Magnetic structure of rhombohedral honeycomb antiferromagnets

We propose to determine details of the magnetic structure in the layered honeycomb easy-plane magnet CoTiO3, with ferromagnetic honeycomb layers stacked antiferromagnetically. In recent inelastic measurements on MERLIN we observed a spin gap at low energies, suggesting a preferential orientation of the moments. This is unexpected as no local anisotropy is allowed for Kramers ions and the three-fold symmetry of the crystal structure ensures that no anisotropic bilinear spin exchanges can select a unique moment orientation in the ab plane. The ground state selection could be due to quantum zero-point fluctuations, here we plan to search for evidence of a substantial reduction of the ordered moment expected if this mechanism was applicable. We also propose complementary and contrasting measurements on the easy-axis FeTiO3, where an almost full ordered moment is expected along the c-axis.