1. Center for Quantum Spintronics, Department of Physics, NTNU-Norwegian University of Science and Technology, NO-7491 Trondheim, Norway
2. Department of Electronic Systems, NTNU-Norwegian University of Science and Technology, NO-7491 Trondheim, Norway
3. Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
4. Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA