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.
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