Cr添加Nd-Fe-B系ナノコンポジット磁石の磁気特性

Abstract

By means of 57Fe Mössbauer spectroscopy, X-ray diffraction and magnetization measurements, the magnetic properties and phase compositions of Cr-added Nd-Fe-B nanocrystalline composite magnets, Nd4.5(Fe1−xCrx)77B18.5(x=0, 0.013, 0.039, 0.065, 0.130, 0.260), have been investigated as a function of the Cr concentration. With increasing Cr content, the coercivity of the specimen increases, but the remanence decreases. In all the specimens, hard magnetic Nd2Fe14B crystallizes and its amount increases with increasing Cr concentration. Such an increase in the amount of the hard magnetic compound results in an increase of coercivity. The amount and kind of the soft magnetic phase formed varies with the Cr content. The specimen with low Cr content consists of both Nd2Fe14B and Fe3B. On the other hand, the specimen which contains a large amount of Cr consists of Nd2Fe14B-(Fe2B, α-Fe) nanocrystalline composite. From the Mössbauer measurements, it is found that the hyperfine magnetic fields of both Nd2Fe14B and α-Fe in the specimens are almost independent of the Cr content. The hyperfine magnetic field of the Fe-B compound decreases as a function of the Cr content, and consequently Fe2B in Nd4.5(Fe0.74Cr0.26)77B18.5 contains a large amount of Cr and becomes non-magnetic at room temperature. These results suggest that the Cr atoms mainly exist in the Fe-B compound, not in the Nd2Fe14B and α-Fe compounds, and reduce the magnetization and the Curie temperature of the Fe-B compound. Moreover, the total amount of the soft magnetic phases decreases with increasing Cr content. These are the origins of the decrease in remanence of Nd-Fe-B-Cr nanocrystalline composite magnets.