As. Wills et al., Magnetic properties of pure and diamagnetically doped jarosites: Model kagome antiferromagnets with variable coverage of the magnetic lattice, PHYS REV B, 61(9), 2000, pp. 6156-6169
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 antiferromagnet
s with which to test suggestions that highly frustrated antiferromagnets ha
ve unconventional magnetic ground states and excitations. In all cases Fe3 ions provide S = 5/2 moments, arranged on the vertices of well-separated k
agome layers, and are coupled through strong antiferromagnetic exchange wit
h 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 a
ll 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-l
ike 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 de
uteronium salt except for the coverage of the magnetic lattice, which is hi
gher (97 +/- 3 %) in the latter than that of the rest of the family (less t
han 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 o
rder in the jarosites, and the material (D3O)Fe3-xAly(OD)(6)(SO4)(2), in wh
ich the coverage of the magnetic lattice is 89 +/- 3 % has been prepared an
d characterized to test this hypothesis. chi(dc) for this material has a cu
sp at 25.5 K, and powder neutron diffraction reveals long-range magnetic or
der at 1.4 K with the same ordering vector as that seen in the other materi
als.