Neutron diffraction and polarimetric study of the magnetic and crystal structures of HoMnO3and YMnO3

We have undertaken a new study of the magnetic and crystalline structures of YMnO 3 and HoMnO 3 using neutron diffraction combined with neutron polarimetry. It is shown how the long-standing problem of distinguishing between magnetic structure models which give nearly identical diffraction intensities may be resolved. The experiments show that the magnetic structure of YMnO 3 has space group P 6 3 ′ (or P 6 3 ) rather than P 6 3 ′ cm ′ (or P 6 3 cm ) with Mn moments of 3.14(3) μ B inclined at 11(1)° to [10.0] (or [12.0] for P 6 3 ) in the (00.1) plane. For HoMnO 3 the experiments confirm the P 6 3 ′ c ′ m symmetry at 50 K and the spin-rotation transition leading to a low-temperature structure with magnetic space group P 6 3 ′ cm ′. In this structure both the Ho1 and Ho2 moments can order with spins parallel to [00.1] in ferromagnetic layers, anti-ferromagnetically coupled. The Ho1 and Ho2 moments in neighbouring layers are found to be oppositely oriented. The ordered moments at 2 K are 3.32(8), 4.17(13) and 1.31(11) μ B on the Mn, Ho1 and Ho2 atoms respectively. No evidence was found, in this zero-field study, for the magnetic transition at ≈5 K reported in the literature (Lottermoser et al 2004 Nature 30 541; Vajk et al 2005 Phys. Rev. Lett. 94 87601). Nuclear structure refinements show that in YMnO 3 at 10 K and in HoMnO 3 at all temperatures below 50 K any deviation of the manganese x parameter from 1/3 is less than 0.0008.