Effect of cation disorder on the magnetic properties of Sr₂Fe_1-xGa_xReO₆ (0 < x < 0.7) double perovskites

The effect of diamagnetic dilution of the Fe sublattice on the structural and magnetic properties of the double perovskite Sr2Fe1-xGaxReO6 (0 < x < 0.7) has been explored by means of x-ray structural analysis, magnetometry, Mossbauer spectroscopy, and band structure calculations. The end member of the solid solution series, Sr2GaReO6, was predicted to be a half-metallic ferromagnet based on ab initio band structure calculations using the WIEN2k software package. Because the Re-containing double perovskites, like the parent compound Sr2FeReO6, rarely exhibit antisite disorder, the appearance of increasing amounts of antisite disorder in Sr2Fe1-xGaxReO6 (0 < x < 0.7) is unexpected. Although the amount of antisite disorder increases with increasing Ga content, it also depends on the sample preparation history. For 0 < x < 0.4 only a tetragonal, magnetically ordered phase was detected by x-ray structural analysis as well as Mossbauer analysis. A phase separation into a tetragonal and a cubic phase for x >= 0.4 is detected by x-ray structural analysis accompanied by the observation of a magnetically ordered and a paramagnetic phase in the corresponding Mossbauer spectra. Below 20% Ga content, Ga statistically dilutes the -Fe-O-Re-O-Fe- double-exchange pathways. Phase separation begins at 20% Ga substitution; between 20% and 40% of Ga content, the paramagnetic Ga-based phase does not contain any Fe. The Fe-containing, paramagnetic cubic phases which can be detected by Mossbauer spectroscopy first appear for x=0.4