Magnetic excitations in the Zn-Mg-Tb icosahedral quasicrystal: An inelastic neutron scattering study

Low-temperature spin dynamics in the face-centered-icosahedral Zn-Mg-Tb quasicrystal has been investigated by inelastic neutron scattering around its spin-glass-like freezing temperature (T_f≃5.8 K). A single broad inelastic peak, centered at ℏω≃2.5 meV, was observed in the inelastic scattering spectrum at temperatures ranging from the base temperature up to ∼20 K. The inelastic peak energy is Q independent, whereas the peak intensity shows weak Q dependence, which is qualitatively similar to that observed for the elastic diffuse scattering. The inelasticity of spin excitations is hardly seen in canonical spin glasses and thus is a distinct feature of the Zn-Mg-Tb quasicrystal. We argue, with an aide of numerical simulation, that this broad inelastic peak can be interpreted as localized collective fluctuations of short-range-ordered spins in a dodecahedral spin cluster. In a much lower-energy region (ℏω<0.8 meV), we observed a strong quasielastic signal, appearing only above T_f; its peak width decreases as the temperature is lowered and vanishes at T_f. It is thus evident that the quasielastic signal corresponds to a slow spin dynamics that freezes at the macroscopic freezing temperature. The coexistence of the inelastic peak and quasielastic signal suggests that the short-range order in the single cluster is robustly formed but is dynamic for T_f<T<20 K. The macroscopic freezing at T_f is, thus, attributed to a random freezing of spin-cluster fluctuations.