  The tunnel magnetoresistance (TMR) is one of the most important spintronic
phenomena but its reduction at finite temperature is a severe drawback for
applications. Here, we reveal a crucial determinant of the drawback, that is,
the $s$-$d$ exchange interaction between conduction $s$ and localized $d$
electrons at interfacial ferromagnetic layers. By calculating the temperature
dependence of the TMR ratio in Fe/MgO/Fe(001), we show that the obtained TMR
ratio significantly decreases with increasing temperature owing to the
spin-flip scattering in the $\Delta_1$ state induced by the $s$-$d$ exchange
interaction. The material dependence of the coupling constant $J_{sd}$ is also
discussed on the basis of a nonempirical method.
