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.
Categories: cond-mat.str-el