IMF2017

Advances in micro-Raman spectroscopy with multiple-wavelength excitations are allowing nondestructive and in situ measurements of chemistry and structures of materials with molecular level sensitivity. A key parameter of a Raman system is excitation wavelength, which affects both depth of penetration into the sample (which is proportional to the laser wavelength) and Raman scattering intensity (inversely proportional to the fourth power of laser wavelength). UV (325 nm) laser excitation Raman spectroscopy has been used successfully to investigate lattice dynamics and phase transitions in thin films and superlattices of Sr- and Ba-titanates. In this presentation, micro-Raman investigations of chemically synthesized nanocrystalline cobalt-substituted (up to 10 atomic%) ZnO samples exhibiting room temperature ferromagnetism are discussed. These studies were carried out using multiple laser excitation wavelengths − 457.9, 532 and 785 nm. The Raman spectra of the samples excited with 457.9 nm show predominantly lattice modes of ZnO because of pre-resonance enhancement. In the Raman spectra of cobalt-doped ZnO samples excited with 532 and 785 nm, clear evidence is found for the presence of Co₃O₄ (an impurity phase not detectable in XRD) in the samples with cobalt exceeding 3%. Results of micro-Raman measurements on ferroelectric Li-doped and Li-implanted ZnO, and (Co, Li) co-implanted ZnO films, which show simultaneous ferromagnetic and ferroelectric properties at room temperature, will be discussed.