Study of the Spin Glass Transition Using Coherent Magnetic X-ray Scattering*

Spin Glass represents a new state of matter which has been intensely studied experimentally and theoretically since the 1970’s. 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. However, the existence of a true second-order phase transition from the paramagnetic to the spin glass state accompanied by critical fluctuations has been difficult to establish experimentally with neutron scattering owing to the long-time scales involved. We show 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. 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 10 secs.