CONTROL OF ELECTRONIC PROPERTIES BY LANTHANIDE SIZE AND MANGANESE OXIDATION STATE IN THE MnIII/MnIV RUDDLESDEN-POPPER PHASES Ln2xSr1+xMn2O7

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 was investigated in the Sr2-xLn1+xMn2O7 composition range (x = 0-0.7) for Ln = Nd, Tb. 33 refs.