NEUTRON-DIFFRACTION STUDY OF THE MAGNETIC-STRUCTURE OF ER2BANIO5

The magnetic ordering of R2BaNiO5 (R Y, Er) has been studied by neutron powder diffraction. The crystal structure of the Er compound was refined at T = 50 K, above the ordering temperature, in the orthorhombic space group Immm. The structure contains isolated chains of flattened NiO6 octahedra along the a axis. For Y2BaNiO5 no magnetic ordering has been detected between room temperature and 1.5 K. For Er2BaNiO5 the neutron diffraction patterns below 33 K show magnetic peaks that reach a maximum in intensity at T = 4 K. The magnetic structure is antiferromagnetic and can be described in terms of a single propagation vector k = [ 1 2,0, 1 2]. The group-theory analysis gives the possible magnetic structures compatible with the crystal symmetry. The structure that leads to a best fit of the neutron data involves only one irreducible representation for both Ni and Er sublattices. Ni atoms are antiferromagnetically coupled along the Ni-O-Ni chains parallel to the a axis direction. The coupling between chains is ferromagnetic along the b direction. The Er sublattice, constituted by two Er atoms per primitive cell, can be described as a (AxAz) mode. The magnetic moments for Ni2+ and Er3+ are 1.54(5) and 7.9(1) μB at T = 4 K, respectively. The thermal evolution of the magnetic moments suggests that the Ni-Er interactions are predominant between 33 and 16 K, and, at lower temperatures, cooperative Er-Er interactions lead to a saturation of the Er3+ ordered moments at T = 4 K. The different behaviour of the Y and Er compounds demonstrates the important role that the presence of a paramagnetic R3+ ion (e. g. Er3+) plays in the three-dimensional ordering of both Ni2+ and R3+ sublattices. © 1990.