  There has been much interest in the injection and detection of spin polarized
carriers in semiconductors for the purposes of developing novel spintronic
devices. Here we report the electrical injection and detection of
spin-polarized carriers into Nb-doped strontium titanate (STO) single crystals
and La-doped STO epitaxial thin films using MgO tunnel barriers and the
three-terminal Hanle technique. Spin lifetimes of up to ~100 ps are measured at
room temperature and vary little as the temperature is decreased to low
temperatures. However, the mobility of the STO has a strong temperature
dependence. This behavior and the carrier doping dependence of the spin
lifetime suggest that the spin lifetime is limited by spin-dependent scattering
at the MgO/STO interfaces, perhaps related to the formation of doping induced
Ti3+. Our results reveal a severe limitation of the three-terminal Hanle
technique for measuring spin lifetimes within the interior of the subject
material.
