[1] Tech Univ Denmark, Mat Res Div, Riso DTU, DK-4000 Roskilde, Denmark
[2] Swiss Fed Inst Technol, Neutron Scattering Lab, CH-5232 Villigen, Switzerland
[3] Paul Scherrer Inst, CH-5232 Villigen, Switzerland
[4] ETH, Solid State Phys Lab, CH-8093 Zurich, Switzerland
[5] Ecole Polytech Fed Lausanne, Lab Quantum Magnetism, CH-1015 Lausanne, Switzerland
[6] DESY, Hamburger Synchrotronstrahlungslabor, D-22603 Hamburg, Germany
[7] Iowa State Univ, Dept Phys & Astron, Ames, IA 50011 USA
[8] Iowa State Univ, Ames Lab, Ames, IA 50011 USA