Magnetism of pure iron jarosites

Stoichiometrically pure jarosites of the formula $A{\mathrm{Fe}}_3\left(\mathrm{OH}{)}_6\right({\mathrm{SO}}_4{)}_2$ 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 $(-829\mathrm{K}<{\Theta }_{\mathrm{CW}}<-812\mathrm{K})$ and transition temperatures for long-range order (LRO) $(61\mathrm{}\mathrm{K}<T_N<65\mathrm{}\mathrm{K})$ 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 $(\sim 53\mathrm{K}<T_D<\sim 58\mathrm{K}),$ the magnitude of which depends on crystallite size. X-ray single crystal analyses of the pure ${\mathrm{Fe}}^{3+}$ jarosite compounds reveal that the kagomé 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 kagomé 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 kagomé 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 kagomé layers of jarosites prepared by the new redox-based hydrothermal methods. These results suggest that the absence of LRO in $\left({\mathrm{H}}_3\mathrm{O}\right){\mathrm{Fe}}_3\left(\mathrm{OH}{)}_6\right({\mathrm{SO}}_4{)}_2$ is due to structural and magnetic disorder arising from proton transfer from the interlayer hydronium ion to the bridging hydroxide ions of the kagomé layers.