Memory and superposition in a spin glass

– Non-equilibrium dynamics in a Ag(Mn) spin glass are investigated by measurements of the temperature dependence of the remanent magnetisation. Using specific cooling protocols before recording the thermo-or isothermal remanent magnetisations on reheating , it is found that the measured curves effectively disclose non-equilibrium spin glass characteristics such as ageing and memory phenomena as well as an extended validity of the superposition principle for the relaxation. The usefulness of this " simple " dc-method is discussed, as well as its applicability to other disordered magnetic systems. Introduction. – The non-equilibrium nature of spin glasses and other glassy systems at low temperature implies that a comprehensive study of the dynamics usually requires sophisticated dc or ac relaxation experiments [1, 2]. Dc relaxation is typically used to investigate the ageing properties in spin glasses. In such experiments, the sample is first cooled from above the transition temperature T g to a constant temperature. After a waiting time t w , a small dc field is applied (or cut off), and the magnetisation is recorded versus time. The relaxation occurs in a characteristic way, and shows a clear dependence on t w [1–3]. Relaxation of the low-frequency ac susceptibility [4–6] is a prime tool to investigate memory effects in glassy magnets. When cooling from above T g , a halt at constant temperature T s <T g is made during t s , allowing the system to relax towards its equilibrium state at T s ; both components of the ac-susceptibility then decay in magnitude. This equilibrated state becomes frozen in on further lowering the temperature, and is retrieved on reheating. The weak low frequency ac-field employed in this kind of experiments does not affect the non-equilibrium processes intrinsic to the sample, but only works as a non-perturbing probe of the system.