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Thick O and thin O soft intergranular phase

Easy axes in neighbouring grains

' Single phase exchange coupled material (Mr > and Hc <)

Thick O and thin O soft intergranular phase

Easy axes in neighbouring grains

Figure 12. Influence on the remanent magnetization and the coercive force of the occurrence of direct exchange coupling (single phase material) and of coupling through intergranular phases having different thicknesses (composite materials).

to an isotropic uniaxial material). The enhancement is linked to the fact that the moments forming the transition structure point away from their associated easy axis direction and closer to the direction of the previous saturation. On the other side, they produce a significant coercivity decrease, related to the fact that they act as very effective nucleation sites (Fig. 12) for the reversal of the magnetization (the reversal process can, alternatively be seen as corresponding to the depinning from the grain boundary region of the domain wall-like structures). The combination of these two effects is, nevertheless interesting in general because it can result in an increase of the maximum energy product, (BH)max, the parameter giving the figure of merit of a permanent magnet material (for a coercive force value sufficiently high so as to effectively support the remanence, the maximum energy product is proportional to ^0M2 /4). Let us finally say that the first single-phase materials exhibiting a significant remanence enhancement contained Si and Al additions, and it was believed that the occurrence of small grains strongly exchange coupled was linked to the presence of those additives. Today it is clear that both the nanostructure and the remanence enhancement are a consequence of a nucleation ruled crystallization process originating with narrow grain boundaries.

Going now into the case of the multiphase nanostructured materials and, as has been discussed previously, the basic idea behind the use of rare-earth transition metal inter-metallic phases for the preparation of permanent magnet materials was the achievement of a combination of high magnetic anisotropy with high magnetization values at room and higher temperatures (a property linked to the large transition metal exchange and Curie temperature). This combination also can be implemented by adequately coupling a high anisotropy phase (as, mainly, a rare-earth transition metal intermetallic) with a phase exhibiting high magnetization (usually a transition metal, a transition metal inter-metallic, or a transition metal-metalloid alloy). The point has been analyzed in detail, and it is well established that the requisites to achieve such a coupling are linked to the nanostructure of the phases. It is necessary to produce an intimate phase mixture, essentially consisting of hard grains embedded into a soft phase and strongly exchange coupled to it. Since the dominating exchange coupling both in the hard phase and in the soft one is the transition metal-transition metal one, the effective magnitude of the intergranular coupling is ruled both by the boundary thickness and by the amount of disorder present at the boundary. In the case of an intense hard-soft coupling, the optimum thickness of the soft phase in between two hard grains corresponds to half the hard-phase domain-wall width.

This optimum value results from a twofold compromise: first, it should correspond to a large polarization of the magnetization of the soft phase along the direction of the magnetization of the neighboring hard grains and, consequently, to a high remanence (this, in principle, could be achieved even with a large soft-phase thickness, provided the interphase coupling could be large enough), and, second, it should preserve, as much as possible, the coercivity of the hard phase (which limits the soft-phase thickness, since it should be kept below the soft-phase exchange correlation length to avoid the easy complete magnetization reversal inside the soft phase, which could propagate into the hard one due to the large interphase coupling). As for the average hard-phase grain dimensions and, again, in order to achieve high remanences, they should be slightly larger than the hard-phase exchange correlation length. There exist a quite broad group of hard-soft composites meeting (to a different extent) the requirements corresponding to the optimized hysteretic behavior. They are based on the Nd2Fe14B and SmCo5 phases and contain as soft phase a-Fe and Fe3B.

To end this section, we will discuss the case corresponding to the presence of antiferromagnetic phases at the intergranular region. It recently has been shown [106] that in mechanically ground samples formed by SmCo and NiO (the antiferromagnetic phase), the intimate mixture of both phases results in a remarkable increase of the coercivity with respect to that measured in the NiO-free SmCo samples. The origin of this increase has been associated with the exchange coupling between the hard and the antiferromag-netic phase that transmits the anisotropy of the latter to the easily reversible moments on the edges and corners of the SmCo grains.

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