The possibility of a very large magnetoresistance in half-metallic oxide systems

The tunnel magnetoresistance (TMR) is analyzed for ferromagnet-insulator-ferromagnet junctions, including novel half-metallic systems with 100% spin polarization. Direct tunneling is compared with the 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 to 4% with Fe-based electrodes. A resonant tunnel diode structure would give a TMR of about 8%. The model applies qualitatively to half-metallics, where the change in resistance in the absence of spin flips may be arbitrarily large and even in the case of imperfect magnetic configurations the resistance change can be several thousand percent. Examples of half-metallic ferromagnetic systems are CrO2/TiO2 and CrO2/RuO2. A discussion of their properties is presented. (C) 1997 American Institute of Physics.