New low frequency ac susceptibility measurements on two different spin glasses show that cooling/heating the sample at a constant rate yields an essentially reversible (but rate dependent) χ(T ) curve; a downward relaxation of χ occurs during a temporary stop at constant temperature (ageing). Two main features of our results are: (i) when cooling is resumed after such a stop, χ goes back to the reversible curve (chaos) (ii) upon re-heating, χ perfectly traces the previous ageing history (memory). We discuss implications of our results for a real space (as opposed to phase space) picture of spin glasses.PACS numbers: 75.50.Lk 75.10.Nr to appear in Phys. Rev. Lett.The dynamic properties of the spin glass phase have been extensively studied by both experimentalists and theorists for almost two decades [1,2]. The observed properties reflect the out-of-equilibrium state of the system: the response to a field variation is logarithmically slow, and, in addition, depends on the time spent at low temperature ("ageing"). Ageing is fully reinitialized by heating the sample above the glass temperature T g . It corresponds to the slow evolution of the system towards equilibrium, starting at the time of the quench below T g . Many aspects of ageing are similar to the"physical ageing" phenomena that have been characterized in the mechanical properties of glassy polymers [3]. In the last few years, some interesting progress in the theoretical understanding of ageing in disordered systems has been achieved [4].From the studies of the critical behaviour at T g [5], it appears that the approach of T g is associated to the divergence of a spin-spin correlation length, as is the case in the phase transition of classical ordered systems. In the spin glass phase, the system is out of equilibrium: as in simple ferromagnets, it is tempting to associate ageing with the progressive growth of a typical domain size towards an equilibrium infinite value. However, this simple picture cannot account for all the experimental observations. In particular, the effect of small temperature cycles (within the spin-glass phase) is rather remarkable [6,7]:• on the one hand, ageing at a higher temperature barely contributes to ageing at a lower temperature. Said differently (as will be discussed again below), the thermal history at sufficiently higher temperatures is irrelevant. This is at variance with a simple scenario of thermal activation over barriers, where the time spent at higher temperature would obviously help the system to find its equilibrium state. Everything happens as if there were strong changes of the free-energy landscape with temperature. This point is suggestive of the "chaotic" aspect of the spin glass phase that has been predicted from mean field theory [8] and from scaling arguments in [9,10].• on the other hand, interesting memory effects concomitantly appear: the state reached by the system at a given temperature can be retrieved after a negative temperature cycle.In the present letter, we describe some new experiments which rev...
The inherent dynamic properties of the ferromagnetic phase of a reentrant ferromagnet, (Fe 0.20 -Ni 0.80 ) 75 P 16 B 6 Al 3 , have been experimentally investigated by low field magnetic relaxation and ac susceptibility measurements. A prominent aging behavior and an apparent fatal fragility to temperature fluctuations imply that the ferromagnetic phase of the reentrant system has a chaotic nature similar to the spin glass phase, but in striking contrast to the robust nature of the regular ferromagnetic phase. [S0031-9007(96)01179-9] PACS numbers: 75.40. Gb, 75.50.Lk Random magnets have been intensively studied in the last couple of decades. An important class of random systems is the bond disordered system in which the magnetic interactions are taken randomly from a distribution of both positive (ferromagnetic) and negative (antiferromagnetic) interactions. If all the interactions are ferromagnetic (FM) the low temperature ordered phase is the ferromagnetic phase. If a few interactions, taken at random, are changed to be antiferromagnetic (AF), the low temperature phase will still have long range ferromagnetic order but some magnetic moments will be frustrated. On increasing the concentration of random AF interactions, frustration increases and, above some concentration, long range ferromagnetic order is no longer favorable. In such a bond disordered system the low temperature phase is the spin glass phase [1].Experimentally, there are numerous examples of compounds where the amount of AF and FM interactions can be continuously varied by changing the concentration of a magnetic ion. One such example is ͑Fe x Ni 12x ͒ 75 P 16 B 6 Al 3 [2]. This system is an amorphous metallic glass with RKKY type of interactions between the magnetic ions. Its magnetic properties are mainly determined by the Fe atoms since the magnetic moment of Ni is quenched due to charge transfer from the metalloids. For concentrations x , 0.17 the system is a spin glass and for x . 0.17 the system shows ferromagnetism. In a range of concentrations 0.17 , x , x RSG the system shows reentrant behavior, i.e., on lowering the temperature the sequence of phases paramagneticferromagnetic-spin glass occurs.In this Letter we study the intrinsic dynamics of the ferromagnetic phase of the reentrant ferromagnet ( Fe 0.20 Ni 0.80 ) 75 P 16 B 6 Al 3 , using low field magnetic relaxation experiments. The main conclusion from the investigation is that the ferromagnetic phase of a reentrant ferromagnet is chaotic, in a similar way as the spin glass phase [3], but in contrast to the robust nature of a nonfrustrated ferromagnetic phase.To recall the characteristic temperature dependence and dynamic nature of the susceptibility of a reentrant ferromagnet the ac susceptibility, x͑T , v͒, of (Fe 0.20 Ni 0.80 ) 75 P 16 B 6 Al 3 is shown in Fig.
Non-equilibrium dynamics in a Ag(11%Mn) spin-glass has been studied by low frequency acsusceptibility and magnetic relaxation experiments. The results unequivocally show that spin structures that memorize the cooling process are imprinted in the system. These imprinted structures disclose themselves through dramatic changes of the dynamics on re-heating the spin-glass through the temperatures where intermittent stops or changes of the cooling rate have been imposed. We can qualitatively interpret our results in terms of the droplet spin-glass model developed by Fisher and Huse [Phys. Rev. B 38 (1988) 373; 386].
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