Double-Q 120 degrees structure in the Heisenberg antiferromagnet on rhombohedrally stacked triangular lattice LiCrO₂

The magnetic ordering of the S=2/3 Heisenberg antiferromagnet on a rhombohedrally stacked triangular lattice LiCrO₂ is studied by susceptibility, neutron diffraction and polarization analysis measurements using a single crystal. The temperature dependence of the susceptibility strongly suggests that a 120 degrees structure is established in each quasi-two-dimensional layer, which is parallel to the c plane. Assuming the 120 degrees structure in which magnetic moments are confined in a plane including the c axis because of an Ising-type anisotropy, neutron scattering shows that the magnetic ordering is a double-Q structure with non-equivalent wave numbers q=(1/3 1/3 0) and (-2/3 1/3 1/2). It is characterized by an alternating sequence of rotational direction of the 120 degrees structure along the c axis.