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 $(T_{\mathrm{f}}\simeq 5.8\mathrm{K})$. A single broad inelastic peak, centered at $\hslash \omega \simeq 2.5\mathrm{meV}$, was observed in the inelastic scattering spectrum at temperatures ranging from the base temperature up to $\sim 20\mathrm{K}$. The inelastic peak energy is $\mathit{Q}$ independent, whereas the peak intensity shows weak $\mathit{Q}$ 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 $(\hslash \omega <0.8\mathrm{meV})$, we observed a strong quasielastic signal, appearing only above $T_{\mathrm{f}}$; its peak width decreases as the temperature is lowered and vanishes at $T_{\mathrm{f}}$. 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 $T_{\mathrm{f}}<T<20\mathrm{K}$. The macroscopic freezing at $T_{\mathrm{f}}$ is, thus, attributed to a random freezing of spin-cluster fluctuations.

• Received 4 October 2005

DOI:https://doi.org/10.1103/PhysRevB.73.054417