Localized Magnetic Excitations in Classical Spin Glasses

Spin glasses (SGs) are ubiquitous in magnetic materials having competing interactions and randomness. Complex magnetic relaxation describes the characteristics of SGs and has been actively studied. However, excitation features in an SG state are little understood. To explore an elementary excitation inherent to the SG state, inelastic neutron scattering (INS) experiments have been performed for classical SG systems with significantly different spin glass transition temperatures (TSG = 0.4 - 32 K), magnetic ionic liquid (C4mimFeCl4), iron aluminosilicate glass (FeO-Al2O3-SiO2), and Cu1-xMnx dilute alloys, mainly using AMATERAS spectrometer at J-PARC. The former two materials are amorphous and the latter is crystalline, where their SG state is well separated from magnetically ordered phases. Bose-scaled localized magnetic excitations were observed in all the materials below TSG. The INS spectrum exhibits a maximum at low energy and a broad tail on the high-energy side. The magnetic excitation in the SG state is highly reminiscent of “boson peak”, local vibrational modes commonly observed in structural glasses. We consider that the Bose-scaled broad excitation peak is a general feature of classical systems with quenched disorder.