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) antiferromagn etic phases with ordering temperatures in the region of 130 K co-exist with phases showing a magnetic response suggestive of superparamagnet ism or the development of small ferromagnetic clusters at high tempera ture. The magnetic transition temperature drops to 20 K in samples con taining smaller, acidic cations (Ln = Gd-Er, Y). In the latter group o f compounds, the transition is from a Curie-Weiss paramagnet to a spin -glass; there is no evidence for long-range magnetic order. This chang e in behaviour can be explained by considering the variation in the re lative strength of superexchange and double exchange interactions as a function of the lanthanide cation. The influence of manganese oxidati on 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 equ al to 0.7) for Ln = Nd, Tb.