Structural and magnetic properties of the solid solution series Sr₂Fe_1-xM_xReO₆ (M = Cr, Zn)

Strong correlations between the electronic, structural and magnetic properties have been found during the study of doped double perovskites Sr2Fe1-xMxReO6 (0 <= x <= 1, M = Zn, Cr). The interplay between the van Hove singularity and the Fermi level plays a crucial role for the magnetic properties. Cr doping of the parent compound Sr2FeReO6 leads to a non-monotonic behaviour of the saturation magnetization and an enhancement for doping levels up to 10%. The Curie temperatures monotonically increase from 401 to 616 K. In contrast, Zn doping leads to a continuous decrease in the saturation magnetization and the Curie temperatures. Superimposed on the electronic effects is the structural influence which can be explained by size effects modelled by the tolerance factor t. For Cr doping levels higher than 40% we observed a tetragonal-cubic transformation. For Zn doped samples the tetragonal distortion linearly increases with the increasing Zn content. The valence state of iron strongly influencing the structure was determined by Mossbauer spectroscopy to be +2.7