Magnetic properties of the double perovskite Sr$_2$FeOsO$_6$: microscopic insights from ab-initio density-functional theory study

Using density-functional theory calculations, we investigated the electronic and magnetic properties of the ordered 3d-5d double perovskite Sr$_2$FeOsO$_6$, which has recently drawn attention for interesting antiferromagnetic (AFM) phase transitions in low temperature observed in experiments. The calculated effective magnetic exchange interactions reveal the importance of long-range super-superexchange interactions in this compound. The competition between the ferromagnetic (FM) Os-O-Fe short-range interaction and AFM Os-O-Fe-O-Os long-range interaction induces strong magnetic frustration along the crystallographic c axis. This strong magnetic frustration is proposed to drive the magnetic phase transition between two AFM phases (AFM1 to AFM2) and related lattice distortion, which were also observed in the experiment. [Ref: Sudipta Kanungo, Binghai Yan, Martin Jansen, and Claudia Felser; Phys. Rev. B 89, 214414 (2014)]