Studies of Multiferroic System of LiCu2O2: II. Magnetic Structures of Two Ordered Phases with Incommensurate Modulations

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Neutron diffraction and 7Li-NMR have been applied to determine the three dimensional magnetic structures of the multiferroic system LiCu2O2, which has four chains (ribbon chains) of edge-sharing CuO4 square planes in a unit cell. First, we have confirmed that there are successive magnetic transitions at $T_{\text{N1}} = 24.5$ K and $T_{\text{N2}} = 22.8$ K. In the $T$ region between $T_{\text{N1}}$ and $T_{\text{N2}}$, the quasi one-dimensional spins ($S = 1/2$) of Cu2+ ions within a chain have a collinear and sinusoidally modulated structure with Cu-moments parallel to the $c$-axis and with the modulation vector along the $b$-axis. At $T < T_{\text{N2}}$, an ellipsoidal helical spin structure with the incommensurate modulation has been found. We present detailed parameters, describing the modulation amplitudes, helical axis vectors as well as the relative phases of the modulations of four ribbon chains, which can well reproduce both the NMR and neutron results, in the two magnetically ordered phases. This finding of the rather precise magnetic structures enables us to discuss the relationship between the magnetic structure and the multiferroic nature of the present system in zero magnetic field, as presented in our companion paper (paper I), and open a way how to understand the reported electric polarization under the finite magnetic field.
2009-08-15