Thin film multiferroic oxides with co-existing ferroelectric and ferromagnetic ordering have attracted much interest in recent years, partly as a result of the enhancements achieved through the adoption of strained thin film geometries. This thesis presents work on two such thin film oxides; lanthanide substituted BiFeO<sub>3</sub> and Fe substituted PbTiO<sub>3</sub>. Coherent magnons and acoustic phonons were impulsively excited and probed in thin films of the room temperature multiferroic Bi<sub>1-x-y</sub>Dy<sub>x</sub>La<sub>y</sub>FeO<sub>3</sub> using femtosecond laser pulses. The elastic moduli of rhombohedral, tetragonal and rare-earth doped BiFeO<sub>3</sub> were determined from acoustic mode frequencies in conjunction with spectroscopic ellipsometry. A weak ferromagnetic order, induced alternately by magnetization in the growth direction or by tetragonality, created a magnon oscillation at 75 GHz, indicative of a Dzyaloshinskii-Moriya interaction energy of 0.31 meV. Bulk crystals and thin films of PbTi<sub>1-x</sub>Fe<sub>x</sub>O<sub>3</sub> (PTFO) are multiferroic, exhibiting ferroelectricity and ferromagnetism at room temperature. Here we report that the Ruddlesden-Popper phase Pb<sub>n+1</sub>(Ti<sub>1-x</sub>Fe<sub>x</sub>)<sub>n</sub>O<sub>3n+1</sub> forms spontaneously during pulsed laser deposition of PTFO on LaAlO<sub>3</sub> substrates. High-resolution transmission electron microscopy, x-ray difraction and x-ray photoemission spectroscopy were utilised to perform a structural and ompositional analysis, demonstrating that n≃8 and x≃0.33. The complex dielectric function of the films was determined from far-infrared to ultraviolet energies using a combination of terahertz time-domain spectroscopy, Fourier transform spectroscopy, and spectroscopic ellipsometry. The simultaneous Raman and infrared activity of phonon modes, and the observation of second harmonic generation, establishes a non-centrosymmetric point group for Pb<sub>n+1</sub>(Ti<sub>0.67</sub>Fe<sub>0.33</sub>)<sub>n</sub>O<sub>3n+1-δ</sub> consistent with ferroelectricity. No evidence of macroscopic ferromagnetism was found in SQUID magnetometry. The ultrafast optical response exhibited coherent magnon oscillations compatible with local magnetic order, and additionally was used to study photocarrier cooling on picosecond timescales. An optical gap smaller than that of BiFeO<sub>3</sub> and long photocarrier lifetimes may make this system interesting as a ferroelectric photovoltaic.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:658478 |
| Date | January 2014 |
| Creators | Doig, Katie I. |
| Contributors | Lloyed-Hughes, James; Radaelli, Paolo |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://ora.ox.ac.uk/objects/uuid:e44e0a2a-a675-4923-90de-a1bdfa24f184 |
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