Studies of multiferroic system LiCu2O2. 1. Sample characterization and relationship between magnetic properties and multiferroic nature

Single-crystal samples of LiCu2O2 with spin 1/2 Cu2+ chains of edge-sharing CuO4 square planes (ribbon chains), have been characterized by X-ray diffraction, thermogravimetric analysis, and magnetic measurements. Neither the atomic deficiency nor the mixing of Cu and Li atoms has been found, indicating that lattice defects conceived as an origin of the reported multiferroic behavior can be excluded. Observations of clear anomalies found in the temperature (T) dependence of the specific heat and neutron integrated intensities of the magnetic Bragg reflections have presented clear confirmations that the system exhibits successive magnetic transitions at TN1 - 24.5 K and TN2 - 22.8 K. The magnetization-magnetic field (H) curve indicates that no spin-flop transition exists, at least, up to H=5.5 T. On the bases of these data and the magnetic structures obtained by our group at temperatures T < TN2 and TN2 < T < TN1, arguments on the multiferroic properties of the system are presented. (author)