Magnetoresistance of Ferromagnetic Tunnel Junctions in the Double-Exchange Model

We conduct a theoretical study of the temperature dependence of the spin polarization ( $P$) and the magnetoresistance (MR) ratio using the double exchange (DE) model for ferromagnetic tunnel junctions with half-metallic systems. It is shown that the strong exchange coupling in the DE model plays an important role in the temperature dependence of both $P$ and the MR ratio; their values can be less than the maximum values expected for half-metallic systems at low temperatures, and the MR ratio decreases more rapidly than $P$ with increasing temperature. The calculated results, however, indicate that the MR ratio may still be large at high temperatures near the Curie temperature.