Magnetic properties of pure and diamagnetically doped jarosites:: Model kagome antiferromagnets with variable coverage of the magnetic lattice

Jarosites are a family of minerals of general formula AFe(3)(OH)(6)(SO4)(2) (where A(+) is typically a univalent cation such as Na+, K+, Rb+, ND4+, Ag+, Tl+, or D3O+). They provide good model Heisenberg kagome antiferromagnets with which to test suggestions that highly frustrated antiferromagnets have unconventional magnetic ground states and excitations. In all cases Fe3+ ions provide S = 5/2 moments, arranged on the vertices of well-separated kagome layers, and are coupled through strong antiferromagnetic exchange with values of the Weiss constants theta of the order of -700 K. We report de magnetic susceptibility (chi(dc)) and powder neutron diffraction studies of materials in which A(+) = Na+, Rb+, ND4+, Ag+, or D3O+ and show that for all materials except the deuteronium (A(+) = D3O+) salt, long-range magnetic order with the in-plane q = 0 spin structure sets in below a temperature T-f of the order of 50 K. (D3O)Fe-3(SO4)(2)(OD)(6) shows only a spin-glass-like transition at T-f congruent to 15 K. There is no obvious difference in the structures of the salts that show magnetic long-range order, and the deuteronium salt except for the coverage of the magnetic lattice, which is higher (97 +/- 3 %) in the latter than that of the rest of the family (less than or equal to 95%, and most typically congruent to 90 %). It is proposed that reduction of the coverage of the magnetic lattice induces long-range order in the jarosites, and the material (D3O)Fe3-xAly(OD)(6)(SO4)(2), in which the coverage of the magnetic lattice is 89 +/- 3 % has been prepared and characterized to test this hypothesis. chi(dc) for this material has a cusp at 25.5 K, and powder neutron diffraction reveals long-range magnetic order at 1.4 K with the same ordering vector as that seen in the other materials.