Magnetic Tunnel Junctions With $\hbox{Co:TiO}_{2}$ Magnetic Semiconductor Electrodes

Spin-polarized tunneling is investigated in magnetic tunnel junctions containing an ultrathin interfacial layer of $\hbox{Co:TiO}_{2}$ magnetic semiconductor. The $\hbox{Co:TiO}_{2}$ layers (0 to 1 nm thick) are inserted at the $\hbox{SrTiO}_{3}/\hbox{Co}$ interface in $\hbox{La}_{0.67}\hbox{Sr}_{0.33}\hbox{MnO}_{3}/\hbox{SrTiO}_{3}/\hbox{Co}$ tunnel junctions. For all junctions we find a negative tunnel magnetoresistance, which decreases upon the insertion of $\hbox{Co:TiO}_{2}$ , while the junction resistance increases strongly. This suggests that the ultrathin $\hbox{Co:TiO}_{2}$ is a paramagnetic insulator that acts as an additional tunnel barrier, in contrast to thick (180 nm) layers grown under comparable conditions, which exhibit metallic impurity band conduction and room-temperature ferromagnetism.