Conical Magnetic Structure and Atomic Displacements in Chiral Helimagnet Yb(Ni,Cu)${}_3$Al${}_9$ in Magnetic Fields along the Helical $c$ Axis

We investigated the conical magnetic state of a uniaxial chiral helimagnet Yb(Ni${}_{1-x}$Cu${}_x$)${}_3$Al${}_9$ induced in magnetic fields applied along the $c$ axis, which coincides with the helical axis at zero field. Using resonant X-ray diffraction, we clearly observed the disappearance of magnetic satellite peaks, corresponding to the transition from the conical to the field-induced ferromagnetic state. The critical fields were determined to be 4 T for $x=0$ and 7 T for $x=0.05$, which were hardly discernible in the magnetization curves. We also found that atomic displacements with the same propagation vector emerge simultaneously with the onset of the conical order. The transition temperature $T_{\text{N}}$ and the critical fields for $H\parallel c$ ($H_{\text{c}}^z$) and $H\perp c$ ($H_{\text{c}}^x$) are discussed on the basis of a mean-field calculation for a simple $q=1$ model of the magnetic structure. We propose that $T_{\text{N}}$ and $H_{\text{c}}^z$ primarily reflect the dominant intralayer exchange interactions within the honeycomb Yb-layer, whereas $H_{\text{c}}^x$ is governed by the much weaker interlayer coupling.