Two-level-systems in spin glasses : a dynamical study of the magnetizations below TG, application to CuMn systems

A model of distributions of thermally activated two-level-systems (T.L.S.) which imposes itself on the basis of the observed relaxations of the magnetization and of the energy, allows, without further assumptions, to predict or to justify the main features of the hysteresis of spin glasses. We discern the processes where the temperature is varied at constant field which lead to a quasi equilibrium from the isothermal processes which lead to a non-equilibrium situation whose description involves classical laws (Rayleigh's laws) in terms of a reduced temperature T/TG. T G as any characteristic temperature in the problem is a function of the time t of the measurement : $$. Energetic considerations fix a higher limit Tc to the domain where this scaling, characteristic of a glass transition, is valid, which permits to reconcile the apparently contradictory data which report a frequency dependent susceptibility maximum in some systems, a frequency independent one in others. It is the existence of well characterized distributions of T.L.S. which is important rather than the microscopical description of the objects themselves which are hidden behind the T.L.S. concept. This is the disorder which is responsible for the character of the distributions hence for the generality of the model