The nucleation process and magnetic interaction in nanocrystalline alloy Nd4Fe77Co3B16

Abstract

The temperature dependence of coercivity of nanocrystalline Nd4Fe77Co3B16 is analyzed in this article. The best linear fit of HC(T)/Ms(T) vs 2K1(T)/μ0Ms2(T) indicates that HC is controlled by a nucleation process in the nanocrystalline two-phase magnet. The effects of additives (Hf, Ga) and magnetic field heat treatment on the microstructure and magnetic interaction are also studied. It is found that the condition of the grain shapes is improved after adding Hf and Ga, but the surfaces are deteriorated by some precipitates present on the boundary. Taking into account the values of α, Neff, the results of transmission electron microscopy (TEM) and Mossbauer spectroscopy, the magnetic field heat treatment not only induces grain refinement but also causes a uniform distribution of the soft and hard phases, and therefore it is a recommendable method for improving the condition of microstructure. From the analysis of high resolution TEM, amorphous phase with scale of <10 Å exists between the different phase’s grains due to the mismatch of their lattices. The existence of amorphous phase reduces the exchange-coupled interaction between the two magnetic phases and results in a relatively low coercivity in the nanocrystalline magnet. The magnetic interaction in Nd2Fe14B/Fe3B+αFe based nanocrystalline magnet was studied using δM plots. The results show that the exchange-coupled interaction is greatly enhanced in the sample annealed with magnetic heat treatment, achieving the highest energy product (BH)max=15.8 MGOe.