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.