Control of electronic properties by lanthanide size and manganese oxidation state in the Mn-III/Mn-IV Ruddlesden-Popper phases Ln(2-x)Sr(1+x)Mn(2)O(7)

The magnetic behaviour of the n = 2 Ruddlesden-Popper phases Sr(2)LnMn(2)O(7) is very sensitive to the Ln(3+) lanthanide cation. In samples with larger, more basic lanthanide cations (Ln = Nd, Pr) antiferromagnetic phases with ordering temperatures in the region of 130 K co-exist with phases showing a magnetic response suggestive of superparamagnetism or the development of small ferromagnetic clusters at high temperature. The magnetic transition temperature drops to 20 K in samples containing smaller, acidic cations (Ln = Gd-Er, Y). In the latter group of compounds, the transition is from a Curie-Weiss paramagnet to a spin-glass; there is no evidence for long-range magnetic order. This change in behaviour can be explained by considering the variation in the relative strength of superexchange and double exchange interactions as a function of the lanthanide cation. The influence of manganese oxidation state on magnetic response is investigated in the Sr(2-x)Ln(1+x)Mn(2)O(7) composition range (0.0 less than or equal to x less than or equal to 0.7) for Ln = Nd, Tb.