Soft magnetic nanocrystalline materials

The microstructure property relationship in soft magnetic nanocrystalline Fe-base materials is surveyed: The basic mechanism is that the magneto-crystalline anisotropy of the small, randomly oriented bcc-Fe grains is averaged out by exchange interaction. The resulting magnetic behaviour can be well described with the random anisotropy model. The latter has been extended to multiphase materials including the case that the grain size exceeds the exchange length and, is able now to describe the hardening mechanism in the overannealed nanocrystalline state due to the precipitation of borides. Further, the crucial role of the coupling mechanism between the grains is discussed. Thus, as a function of temperature, a single sample combines a variety of phenomena ranging from soft, over hard to finally superparamagnetic behaviour. Finally it is shown, how the structural phases present lead to low or vanishing magnetostriction; the random anisotropy effect guarantees an essentially isotropic behaviour comparable to the amorphous case. Together with the suppressed magneto-crystalline anisotropy, the low magnetostriction provides the basis for the superior soft magnetic properties observed in particular compositions.