Novel Quantum Spin Assisted Tunneling in Half Metallic Manganite Tunnel Junctions

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.