Factors Governing the Propagation Direction and Spin-Rotation Plane of Noncollinear Magnetic Structures: A Helix vs Cycloid in Doubly Ordered Perovskites NaYMnWO and NaYNiWO.

The magnetic structure of NaYMnWO was determined by neutron powder diffraction measurements. Below 9 K, the magnetic structure is a helix to wave vector = (0, 0.447, /), in contrast with NaYNiWO, which shows a transverse spin density wave with = (0.47, 0, 0.49). By analyzing the differences in the spin exchanges of NaYMnWO and NaYNiWO, and in the magnetic anisotropies of the Mn (, = /) and the Ni (, = 1) ions, we show what factors govern the propagation direction of a noncollinear magnetic structure and whether it becomes a helix or a cycloid.