1. Biophysics Group, Department of Physics and Astronomy, University College London, London WC1E 6BT, United Kingdom
2. UCL Healthcare Biomagnetics and Nanomaterials Laboratories, 21 Albemarle Street, London W1S 4BS, United Kingdom
3. Department of Chemical Engineering, University College London, London WC1E 7JE, United Kingdom
4. International Iberian Nanotechnology Laboratory, Braga 4715-330, Portugal
5. Superconductivity and Magnetism Group, Physics Department, University of Warwick, Coventry CV4 7AL, United Kingdom