What neutrons do tell us about the nature of (spin) glasses?

A common feature of all glassy systems is the decisive role of dynamics in the evolution of macroscopic properties, while the short and eventually partial long-range order shows little difference between the different phases. Our recent neutron scattering results on a Cu-Mn spin glass dramatically illustrate this point, and provide unambiguous evidence against recent suggestions that behaviour in Cu-Mn is driven by spin density waves rather than glassy dynamics. Recent findings in Au-Fe provide the first circumstantial evidence for the theoretically predicted extremely long correlation lengths at the spin-glass transition. Recent experimental advances opened up the way to investigate dynamics in glasses and liquids on extended length scales, where only spin correlations could be studied by now. (C) 2000 Elsevier Science B.V. All rights reserved.