Abstract
NMR investigation of magnetic structure and phase transitions in two isostructural quasi-one-dimensional cuprates LiCu 2O 2 and NaCu 2O 2 has been performed. LiCu 2O 2 exhibits a unique sequence of phase transitions at T c = 2 4 , 22.5 and 9 K. NMR evidence of low temperature incommensurate (IC) in-chain spin structure in LiCu 2O 2 has been obtained. It is shown that below the magnetic ordering temperature T c = 2 4 K the 7Li 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 θ and φ shows good agreement with experimental 7Li 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 LiCu 2O 2 at T < 9 K . Therefore the transition at 9 K can be assigned to freezing out the phason-like fluctuations. First 23Na NMR measurements confirmed the existence of IC magnetic structure in NaCu 2O 2 at 3 K.
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