Crystal structure and magnetism of YbFeMnO 5: A neutron diffraction and Mössbauer spectroscopy study

Abstract

We have studied the crystal structure and magnetic properties of Y bFeMnO 5 obtained by substituting Fe 3+ for Mn 3+ in the parent Y bMn 2O 5 compound, through x-ray (XRD) and neutron (NPD) powder diffraction, magnetometry and Mössbauer spectroscopy. The samples were prepared in polycrystalline form by a soft chemistry route, followed by thermal treatments under high-oxygen pressure. The Rietveld analysis of diffraction data shows that Y bFeMnO 5 is isostructural with the oxides of stoichiometry RMn 2O 5 (R=rare earth, Y or Bi); the crystal structure is orthorhombic, Pbam space group, formed by chains of edge-sharing Mn 4+O 6 octahedra linked together by dimer groups of square pyramids Fe 3+O 5 and Y b 3+O 8 scalenohedra. A low level of disorder was established between the two transition metal positions 4 f and 4 h, occupied ideally by Mn 4+ and by Fe 3+: about 6% of Mn cations is replaced by Fe and 16% of Fe by Mn. Mössbauer spectroscopy data confirm the existence of two distinct crystallographic sites for Fe 3+. One of them corresponds to almost regular octahedra (at 4 f positions), characterized by nearly equal Mn/Fe–O distances of 1.890 Å at R T (from NPD data), giving a quadrupole doublet in the Mössbauer spectra at R T , broadened by the Fe/Mn disorder over this site. The second environment for Fe 3+ contributes to a less broadened, but more intensive doublet in the Mössbauer spectra, which corresponds to a distorted square pyramid Fe 3+O 5 (at 4 h sites), for which NPD data demonstrates an axial distortion with three sets of Fe–O distances at 2.010(2) Å, 1.859(5) Å and 1.925(3) Å. Magnetic studies and the thermal evolution of the NPD patterns show that below a transition temperature T c ∼ 178 K a long-range magnetic order is developed, resolved from NPD data as a ferrimagnetic structure with propagation vector k=0. The spin arrangements for the Mn 4+ ions ( 4 f site) and Fe 3+ ions ( 4 h site) are given by the basis vectors ( 0 , 0 , F z ) and ( 0 , 0 , F z ′ ), respectively. The Mn and Fe ordered magnetic moments are oriented along the c axis. At lower temperatures, the magnetic moment of the Y b 3+ cation also participates in the magnetic structure, adopting a parallel arrangement with the Mn 4+ spins; at T=1.8 K the magnitudes of the magnetic moments of the effective scatterers are 2.00(8), −3.04(17) and 0.35(14) μ B, over the 4 f , 4 h and 4 g sites respectively.