Helicoidal magnetic order in a clean copper oxide spin chain compound

squares have been shown toexhibit quasi-one-dimensional behavior. Two classes ofcopper oxide spin chain materials are known. Com-pounds in which adjacent squares share their corners areexcellent realizations of the one-dimensional (1D) spin-1/2 Heisenberg Hamiltonian [1, 2, 3]. Linear Cu-O-Cubonds along the spin chains give rise to a large antifer-romagnetic nearest-neighbor exchange coupling. In com-pounds built up of edge-sharing squares, on the otherhand, the Cu-O-Cu bond angle is nearly 90