EPR and optical measurements of weakly doped LiNbO3:: Er

Optical spectroscopy measurements of the congruent LiNbO3 (LN) single crystals, weakly doped with Er (0.1-0.3wt%) and Er (0.3 wt%) and Yb (0.5 wt%), have been carried out. The shape of the optical absorption and additional absorption bands registered after,gamma-irradiation suggests the presence of Er3+ ion pairs. EPR investigations were performed for LN single crystal doped with Er (0.1 wt%). Unusual behaviour of the temperature dependence of the intensity and linewidth of the main EPR line-corresponding to the fine transition of Er-even(3+) ions, is reported. The main EPR line appears to be a superposition of several paramagnetic centres originating from isolated Er-even(3+) ions and Er-even(3+)-Er-even(3+) pairs of ions. In low temperature region (below 20K), the main EPR signal is dominated by signals arising from Er-even(3+)-Er-even(3+) pairs of ions. The inverse intensity of the EPR line at low temperature region fulfils the Curie-Weiss law and enabled to determine the Curie-Weiss constant theta = 1.5 +/- 0.5K. The positive sign of 0 suggests that the ferromagnetic interactions arise in the system of Er-even(3+)-Er-even(3+) ion pairs in LN. Our results suggest that the distribution of Er ions in congruent LN is not homogeneous and Er impurity ions can form clusters in host lattice even in the case of weak erbium doping. (C) 2007 Elsevier B.V. All rights reserved.