Magnetoresistance of magnetic tunnel junctions in the presence of a nonmagnetic layer

We find that electron propagation across a uniform nonmagnetic metallic layer d(i) in a magnetically layered structure is coherent up to a critical length d(c). This provides a possible explanation for the slow decay of the magnetoresistance of magnetic tunnel junctions when a layer of Cu or Ag is inserted between the magnetic electrode and the barrier. However, for many (most) structures the roughness of the interfaces breaks the coherence for d(i) << d(c). While the loss of coherence is not fatal for the magnetoresistance of metallic multilayers, it is for tunnel junctions. The quantum well states arising from the insertion of a nonmagnetic layer in a magnetic tunnel junction do not promote magnetoresistance.