Point-Contact Andreev Reflection in Ferromagnet/Superconductor Ballistic Nanojunctions

A critical analysis of point-contact Andreev reflection (PCAR) in ferromagnet/superconductor ballistic junctions is presented, based on tight-binding Hamiltonians with s, p and d orbitals and material-specific parameters. By accurately modeling the band structure of the bulk materials, we show that to reproduce the measured differential conductance of Cu/Pb and Co/Pb nanocontacts^{1, 2} one needs a detailed understanding of interface parameters which goes beyond simple averages of bulk matrix elements. As an example of how such a modification of surface parameters can affect spin-polarized transport, we explore the effect of an enhancement of the surface magnetization and show that this can significantly improve agreement with experiment. We also demonstrate that in general the spin polarization P of the junction may be different from the bulk polarization P of the magnetic metal used. For bulk Co we find P = −0.4, whereas for the Co/Pb interface P = +0.4 and for the Co/Ir interface P = −0.01.