Long-range ordering of reduced magnetic moments in the spin-gap compound CeOs_2Al_10 as seen via muon spin relaxation and neutron scattering

We have carried out neutron diffraction, muon spin relaxation (mu SR), and inelastic neutron scattering (INS) investigations on a polycrystalline sample of CeOs2Al10 to investigate the nature of the phase transition observed near 29 K in the resistivity and heat capacity. Our mu SR data clearly reveal coherent frequency oscillations below 28 K, indicating the presence of an internal field at the muon site, which confirms the long-range magnetic ordering of the Ce moment below 28 K. Upon cooling the sample below 15 K, unusual behavior of the temperature-dependent mu SR frequencies may indicate either a change in the muon site, consistent with the observation of superstructure reflections in electron diffraction, or a change in the ordered magnetic structure. Neutron diffraction data do not reveal any clear sign of either magnetic Bragg peaks or superlattice reflections. Furthermore, INS measurements clearly reveal the presence of a sharp inelastic excitation near 11 meV between 5 and 26 K, due to opening of a gap in the spin-excitation spectrum, which transforms into a broad response at and above 30 K. The magnitude of the spin gap (11 meV) as derived from the INS peak position agrees very well with the gap value as estimated from the bulk properties.