Incommensurate helix magnetic order in quasi-1D chain cuprates LiCu2O2 and NaCu2O2 as seen by NMR

NMR investigation of magnetic structure and phase transitions in two isostructural quasi-one-dimensional cuprates LiCu2O2 and NaCu2O2 has been performed. LiCu2O2 exhibits a unique sequence of phase transitions at T-c = 24, 22.5 and 9 K. NMR evidence of low temperature incommensurate (IC) in-chain spin structure in LiCu2O2 has been obtained. It is shown that below the magnetic ordering temperature T-c = 24 K the Li-7 NMR lineshape is characteristic for an IC static modulation of the local magnetic field caused by the helical spin structure of Cu magnetic moments twisted along the chain axis. The proposed helix structure model involving modulation of both helix angles theta and phi shows good agreement with experimental Li-7 line profile. The absence of the central peak at 4.2 K provides unambiguous evidence that the plane wave IC regime is well established for helix magnetic structure of LiCu2O2 at T<9K. Therefore the transition at 9 K can be assigned to freezing out the phason-like fluctuations. First Na-23 NMR measurements confirmed the existence of IC magnetic structure in NaCu2O2 at 3 K. (C) 2005 Elsevier B. V. All rights reserved.