  We report low-temperature, high-field magnetotransport measurements of SrTiO3
gated by an ionic gel electrolyte. A saturating resistance upturn and negative
magnetoresistance that signal the emergence of the Kondo effect appear for
higher applied gate voltages. This observation, enabled by the wide tunability
of the ionic gel-applied electric field, promotes the interpretation of the
electric field-effect induced 2D electron system in SrTiO3 as an admixture of
magnetic Ti3+ ions, i.e. localized and unpaired electrons, and delocalized
electrons that partially fill the Ti 3d conduction band.
