Role of the barrier in spin-dependent tunneling addressed with superconductor spectroscopy

To establish the role of the barrier material in spin-polarized tunneling, the authors directly measure the sign and magnitude of the tunneling spin polarization in Al/barrier/ferromagnet junctions with different barriers using the Zeeman-split superconducting d. of states of the Al electrode. It is shown that clear Zeeman splitting is difficult to obtain with heavy metal oxide barriers, such as HfOx and TaOx, due to a large spin-orbit scattering rate most likely induced by the heavy atoms at the Al/barrier interface. Junctions with MgO barriers, however, show clear Zeeman splitting and a tunneling spin polarization of +30% with both Co and Fe as the top electrode, a no. which significantly differs from +40% found for our AlOx junctions. We claim that this barrier dependence originates from the electronic structure of the barrier/ferromagnet interface. The pos. tunneling polarization is consistent with the presumption that due to the absence of d orbitals in the MgO barrier, tunneling is dominated by electrons with s character. [on SciFinder (R)