Neutron diffraction and polarimetric study of the magnetic and crystal structures of HoMnO3 and YMnO3

We have undertaken a new study of the magnetic and crystalline structures of YMnO3 and HoMnO3 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 YMnO3 has space group P6 '(3) (or P6(3)) rather than P6 '(3)cm ' (or P6(3)cm) with Mn moments of 3.14(3) B inclined at 11(1). to [10.0] (or [12.0] for P63) in the (00.1) plane. For HoMnO3 the experiments confirm the P6 '(3)c ' m symmetry at 50 K and the spin-rotation transition leading to a low-temperature structure with magnetic space group P6 '(3)cm '. In this structure both the Ho1 and Ho2 moments can order with spins parallel to [00.1] in ferromagnetic layers, antiferromagnetically 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) mu(B) on the Mn, Ho1 and Ho2 atoms respectively. No evidence was found, in this zero-field study, for the magnetic transition at approximate to 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 YMnO3 at 10 K and in HoMnO3 at all temperatures below 50 K any deviation of the manganese x parameter from 1/3 is less than 0.0008.