1. Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, California 94720, USA
2. Department of Electricity and Electronics, University of the Basque Country, Leioa 48940, Spain
3. Department of Immunology, Microbiology and Parasitology, University of the Basque Country, Leioa 48940, Spain
4. SGIKER Magnetic Measurements, University of the Basque Country, Leioa 48940, Spain
5. Basque Center for Materials, Applications and Nanostructures (BCMaterials), UPV/EHU Science Park, Leioa 48940, Spain
6. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA