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Magnetic viscosity measurements performed on nanocrystalline SmFeCo spring magnets with different Fe content are analyzed by the Preisach hysteresis model. The model gives a unified description of hysteresis and relaxation properties. Comparison between calculated and measured viscosity permits one to estimate the aftereffect activation volume r. Calculated [v.sup.1/3] values are approximately equal to the average grain size of the hard phase. Index Terms - Aftereffect, hysteresis models, nanocrystalline hard materials, thermal activation.

Research into the interaction between activation volumes and microstructure in nanocrystalline Fe73.5Cu1Nb3Si13.5B9 (Fimemet) alloys is presented. It is suggested that there is a broad distribution of activation volumes controlled by structural disorder.

We present experiments on nanocrystalline materials where thermal activation gives rise to dynamic effects on hysteresis curves. A scheme of interpretation is presented and the typical activation volume is found to be about 100 nm. Index Terms - Thermal activation. Dynamic hysteresis loops. Nanocrystalline soft materials. Magnetic hardening.

Magnetic viscosity measurements on NdFeB spring-magnets are interpreted by means of a thermodynamic generalization of the Preisach hysteresis model. The model correctly predicts the observed decay of magnetization and clarifies the connection between anomalous, non-monotone after-effect and applied field history. Comparison between theory and experiments permits one to estimate the after-effect activation volume, [d.sup.3] [approximately equal to] [(12 nm).sup.3], and the attempt frequency 1/[[Tau].sub.r] [approximately] [10.sup.10] [s.sup.-1].

A thermodynamic model of hysteresis is presented based on an internal variable reinterpretation of the Preisach model. The model is applied to the prediction of nonmonotonic heat exchange along the hysteresis loop measured for ferromagnets.

Magnetic properties of nanocrystalline Finemet ribbons with composition [Fe.sub.73.5] [Cu.sub.1] [Nb.sub.3] [Si.sub.13.5] [B.sub.9] are studied. Six different samples annealed for 1 hour at different temperatures are studied. Measurement of hysteresis loops and of relaxation, performed at temperature above the Curie point of the amorphous phase, are presented and discussed. Index Terms--Nanocrystalline soft materials, thermal relaxation, hysteresis, superparamagnetism.

A vector model of magnetic hysteresis in biaxial anisotropy structures is presented, in which magnetoelastic effects are explicitly taken into account. The magnetization vector of individual grains is forced to lie along one of the easy axis, and jumps from one easy direction to another controlled by the sum of the external field and magnetoelastic energy.