Origin of neutron magnetic scattering in antisite-disordered Sr2FeMoO6 double perovskites

Antisite disordering in Sr2FeMoO6 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 Sr2FeMoO6 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 TC=415 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 LaFeO3 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