  The perovskite rare-earth titanates are model Mott insulators with magnetic
ground states that are very sensitive to structural distortions. These
distortions couple strongly to the orbital degrees of freedom and, in
principle, it should be possible to tune the superexchange and the magnetic
transition with strain. We investigate the representative system
(Y,La,Ca)TiO$_3$, which exhibits low crystallographic symmetry and no
structural instabilities. From magnetic susceptibility measurements of the
Curie temperature, we demonstrate direct, reversible and continuous control of
ferromagnetism by influencing the TiO$_6$ octahedral tilts and rotations with
uniaxial strain. The relative change in $T_C$ as a function of strain is well
described by ab initio calculations, which provides detailed understanding of
the complex interactions among structural, orbital and magnetic properties in
rare-earth titanates. The demonstrated manipulation of octahedral distortions
opens up far-reaching possibilities for investigations of electron-lattice
coupling, competing ground states, and magnetic quantum phase transitions in a
wide range of quantum materials.
