Conventional and unconventional orderings in the jarosites

The jarosites make up the most studied family of {\it kagom'e} antiferromagnets. The flexibility of the structure to substitution of the A and B ions allows a wide range of compositions to be synthesised with the general formula AB3(SO4)2(OH)6 (A = Na${}^{+}$, K${}^{+}$, Ag${}^{+}$, Rb${}^{+}$, H3O${}^{+}$, NH4${}^{+}$, 1/2Ba${}^{2+}$, and 1/2Pb${}^{2+}$; B = Fe${}^{3+}$, Cr${}^{3+}$, and V${}^{3+}$). Additional chemical tuning of the exchange between layers is also possible by substitution of the (SO4)${}^{2-}$ groups by (SeO4)${}^{2-}$ or (CrO4)${}^{2-}$. Thus, a variety of S = 5/2, 3/2, and 1 systems can be engineered to allow study of the effects of frustration in both the classical and more quantum limits. Within this family both conventional long-ranged magnetic order and more exotic unconventional orderings have been found. This article reviews the different types of magnetic orderings that occur and examines some of the parameters that are their cause.