We acknowledge useful discussions with R. Baumbach, C. Pfleiderer, M. Garst, M. Votja, P. Böni and J. M. Lawrence. Work at Los Alamos National Laboratory (LANL) was performed under the auspices of the US Department of Energy. LANL is operated by Los Alamos National Security for the National Nuclear Security Administration of DOE under contract DE-AC52-06NA25396. Research supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under the project ‘Complex Electronic Materials’ (material synthesis and characterization) and the LANL Directed Research and Development program (neutron scattering, development of the spin wave model, mean-field computation and development of analysis software). Research conducted at Oak Ridge National Laboratory’s (ORNL) Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, US Department of Energy. Experiments at the ISIS Pulsed Neutron and Muon Source were supported by a beam time allocation from the Science and Technology Facilities Council. We acknowledge the support of the National Institute of Standards and Technology, US Department of Commerce, in providing the neutron research facilities used in this work.