To conclude this chapter, and in order to highlight the most relevant point in our exposition, we would like to come back to an idea already mentioned in the introduction: the crucial role on the achievement of controlled (and optimized) hysteretic properties of the grain boundary regions.

As we have discussed, the occurrence of strong exchange intergranular coupling in nanocrystalline systems having large exchange correlation lengths, results in a dramatic softening of the material arising from the effective average of the local anisotropies. Differently from this, in the case of the single phase, high anisotropy materials, used to produce permanent magnets, it is highly desirable to reduce as much as possible the intergranular exchange in order to hinder the propagation of a local reversal induced by a moderate demagnetizing field at an anisotropy defect. In the case of the recording media, the role of the exchange coupling is even more subtle: it should, in principle, be kept as reduced as possible in order to reduce the bit size, but a moderate amount of coupling could also account for an improvement in the signal-to-noise properties of the material. Finally, and now in relation to composite nanocrystalline materials, it again is the interphase exchange magnitude that is the main factor ruling the achievement from the properties of the individual phases, enhanced hysteretic behavior.

Although all these effects (and many other similar) are qualitatively well understood, we are still today quite far from effectively tailoring the grain boundary properties to meet the different requirements involved in an optimization process. Thus, it is a clear priority for the research in the field of the nanocrystalline materials to develop experimental techniques to locally determine the grain boundary structure and properties and, in parallel to this, to implement realistic quantitative simulations of the influence on the global properties of the intergranular regions. If these goals are reasonably achieved, it is possible to foresee the possibility of getting, both from single phase and composite materials, ranges of hysteretic properties much broader than those available today.

0 0

Post a comment