| Summary: | Regent theoretical interest in Kagomé antiferromagnets arose from the prediction that they possess a new type of magnetic ground state in which there is no magnetic long-range order even at <I>T =</I> 0. Prior to this work no detailed investigations had bee made of the magnetic properties of real materials that provide models for this class of magnet. Members of the series AB<SUB>3</SUB>(SO<SUB>4</SUB>)<SUB>2</SUB>(OH)<SUB>6</SUB> (A<SUP>+</SUP> = Na<SUP>+</SUP>, K<SUP>+</SUP>, Rb<SUP>+</SUP>, Ag<SUP>+</SUP>, NH<SUB>4</SUB><SUP>+</SUP>, and H<SUB>3</SUB>O<SUP>+</SUP>; B<SUP>3+</SUP> = V<SUP>3+</SUP>, Cr<SUP>3+</SUP>, and Fe<SUP>3+</SUP>) have been studied using a combination of experimental techniques- ac and dc susceptibility, specific heat, neutron diffraction, and MuSR. The hydronium compounds (A<SUP>+</SUP> = H<SUB>3</SUB>O<SUP>+</SUP>; B<SUP>3+</SUP> = V<SUP>3+</SUP>, Cr<SUP>3+</SUP>, and Fe<SUP>3+</SUP>) have been shown to provide the best model Kagomé antiferromagnets with <I>S </I>= 1, 3/2 and 5/2 respectively. (H<SUB>3</SUB>O)Fe<SUB>3</SUB>(SO<SUB>4</SUB>)<SUB>2</SUB>(OH)<SUB>6</SUB> has been shown to possess only short-range magnetic order with a correlation length of 19 ± 2Å, below a spin glass transition temperature, <I>T<SUB>f</SUB></I> 17K. Unlike normal spin glasses, muon spin relaxation measurements show that this magnetic phase is dynamic below <I>T<SUB>f</SUB></I> (H<SUB>3</SUB>O)Fe<SUB>3</SUB>(SO<SUB>4</SUB>)<SUB>2</SUB>(OH)<SUB>6</SUB> is a spin fluid: its dynamic and thermodynamic behaviour are completely unlike those seen in normal spin glass or long-range ordered ground states; the specific heat varies as <I>T<SUP>2</SUP></I> below <I>T<SUB>f</SUB></I>, rather than the linear <I>T</I> dependence observed in canonical spin glasses. Deliberate reduction of the concentration of magnetic atoms destabilises the spin fluid ground state and induces the formation of long-range order of the type seen in the other less magnetically concentrated members of the series. This is the first example of a system in which diamagnetic dilution causes it to order classically. (H<SUB>3</SUB>O)Cr<SUB>3</SUB>(SO<SUB>4</SUB>)<SUB>2</SUB>(OH)<SUB>6</SUB> has a broad transition to a dynamic short-range ordered state at 30K, similar to that seen in (H<SUB>3</SUB>O)Fe<SUB>3</SUB>(SO<SUB>4</SUB>)<SUB>2</SUB>(OH)<SUB>6</SUB>, but with no observed spin glass behaviour. A weak ferromagnetic transition at 2.2K is ascribed to a canting of antiferromagnetically coupled sublattices. Specific heat and dc susceptibility measurements show that (H<SUB>3</SUB>O)V<SUB>3</SUB>(SO<SUB>4</SUB>)<SUB>2</SUB>(OH)<SUB>6</SUB> undergoes a sharp antiferromagnetic transition at 21K to an ordered ground state. However, neutron diffraction data taken at 4.2K reveals no evidence for long-range magnetic order. It is suggested that the magnetic ground state could be a highly correlated spin fluid.
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