Magnetic boson peak in classical spin glasses

We revisited spin dynamics in archetypical classical spin-glass (SG) systems, such as Cu1-xMnx (x = 0.017, 0.034, and 0.067) dilute alloys and iron aluminosilicate glass, using a modern neutron scattering spectrometer with high neutron flux. The former is crystalline, and the latter is amorphous, where their SG state is well separated from magnetically ordered phases. Bose-scaled localized magnetic excitations were observed in both compounds below the spin-freezing temperature (Tf). The spectrum exhibits a maximum at low energy and a broad tail on the high-energy side. The excitation energy tends to be higher for the material with higher Tf. Above Tf, the spectrum considerably changes with temperature, thereby indicating the emergence of the magnetic relaxation process. The magnetic excitation in the SG state has much in common with the boson peak in structural glasses. We consider that the Bose-scaled broad excitation peak is an elementary excitation inherent in disordered systems.