Spin and orbital ordering in TlMnO3: Neutron diffraction study

Crystal and magnetic structures of the high-pressure stabilized perovskite phase of TlMnO3 have been studied by neutron powder diffraction. The crystal structure involves two types of primary structural distortions: a(+) b(-) b(-)octahedral tilting and antiferrodistortive type of orbital ordering, whose common action reduces the symmetry down to triclinic P (1) over bar The orbital pattern and the way it is combined with the octahedral tilting are different from the family of LnMnO(3) (Ln = lanthanide or Y) manganites who share with TlMnO3 the same tilting scheme. The experimentally determined magnetic structure with the k = (1/ 2,0,1/ 2) propagation vector and P-S (1) over bar symmetry implies anisotropic exchange interactions with a ferromagnetic coupling within the (1,0, (1) over bar planes and an antiferromagnetic one between them ( A type). The spins in the primary magnetic mode were found to be confined close to the (1,0, (1) over bar) plane, which underlines the predominant role of the single ion anisotropy with the local easy axes of Mn3+ following the Jahn- Teller distortions of the octahedra. In spite of the same octahedral tilting scheme in the perovskite structures of both LnMnO(3) and TlMnO3 manganites, a coupling of the secondary ferromagnetic component to the primary A-type spin configuration through antisymmetric exchange interaction is allowed in the former and forbidden in the latter cases.