| Summary: | The magnetic behaviour of the n= 2 Ruddlesden-Popper phases Sr2LnMn2O7 is very sensitive to the Ln3+ 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 Sr2-xLn1+xMn2O7 composition range (0.0≤x≤0.7) for Ln=Nd,Tb.
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