  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 PrAu2Si2, a stoichiometric compound with a
well-ordered crystal structure, was reported to exhibit spin glass freezing. In
this article, we report on inelastic neutron scattering measurements of the
crystal field excitations, which show that PrAu2Si2 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.
