AGING IN SPIN-GLASSES AS A RANDOM-WALK - EFFECT OF A MAGNETIC-FIELD

We investigate the role of a magnetic field on the aging properties of spin glasses, focusing both on ac susceptibility measurements with an additional field variation and on thermoremanent magnetization (TRM) relaxation (to which the field variation is inherent). We propose a scaling of the TRM decay curves for various amplitudes of the magnetic field. Regarding aging as a random walk in a set of traps, we analyze the transient modification of the dynamics after a field change as twofold: an average reduction of relaxation times, well descried by a Zeeman reduction of the energy barriers (hard traps), together with an ''emptying out'' of some other relaxation modes (fragile traps) corresponding to a partial quench effect. The fragile traps are associated with the longest time scales, that is to processes involving a large number of spins. a quantitative analysis of the results allows us to estimate a typical number of spins involved in both types of traps, as well as to discuss the growth of the energy barriers as a function of the number of spins.