Spin-glass order induced by dynamic frustration

Abstract

Disorder and geometric frustration usually lead to magnetic spins that point in random directions, as in a spin glass. So how can spin-glass behaviour emerge in a well-ordered system without static frustration? The presence of ‘dynamic frustration’ may explain the situation. Spin glasses are systems whose magnetic moments freeze at low temperature into random orientations without long-range order1. 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 reported2 to show spin-glass freezing. Here, we report on inelastic neutron scattering measurements of 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.