber, Mads Christof UL

Doctoral thesis (2016)

The work of this thesis stands in the framework of the understanding of multiferroics and light induced effects in these materials, more specifically in rare-earth and bismuth ferrites. Iron-based ... [more ▼]

The work of this thesis stands in the framework of the understanding of multiferroics and light induced effects in these materials, more specifically in rare-earth and bismuth ferrites. Iron-based materials offer the advantage of a high magnetic-ordering temperature, commonly well-above room temperature. To understand the coupling between magnetism and crystal lattice and the interaction of a material with light, knowledge about the crystal structure and electronic band structure, respectively, is crucial. In the first part of this work, the structural properties of six rare-earth orthoferrites RFeO3 (R = La, Sm, Eu, Gd, Tb, Dy) are analyzed by Raman scattering (RS). Polarization dependent RS of SmFeO3 and the comparison with first-principle calculations enable the assignment of the measured phonon modes to vibrational symmetries and atomic displacements. This allows correlating the phonon modes with the orthorhombic structural distortions of RFeO3 perovskites. In particular, the positions of two specific Ag modes scale linearly with the two FeO6 octahedra tilt angles, allowing the distortion to be tracked throughout the series. At variance with literature, we find that the two octahedra tilt angles scale differently with the vibration frequencies of their respective Ag modes. This behavior, as well as the general relations between the tilt angles, the frequencies of the associated modes, and the ionic radii are rationalized in a simple Landau model. The precise knowledge about the lattice vibration is used in the second part of the work to investigate the impact of magnetic transitions on crystal lattice of SmFeO3. SmFeO3 stands out of the rest of the rare-earth orthoferrites for its comparably high magnetic transition temperatures. While tuning the temperature through magnetic transitions, the structural properties are probed by RS complemented by resonant ultrasound spectroscopy and linear birefringence measurements. During the Fe3+-spin reorientation phase, we find an important softening of the elastic constants in the resonant-ultrasound spectra. Towards lower temperatures the Sm3+-spins order; this ordering is clearly represented in the Raman spectra in the form of changes of the evolution of certain vibrational bands and additional bands appear in the spectra. The knowledge about the vibrational displacements of the Raman bands allows an investigation of the anomalies related to the Sm3+-spin ordering. Bismuth ferrite can be seen as the role model multiferroic material since it is one of the few room temperature multiferroics with a strong electric polarization. The ferroelectric and magnetic properties have been studied in great detail. In addition to the multiferroic properties, photo-induced phenomena have renewed the interest in BiFeO3. However, for the understanding and tuning of photo-induced effects a profound knowledge of the electronic band structure is important. Despite the extensive study of BiFeO3, the understanding of the electronic transitions remains very limited. In the third part of the thesis, the electronic band structure of BiFeO3 is investigate using RS with twelve different excitation wavelengths ranging from the blue to the near infrared. Resonant Raman signatures (RRS) can be assigned to direct and indirect electronic transitions, as well as in-gap electronic levels, most likely associated with oxygen vacancies. RRS allows to distinguish between direct and indirect transitions even at higher temperatures. Thus, it is found that the remarkable and intriguing variation of the optical band gap with temperature can be related to the shrinking of an indirect electronic band gap, while the energies for direct electronic transitions remain nearly temperature independent. [less ▲]

Detailed reference viewed: 484 (18 UL)