Magnetic transitions in double perovskite Sr2FeRe1-xSbxO6 (0≤x≤0.9)

The double perovskites Sr2FeMO6 (M=Re,Mo) belong to the important class of half-metallic magnetic materials. In this study we explore the effect of replacing the electronic 5d buffer element Re with variable valency by the main group element Sb with fixed valency. X-ray diffraction reveals Sr2FeRe1-xSbxO6 (0 < x < 0.9) to crystallize without antisite disorder in the tetragonally distorted perovskite structure (space group I4/mmm). The ferrimagnetic behavior of the parent compound Sr2FeReO6 changes to antiferromagnetic upon Sb substitution as was determined by magnetic susceptibility measurements. Samples up to a doping level of 0.3 are ferrimagnetic, while Sb contents higher than 0.6 result in an overall antiferromagnetic behavior. Fe-57 and Sb-121 Mossbauer spectroscopy specifies the valence state of Sb to be +5 within the whole range of substitution whereas the Fe valence state changes from +2.7 for the parent compound to +2.9 for Sr2FeRe0.1Sb0.9O6. Accordingly, Fe adopts the role of an electronic buffer element from Re upon heavy Sb doping. Additionally, Fe-57 Mossbauer results show a coexistence of ferri- and antiferromagnetic clusters within the same perovskite-type crystal structure in the Sb substitution range 0.3 < x < 0.8, whereas Sr2FeReO6 and Sr2FeRe0.9Sb0.1O6 are "purely" ferrimagnetic and Sr2FeRe0.1Sb0.9O6 contains antiferromagnetically ordered Fe sites only. Consequently, a replacement of the Re atoms by a nonmagnetic main group element such as Sb blocks the superexchange pathways -Fe-O-Re(Sb)-O-Fe- along the crystallographic axis of the perovskite unit cell and destroys the itinerant magnetism of the parent compound.