Composite Materials

Few micromagnetic calculations have been performed in soft nanostructured magnetic materials. One of the possible explanations is that due to the small value of the anisotropy, the discretization length in soft magnetic materials is so small that the possible total size of the calculations is very limited. In Ref. [123], the micromagnetic simulations have been performed for magnetic permeability of composite soft materials (thoroughly mixing iron powder). The initial susceptibility was shown to increase as a function of packing density in a highly nonlinear manner and was shown to decrease as a function of the intrinsic susceptibility. Interestingly, the computational results were significantly different from the mean-field approximation, which indicates an important contribution of the nearest-neighbor interactions.

At the same time, the nanostructured soft-hard magnetic systems such as, for example, exchange-spring permanent magnets, have become a topic of interest since a mixture of hard and soft phases reduces the rare-earth contest while preserving good magnetic properties. Reversal mechanisms in these systems can be interpreted schematically as two-step processes: the initial magnetization reversal is dominated by the reversible nucleation in the soft phase, while irreversible processes, which characterize the coercivity mechanisms, occur at larger field values and are determined by the hard phase. The micromagnetic calculations (e.g. Figs. 13, 17, 18) have been performed to predict qualitatively the ratio between soft and hard phases optimal to establishing a high coercivity and remanence. By using a 1D micromagnetic model, Kneller and Hawig [157] found that the optimum microstructure for improved magnetic properties of a hard magnet would be hard grains embedded in a magnetically soft matrix with lateral dimensions of both phases about equal to the domain-wall width of the hard phase.

Later simulations [111] (Fig. 13) in soft-hard composites (grains of a-Fe and Fe14Nd2B) showed that the rema-nence was increased as a function of the volume fraction of magnetically soft grains, and this enhancement is higher for smaller grains. Remarkably, unlike pure hard nano-structured materials, the coercivity, although reduced, was high (of the order of 1 T; Fig. 18) for microstructures that have magnetically soft phase grain smaller than twice the domain-wall width of the hard grain (10-nm grains in the case described above). For NdFeB magnets, coercivity and remanence [158] also were calculated by using soft magnetic grain of a-Fe surrounded by hard magnetic grains with random easy axes. If the intergrain exchange constant were reduced by 1/10th of its bulk value, irreversible switching of the magnetization was shown to shift by more than 259 kA/m toward higher values of the opposite field, since the grains become more decoupled. Figure 18 presents results of micromagnetic simulations for different soft-hard contents and different grain sizes by using a micromagnetic model similar to that of Figure 17.

0 0

Post a comment