Andreev-reflection spectroscopy of ferromagnets: the impact of Fermi surface mismatch

TL;DR

This study investigates Andreev-reflection spectra at superconductor-ferromagnet interfaces, highlighting the importance of Fermi surface mismatch and lifetime effects in accurately determining magnetic polarization.

Contribution

It introduces the role of Fermi surface mismatch and lifetime effects in interpreting Andreev-reflection spectra, improving the accuracy of polarization measurements.

Findings

Co polarization agrees with previous observations

Lifetime effects can describe spectra as well as polarization

Fermi surface mismatch influences Andreev reflection measurements

Abstract

We have investigated point contacts between a superconductor (Nb, AuIn2) and a normal metal (ferromagnetic Co, non-magnetic Cu). The observed Andreev-reflection spectra were analyzed using the modified BTK theory including spin-polarization effects. This resulted in a polarization of Co that agrees with observations by others, but lifetime effects describe the spectra equally well. On the other hand, the spectra with non-magnetic Cu can be well described using the spin-polarization model. The ambiguity between polarization and lifetime interpretation poses a dilemma which can be resolved by considering the normal reflection at those interfaces due to Fermi surface mismatch. Our data suggest that Andreev reflection at Nb - Co contacts does deliver the true magnetic polarization of Co only when lifetime effects and the above mentioned intrinsic normal reflection are included.