  BaRuO3 having five different crystal structures has been synthesized by
varying the pressure while sintering. Contrary to the other phases being
nonmagnetic, the cubic perovskite phase synthesized recently shows an itinerant
ferromagnetic character. We investigated this ferromagnetic BaRuO3 using first
principles calculations. A few van Hove singularities appear around the Fermi
energy, causing unusually high magnetovolume effects of $\Delta M/\Delta a$ ~
4.3 $\mu_B$/\AA as well as a Stoner instability [IN(0) ~ 1.2]. At the optimized
lattice parameter a, the magnetic moment M is 1.01 $\mu_B$ in the local spin
density approximation. When spin-orbit coupling is included, the topologies of
some Fermi surfaces are altered, and the net moment is reduced by 10% to a
value very close to the experimentally observed value of ~ 0.8 $\mu_B$. Our
results indicate that this ferromagnetism is induced by the Stoner instability,
but the combined effects of the p-d hybridization, the magnetovolume, and the
spin-orbit coupling determine the net moment. In addition, we briefly discuss
the results of the tight-binding Wannier function technique.
