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Zusammenfassung:
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.
