  Double perovskites Sr2FeOsO6 and Ca2FeOsO6 show puzzling magnetic properties,
the former a low-temperature antiferromagnet while the later a high-temperature
insulating ferrimagnet. Here, in order to understand the underlying mechanism,
we have investigated the frustrated magnetism of A2FeOsO6 by employing density
functional theory and maximally-localized Wannier functions. We find that
lattice distortion enhances the antiferromagnetic nearest-neighboring Fe-O-Os
interaction but weakens the antiferromagnetic interactions through the
Os-O-O-Os and Fe-O-Os-O-Fe paths, which is responsible for the magnetic
transition from the low-temperature antiferromagnetism to the high-temperature
ferrimagnetism with the decrease of the radius of the A2+ ions. We also discuss
the 5d3-3d5 superexchange and propose such superexchange is intrinsically
antiferromagnetic instead of the expected ferromagnetic. Our work illustrate
that the magnetic frustration can be effectively relieved by lattice
distortion, which provides another dimension to tune the complex magnetism in
other 3d-5d (4d) double perovskites.
