A microscopic mechanism for rejuvenation and memory effects in spin glasses

Aging in spin glasses (and in some other systems) reveals astonishing effec ts of 'rejuvenation and memory' upon temperature changes. In this paper, we propose microscopic mechanisms (at the scale of spin-spin interactions) wh ich can be at the origin of such phenomena. Firstly, we recall that, in a f rustrated system, the effective average interaction between two spins may t ake different values (possibly with opposite signs) at different temperatur es. We give simple examples of such situations, which we compute exactly. S uch mechanisms can explain why now ordering processes (rejuvenation) seem t o take place in spin glasses when the temperature is lowered. Secondly, we emphasize the fact that inhomogeneous interactions do naturally lead to a w ide distribution of relaxation times for thermally activated flips. 'Memory spots' spontaneously appear, in the sense that the flipping time of some s pin clusters becomes extremely long when the temperature is decreased. Such memory spots are capable of keeping the memory of previous ordering at a h igher temperature while now ordering processes occur at a lower temperature . After a qualitative discussion of these mechanisms, we show in the numeri cal simulation of a simplified example that this may indeed work. Our concl usion is that certain chaos-like phenomena may show up spontaneously in any frustrated and inhomogeneous magnetic system, without impeding the occurre nce of memory effects.