Spin glass experiments
Résumé
A spin glass is a diluted magnetic material in which the magnetic moments are randomly interacting, with a huge number of metastable states which prevent reaching equilibrium. Spin-glass models are conceptually simple, but require very sophisticated treatments. These models have become a paradigm for the understanding of glassy materials and also for the solution of complex optimization problems. After cooling from the paramagnetic phase, the spin glass remains out of equilibrium, and slowly evolves. This aging phenomenon corresponds to the growth of a mysterious "spin-glass order", whose correlation length can be measured. A cooling temperature step during aging causes a partial "rejuvenation", while the "memory" of previous aging is stored and can be retrieved. Many glassy materials present aging, and rejuvenation and memory effects can be found in some cases, but they are usually less pronounced. Numerical simulations of these phenomena are presently under active development using custom-built supercomputers. A general understanding of the glassy systems, for which spin glasses bring a prominent insight, is still under construction.