Scanning probe microscopies applied to the study of the domain wall in a ferroelectric crystal.

Scanning near-field optical microscopy is capable of measuring the topography and optical signals at the same time. This fact makes this technique a valuable tool in the study of materials at nanometric scale and, in particular, of ferroelectric materials, as it permits the study of their domains structure without the need of chemical etching and, therefore, not damaging the surface (as will be demonstrated later). We have measured the scanning near-field optical microscopy transmission, as well as the topography, of an RbTiOPO(4) single crystalline slab, which exhibits two different of macroscopic ferroelectric domains. A chemical selective etching has been performed to distinguish between them, obtaining areas with a noticeable roughness (C(-) domain) in comparison with the original flat aspect of the other ones (C(+) domain). The effects of the selective chemical etching have been quantified in topographic images obtained by means of our fibre tip probe, and have been compared to topographic images obtained by Atomic Force Microscopy, with a better resolution. The near-field optical transmission images recorded have been obtained under different excitation wavelengths. These images are modulated by the light scattering due to the grains at the rough surface, which depends on the excitation wavelength used. In addition, they show a significant optical contrast due to the variations of the dielectric constant on the proximity of the ferroelectric domain wall.

Excitations of the orbital order in RMnO3 manganites: light scattering experiments.

Raman spectroscopy was used to seek out the orbital excitations in orbitally ordered RMnO3. The high energy Raman peaks are found to correspond to second and third order phonons which couple in some degree to the orbital order. Franck-Condon-type Raman resonance of multiphonons is observed but the cross sections are smaller than expected. A surging emission band at 1.5 eV, detected at high temperatures, disappears when the orbital order is established. The crossing of the configurational coordinate diagrams of the localized 3d Mn lowest levels, in the orbitally ordered state, originates the nonradiative relaxation and the quenching of the luminescence.