Tuning charge transport properties and magnetic order in the metallic quantum multiferroic candidate EuTiO_3-δ

EuTiO₃ has emerged as a promising quantum multiferroic candidate, with functional properties that are tunable via element substitution and strain. Notably, this perovskite not only exhibits a ferroelectric soft mode that does not quite reach zero energy, making it an incipient ferroelectric, but also antiferromagnetic order associated with Eu moments, with a Néel temperature of T_N ~ 5.5 K. The proximity to both ferroelectric and magnetic transitions at zero temperature makes EuTiO₃ a compelling candidate quantum critical system. In this work, we use oxygen reduction to achieve a higher oxygen deficiency level than previously reported. The resultant increase in carrier concentration drastically lowers the resistivity of EuTiO_3-δ, and notably induces a transition from antiferromagnetic to ferromagnetic order, with a maximum Curie temperature of T_C ~ 11 K. Based on complementary DFT simulations, we attribute this ground-state change to a large change of the nearest-neighbor exchange constant J₁. Our comprehensive charge-transport measurements indicate that metallic EuTiO_3-δ is best described by a three-band model, consistent with the band structure calculations. These findings demonstrate that oxygen-vacancy oping enables the simultaneous tuning of charge-transport properties and magnetic order in EuTiO_3-δ.