Magnetism of pure iron jarosites

Stoichiometrically pure jarosites of the formula $A{\mathrm{Fe}}_{3}(\mathrm{OH}{)}_{6}({\mathrm{SO}}_{4}{)}_{2}$ with $A={\mathrm{Na}}^{+},$ ${\mathrm{K}}^{+},$ ${\mathrm{Rb}}^{+},$ and ${\mathrm{NH}}_{4}^{+}$ have been afforded by a newly developed redox-based, hydrothermal method. The jarosites exhibit an intralayer antiferromagnetic exchange interaction $(\ensuremath{-}829\mathrm{K}l{\ensuremath{\Theta}}_{\mathrm{CW}}l\ensuremath{-}812\mathrm{K})$ and transition temperatures for long-range order (LRO) $(61\mathrm{K}l{T}_{N}l65\mathrm{K})$ that are essentially insensitive to the size of the ${A}^{+}$ ion. A cusp at ${T}_{N}$ 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 $(\ensuremath{\sim}53\mathrm{K}l{T}_{D}l\ensuremath{\sim}58\mathrm{K}),$ the magnitude of which depends on crystallite size. X-ray single crystal analyses of the pure ${\mathrm{Fe}}^{3+}$ jarosite compounds reveal that the kagom\'e layers are structurally invariant with those of their ${\mathrm{Cr}}^{3+}$ and ${\mathrm{V}}^{3+}$ 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 ${M}^{3+}$ d-electron count. Infrared studies show the presence of ${\mathrm{H}}_{2}\mathrm{O}$ within the kagom\'e layers of alkali metal and hydronium ion ${\mathrm{Fe}}^{3+}$ jarosites prepared by conventional precipitation methods; conversely, ${\mathrm{H}}_{2}\mathrm{O}$ 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 $({\mathrm{H}}_{3}\mathrm{O}){\mathrm{Fe}}_{3}(\mathrm{OH}{)}_{6}({\mathrm{SO}}_{4}{)}_{2}$ 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.