VALIDITY OF THE JULLIERE MODEL OF SPIN-DEPENDENT TUNNELING

We consider spin-dependent tunneling between two ferromagnets separate d by a simple step barrier, and examine four models for the magnetocon ductance ratio Delta G/G: A model due to Julliere which characterizes the magnetoconductance solely in terms of the tunneling spin polarizat ion, a model due to Slonczewski which provides an approximate expressi on for the magnetoconductance of free electrons tunneling through a ba rrier, the exact expression for the magnetoconductance of free electro ns tunneling through a barrier, and the numerical calculation of the m agnetoconductance of band electrons in iron tunneling through a barrie r. We find that the Julliere model does not accurately describe the ma gnetoconductance of free electrons tunneling through a barrier. Althou gh Slonczewski's model provides a good approximation to the exact expr ession for free electrons in the limit of thick barriers, we find that the tunneling of band electrons shows features that are not described well by any free electron picture and which reflect the details of th e band structure of iron at the Fermi energy. [S0163-1829(97)05641-5].