Magnetic Structure of K 2 NiF 4 -type Iron(III) Oxide Halides
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Magnetic Structure of K2NiF4-type Iron(III) Oxide Halides Andrew L. Hector School of Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ, UK
ABSTRACT Materials with the K2NiF4 structure and a magnetic atom on the B site are examples of a pseudo-2D square lattice. The nearest neighbour interactions are very strong and, with iron(III), usually lead to xy antiferromagnetic coupling. We have used powder neutron diffraction at 2 K to study the magnetic structures of A2FeO3X where A is Ca or Sr and X is Cl or Br. There are two common magnetic structures for K2NiF4-type xy antiferromagnets, those of La2NiO4 and La2CuO4, here we find the latter. In Sr2FeO3F we find a more complex situation. The reflections corresponding to the La2CuO4 magnetic structure are present from 2 K to ambient temperature, but a further set of peaks decays from 2 K to around 60 K. These correspond to a magnetic structure with a doubled c-axis. This is due to a magnetic structure in which La2NiO4and La2CuO4-type stacking alternate. To our knowledge this is the only example of this magnetic structure type. INTRODUCTION The K2NiF4 structure type consists of Perovskite layers separated by Rocksalt layers and is thus magnetically an example of a pseudo-2D square lattice. The A2FeO3X materials, including Sr2FeO3F, have oxide/halide ordering in the rocksalt part of the structure. Oxide ions bridge the iron sites within the perovskite layers but the rocksalt layers have alternate AX and AO composition. The Fe-X bonding interaction is very weak, having a bond valence sum of 0.06-0.07 in Sr2FeO3X (X = F, Cl, Br), effectively resulting in the square pyramidal FeO5 polyhedra as shown. The layers are also no longer separated by half a unit cell along c and the body centering is lost, the new space group is P4/nmm.1 Magnetic interactions between nearest neighbor iron sites in the perovskite layer are very strong, and in K2NiF4-type materials superexchange through O often results in antiferromagnetic coupling.2 In the ordered state the moments can point along the z-axis (Ising antiferromagnetism), as found in K2NiF4 and Ca2MnO4,3 or can be confined to the xy plane (xyantiferromagnetism) in most cuprate oxides and La2NiO4.4 The interplane coupling is typically much weaker, but nonetheless 3-dimensional (3D) ordering is often found and thus neutron diffraction becomes a useful probe of magnetic structure. Previous studies of magnetic structure of CaLaFeO4,5 SrLaFeO46 and our initial work on SrFeO3X materials1 indicate xyantiferromagnetic behavior in all of the iron(III) materials. Fig.1 shows a possible pathway for superexchange interactions between layers. The square lattice with a half unit cell translation along [110] between consecutive layers results in a
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Figure 1. The anion-ordered K2NiF4 structure found in A2FeO3X phases, with the superexchange pathway between iron atoms in adjacent layers (iron atoms shown in brown, oxygen in red, halogen in green and calcium/strontium in grey). possible source of frustration where th
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