  Atomistic defect engineering through the pulsed laser epitaxy of perovskite
transition metal oxides offers facile control of their emergent
opto-electromagnetic and energy properties. Among the various perovskite
oxides, EuTiO3 exhibits a strong coupling between the lattice, electronic, and
magnetic degrees of freedom. This coupling is highly susceptible to atomistic
defects. In this study, we investigated the magnetic phase of EuTiO$_3$
epitaxial thin films via systematic defect engineering. A magnetic phase
transition from an antiferromagnet to a ferromagnet was observed when the unit
cell volume of EuTiO3 expanded due to the introduction of Eu-O vacancies.
Optical spectroscopy and density functional theory calculations show that the
change in the electronic structure as the ferromagnetic phase emerges can be
attributed to the weakened Eu-Ti-Eu super-exchange interaction and the
developed ferromagnetic Eu-O-Eu interaction. Facile defect engineering in
EuTiO$_3$ thin films facilitates understanding and tailoring of their magnetic
ground state.
