Helical magnetic structures and their responses to external magnetic fields in Yb(Ni1-xCux)3Al9 with a chiral crystal structure of the space group R32 have been investigated by resonant X-ray diffraction. It is shown that the crystal chirality is reflected in the helicity of the magnetic structure by a one-to-one relationship, indicating that an antisymmetric exchange interaction mediated via the conduction electrons exists. When a magnetic field is applied perpendicular to the helical axis (c axis), the second-harmonic peak of (0; 0; 2q) develops with increasing field. The third-harmonic peak of (0; 0; 3q) has also been observed for the x=0.06 sample. This result provides strong evidence for the formation of a chiral magnetic soliton lattice state, a periodic array of chiral twist of spins, which has been suggested by the characteristic magnetization curve. The helical ordering of magnetic octupole moments accompanying the magnetic dipole order has also been detected.

This work was supported by JSPS KAKENHI Grant Numbers 15K05175, 26400333, 16H01073, 15H05885, 17H02912, and 16KK0102. The synchrotron radiation experiment was performed under Proposals No. 2015B3711, No. 2016A3761, No. 2016B3762, and No. 2017A3787 at BL22XU of SPring-8. We also acknowledge support by the JSPS Core-to-Core Program, A. Advanced Research Networks.