J. Phys. I France
Volume 2, Numéro 12, December 1992
Page(s) 2299 - 2309
DOI: 10.1051/jp1:1992282
J. Phys. I France 2 (1992) 2299-2309

Crossover from equilibrium to nonequilibrium dynamics in a reentrant AuFe ferromagnet

P. Mitchler, R. M. Roshko and W. Ruan

Department of Physics, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2

(Received 26 May 1992, accepted in final form 11 August 1992)

Measurements of the decay of the low field thermoremanent magnetization, over four decades of observation time, 6 s $\le t\le 10^4$ s, as a function of temperature T and wait time $t_{\rm w}$, have been performed on two AuFe alloys located on either side of the multicritical point : a Au 90Fe 10 spin glass and a strongly bond-disordered reentrant Au 83Fe 17 ferromagnet. The spin glass relaxation isotherms exhibit the canonical symptoms of nonequilibrium ordering : below $T_{\rm SG}$, the isotherms are characterized by an inflection point, on a logarithmic time perspective, which is sensitive to the wait time $t_{\rm w}$, display a memory for a history of field reversals, and are all compatible with an analytical representation based on a stretched exponential function, $M_{\rm R}(t)=M_0+M_1\exp [-(t/\tau)^{1-n}]$, with $\tau=t_{\rm infl}$ and a weakly temperature and wait time dependent exponent $n\cong 0.7$. The reentrant ferromagnet exhibits a temperature-driven crossover between two distinct relaxation regimes : a "high temperature" regime coincident with the ferromagnetic phase, characterized by an extremely rapid approach to equilibrium, negligible wait time effects, and a weak power law decay, $M_{\rm R}(t)=M_0+M_1t^{-m}$, with a weakly temperature dependent exponent $0.04\le m\le 0.08$, consistent with the predictions of droplet fluctuation models of random Ising ferromagnets, and a "low temperature" regime coincident with the reentrant phase, with nonequilibrium, age-dependent dynamics which are virtually indistinguishable from those in the "pure" spin glass phase and describable by the same stretched exponential relaxation function. This study represents the first systematic comparison of relaxation dynamics in the reentrant and ferromagnetic phases of a system with sequential transitions, and provides compelling evidence for the equivalence of the reentrant and spin glass phases.

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