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 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.