The magnetic structures of the mixed layer pnictide oxide compounds Sr(2)Mn(3)Pn(2)O(2) (Pn=As, Sb)

Neutron powder diffraction results are reported for the compounds Sr(2)Mn(3)Pn(2)O(2) (Pn = As, Sb), and the magnetic structures are determined. The chemical structure can be described as a 1:1 intergrowth of Mn(2)Pn(2)(2-) layers of the BaAl4 type with MnO22- layers isostructural with the CuO2 planes found in cuprate superconductors. The Sr2+ cations separate the two layers. Two independent magnetic sublattices exist, Mn(1) and Mn(2), associated with the MnO22- and Mn(2)Pn(2)(2-) layers respectively. Both sublattices have tetragonal symmetry, Mn(2) being primitive and Mn(1) being centered. The interlayer spacing within each sublattice is about 10 Angstrom and between the sublattices it is about 5 Angstrom. Owing to the centering of the Mn(1) sublattice, interactions between the nearest MnO22- layers cancel, as do interactions between the Mn(1) and Mn(2) sublattices. The Mn(2) sublattice is found to order at the relatively high temperatures of about 340 K (As) and 300 K (Sb) in a type G(z) structure, k = (000) with a saturation moment of 3.4 mu(B) at 4 K. At low temperatures new reflections of magnetic origin appear which can be assigned to the Mn(1) sublattice, but the details are different for each compound. The Mn(1) moments for Sr2Mn3Sb2O2 order below 65 K with k = (1/21/20) along (100), i.e. normal to the Mn(2) spins, with a significantly larger moment of 4.2 mu(B). In Sr2Mn3As2O2 one sees much weaker reflections below 75 K, the most prominent of which shows a Warren line shape characteristic of two-dimensional correlations only. Thus it appears that the Mn(1) sublattice may not order in the long range sense down to 4K in Sr2Mn3As2O2. The properties of the individual sublattices of these manganese pnictide oxides are compared with similar magnetic sublattices of other compounds.