Non-equilibrium relaxation dynamics in the spin glass and ferromagnetic phases of CrFe

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Abstract
Measurements of the decay of the thermoremanent magnetization have been performed, over four decades of observation time (2s≤ t≤ 104 s), on a Cr83Fe17 spin glass and a Cr78Fe22 re-entrant ferromagnet as a function of both temperature and the age of the system. The spin-glass relaxation isotherms exhibit temperature-dependent and wait-time-dependent inflection points on a logarithmic time perspective, and are all accurately described, over the entire observation window, by the product of a power law and a stretched exponential superposed on a constant baseline. Temperature cycling experiments provide compelling evidence for the existence of an overlap length and thus support droplet/domain models of non-equilibrium dynamics. In the ferromagnet, two distinct thermal relaxation regimes are distinguishable: a low-temperature regime (T>35K), where the isotherms exhibit the non-equilibrium, age-dependent, characteristics of a spin glass and are described analytically by a ‘pure’ stretched exponential superposed on a constant term, and a high-temperature regime (T≥40K), where the relaxation is characteristic of thermal equilibrium and is accurately represented by a weak power-law decay. Comparison with the relaxation dynamics of a ‘pure’ Pdg98·6Fe1·4 Heisenberg ferromagnet confirms that the latter behaviour is symptomatic of ferromagnetic ordering, while the former provides convincing support for a re-entrant collapse into a spin-glass-like ground state.