Origin of tunneling spin polarization

Full text: The relationship of the spin polarization of current tunneling from a ferromagnetic material through a tunnel barrier to its magnetization is of considerable current interest, particularly with regard to the development of magnetic tunnel junctions with high tunneling magnetoresistance suitable for magnetic memory and sensor applications 1. The tunneling spin polarization (TSP) can be directly measured using superconducting tunneling spectroscopy (STS) or inferred from measurements of tunneling magnetoresistance (TMR). The TSP depends not only on the ferromagnetic material but also on the tunneling barrier. Using crystalline (100) oriented MgO tunnel barriers and bcc CoFe electrodes, TSP values exceeding 85% are found from STS studies at 0.25 K with TMR values greater than 300% at 5K2. The same ferromagnetic electrodes using conventional amorphous alumina tunnel barriers exhibit TSP values of only ∼50%. In these cases the sign of TSP is positive corresponding to the preferential tunneling of majority spin electrons. However, using magnetic electrodes formed from the ferrimagnetic rare-earth transition-metal alloys, such as Co-Gd, we show that the sign of the TSP and TMR can be either positive or negative depending on the composition of the alloy and temperature. For alloys of Co and Fe with Pt and Pd we show that the TSP is strongly influenced by bonding at the tunnel barrier/ ferromagnetic electrode interface. In particular, we compare results for CoPt alloys with AlN and Al2O3 tunnel barriers. For AlN barriers the TSP decreases linearly with increasing Pt content whereas for Al2O3 tunnel barriers the TSP has only a weak dependence on Pt content for up to ∼50 atomic % Pt. These results can be understood as a consequence of stronger Co - oxygen bonds as compared to Pt - oxygen bonds. References: 1. S. S. P. Parkin, X. Jiang, C. Kaiser, et al., Proc. IEEE 91, 661 (2003). 2 . S. P. Parkin, C. Kaiser, A. Panchula, et al., Nature Mater. 3, 862 (2004)