1. Department of Engineering Materials, University of Sheffield, Sir Robert Hadfield Building, Portobello Street, Sheffield S1 3JD, UK.
2. Department of Physics, University of Durham, South Road, Durham DH1 3LE, UK.
3. Blackett Physics Laboratory, Imperial College London, Prince Consort Road, London SW7 2BW, UK.
4. Durham Magneto Optics Ltd., The University of Durham, Durham DH1 3HP, UK.