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 $d_i\ll d_c$. 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.