  Strontium titanate (SrTiO$_3$) is a foundational material in the emerging
field of complex oxide electronics. While its electronic and optical properties
have been studied for decades, SrTiO$_3$ has recently become a renewed
materials research focus catalyzed in part by 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 in these effects. Moreover, recent
signatures of magnetization in gated SrTiO$_3$ have further galvanized interest
in the emergent properties of this nominally nonmagnetic material. Here we
observe an optically induced and persistent magnetization in oxygen-deficient
SrTiO$_{3-\delta}$ using magnetic circular dichroism (MCD) spectroscopy and
SQUID magnetometry. This zero-field magnetization appears below ~18K, persists
for hours below 10K, and is tunable via the polarization and wavelength of
sub-bandgap (400-500nm) light. These effects occur only in oxygen-deficient
samples, revealing the detailed interplay between magnetism, lattice defects,
and light in an archetypal oxide material.
