  The electron tunneling in half metallic manganite tunnel junctions is studied
by using a quantum mechanically treated double exchange model. We show that the
stimulation of spin excitations, caused by the strong Hund's coupling between
the conduction $e_g$ electrons and the localized quantum spins of the manganite
ions, would assist electrons to tunnel through the junction even with the
antiparallel aligned magnetizations of the electrodes. This mechanism gives
rise to an extra tunnel conductance, in addition to that predicted by
Julliere's model. Our theory is in good agreement with the voltage dependence
of the tunnel conductance in manganite tunnel junctions observed by
experiments.
