Magnetic structures of the three-dimensional Heisenberg antiferromagnets K₂FeCl₅.D₂O and Rb₂FeCl₅.D₂O

Abstract

The crystal and magnetic structures of K2FeCl5.D2O and Rb2FeCl5/D2O have been determined by neutron powder diffraction. The crystal structure of K2FeCl5.D2O has been redetermined using single-crystal diffraction. All compounds crystallize with the orthorhombic space group Pnma. The magnetic structure of both deuterated compounds can be described by ferromagnetic planes perpendicular to the b axis coupled antiferromagnetically to each other. The critical behaviour of the staggered magnetization below TN=14.2 K and 10.0 K for K2FeCl5.D2O and Rb2FeCl5.D2O, respectively, can be described by a power law for reduced temperature 0.03<t<0.2 by a critical exponent beta =0.35+or-0.02.

Low-field remanent magnetization inRb2FeCl5⋅H

“…The recent determination of the magnetic structure of K 2 FeCl 5 •H 2 O excludes the piezomagnetic origin, at least in the Fe/In systems, since the symmetry of the magnetic point group (mmm) is not consistent with piezomagnetism. 12 We believe that the presence of M r at very low magnetic fields is an entirely new feature of low-anisotropy antiferromagnets bearing a certain degree of disorder.…”

“…25 Recently, the magnetic structure of Rb 2 FeCl 5 •D 2 O has been obtained by means of neutron diffraction experiments. 12 The sublattice magnetization was measured from the temperature dependence of the integrated intensities of the magnetic Bragg peaks. The result is compared in Fig.…”

“…Magnetically, all these materials are easy-axis antiferromagnets with two collinear interpenetrating sublattices. 12,14 The single ion spin value for all of these systems is Sϭ 5 2 in a 5 A 1g ground state. The ordering temperature for Rb 2 FeCl 5 •H 2 O is T N ϭ10 K and the ratio between the anisotropy field and the exchange field is ␣ϭH A /H E ϭ3.4ϫ10 Ϫ3 .…”

Understanding magnetic interactions in the seriesA2FeX5⋅H2O(A=

“…The existence of such a spin delocalization, which would strengthen the interaction between magnetic orbitals localized in different octahedra, is also supported by a reduced magnetic moment observed at the iron site in a powderneutron-diffraction experiment on K 2 FeCl 5 ·H 2 O. 26 In this paper we report the analysis of the spin-density distribution in K 2 FeCl 5 ·H 2 O and Rb 2 FeBr 5 ·H 2 O using two complementary methods: polarized neutron diffraction ͑PND͒ and density-functional-theory ͑DFT͒ calculations. This analysis will contribute to elucidation of two important open questions necessary to understand the mechanism of the magnetic interactions in these compounds: the extent to which the spin is delocalized from the iron atom toward the halogen atoms and the oxygen atom and, related to the first point, the reason why magnetic interactions are stronger in the bromide derivatives than in the chloride derivatives.…”

“…In the case of the studied compounds, the nuclear structure of K 2 FeCl 5 ·H 2 O was already known from a single-crystal x-ray- 42 and powder-neutron-diffraction experiments, 26,43 while the nuclear structure of Rb 2 FeBr 5 ·H 2 O was known only from x-ray experiments at room temperature. 17 Since the hydrogen atoms are supposed to play an important role in the magnetic interactions of these compounds, their location is of particular interest.…”