Exchange-coupling interaction and effective anisotropy of NdFeB nanocomposite permanent magnetic materials

Taking Nd2Fe14B/alpha-Fe as a typical sample and adopting the cubic grain model, we investigated the exchange - coupling interaction between different magnetic grains and the effective anisotropy of nanocomposite permanent materials in this paper. The effective anisotropy of nanocomposite permanent materials, K-eff, is equal to statistical mean values of the effective anisotropy of magnetically soft and hard grains. Every grain's anisotropy is determined by the anisotropy constants of the coupled part at grain boundaries and the internal uncoupled part. The calculated results reveal that the distributions of soft and hard grain sizes influence the effective anisotropy. When the sizes of magnetically soft and hard grains are identical, the effective anisotropy decreases with the reduction of grain size and hard phase component, and decreases rapidly while the grain size is less than 20nm. When the soft-grain sizes are of some definite values, K-eff shows a maximum value with the variation of the hard-grain size. The hard - grain sizes corresponding to the maximum of K-eff decrease with the increase of soft phase components and the reduction of soft-grain sizes. The grain size dependence of K-eff is basically similar to that of coercivity. The decrement of coercivity for nanocomposite permanent materials is mainly due to the reduction of effective anisotropy.