  Using density-functional theory calculations, we investigated the electronic
structure and magnetic exchange interactions of the ordered 3d-5d double
perovskite Sr2FeOsO6, which has recently drawn attention for interesting
antiferromagnetic transitions. Our study reveals the vital role played by
long-range magnetic exchange interactions in this compound. The competition
between the ferromagnetic nearest neighbor Os-O-Fe interaction and
antiferromagnetic next nearest neighbor Os-O-Fe-O-Os interaction induces strong
frustration in this system, which explains the lattice distortion and magnetic
phase transitions observed in experiments.
