Experimental studies of the spin glass dynamics: Towards a better understanding of the spatial correlations in the spin glass phase?

Abstract

Dynamical studies of spin glasses (from susceptibility, relaxation, noise measurements) are reviewed. Even if it is not a priori obvious to extract a universal behavior from the great amount of existing data and the variety of the fits proposed, some general features are emerging from the dynamics of 3D macroscopic spin glasses. One of them is the existence of a very wide spectrum of relaxation times without any measurable low frequency cutoff in the low temperature spin glass phase in the limit of infinite waiting times. This result is compatible with an algebraic (involving a very small exponent) or a logarithmic decay of the two-spin time correlation function at low temperature. On the other hand, there is no direct way to measure the spatial correlation function Γ( R) of the spin glass order parameter which involve four spin correlation products. There are however some indirect indications, coming from the singular behavior of the nonlinear magnetisation, that Γ( R) decays also algebraically without any long length cutoff in the low temperature spin glass phase. This leads to the problem of the interplay between spatial and time scales in the system. We discuss some experiments (finite size effects, resistance noise) recently attempted to shed light on this question.