Tunneling of electrons in conventional and half-metallic systems: Towards very large magnetoresistance

The tunnel magnetoresistance (TMR) is analyzed for ferromagnet-insulator-ferromagnet junctions, including half-metallic systems. Direct tunneling in a corrected standard model is compared with impurity-assisted and resonant TMR. Direct tunneling in iron group systems leads to about a 20% change in resistance, as observed experimentally. Impurity-assisted tunneling decreases the TMR down to 4% with Fe-based electrodes, and spin-flip scattering from defect states will further reduce this value. A resonant tunneling diode structure would give a TMR of about 8%. The model applies qualitatively to half-metallics with 100% spin polarization, where the change in resistance in the absence of spin-flips may be arbitrarily large. Even in the case of imperfect magnetic configurations the resistance change can be a few 1000 percent. Examples of half-metallic systems are CrO${}_2$/TiO${}_2$ and CrO${}_2$/RuO${}_2$, and a brief account of their peculiar band structures is presented.