1. Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
2. Elettra Sincrotrone Trieste S.C.p.A, 34149 Basovizza, TS, Italy
3. Institute of Applied Physics, University of Bern, 3012 Bern, Switzerland
4. Istituto Officina dei Materiali CNR, 34149 Basovizza, TS, Italy
5. JILA and Department of Physics, University of Colorado, Boulder, Colorado 80309, USA
6. Paul Scherrer Institute, Villigen 5232, Switzerland
7. Department of Physics, University of Trento, I-38123 Povo, TN, Italy
8. Applied Physics Department, CINVESTAV-Unidad Mérida, Mérida, Yucatán 97310, Mexico