Relaxation dynamics in some reentrant disordered magnetic systems, FeNiCr, FeNiMn, CrFe

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Li, Dawei
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The relaxation response of three reentrant disordered ferromagnetic systems Fe$\sb{0.65}$Ni$\sb{0.35-x}$Cr$\sb{x}$ $(x=0.11,\ 0.12),$ Cr$\sb{1-x}$Fe$\sb{x}$ $(x=0.21),$ and (Fe$\sb{0.65}$Ni$\sb{0.35})\sb{1-x}$Mn$\sb{x}$ $(x=0.118),$ have been investigated over a range of temperatures both below and above their glass temperatures. The measurements were performed with a home-made variable temperature, variable frequency SQUID dc-susceptometer with a temperature range from 4.2 K to 250 K and a dc-field range up to 100 Oe. The relaxation response was measured using a variety of experimental procedures. In the simplest procedure, the sample was field cooled from a reference temperature in the paramagnetic regime to the measurement temperature, held at fixed temperature for a waiting time $t\sb{w},$ after which the field was removed and the response was measured over an observation time $2s\le t\le10\sp4s.$ In more complicated procedures, the temperature was cycled or shifted during the waiting time $t\sb{w}$ or subjected to a field change of varying amplitude. All samples investigated here were characterized by two thermally distinct relaxation regimes, a high temperature regime of equilibrium, power law dynamics which coincided with the ferromagnetic regime, and a low temperature regime of nonequilibrium, age-dependent dynamics which coincided with the reentrant glass phase. The relaxation isotherms in both regimes were fitted to specific functional forms predicted by various models of slow relaxation in disordered systems including Fisher and Huse's droplet scaling theory of domain growth, heirarchically constrained dynamics, Bouchaud's theory of random traps, an Elementary Decay Model based on a stochastic distribution of activation energies, and a percolation theory for relaxation of dispersive excitations within finite domains. The fitting parameters extracted from these fits provided detailed information on the organization of metastable states in the configuration space of a structurally disordered system, on their evolution with temperature, and on their fragility with response to field and temperature fluctuations, and allow us to compare the various theoretical approaches to slow relaxation, establish possible correlations and expose inconsistencies.