Magneto-optical spectroscopy and domain imaging of functional oxides /

• Autores: Blai Casals Montserrat
• Directores de la Tesis: Gervasi Herranz Casabona (dir. tes.), Javier Rodríguez Viejo (tut. tes.)
• Lectura: En la Universitat Autònoma de Barcelona ( España ) en 2017
• Idioma: español
• Tribunal Calificador de la Tesis: Andrei I. Kirilyuk (presid.), Antonio García Martín (secret.), John Martin Gregg (voc.)
• Programa de doctorado: Programa de Doctorado en Ciencia de Materiales por la Universidad Autónoma de Barcelona
• Materias:
  • Física
    • Electromagnetismo
      • Magnetismo
    • Óptica
      • Propiedades ópticas de los sólidos
• Enlaces
  • Tesis en acceso abierto en:  DDD  TDX 
• Resumen

  • In this Thesis we have investigated the physical properties of different systems based on functional transition metal oxides. These materials show a large variety of properties –magnetism, ferroelectricity, superconductivity– that makes them interesting to explore novel devices beyond today's electronic and photonic technologies. The driving force of our study has been the understanding of the fundamental physical processes that explain a series of observed phenomena. In some cases, our interest has been to understand why a particular material shows an atypically large magneto-optical activity; in other cases, why and how ferroelastic domains move under electric fields; finally, we have turned our attention to surface acoustic waves and their interaction with magnetic materials. In spite of the disparity of materials and properties there is a common thread: our most important methodology to research these systems has been the use of light and, more specifically, the exploitation of magneto-optical spectroscopy and optical imaging. At the same time, efforts have been aimed at building, wherever possible, appropriate theoretical models that describe the experimental data.

    In the following, a concise and brief account is given of the most relevant outcomes of the research described in this Thesis:

    (i) We have determined the intrinsic contribution to the magneto-optical activity of polarons in manganites. Our study has revealed a large magneto-optical response in the visible, almost two orders of magnitude larger than the background response of the material and comparable to photonic- or plasmonic-mediated magneto-optical enhancement. Additionally, we have identified the photoinduced electronic transitions responsible for the intrinsic magneto-optical activity of self-trapped polarons. This finding opens new perspectives to explore other pathways to obtain large magnetoelectric effects, using magneto-optics instead of magnetic properties.

    (ii) We have analyzed the incorporation of Cerium into Yittrium Iron Garnet (YIG) and the consequences of this doping on the magnetic and electronic properties of YIG. Summarizing, our results show that Ce-doping triggers a selective charge transfer from Ce to the Fe tetrahedral sites in the YIG structure. This, in turn, causes a disruption of the electronic and magnetic properties of the parent compound, reducing the exchange coupling between the Ce and Fe magnetic moments and causing atypical magnetic behavior. Our findings represent an important step forward for the comprehension of the physical processes that determine the optical properties of YIG-based compounds. This specially relevant, taking into account that these materials are nowadays present in commercial devices in optical communication technologies.

    (iii) We have used optical and magneto-optical imaging to analyze the spatial distributions of ferroelastic twin domains in SrTiO3 crystals under the application of in-situ applied electric fields. Our work has enabled us to identify the sign of the anisotropy of the low-temperature dielectric behavior of SrTiO3. Interestingly, the theoretical frame that we have developed to describe this anisotropy indicates the essential role of the emergence of an antiferroelectric displacement of the Ti ions that couples to polar and antiferrodistortive lattice modes. Our observations are very relevant for applications where understanding and controlling the distribution of all types of ferroelastic domains is essential for nanotechnology design.

    (iV) Finally, we have used magneto-optical microscopy to access the magnetic properties of the individual piezoelectric/magnetic microstructured magnetoelectric devices. Specifically, we have studied the effects of surface acoustic waves (SAW) propagating on a piezoelectric LiNbO3 on the magnetic properties of microstructured Pt/Co/Pt squares with perpendicular magnetic anisotropy. Our results show that SAW can induce large changes in the magnetic coercive field, up to 80 % of the initial value. By using a thermal proximity scanning probe we have shown that the changes in the magnetic properties are largely due to an intrinsic SAW-induced heat dissipation.