Spin-glass order induced by dynamic frustration

Spin glasses are systems whose magnetic moments freeze at low temperature into random orientations without long-range order. It is generally accepted that both frustration and disorder are essential ingredients in all spin glasses, so it was surprising that PrAu"2Si"2, a stoichiometric compound with a well-ordered crystal structure, was reported to show spin-glass freezing. Here, we report on inelastic neutron scattering measurements of crystal-field excitations, which show that PrAu"2Si"2 has a singlet ground state and that the exchange coupling is very close to the critical value to induce magnetic order. We propose that spin-glass freezing results from dynamic fluctuations of the crystal-field levels that destabilize the induced moments and frustrate the development of long-range magnetic correlations. This novel mechanism for producing a frustrated ground state could provide a method of testing the concept of avoided criticality in glassy systems.