Magnetic Properties of a Homologous Series of Vanadium Jarosite Compounds

Redox-based, hydrothermal synthetic methodologies have enabled the preparation of a new series of stoichiometrically pure jarosites of the formula, AV3(OH)6(SO4)2 with A = Na+, K+, Rb+, Tl+, and NH4 +. These jarosites represent the first instance of strong ferromagnetism within a Kagomé layered framework. The exchange interaction, which is invariant to the nature of the A+ ion (ϑCW ≈ +53(1) K), propagates along the d2 magnetic sites of the triangular Kagomé lattice through bridging hydroxyl groups. An analysis of the frontier orbitals suggests this superexchange pathway to possess significant π-orbital character. Measurements on a diamagnetic host jarosite doped with magnetically dilute spin carriers, KGa2.96V0.04(OH)6(SO4)2, reveal significant single-ion anisotropy for V3+ ion residing in the tetragonal crystal field. This anisotropy confines the exchange-coupled moments to lie within the Kagomé layer. Coupling strengths are sufficiently strong to prevent saturation of the magnetization when an external field is applied orthogonal to the Kagomé layer. Antiferromagnetic ordering of neighboring ferromagnetic Kagomé layers becomes dominant at low temperatures, characteristic of metamagnetic behavior for the AV3(OH)6(SO4)2 jarosites. This interlayer exchange coupling decreases monotonically with increasing layer spacing along the series, A = Na+, K+, Rb+, NH4 +, and Tl+, and it may be overcome by the application of external field strengths in excess of ∼6 kOe.