Helical ground state and weak ferromagnetism in the edge-shared chain cuprate NaCu2O2

We report on susceptibility, magnetization, 23Na NMR, and specific heat data of the spin-chain material NaCu2O2 in the paramagnetic and ordered phases. Below 13 K, where a sharp field-dependent specific heat peak appears, the NMR lineshape points to an incommensurate static modulation of the local magnetic field consistent with a spiral state of the Cu magnetic moments. At 2 K weak ferromagnetism with an ordered moment of about 4*10- 3muB has been observed. LDA-based estimates of exchange integrals reveal a large inchain frustration leading to a magnetic spiral.