Abstract
Multiferroic BiFeO3 crystals were investigated by means of micro-Raman spectroscopy using the laser wavelengths of 442 nm (resonant conditions) and 633 nm (non-resonant conditions). The azimuthal angle dependence of the intensity of the Raman modes allowed their symmetry assignment. The experimental data are consistent with a simulation based on Raman tensor formalism. Mixed symmetries were taken into account, considering the orientation of the crystal optic axis along a pseudo-cubic <111> direction. The strong anisotropic intensity variation of some of the polar Raman modes was used for line scans and mappings in order to identify ferroelastic domain patterns. The line scans performed with different excitation wavelengths and hence different information depths indicate a tilt of the domain walls with respect to the sample surface. The domain distribution found by Raman spectroscopy is in very good agreement with the finding of electron back scattering diffraction.
Highlights
Due to its multiferroic properties at room temperature, the visible-light photovoltaic effect, and related potential applications, BiFeO3 (BFO) attracts a lot of scientific interest[1,2,3,4]
In this work we present a method based on the Raman tensor formalism, which allows an assignment of the BFO Raman modes of pure as well as mixed character/symmetry
Piezoresponse force microscopy indicates that for both domains the polarisation points downwards along pc diagonal, which means that 71° domain walls should be formed[31,33,34]
Summary
Due to its multiferroic properties at room temperature, the visible-light photovoltaic effect, and related potential applications, BiFeO3 (BFO) attracts a lot of scientific interest[1,2,3,4]. It should be mentioned that the Raman tensor approach was only applied until now for pure (not mixed) modes This restriction corresponds to the cases θ = 0° and θ = 90° (where θ is the angle between the phonon propagation and optic axis), i.e. the case of (111)pc oriented BFO. The method presented in this work analyses the in-plane (i.e. in the crystal surface plane) variation of the Raman intensities upon azimuthal rotation of a single crystal instead of the phonon frequency dispersion (which was reported in ref.[24]) Since it is based on the Raman intensity analysis, the Raman tensor method presented here is highly sensitive to the in-plane orientation of the optic axis projection onto the sample surface for any angle θ between the phonon propagation (laser beam direction) and the optic axis. Thanks to the sensitivity of the Raman signal to the domain orientation that is combined with the depth information when using two appropriate laser wavelengths for excitation, Raman spectroscopy can be used for domain imaging in BFO, as an alternative to other methods such as birefringence[29] or circular dichroism photoemission[30]
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