  The ability to tune exchange (magnetic) interactions between 3d transition
metals in perovskite structures has proven to be a powerful route to discovery
of novel properties. Here we demonstrate that the introduction of 3d-5d
exchange pathways in double perovskites enables additional tunability, a result
of the large spatial extent of 5d wave functions. Using x-ray probes of
magnetism and structure at high pressure, we show that compression of Sr2FeOsO6
drives an unexpected continuous change in the sign of Fe-Os exchange
interactions and a transition from antiferromagnetic to ferrimagnetic order. We
analyze the relevant electron-electron interactions, shedding light into
fundamental differences with the more thoroughly studied 3d-3d systems.
