  We inserted non-magnetic layers of Au and Cu into sputtered AlOx-based
magnetic tunnel junctions and Meservey-Tedrow junctions in order to study their
effect on tunnelling magnetoresistance (TMR) and spin polarization (TSP). When
either Au or Cu are inserted into a Co/AlOx interface, we find that TMR and TSP
remain finite and measurable for thicknesses up to several nanometres.
High-resolution transmission electron microscopy shows that the Cu and Au
interface layers are fully continuous when their thickness exceeds ~3 nm,
implying that spin-polarized carriers penetrate the interface noble metal to
dis- tances exceeding this value. A power law model based on exchange
scattering is found to fit the data better than a phenomenological exponential
decay. The discrepancy between these length scales and the much shorter ones
reported from x-ray magnetic circular dichroism studies of magnetic
proximitization is ascribed to the fact that our tunnelling transport
measurements selectively probe s-like electrons close to the Fermi level. When
a 0.1 nm thick Cu or Au layer is inserted within the Co, we find that the
suppression of TMR and TSP is restored on a length scale of <=1 nm, indicating
that this is a sufficient quantity of Co to form a fully spin-polarized band
structure at the interface with the tunnel barrier.
