We have studied optical and Raman spectroscopy of single crystal LuMnO3 at temperatures between 4 and 300 K. A symmetry allowed on-site Mn d-d transition near 1.7 eV is observed to blue shift (~0.1 eV) in the antiferromagnetic state due to Mn-Mn superexchange interactions. Similar anomalies are observed in the temperature dependence of the IR active TO phonon frequencies which arise from spin-phonon interaction. We find that the known anomaly in temperature dependence of the quasi-static dielectric constant ε0 below the TN ~ 90 K is overwhelmingly dominated by the phonon contributions .
All Raman phonons are resonant to an excitation energy just below the optical feature. The intensity of the low-energy modes, including the Lu (Y) phonon, correlate strongly with TN. Photoluminescence measurements show a spectral feature at ~1.4 eV, which corresponds to the radiative recombination to a Jahn-Teller split level in the optically excited state . Intensities of the narrow Raman modes and of a broad scattering anti-correlate with temperature around TN, and the photoluminescence intensity drops dramatically above TN. Both these observations reveal an important role of spin fluctuations in relaxation process in this frustrated magnetic system.
We have shown that optical measurements give important information about the exchange interactions, the spin-lattice interactions and the interaction between the ferroelectricity (Lu or Y ion) and magnetism (Mn ion) in hexagonal manganites.
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