Antisite disordering in ${\mathrm{Sr}}_2{\mathrm{FeMoO}}_6$ double perovskites (containing Mo atoms at Fe positions, and vice versa) has recently been shown to have a dramatic influence in their magnetic and magnetotransport properties. In the present paper, two polycrystalline ${\mathrm{Sr}}_2{\mathrm{FeMoO}}_6$ samples showing different degrees of antisite disorder (a nominally “ordered” sample with ∼70% of cationic ordering and a nominally “disordered” sample with ∼18% of cationic ordering) have been examined by magnetic measurements and neutron powder diffraction techniques in the 15–500 K temperature range. Our main finding is that the “disordered” sample exhibits a strong magnetic scattering (noticeable even at 500 K), comparable to that displayed by the “ordered” one below $T_C=415\mathrm{}\mathrm{K}.$ For the “disordered” sample, the magnetic scattering exhibited on low-angle Bragg positions, is not to be ascribed to a (nonexistent) ferrimagnetic ordering: our results suggest that it originates upon naturally occurring groups of Fe cations in which strong antiferromagnetic (AFM) Fe-O-Fe superexchange interactions are promoted, similar to those existing in the ${\mathrm{LaFeO}}_3$ perovskite. These Fe groups are not magnetically isolated, but coupled by virtue of Fe-O-Mo AFM interactions, which maintain the long-range coherence of this AFM structure. Susceptibility measurements confirm the presence of AFM interactions below 770 K.

• Received 17 April 2001

DOI:https://doi.org/10.1103/PhysRevB.65.104426