Magnetic Field Effects on Spin Glass Dynamics*

We have measured magnetic field effects on the dynamics of Cu_0.89Mn_0.11 thin film spin glasses of 20 nm thickness arranged in a multilayer structure. We extract the maximum free energy barrier height, Δ_max , by allowing the perpendicular correlation length to reach the film thickness L. This fixes the correlated region in the spin glass, and results in activated dynamics with activation energy Δ_max. The magnitude of Δ_max is extracted by measuring the time dependence of the irreversible magnetization after the perpendicular correlation length has reached L. A new experimental protocol was invoked that accounts for the uncertainty in the time dependence of the field cooled magnetization. The change in the magnitude of Δ_max with change in magnetic field generates the number of correlated spins. We find the correlated region to be of a pancake-like shape because of d = 2 dynamics after the perpendicular correlation length has reached L.