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

Measurements of the decay of the thermoremanent magnetization have been performed, over four decades of observation time (2s less-than-or-equal-to t less-than-or-equal-to 10(4) 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<35 K), 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 greater-than-or-equal-to 40 K), 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' Pd98.6Fe1.4 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.