Aging phenomena in spin glass and ferromagnetic phases : domain growth and wall dynamics

PACS. 02.75.50.Lk – Spin glasses and other random magnets. PACS. 03.75.10.Nr – Spin-glass and other random models. Abstract. – We compare aging in a disordered ferromagnet and in a spin glass, by studying the different phases of a reentrant system. We have measured the relaxation of the low-frequency ac susceptibility χ, in both the ferromagnetic and spin-glass phases of a CdCr1.9In0.1S4 sample. A restart of aging processes when the temperature is lowered ('chaos-like' effect) is observed in both phases. The memory of previous aging at a higher temperature can be retrieved upon reheating , but in the ferromagnetic phase it can rapidly be erased by the growth of ferromagnetic domains. We interpret the behaviour observed in the ferromagnetic phase in terms of a combination of domain growth and pinned wall reconformations, and suggest that aging in spin glasses is dominated by such wall reconformation processes. Introduction. – In a spin glass, cooling below T g is the starting point of the slow 'aging' evolution. The ac susceptibility relaxes downwards with time after the initial quench (the 'age') (see e.g. [1] and references therein). A natural basis for the interpretation of aging is the idea of a slow domain growth of a spin-glass type 'ordered phase', as was developed in [2, 3]. However, several non-trivial experimental features of aging in spin glasses are hard to interpret within a simple domain growth scenario [4, 5, 6]. Namely, after aging at a given temperature, aging is reinitialized by a further temperature decrease (1), and the memory of previous aging can be retrieved upon heating back. On the other hand, domain growth phenomena should be more directly relevant to the case of ferromagnets. Our motivation in the present work is to compare aging in these systems and in spin glasses, having in mind the discussion of a consistent picture of aging in the latter, more complex case.