Neutron Diffraction Study of 1D Quantum Spin System Li2ZrCuO4 with Incommensurate Magnetic Structure

Magnetic structure has been studied by using neutron diffraction for quasi one-dimensional quantum spin system Li2ZrCuO4 with CuO2 ribbon chains which are formed of edge-sharing CuO4 square planes. Due to the geometrical characteristic of the crystal structure of CuO2 ribbon chains, the nearest-neighbor exchange interaction J1 between spins is ferromagnetic, and the second neighbor interaction J2 is antiferromagnetic. If the spin system exhibits a magnetic transition under these conditions, it has often a helical magnetic structure, and is often accompanied with ferroelectricity, which is called multiferroic behavior. We have found that LiVCuO4 and PbCuSO4(OH)2 with the CuO2 ribbon chains show the helical magnetic structure and multiferroic behavior [1,2]. The CuO2 ribbon system Li2ZrCuO4 exhibits the antiferromagnetic transition at TN~7 K [3], and the magnetic structure of this system was proposed to be incommensurate helical type by Li-NMR measurements [4]. In Li2ZrCuO4 however, the magnetic transition is not accompanied with ferroelectricity [4]. Here, the powder neutron diffraction measurement has been carried out for Li2ZrCuO4 using the high-resolution powder diffractometer (HRPD) installed at JRR-3 in Tokai. At 3K, we observed magnetic superlattice reflections at Q = (0, k, l±δ) and (h, 0, l±δ) (h, k and l = even) with δ~0.488. By the combined studies of neutron diffraction and magnetization data, the magnetic structure of Li2ZrCuO4 is proposed. On the bases of the obtained data, reasons for absence of ferroelectricity in Li2ZrCuO4 are discussed.