A double-spiral spin ordering in the helimagnet YBaCuFeO${}_5$

Abstract

Materials with a spiral spin ordering always show a rich phase diagram and can be a playground for studying the exotic physical properties associated with spiral magnetism. Using neutron elastic and resonant x-ray scattering on a high-quality single crystal YBaCuFeO${}_5$, we demonstrate YBaCuFeO${}_5$ to be a helimagnet consisting of a double-spiral spin ordering. YBaCuFeO${}_5$ undergoes a commensurate to incommensurate magnetic phase transition at $T_{N2}$$\sim$ 175 K, and the incommensurate phase consists of two spin-ordered components. Both components have different periodicities but with the same propagating direction along the {\it c}-axis below $T_{N2}$. Using resonant x-ray scattering at the Fe and Cu \textit{K}-edges, we further demonstrate that both spiral spin orderings result from the Fe${}^{3+}$ and Cu${}^{2+}$, respectively, forming a double-spiral spin ordering structure. This can be understood to be caused by the coupling between both sublattices of Fe${}^{3+}$ and Cu${}^{2+}$ with the atomic lattice.

29 pages, 6 figures