Synthesis, structural study and magnetic properties of TbFeMnO 5

Abstract

We describe the preparation and characterization of a new ferrimagnetic oxide of composition TbFeMnO 5. It has been prepared by a soft-chemistry procedure, involving the thermal treatment at moderate temperatures (600 °C) of citrate precursors, subsequently annealed at 900 °C under high oxygen pressure (200 bar). The crystallographic and magnetic structures have been studied by neutron powder diffraction (NPD) experiments, in complement with magnetization and susceptibility measurements. TbFeMnO 5 is an orthorhombic, Pbam space group; the crystal structure contains infinite chains of Mn 4+O 6 edge-sharing octahedra running along the c axis and interconnected by Fe 3+O 5 pyramids and TbO 8 units. Some anti-site disorder is observed from the NPD refinements: about 9% of the 4 f octahedral sites are occupied by Fe 3+ cations and about 18% of the pyramidal 4 h positions contain Mn 4+. These structural features are confirmed by Mössbauer spectroscopy: two sites for Fe 3+ are identified; one of them corresponds to almost regular octahedra (at 4 f positions), characterized by nearly equal Mn/Fe–O distances of 1.906Å at RT (from NPD data), giving a quadrupole doublet in the Mössbauer spectra at RT; the second contributes to a more intensive doublet corresponding 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 Mn/Fe–O distances at 1.981(4), 1.851(5) and 1.944(3) Å. Both doublets are broadened by the Fe/Mn disorder over these sites. TbFeMnO 5 undergoes long-range magnetic ordering below T C ≈ 175 K , as shown from magnetization, NPD and Mössbauer spectroscopy data. The propagation vector k = 0 defines a ferrimagnetic structure that initially concerns the Mn 4+ and Fe 3+ cations, in a magnetic arrangement defined by the basis vectors ( 0 , 0 , F z ) and ( 0 , 0 , F z ′ ) respectively. The Tb 3+ sublattice is ordered at 2 K with a spin arrangement given by ( 0 , 0 , F z ″ ) . The refined magnetic moments at this temperature are 2.66 ( 12 ) μ B for the Mn 4+, − 0.8 ( 2 ) μ B for the Fe 3+ and − 2.0 ( 3 ) μ B for the Tb 3+ cations.