Glassy Dynamics: Spin Glasses at the Mesoscale

Spin glasses are an exemplar of glassy dynamics, exhibiting aging, freezing, and other correlation properties. The lower critical dimension is < 3 so that, by working with CuMn/Cu multilayers at the mesoscale (CuMn thicknesses L less than ~ 20 nm), the spin glass can be frozen into a metastable state. The perpendicular component of the correlation length grows to L but the parallel component of the correlation length grows to the d = 2 correlation length. The combination of the two conditions leads to a “pancake-like” correlated volume. Comparison of mesoscale and bulk waiting time effects supports this interpretation. Sessation of correlation length growth results in a largest free energy barrier of height independent of T. Metastable states occupancies cause the largest barrier to dominate, leading to glassy dynamics. The L-dependence of the largest barrier height distinguishes between two competing theories for spin glass dynamics: activated vs power law dynamics, the latter in better agreement with experiment. The largest barrier height’s change with temperature and magnetic field exhibits temperature chaos and determines the size of the correlated regions, respectively.