Spin tunneling in ferromagnet-insulator-ferromagnet junctions

We have applied Buttiker's gauge invariant, charge conserving de transport theory to investigate the spin-polarized tunneling in ferromagnet-insulator -ferromagnet junctions at finite temperatures. It is observed that the spin polarization direction and temperature have remarkable effects on the diff erential conductance as well as the derivative of the conductance. At low t emperatures the quantum resonant tunneling is dominant, while the thermal a ctivated process plays a great role at high temperatures. A so-called spin- valve phenomenon is clearly seen at low temperatures. The results for asymm etric and symmetric junctions differ. The quantum electrochemical capacitan ce for such a junction is also calculated. Our findings are expected to be confirmed experimentally. (C) 2000 Elsevier Science B.V. All rights reserve d.