Glassy behavior : chaos and other problems in spin glasses

Spin glasses (SG) exhibit many of the common properties of glassy behavior: a phase transition at T [subscript g], gradual freezing below T [subscript g], and an out-of-equilibrium aging sensitive to initial conditions. The work in this dissertation focuses on four effects: glassy dynamics in SG thin films, magnetic field effects on SG dynamics, classification of aging rates of the SG correlation length, and SG temperature chaos. The experiments were performed on the canonical metallic SG, CuMn, studied in multilayer thin films, and on a CuMn single crystal. The beauty of working with a thin film geometry lies with the growing correlation length ξ⊥(t) saturating at the thin film thickness L on laboratory time scales, thereby creating a quasi-equilibrium state. From studying the crossover dynamics on 4.5, 9 and 20 nm thin films, a thermal activation picture was developed to describe the dynamics after dimensional crossover: from ξ⊥(t) < L (three dimensional) to ξ⊥(t) = L (two dimensional). The maximum free energy barrier height governing the dynamics scales as ℓnL, compatible with power law dynamics of the Parisi mean field solution, and not the activated dynamics of the droplet model of Fisher and Huse. The freezing temperature T [subscript f] < T [subscript g] in thin films was studied systematically and related to the observation time scale. The free energy barrier heights are reduced by an external magnetic field, enabling us to obtain the number of correlated spins in the 20 nm CuMn multilayer sample. The geometry of the correlated spins is found to be “pancake-like” or cylindrical in the thin films. In order to remove the finite size constraint on the growth of SG correlation length ξ⊥(t), aging rates were extracted from long time aging experiments on a single crystal CuMn sample. The extracted correlation length is roughly 200 lattice spacing, the largest ever achieved. The results were compared with large scale numerical simulation from the Janus Collaboration, and an almost perfect match was found. The aging rates slows down as the SG correlation length increases, indicated by T = 0 fixed point dynamics. Benefiting from quasi-equilibrium states in thin films after dimensional crossover, temperature chaos, the sensitivity of spin glass equilibrium state to external temperature changes, was explored in the 9 nm and 20 nm multilayer CuMn thin films. The results have been rather counterintuitive. The observable effective waiting time differs with respect to different cooling protocols. A deeper understanding of temperature chaos requires further investigation.