Control of electronic properties by lanthanide size and manganese oxidation state in the MnIII/MnIV Ruddlesden–Popper phases Ln2?xSr1+xMn2O7

The magnetic behaviour of then=2 Ruddlesden–Popper phases Sr2 LnMn2 O7 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-x Ln1+x Mn2 O7 composition range (0.0?x?0.7) for Ln=Nd, Tb.