  In the 1970s a new paradigm was introduced that interacting quenched systems,
such as a spin-glass, have a phase transition in which long time memory of
spatial patterns is realized without spatial correlations. The principal
methods to study the spin-glass transition, besides some elaborate and elegant
theoretical constructions, have been numerical computer simulations and neutron
spin echo measurements . We show here that the dynamical correlations of the
spin-glass transition are embedded in measurements of the four-spin
correlations at very long times. This information is directly available in the
temporal correlations of the intensity, which encode the spin-orientation
memory, obtained by the technique of resonant magnetic x-ray photon correlation
spectroscopy (RM- XPCS). We have implemented this method to observe and
accurately characterize the critical slowing down of the spin orientation
fluctuations in the classic metallic spin glass alloy Cu(Mn) over time scales
of 1 to 1000 secs. Our method opens the way for studying phase transitions in
systems such as spin ices, and quantum spin liquids, as well as the structural
glass transition.
