  Different mechanisms of spin-dependent tunneling are analyzed with respect to
their role in tunnel magnetoresistance (TMR). Microscopic calculation within a
realistic model shows that direct tunneling in iron group systems leads to
about a 30% change in resistance, which is close but lower than experimentally
observed values. The larger observed values of the tunnel magnetoresistance
(TMR) might be a result of tunneling involving surface polarized states. It is
found that tunneling via resonant defect states in the barrier radically
decreases the TMR by order of magnitude. One-magnon emission is shown to reduce
the TMR, whereas phonons increase the effect. The inclusion of both magnons and
phonons reasonably explains an unusual bias dependence of the TMR. The model
presented here is applied qualitatively to half-metallics with 100% spin
polarization, where one-magnon processes are suppressed and the change in
resistance in the absence of spin-mixing on impurities 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
CrO2/TiO2 and CrO2/RuO2.
