Magnetism of pure iron jarosites

Stoichiometrically pure jarosites of the formula AFe₃(OH)₆(SO₄)₂ with A=Na⁺, K⁺, Rb⁺, and NH₄⁺ have been afforded by a newly developed redox-based, hydrothermal method. The jarosites exhibit an intralayer antiferromagnetic exchange interaction (-829K<ΘCW<-812K) and transition temperatures for long-range order (LRO) (61K<TN<65K) that are essentially insensitive to the size of the A⁺ ion. A cusp at TN in the ac susceptibility curve is frequency independent. The origin of LRO is consistent with coupling of jarosite layers exhibiting a net magnetization, which arises from an anisotropy developed, most likely, from the Dzyaloshinsky-Moriya (DM) interaction. A canted intralayer spin structure, which is a consequence of the DM interaction, is signified by a remanent magnetization (~53K<TD<~58K), the magnitude of which depends on crystallite size. X-ray single crystal analyses of the pure Fe³⁺ jarosite compounds reveal that the kagom\'e layers are structurally invariant with those of their Cr³⁺ and V³⁺ relatives. This structural homology allows the sign and magnitude of exchange coupling within kagom\'e layers to be correlated to the different orbital parentages engendered by the M³⁺ d-electron count. Infrared studies show the presence of H₂O within the kagom\'e layers of alkali metal and hydronium ion Fe³⁺ jarosites prepared by conventional precipitation methods; conversely, H₂O is absent within the kagom\'e layers of jarosites prepared by the new redox-based hydrothermal methods. These results suggest that the absence of LRO in (H₃O)Fe₃(OH)₆(SO₄)₂ is due to structural and magnetic disorder arising from proton transfer from the interlayer hydronium ion to the bridging hydroxide ions of the kagom\'e layers.