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Theory of tunneling magnetoresistance of Fe/GaAs/Fe(001) junctions

We investigate theoretically spin-dependent transport through an epitaxial Fe/GaAs/Fe(001) tunnel junction with and without spin-orbit interaction. Calculations neglecting spin-orbit interaction and the effect of d orbitals on the GaAs band structure predict that the tunneling magnetoresistance (TMR) should increase with increasing thickness of GaAs barrier and approach values close to the perfect spin-valve limit. However, when d orbitals and, in particular, spin-orbit interaction is included the TMR ratio saturates rapidly with GaAs thickness to a rather modest value of about 30% when the Fermi level E F lies in the middle of the GaAs gap. This unexpectedly small value cannot be explained by spin-orbit interaction alone. It is shown that the underlying reason for this is the presence of a resonance in the minority-spin band structure of the Fe/GaAs/Fe trilayer lying close to the center of the gap. Investigation of the dependence of the TMR on the height of the GaAs barrier (position of the Fermi energy EF in the gap) shows that the TMR of a perfect junction is strongly enhanced when EF lies at the resonance in the minority-spin channel. However, we show that any small asymmetry of the junction removes the TMR peak and reduces the TMR to small values, of the order of 50%, for a rather large interval of values of E F in the vicinity of the middle of the GaAs gap. We thus conclude that the spin-orbit coupling leads to saturation of the TMR with GaAs thickness, but the saturation value is determined by the presence of the resonance.