Connection between hysteresis and thermal relaxation in magnetic materials

The connection between hysteresis and thermal relaxation in magnetic materi als is studied from both the experimental and the theoretical viewpoint. Hy steresis and viscosity effects are measured in Finemet-type nanocrystalline materials above the Curie temperature of the amorphous phase, where the sy stem consists of ferromagnetic nanograins imbedded in a paramagnetic matrix . The hysteresis loop dependence on field rate, the magnetization time deca y at different constant fields, and the magnetization curve shape after fie ld reversal are all consistent with a single value of the fluctuation field H-f similar or equal to 8 A m(-1) (at 430 degrees C). In addition, it is s hown that all data collapse onto a single curve M(H-ath), when magnetizatio n is plotted as a function of a properly defined field H-ath, dependent on lime and held 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 pred icts data collapse onto a single curve M(H-ath) are discussed.