Quasicrystals have distinct spatial symmetry characterized by highly-ordered but non-periodic (quasiperiodic) atomic structure, which differs both from the periodic and random structures. Ordering and excitations of quasiperiodically arranged magnetic moments (spins) are yet fundamental open problems, despite the intensive efforts continuously made since the discovery of the quasicrystal. In this talk I will present recent development of understanding on the static and dynamic spin structures in quasicrystalline magnets, using the extensively studied Zn-Mg-RE systems as typical examples.
In magnetization measurements, the Zn-Mg-RE quasicrystals all show spin-glass-like behavior, indicating random freezing of spins at low temperatures, however, well-defined short-range order have been observed in the neutron scattering. The inelastic response of Zn-Mg-RE falls into two classes: For RE = Tb and Dy, a broad inelastic peak has been observed with very weak Q dependence, suggesting an existence of the strongly localized collective excitation modes. On the other hand, for RE = Ho, temperature-independent S(Q, h ω ) was observed in the neutron-energy-gain side (h ω >0) for an incredibly large temperature range (up to 200 K!). The anomalous spin fluctuations may be related to criticality of electron wave functions in the quasiperiodic lattice.