  The perovskite ruthenates (ARuO3, A = Ca, Ba, or Sr) exhibit unique
properties owing to a subtle interplay of crystal structure and electronic-spin
degrees of freedom. Here, we demonstrate an intriguing continuous tuning of
crystal symmetry from orthorhombic to tetragonal (no octahedral rotations)
phases in epitaxial SrRuO3 achieved via Ba-substitution (Sr1-xBaxRuO3 with 0 <
x < 0.7). An initial Ba-substitution to SrRuO3 not only changes the
ferromagnetic properties, but also tunes the perpendicular magnetic anisotropy
via flattening the Ru-O-Ru bond angle (to 180{\deg}), resulting in the maximum
Curie temperature and an extinction of RuO6 rotational distortions at x = 0.20.
For x > 0.2, the suppression of RuO6 octahedral rotational distortion
dominantly enhances the ferromagnetism in the system, though competing with the
impact of the RuO6 tetragonal distortion. Further increasing x > 0.2 gradually
enhances the tetragonal-type distortion, resulting in the tuning of Ru-4d
orbital occupancy and suppression of ferromagnetism. Our results demonstrate
that isovalent substitution of the A-site cations significantly and
controllably impacts both electronic and magnetic properties of perovskite
oxides.
