Persistent Optically Induced Magnetism in Oxygen-Deficient Strontium Titanate

Strontium titanate (SrTiO$_3$) is a foundational material in the emerging field of complex oxide electronics. While its electronic, optical, and lattice properties have been studied for decades, SrTiO$_3$ has recently become a renewed focus of materials research owing to the discovery of magnetism and superconductivity at interfaces between SrTiO$_3$ and other oxides. The formation and distribution of oxygen vacancies may play an essential but as-yet-incompletely understood role. Here we observe an \textit{optically induced} and \textit{persistent} magnetization in slightly oxygen-deficient bulk SrTiO$_{3-\delta}$ crystals using magnetic circular dichroism spectroscopy and SQUID magnetometry. The optically induced magnetization appears below $\sim$18 K, persists for hours below 10 K, and is tunable via the polarization and wavelength of sub-bandgap (400-500 nm) light. These effects, which only occur in oxygen-deficient samples, reveal a detailed interplay between defects, magnetism, and light in oxide materials.