Neutron diffraction study of the magnetic structure of Er2BaNiO5

Abstract

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 = [12,0,12]. 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.