1. E. Y. Tsymbal is in the Department of Physics and Astronomy, Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, NE 68588, USA.
2. H. Kohlstedt is at the Institut für Festkörperforschung, Forschungszentrum Jülich, D-52425 Jülich, Germany, and the Department of Materials Science and Engineering, University of California, Berkeley, CA 94720, USA.