Magnetic structure of the edge-sharing copper oxide chain compound ${\text{NaCu}}_2{\text{O}}_2$

Single-crystal neutron diffraction has been used to determine the incommensurate magnetic structure of ${\text{NaCu}}_2{\text{O}}_2$, a compound built up of chains of edge-sharing ${\text{CuO}}_4$ plaquettes. Magnetic structures compatible with the lattice symmetry were identified by a group-theoretical analysis and their magnetic structure factors were compared to the experimentally observed Bragg intensities. In conjunction with other experimental data, this analysis yields an elliptical helix structure in which both the helicity and the polarization plane alternate among copper-oxide chains. This magnetic ground state is discussed in the context of the recently reported multiferroic properties of other copper-oxide chain compounds.

Figure 1

(Color online) Powder diffraction data on ${\text{NaCu}}_2{\text{O}}_2$ taken on D2B with a neutron wavelength $\lambda =1.596\text{Å}$ at $T=1.5\text{K}$. The line through the data is the result of a standard Rietveld refinement. The lower curve indicates the corresponding residuals. Tick marks indicate the positions of nuclear Bragg reflections.

Figure 2

Temperature dependence of the intensity of the magnetic Bragg reflection $\left(0.5,-0.228,-1\right)$ in ${\text{NaCu}}_2{\text{O}}_2$, taken on the Laue diffractometer Vivaldi. The inset shows a rocking curve taken at $T=2\text{K}$ on the four-circle diffractometer D10.

Figure 3

(Color online) Proposed magnetic structures for ${\text{NaCu}}_2{\text{O}}_2$. Model 3 yields competitive residuals in the refinement of the neutron-diffraction data (Table ) and is consistent with other experimental data on ${\text{NaCu}}_2{\text{O}}_2$ (see the text). The arrows indicate the sense of rotation of helices on different copper-oxide chains.