  The connection between hysteresis and thermal relaxation in magnetic
materials is studied from both the experimental and the theoretical viewpoint.
Hysteresis and viscosity effects are measured in Finemet-type nanocrystalline
materials above the Curie temperature of the amorphous phase, where the system
consists of ferromagnetic nanograins imbedded in a paramagnetic matrix. The
hysteresis loop dependence on field rate, the magnetization time decay at
different constant fields and the magnetization curve shape after field
reversal are all consistent with a single value of the fluctuation field Hf =
8A/m (at 430 C). In addition, it is shown that all data collapse onto a single
curve M(Hath), when magnetization is plotted as a function of a properly
defined field Hath, dependent on time and field rate. Experimental data are
interpreted by assuming that the system consists of an assembly of elementary
bistable units, distributed in energy levels and energy barriers. The
approximations under which one predicts data collapse onto a single curve
M(Hath) are discussed.
