The extra-dimension W = kT Ln (t/τ0) of phase space below the spin glass transition : an experimental study of the relaxation of the magnetization at constant field in CuMn

We present experimental evidence obtained in a CuMn spin glass below Tc showing that for a given history of the sample, the totality of the time t and temperature T dependence of the relaxation in a given field can be condensed into a unique master curve of the variable Wc = T Ln (t/τ0). We discuss the implications from a practical and a fundamental point of view. Particular emphasis is put upon the experimental aspects : the predictive power of this scaling is stressed as well as the considerations that experimentalists should not ignore.Nos mesures de l'alimentation hors-équilibre d'un verre de spin CuMn à T < Tc montrent que, pour une histoire donnée de l'échantillon et dans un champ donné, l'intégralité de la dépendance en temps t et température T des relaxations peut être représentée par une courbe maîtresse unique dans un diagramme M(T Ln (t/τ0)) . Nous discutons les implications d'un point de vue pratique et fondamental. Nous soulignons la puissance de prédiction de cette loi ainsi que les servitudes que les expérimentateurs ne doivent pas ignorer

Erasing the glassy state in magnetic fine particles

BaFe10.4Co0.8Ti0.8O19 magnetic fine particles exhibit most of the features attributed to glassy behavior, e.g., irreversibility in the hysteresis loops and in the zero-field-cooling and field-cooling curves extends up to very high fields, and aging and magnetic training phenomena occur. However, the multivalley energy structure of the glassy state can be strongly modified by a field-cooling process at a moderate field. Slow relaxation experiments demonstrate that the intrinsic energy barriers of the individual particles dominate the behavior of the system at high cooling fields, while the energy states corresponding to collective glassy behavior play the dominant role at low cooling fields