[1] UCL, London Ctr Nanotechnol, London WC1E 6BT, England
[2] UCL, Dept Phys & Astron, London WC1E 6BT, England
[3] Diamond Light Source Ltd, Didcot OX11 0DE, Oxon, England
[4] Univ Bristol, Interface Anal Ctr, Royal Commiss Exhibit Res Fellow 1851, Bristol BS2 8BS, Avon, England
[5] Deutsch Elektronen Synchrotron DESY, D-22607 Hamburg, Germany
[6] Ecole Polytech Fed Lausanne, ICMP, Lab Quantum Magnetism, CH-1015 Lausanne, Switzerland
[7] Paul Scherrer Inst, Neutron Scattering Lab, CH-5232 Villigen, Switzerland
[8] Univ Geneva, DPMC MaNEP, CH-1211 Geneva, Switzerland
[9] Natl Inst Mat Sci, Tsukuba, Ibaraki 3050044, Japan
[10] Univ Edinburgh, Sch Phys, Scottish Univ Phys Alliance, Edinburgh EH9 3JZ, Midlothian, Scotland